Marine Hydrogen Pressure Reducing Valves: A Review on Multi-Physics Coupling, Flow Dynamics, and Structural Optimization for Ship-Borne Storage Systems

Abstract

As a zero-carbon energy carrier, hydrogen is playing an increasingly vital role in the decarbonization of maritime transportation. The hydrogen pressure reducing valve (PRV) is a core component of ship-borne hydrogen storage systems, directly influencing the safety, efficiency, and reliability of hydrogen-powered vessels. However, the marine environment—characterized by persistent vibrations, salt spray corrosion, and temperature fluctuations—poses significant challenges to PRV performance, including material degradation, flow instability, and reduced operational lifespan. This review comprehensively summarizes and analyzes recent advances in the study of high-pressure hydrogen PRVs for marine applications, with a focus on transient flow dynamics, turbulence and compressible flow characteristics, multi-stage throttling strategies, and valve core geometric optimization. Through a systematic review of theoretical modeling, numerical simulations, and experimental studies, we identify key bottlenecks such as multi-physics coupling effects under extreme conditions and the lack of marine-adapted validation frameworks. Finally, we conducted a preliminary discussion on future research directions, covering aspects such as the construction of coupled multi-physics field models, the development of marine environment simulation experimental platforms, the research on new materials resistant to vibration and corrosion, and the establishment of a standardized testing system. This review aims to provide fundamental references and technical development ideas for the research and development of high-performance marine hydrogen pressure reducing valves, with the expectation of facilitating the safe and efficient application and promotion of hydrogen-powered shipping technology worldwide.

1. Introduction

Shipping, as the core transportation method for global trade, not only promotes economic development but also brings significant environmental challenges. The industry’s reliance on fossil fuels leads to substantial emissions of greenhouse gases and air pollutants, driving an urgent need for a clean energy transition [1,2,3]. Hydrogen, with its advantages of renewability, zero carbon emissions, high energy density, and high conversion efficiency, is widely regarded as a strategic direction for decarbonizing maritime transportation and achieving carbon neutrality goals [4,5,6,7]. This potential is being actively unlocked through international initiatives and demonstration projects, such as those by the European Union, the International Maritime Organization (IMO), and the U.S. Department of Energy [8,9,10,11,12,13]. The current status of international hydrogen energy application, as depicted in Figure 1, underscores the growing global momentum.

The transportation sector for hydrogen-based fuels is undergoing a multi-dimensional evolution. As shown in

Table 1. Designs of liquefied hydrogen tankers expected to be commercially applied before 2030. Source: IEA Hydrogen Projects database (October 2024).

Company	H2 Cargo Containment	Country	Approval in Principle	Volume (m2)
Korea Shipbuilding & Offshore Engineering, Hyundai Mipo Dockyard	Spherical	Korea	Korean Register of Shipping (KRS) DNV	20,000
Samsung Heavy Industries	Type C	Korea	ABS	20,000
Houlder, Shell, CB&I	Spherical	United Kingdom	DNV (H2 containment)	20,000
C-Job Naval Architects, LH2 Europe	Spherical	Netherlands	-	37,500
TotalEnergies, GTT, LMG Marin, Bureau Veritas	Membrane	France	Bureau Veritas	150,000
Kawasaki Heavy Industries (KHI)	Spherical (technological development completed)	Japan	Nippon Kaiji Kyokai (ClassNK)	160,000
Samsung Heavy Industries	Membrane	Korea	Lloyd’s Register	160,000
GasLog	NA	United States	NA	NA

, several designs for large-capacity liquid hydrogen tankers, employing different containment technologies (spherical, Type C, membrane), are expected to be commercially available before 2030, highlighting the ongoing technological development in this field [14,15,16,17]. To support the safe operation of these vessels, international institutions have taken the lead in introducing regulatory guidelines, such as the “Hydrogen Fuel Vessels Specification” issued by DNV [18].

The practical application of hydrogen energy in shipping hinges on overcoming several core technical hurdles, primarily the development of robust ship-borne fuel cell systems and high-pressure, lightweight hydrogen storage systems. The latter integrates critical components such as storage tanks, delivery pipelines, and pressure regulation devices. The entire design and certification process must adhere to stringent international standards, such as the IEC 62282 series, to ensure safety [19]. At the heart of this system is the high-pressure hydrogen PRV, a component whose performance directly determines the overall reliability, operational efficiency, and safety level of the hydrogen power system [20,21,22]. A PRV failure can lead to reduced fuel cell lifespan, efficiency loss, equipment damage, and pose a direct threat to ship navigation safety [23,24,25,26].

Figure 2 presents a technology roadmap for developing Marine hydrogen storage systems, outlining the evolution of key areas and defining phased goals for technological advancement. In line with the technology roadmap, this review will focus on the pivotal performance challenges for PRVs in high-pressure hydrogen systems, specifically in the domains of flow field control, structural optimization, and system integration. However, the development of PRVs for marine hydrogen systems faces severe challenges, which fundamentally stem from the unique and harsh operating environment of ships and are also constrained by the dual nature of hydrogen’s physical and chemical properties [27,28,29,30,31]. Factors such as continuous vibration, highly corrosive salt spray atmospheres, and temperature fluctuations interweave to form a series of complex and tightly coupled multi-physical field problems, directly threatening the working performance and service life of the valves.

Therefore, this review is dedicated to a systematic analysis of recent advances in marine high-pressure hydrogen PRVs. To gain a deeper understanding of these issues, this study will adopt a new perspective of multi-physics field coupling and will no longer be confined to the terrestrial environment. By synthesizing knowledge from theoretical modeling, numerical simulations, and experimental studies, this work aims to identify key bottlenecks and propose a foundational technical road-map. The goal is to support the development of high-performance, marine-adapted hydrogen pressure reducing valves, thereby accelerating the safe and efficient adoption of hydrogen energy in global shipping.

2. Current Challenges for Marine Hydrogen Pressure Reducing Valves

The development of high-performance PRVs for marine service is hindered by a series of interconnected challenges. These are not merely extensions of terrestrial problems but are fundamentally dictated by the synergistic combination of the marine environment and hydrogen’s intrinsic properties. This section delineates these primary challenges, framing them as inherent multi-physics phenomena that must be addressed for safe and reliable operation.

2.1. Synergistic Material Degradation: Hydrogen Embrittlement and Salt Spray Corrosion

The structural integrity of PRV components is severely threatened by the synergistic effect of hydrogen embrittlement and salt spray corrosion, a coupling phenomenon far more severe than in land-based systems. Hydrogen atoms diffuse into the metal lattice, reducing ductility and fracture toughness. Simultaneously, the chloride-laden marine atmosphere attacks protective surface layers, leading to pitting. These pits act as stress concentrators, dramatically accelerating crack initiation and propagation under cyclic loading, thereby drastically reducing component lifespan.

2.2. Weak Anti-Vibration Performance Under Persistent Marine Dynamics

Ship-borne PRVs are subjected to persistent, low-frequency vibrations originating from the main engine, auxiliary machinery, and wave-induced hull motions. These external excitations couple synergistically with the valve’s intrinsic transient phenomena, such as pressure surges during opening/closing cycles and flow instabilities. This can amplify pressure fluctuations, exacerbate valve core oscillations, accelerate mechanical fatigue, and potentially trigger high-risk phenomena like overpressure, directly threatening pressure stability and structural integrity.

2.3. Poor Temperature Adaptability and Joule–Thomson Effect

The temperature plunge during hydrogen decompression, primarily driven by the Joule–Thomson (J-T) effect, is particularly pronounced for cryogenic hydrogen operations where the J-T coefficient is positive, leading to strong cooling. This can readily cause component freezing, flow path blockage due to ice formation from ambient moisture, or embrittlement failure of sealing materials. Compounding this challenge, existing temperature compensation mechanisms are not yet perfected for the rapid and large temperature swings encountered in marine applications.

2.4. Issues with Dynamic Response and Pressure Stability Under Transient Loads

The dynamic operational profile of a ship, including frequent load variations in the fuel cell and the vessel’s own motions, demands a PRV with a faster and more robust dynamic response than typically required in stationary applications. Sluggish response or instability under these transient conditions can lead to pressure overshoots or undershoots, compromising fuel cell performance and system safety.

2.5. Insufficient Environmental Adaptability and Standardization Gaps

The highly corrosive salt spray atmosphere imposes stringent requirements on materials and coatings. Furthermore, standardized testing protocols that specifically replicate the combined effects of marine vibrations, corrosive atmosphere, and hydrogen service for PRVs are still lacking. International standards are gradually perfecting the relevant tests, but a comprehensive, marine-adapted validation framework is yet to be fully established.

In conclusion, overcoming these challenges requires understanding them not as isolated issues but as intercoupled multi-physics phenomena. The subsequent chapters of this review will systematically analyze existing research progress through this critical lens, focusing on how these fundamental challenges manifest in and are addressed through studies on flow dynamics and structural design.

3. Scope and Methodology

This study systematically reviews and summarizes recent advances in PRVs for marine hydrogen systems. To ensure systematic, transparent, and reproducible analysis, a structured literature screening and review approach is adopted. The literature search primarily utilizes academic databases such as Web of Science, Scopus, and Google Scholar, with a focus on core research from the past decade. Screening is conducted according to predefined criteria: the literature must be highly relevant to high-pressure hydrogen relief processes, especially addressing challenges specific to the marine environment; the research should address key issues such as transient flow and structural optimization; in terms of publication type, priority is given to original journal articles with clear methodologies, while theoretical modeling and experimental studies are also considered.

Based on this, an analytical framework has been constructed around five core themes—transient flow, turbulence and compressibility, parameter optimization, valve core geometry, and multi-stage throttling strategies—with the aim of systematically investigating the various technical challenges. Through a systematic assessment of these themes, this study aims to present a clear picture of current technological frontiers for researchers and engineers, identify limitations in existing research, and suggest future directions—specifically, the development of an integrated modeling and validation framework capable of addressing these interrelated factors.

4. Research on the Flow Field of Hydrogen Pressure Reducing Valves—Dynamics

This section systematically reviews the flow field dynamics within PRVs, structured around five core research themes: transient flow, turbulence and compressibility, parameter optimization, valve core geometry, and multi-stage throttling strategies. The discussion incorporates original synthesis and critical analysis to identify research gaps and propose future directions. Selected literature is evaluated based on its representativeness within each thematic area, methodological innovation—such as the use of advanced Computational Fluid Dynamics (CFD) techniques, experimental validation, or novel geometric designs—and relevance to addressing marine-specific challenges including vibration resistance, hydrogen embrittlement, and dynamic response under transient conditions. The aim is to offer a focused and in-depth overview of current research progress and technical trends in PRV flow field analysis, providing a solid foundation for further innovation and application in marine hydrogen systems.

4.1. Transient Flow Dynamics Under Marine-Induced Vibrations

The analysis of transient flow within PRVs is critical for safety and stability, given hydrogen’s significant flammable and explosive properties which impose strict requirements on system transient response capability [32,33,34,35]. In marine applications, this analysis takes on an added layer of complexity. Ship-borne PRVs are subjected to persistent, low-frequency vibrations from propulsion systems and wave-induced hull motions. These external excitations couple synergistically with the intrinsic transient phenomena of the valve itself—such as pressure surges during opening/closing cycles and flow instabilities during rapid load changes [36,37,38]. This coupling can amplify pressure fluctuations, exacerbate valve core oscillations, accelerate mechanical fatigue, and potentially trigger high-risk phenomena like overpressure and water hammer, thereby posing a direct threat to the pressure stability and structural integrity that are paramount for safe maritime operation.

Foundational research by Yin et al. [39,40,41] early on highlighted the importance of pressure feedback loops and two-stage pressure reduction, revealing supersonic hydrogen velocities. Internationally, studies on safety valve discharge [42], PRV oscillations at low flows [43], and dynamic simulation of regulating valves [36] provided valuable indirect insights. However, the applicability of these findings to the marine context is often limited by the assumption of a stable, non-vibrating base. For instance, Qian et al. [38] conducted a detailed dynamic characteristic analysis and valve core optimization design for the secondary hydrogen pressure reducer, as shown in Figure 3.

Their research indicates that the optimized valve core structure can reduce the amplitude of displacement oscillation and enable a more rapid and stable opening under transient flow conditions. However, a critical gap from a marine perspective is that these simulations were performed under land-based, static conditions. The efficacy of such optimizations remains unproven under the broad-spectrum, random vibrations characteristic of a ship’s engine room, where resonant frequencies could potentially destabilize the valve core in ways not predicted by their models.

Similarly, Wang et al. [44] provided valuable insights into the transient depressurization stages of a spring-loaded hydrogen valve (SHDV) using fluid–structure interaction (FSI) and dynamic mesh methods. Their study elegantly divided the process into rapid response, closing, and equilibrium stages, and investigated the effect of valve core angle on adverse pressure gradients, as shown in Figure 4.

In Figure 4, (A) shows the results of SHDV transient pressure reduction characteristics at different valve core angles, and (B) shows the evolution results of adverse pressure gradients at different valve core angles during the equilibrium stage. They clearly divided the transient depressurization process of SHDV into three typical stages: the rapid response stage, the closing stage, and the equilibrium stage. The study found that the change in the valve core angle significantly affects the pressure and flow fluctuation characteristics during the closing and equilibrium stages. Specifically, as the valve core angle decreases, the pressure downstream along the flow direction (y-direction) of the valve core gradually increases, thereby significantly reducing the overall adverse pressure gradient. This research elucidates the mechanism by which structural parameters affect the characteristics of the transient process stages.

Nevertheless, their FSI model idealized the structural damping and did not incorporate the actual vibration profile of a marine environment. Consequently, while the reported reduction in adverse pressure gradients with smaller valve core angles is insightful, its contribution to long-term pressure stability under real-world ship vibrations cannot be guaranteed.

Other scholars’ work further illuminates various facets of transient valve behavior but shares this common contextual limitation. The stability analysis of spring-loaded valves by Jian et al. [45] concludes that increasing spring stiffness can enhance stability, a principle that may require re-evaluation when the valve is simultaneously subjected to the low-frequency, high-amplitude motions induced by heavy seas. Ye et al. [46] found that increasing the valve core angle intensified the impact of high-pressure hydrogen flow on the valve core in a check valve, a finding that could be critically amplified under the dynamic tilting of a ship. The advantages of Large Eddy Simulation (LES) in accurately predicting turbulent structures, as shown by Taghinia [47] for a homogenizer valve, are clear for capturing transient details, yet its application to marine PRV dynamics under vibration is scarce. Studies on the steady-state and transient performance of other valve types [48], cavitation in water systems [49], and the development of constant pressure ratio valves [50] provide useful methodological references but do not bridge the gap to the marine operational environment.

In conclusion, while the body of work on transient flow provides essential methodologies and qualitative trends, there is a pronounced scarcity of studies that explicitly model or experimentally validate PRV performance under simulated marine vibration spectra. Those promising optimization strategies reported in the literature, such as significantly reduced displacement fluctuations, etc., must all be re-evaluated with ocean vibration as the core input parameter. Future research must prioritize the development of high-fidelity, coupled FSI models that incorporate real ship vibration data and experimental validation on shake tables to ensure that dynamic stability is achieved not just in the laboratory, but in the demanding reality of maritime service.

4.2. Turbulence, Compressible Flow and the Marine Acoustic Environment

The management of turbulence and compressible flow in marine hydrogen PRVs is paramount, as it directly governs two critical operational parameters in the shipboard environment: energy efficiency and acoustic signature. The high-Mach number flows, shock waves, and intense turbulent dissipation inherent in hydrogen decompression are significant sources of energy loss and broadband aerodynamic noise. This intrinsic valve noise can couple with the ship’s structural vibrations, potentially leading to unacceptable cabin noise levels for crew comfort and interfering with sensitive onboard equipment. Therefore, optimizing for low turbulence intensity and controlled compressible flow dynamics is not merely a performance enhancement but a necessity for maritime integration.

CFD studies have been instrumental in decoding these complex flows. Fundamental research on steady flow characteristics [51], Mach number distribution in multi-stage perforated plates [52], and transmission loss analysis [53] has laid the groundwork for understanding energy dissipation in throttling devices. For instance, Huovinen et al. [54] conducted numerical and experimental studies on throttle valve turbulence, while Modesti and Pirozzoli [55] analyzed Mach number effects in compressible channel flows, providing foundational insights into the flow regimes relevant to hydrogen decompression. Building on this, Chen et al. [56] provided a detailed parametric analysis of a multi-stage high-pressure reducing valve (MSHPRV), demonstrating how structural parameters like the number of perforated plates and their diameter influence Mach number distribution and turbulent dissipation rate (ε). Their finding that increasing the number of perforated plate stages promotes subsonic flow and reduces energy consumption is a key design insight. Complementary studies, such as the numerical analysis of pressure loss through perforated plates under turbulent conditions by Özahi [57], further validate the importance of geometric optimization for minimizing energy loss.

However, a pivotal limitation of these studies from a marine perspective is their confinement to idealized, non-corrosive environments. The research by Jin et al. [51] on a high multi-stage PRV for refueling stations, which analyzed the flow fields of both superheated steam and hydrogen, as shown in Figure 5.

Figure 5 reveals that there are significant similarities in their pressure-speed trends, indicating that it is feasible to conduct preliminary research using other fluids. However, this analogy is no longer applicable when it comes to the influence on the interaction between materials and surfaces. In the marine atmosphere, salt aerosol ingress and deposition can significantly alter the surface roughness of internal flow passages. As studies on complex transfer phenomena in structured reactors [58] and pressure loss in porous plates [57] imply, surface characteristics directly impact flow resistance and turbulence generation. This marine-induced roughening can trip boundary layer transitions earlier, modify vortex shedding dynamics, and ultimately alter the acoustic signature and turbulent dissipation rates from the CFD-predicted values obtained from smooth-walled models. Consequently, the energy-saving benefits attributed to an optimized multi-stage design may be partially eroded in long-term marine service.

The choice of turbulence modeling methodology itself introduces uncertainty for marine design. While Reynolds-Averaged Navier–Stokes (RANS) models, as used by Chen et al. [56] and Qian et al. [52], offer computational efficiency for parametric screening, they are known to smooth transient fluctuations and underestimate peak turbulent dissipation in shear layers. In contrast, Large Eddy Simulation (LES), as demonstrated by Taghinia [47] for a homogenizer valve and by Ingenito et al. [59] for turbulent hydrogen supersonic combustion, excels at capturing the coherent vortex structures and unsteady forces that are direct drivers of noise and vibration. The quantitative differences in predictions—such as RANS potentially underestimating peak Mach numbers by 15–20%—mean that a design optimized solely with RANS might overlook high-frequency acoustic modes that could become problematic when superimposed on the ship’s existing vibration spectrum.

Furthermore, the evaluation of performance often lacks the context of long-term marine operational cycles. Studies on the exergetic sustainability of high-pressure hydrogen gas compression [60] rightly highlight the importance of energy efficiency at a system level. However, the long-term evolution of this efficiency as valve internals corrode and surface roughness increases remains unquantified. Similarly, the analysis of three-dimensional incompressible turbulence in valves by An et al. [61] provides a robust methodology, but its assumption of incompressibility limits its direct applicability to the highly compressible hydrogen flows in PRVs. The assumption of steady-state operation in many analyses also fails to capture the transient shocks and flow instabilities that may be triggered by sudden ship motions, a phenomenon that transient-capable methods like LES are better suited to address.

In summary, the current understanding of turbulence and compressible flow in hydrogen PRVs, while advanced, is built upon a foundation that largely excludes marine environmental stressors. The critical gap lies in quantifying the synergistic effect of marine degradation (surface roughening due to corrosion) and operational vibration on the turbulent flow field and its acoustic and energetic byproducts. Future research must integrate material degradation models with high-fidelity transient CFD (e.g., LES) and validate these coupled models against experimental data obtained from valves subjected to combined flow, vibration, and accelerated salt spray testing to reliably predict performance and noise characteristics throughout a valve’s maritime lifecycle.

4.2.1. Multi-Physics Coupling Mechanisms

The complex flow inside the PRV is a typical multi-physics coupling problem, involving close interactions among fluid dynamics, structural mechanics, thermal effects, and material chemistry. To accurately predict the performance and lifespan of valves in harsh environments such as ships, it is necessary to deeply understand the inherent laws of these coupling mechanisms. Although the multi-physics coupling methods have been widely recognized in theory, there is still a significant gap between high-precision simulation and adequate experimental verification in practical engineering applications, resulting in many uncertainties in the existing models when predicting real working conditions.

The current research faces several major bottlenecks. Firstly, FSI simulations often struggle to converge due to the strong nonlinearity of transient hydrogen flow and the resulting structural vibrations. For instance, although Wang et al. [44] used dynamic mesh technology to simulate the valve core response, their model’s prediction of fatigue risks under marine conditions remains unreliable because they did not incorporate actual ship vibration data. Secondly, in the field of thermal-fluid-solid coupling, although Jin et al. [51] successfully predicted the flow field distribution within a multi-stage depressurization device through numerical simulation, the lack of actual measurements of the valve inner wall temperature makes it impossible to verify whether the local low temperatures caused by gas expansion would lead to the embrittlement of sealing materials or harmful thermal stress, highlighting the disconnection between simulation predictions and experimental validation.

The existing literature on multi-stage throttling structures generally adopts a series of simplified assumptions, which limit the predictive accuracy of the models in the marine environment. Most studies use steady-state simulations to optimize geometric parameters, but they ignore the pressure shock and flow instability caused by valve opening and closing or ship movement. Structural analysis often assumes the valve body to be rigid, failing to consider the mutual feedback between unsteady flow forces and structural vibration, thereby masking the fatigue risks that may be induced by vortex-induced vibration at a specific flow rate. Thermal boundary conditions are often simplified to adiabatic walls or simple convection, underestimating the local low temperature caused by the Joule–Thomson effect, while the assumption of single-phase flow completely ignores the risk of blockage and erosion caused by ice particles formed by water vapor condensation and freezing in a high-humidity marine environment. These simplifications collectively lead to overly optimistic estimates of valve performance and significantly underestimate the failure risk of the valve in a real marine hydrogen system.

Previous studies have quantitatively revealed the potential errors of these simplifications through specific cases. Beune et al. [42] found in their CFD simulation of a 600 bar safety valve that the standard k–ε turbulence model seriously underestimated the position and intensity of the shock wave inside the valve, and the predicted emission coefficient deviated from the experimental value by up to 15–20%, mainly due to the model’s insufficient ability to predict the interaction between shock waves and boundary layers and flow separation. Huovinen et al. [54]’s research compared the performance of RANS and LES in throttling valves, pointing out that RANS can capture the average flow field but completely misses the key transient vortex structures that cause broadband noise and high-frequency structural loads, and its predicted pulsating pressure amplitude is one order of magnitude lower than the LES results. The Jin team’s case further demonstrated that models using a steady-state single-phase flow assumption cannot reproduce the intermittent flow blockage phenomenon observed in subsequent experiments caused by water vapor freezing due to J-T cooling, this “functional failure” risk caused by ignoring multiphase transitions has become a major blind spot in the prediction of the reliability of actual systems by numerical models.

At present, several multi-physics challenges related to the PRVs of ship hydrogen systems have not been effectively addressed, and it is difficult to quantitatively predict them. These issues include: the interaction between transient shock waves and boundary layers under ship dynamics conditions and its impact on flow separation and structural loads; the local low temperature caused by J-T cooling effect and the thermal-mechanical fatigue environment generated by the superimposition of ship high-frequency mechanical vibration; the synergistic material degradation mechanism under the combined action of tensile stress, hydrogen diffusion, and salt spray electrolyte; and the solid–gas two-phase flow behavior caused by ice particle formation during transient operation and its blocking and erosion risks.

To address these challenges, future research needs to be carried out simultaneously at multiple levels. In terms of modeling theory, a coupled theoretical framework capable of simultaneously describing transient flow, structural dynamics, temperature field evolution, and hydrogen-induced material damage should be developed, with particular attention to the bidirectional feedback mechanism among the physical fields. In terms of numerical methods, high-fidelity turbulence models and multi-scale methods should be combined to accurately capture complex phenomena such as shock/boundary layer interaction, vortex shedding, and energy dissipation in high-pressure and high-Mach hydrogen flow. At the engineering design level, emphasis should be placed on the collaborative optimization of materials and structures, such as using anti-hydrogen embrittlement alloys, developing adaptive sealing materials, and considering vibration and thermal stress distribution in the structural layout to enhance the reliability and durability of the valves under extreme conditions. In summary, by integrating multi-physics modeling, high-performance numerical simulation, and material-structure innovative design, the overall performance and service life of high-pressure hydrogen pressure reducers in complex coupled environments can be significantly improved.

4.2.2. Comparison of Turbulence Modeling Methods and Their Impact on Prediction Results

Current research indicates that the choice of turbulence simulation methods—primarily between RANS and LES—fundamentally influences prediction results and limits the comparability of outcomes across different studies. This discrepancy stems from their distinct approaches to handling turbulence, particularly in simulating key flow phenomena in safety valves, such as shock/boundary layer interaction, transient vortex shedding, and turbulent energy dissipation.

Chen et al. [56] and Qian et al. [52] adopted RANS models (e.g., RNG k–ε, Realizable k–ε, SST k–ω) for their computational efficiency. However, these models inherently smooth transient fluctuations and blur sharp gradients due to their reliance on isotropic turbulence assumptions. Quantitative comparisons with high-fidelity methods such as LES reveal that RANS models may underestimate the peak Mach number downstream of throttling elements by approximately 15–20%, and the turbulent dissipation rate (ε) in highly unsteady regions (e.g., shear layers and recirculation zones) by up to an order of magnitude. These discrepancies stem from the inability of RANS to resolve large-scale, energy-carrying vortices responsible for dominant dissipation mechanisms. Relevant studies provide specific quantitative evidence: for instance, although Qian et al. [52] analyzed Mach number distributions using RANS in their study of multi-stage orifice plates, a comparative study revealed that such models may underestimate the peak Mach number downstream of the throttling element by 15–20% compared to high-resolution simulations like LES, due to the smoothing of sharp gradients at shock waves. In high-Mach-number hydrogen flows, this typically manifests as underestimation of the peak Mach number downstream of the throttling element, along with significant underestimation of the turbulent dissipation rate in shear layers and recirculation zones. Specifically, Chen et al. [56] reported turbulent dissipation rates in their multi-stage valve study using the RNG k–ε model; however, comparisons with high-fidelity methods generally show that RANS considerably underestimates the value of ε in critical regions such as shear layers—by up to an order of magnitude in highly unsteady regions—because it fails to resolve large-scale, energy-carrying vortices responsible for the main dissipation. Consequently, RANS predictions of unsteady fluid forces and noise acting on the valve core, as well as assessments of fatigue or material failure risks, may represent only lower-bound estimates. Designs optimized solely based on RANS may overlook key high-frequency vibration modes or local energy dissipation hotspots, thereby increasing the risk of fatigue or material failure.

In contrast, LES directly resolves large-scale, energy-carrying vortices and only models small-scale and subgrid-scale motions. As demonstrated by Taghinia [47] in a study of a high-pressure homogenizing valve, LES excels at capturing complex details of transient dynamics, coherent vortex structures, and shock/wake interactions. This high-fidelity approach typically predicts higher and more accurate maximum Mach numbers. The 15–20% discrepancy in peak Mach number predictions between RANS and LES, as mentioned earlier, is a typical quantitative example in compressible flow shock comparison studies. Moreover, LES reveals a more complex and spatially inhomogeneous distribution of turbulent dissipation, enabling accurate capture of intense dissipation in local vortex structures. The ability of LES to resolve the turbulent time spectrum makes it indispensable for predicting flow-induced vibrations, dynamic structural responses, and aerodynamic noise spectral characteristics. However, this high precision comes at the cost of extremely high computational resources, requiring very fine temporal and spatial resolution. For instance, a fully resolved LES of a complex internal flow field may require one to three orders of magnitude more computational resources (CPU time and memory) than a steady-state RANS simulation of the same geometry. Additionally, the accuracy of LES is sensitive to the choice of subgrid-scale models, especially in near-wall regions where the grid resolution required to capture small-scale structures approaches that of direct numerical simulation (DNS).

High computational costs often compel researchers to introduce additional simplifying assumptions when studying complex multi-stage geometries. For example, in Taghinia et al. [48] analysis of a high-pressure homogenizer valve, the use of RANS under the isotropic turbulence assumption may lead to an overestimation of the turbulent dissipation rate in the shear layer by up to an order of magnitude—compared to what a more accurate LES would reveal. This discrepancy directly affects the accuracy of noise and vibration predictions. Therefore, design decisions based solely on the optimization of parameters such as the optimal number of stages or orifice size may fail to effectively suppress high-frequency noise and structural resonance under actual ship conditions with significant unsteady flow.

These methodological differences directly impact the credibility and applicability of simulation results in design optimization. For instance, Chen et al. [56] and Qian et al. [52] conducted studies that utilized RANS models (such as RNG k–ε) to optimize multi-stage throttling structures. However, comparative studies with high-resolution methods (such as LES) demonstrated that due to RANS over-smoothing of sharp gradients at shock waves, it typically underestimates the peak Mach number downstream of the throttling elements by approximately 15–20%. More importantly, in highly unsteady regions such as shear layers and recirculation zones, the predicted values of turbulent dissipation rate ε by RANS may be underestimated by an order of magnitude, as it cannot resolve the large-scale, energy-carrying vortices responsible for the main energy dissipation. Therefore, designs based on RANS optimization may overlook critical high-frequency vibration modes or local energy dissipation ‘hotspots’, thereby underestimating the risk of fatigue or material failure. In contrast, Taghinia [47] conducted a LES study on the high-pressure homogenizer valve and demonstrated that this method can precisely capture the complex details of transient dynamics, coherent vortex structures, and shock/wake interactions. LES is typically capable of predicting higher and more accurate maximum Mach numbers. The aforementioned 15–20% difference in peak Mach number prediction between RANS and LES is a typical quantitative example in compressible flow shock comparison studies. Moreover, LES reveals a more complex and spatially non-uniform turbulent dissipation distribution, thereby enabling accurate capture of intense dissipation within local vortex structures.

Therefore, when interpreting any CFD study, it is necessary to conduct a background analysis considering its methodological limitations. When converting simulation results into design guidelines, it is essential to clearly define the applicable scope and impact of the selected turbulence model. Differences between studies may not necessarily reflect errors but often indicate the different problems addressed by each method: RANS is suitable for efficient analysis of mean performance, while LES is appropriate for high-fidelity studies of transient and acoustic phenomena. Future research should give priority to using high-precision experimental data (such as particle image velocimetry measurements under typical pressure conditions or laser-induced fluorescence) for rigorous verification, in order to quantify these differences and establish the optimal practice plan. Until such validations are complete, cross-comparisons between studies should be conducted cautiously, focusing on consistency in trends rather than absolute values.

4.2.3. Equation of State Selection and Joule–Thomson Effect

For CFD analysis of marine high-pressure hydrogen pressure relief valves (PRVs)—especially those handling cryogenic liquid hydrogen—the choice of an accurate Equation of State (EOS) is essential for reliable prediction of thermophysical properties. Hydrogen exhibits strong real-gas behavior near its critical point (T_c = 33.18 K, P_c = 1.315 MPa) and under cryogenic conditions. Under such states, applying ideal gas laws or simplistic cubic EOS (e.g., van der Waals) can lead to significant errors in predicting key properties such as density, enthalpy, and, crucially, theJ-T coefficient (μ_JT = (∂T/∂P)_H).

The J-T coefficient determines the direction of temperature change during isenthalpic throttling. Hydrogen has a distinctive J-T inversion curve: at ambient temperatures (typically above ~200 K) and moderate pressures, μ_JT is negative, causing temperature rise upon expansion. In contrast, at cryogenic temperatures (e.g., in the liquid hydrogen regime), μ_JT becomes positive, resulting in significant temperature drop. Accurately capturing this sign reversal is vital for identifying risks such as localized cryogenic temperatures, ice formation, frost buildup, and potential auto-refrigeration-induced boil-off.

An inaccurate EOS—such as a standard Peng–Robinson model not calibrated for hydrogen’s specific behavior—can misrepresent the J-T inversion curve and the magnitude of μ_JT. This may cause fundamental errors in thermal field prediction: underestimating cooling at low temperatures can obscure dangers such as valve core or seal embrittlement and blockages, while erroneous heating predictions may overlook material degradation risks in other areas.

Thus, to ensure simulation fidelity, future CFD studies should adopt high-precision EOS such as:

(1)

Multi-parameter helmholtz free energy formulations implemented in the NIST REFPROP database, which provide high accuracy across a wide range of temperatures and pressures, including the liquid hydrogen domain.

(2)

Specially calibrated cubic EOS with parameters rigorously optimized for hydrogen.

Only with a validated, high-fidelity EOS can CFD models reliably predict J-T-induced low-temperature zones, accurately locating potential freezing points, frost formation, and flash evaporation risks when local temperatures fall below the saturation curve. This forms an indispensable theoretical basis for the safe design and material selection of marine hydrogen systems under transient conditions (e.g., startup or sudden load changes). Future research should explicitly report the EOS used and its validation process, establishing this as a fundamental prerequisite for ensuring the reliability of CFD results.

4.3. Parameter Analysis and Optimization for Robust Marine Operation

Parameter optimization for marine hydrogen PRVs must transcend the pursuit of peak efficiency under static conditions and instead prioritize performance robustness and stability under dynamic seafaring loads. The challenges of hydrogen storage and transportation, including the need for precise control and compatibility [62], are amplified in the marine context. The geometric and operational parameters that govern valve performance—such as orifice diameters, spring stiffness, and valve core angles—are typically optimized based on deterministic simulations and steady-state experiments [63,64]. However, the marine environment introduces significant stochasticity: persistent vessel vibrations can cause micromovements in assembled components, altering effective clearances; fluctuating thermal conditions can change material properties; and system-level demands require careful parameter matching [65]. An optimization that is optimal in a land-based lab may thus become suboptimal or even unstable when deployed at sea, where parameters effectively operate within a range of uncertainty.

Existing parametric studies provide an invaluable database and demonstrate clear influence trends, forming a crucial foundation for any design process. The work of Hou et al. [66] and Chen et al. [67] systematically delineates how parameters like the relative angle of porous covers, the number of orifice plates, and their diameters affect internal velocity fields, throttling characteristics, and energy consumption. Similarly, studies on labyrinth-type regulators (π-HPRV) by Li et al. [68] and conical throttling valves by Yu et al. [64] quantitatively linked structural parameters to flow uniformity and temperature distribution. Beyond specific valve components, research on system-level parameter matching, such as the interaction between blowers and back-pressure valves in fuel cell vehicles [69] and the optimization of pressure regulation in water networks [70,71,72,73], provides valuable interdisciplinary insights into the importance of integrated parameter tuning. The broader context of hydrogen infrastructure development and the multi-criteria decision-making involved in its storage and transport [74] further underscore the systemic importance of parameter optimization.

However, from the perspective of marine engineering, these studies share a key flaw: their optimization processes did not take the ship’s vibration spectrum and corrosive environment as core input variables. For instance, the optimal perforated plate diameter determined by Chen et al. [67] was obtained under static conditions. It can be inferred that at specific vibration frequencies, this geometry might couple with the acoustic modes of the valve cavity, intensifying noise or triggering resonance, a phenomenon that cannot be captured in the standard optimization process. Similarly, studies on leakage quantification [75] and sealing performance under extreme temperatures have revealed the sensitivity of seals to gap and material properties—parameters that are directly affected by marine vibrations and thermal cycling disturbances but are often treated as fixed in land-based optimization.

This limitation highlights a deeper methodological gap. Most parametric CFD studies lack systematic uncertainty quantification (UQ) and global sensitivity analysis (GSA). Although we based our deterministic simulation data on the aforementioned literature and summarized the predicted uncertainty intervals of outlet pressure and temperature caused by changes in key parameters in

Table 2. Overview of the Prediction Uncertainty Range of Hydrogen Pressure Reducing Valve Outlet Pressure and Temperature in Response to Changes in Key Parameters.

Key Parameter	Parameter Variation Range	Impact on Export Pressure (Range of Uncertainty)	Influence on Outlet Temperature (Uncertainty Range)	Main Sources of Literature	Remarks Content
inlet pressure	10–70 Mpa	±(0.5–1.2) Mpa	±(2–8) K	[43,51,67]	The J-T effect is prominent under high pressure, and the temperature fluctuation is significant.
Valve core opening degree	20–100%	±(0.3–0.8) Mpa	±(1–5) K	[44,77,78]	At small opening degrees, the flow field becomes unstable and the pressure fluctuations intensify.
inlet temperature	253–313 K	±(0.1–0.4) Mpa	±(3–10) K	[51,68]	The J-T cooling effect is stronger at low temperatures.
Diameter of the porous plate	2–6 mm	±(0.2–0.6) Mpa	±(1–4) K	[66,67]	The smaller the aperture, the greater the pressure drop and the more significant the temperature drop.
Valve core cone angle	30–60°	±(0.4–1.0) Mpa	±(2–6) K	[44,77]	Small angles are prone to causing turbulence and have poor pressure stability.

, the inputs in the real marine environment (such as the imported pressure affected by oscillation) are inherently uncertain. This requires the use of UQ techniques such as Monte Carlo simulation or polynomial chaos expansion to quantify how input uncertainties are transmitted to key outputs, as suggested in the broader energy system assessment methods [76]. Additionally, GSA (such as using Sobol indices) can identify which parameters dominate the performance fluctuations under marine environment disturbances, enabling designers to focus on strengthening these key aspects, which is a missing, crucial step related to reliability in current PRV parameter research.

As a preliminary attempt in this direction, this study, based on numerical simulations and experimental data from the literature, summarizes in Table 2 the predicted uncertainty intervals of key parameters within typical variation ranges for the outlet pressure and temperature, aiming to provide intuitive references for engineering applications.

Sensitivity Analysis of Key Parameters on the Outlet Pressure and Temperature

As shown in Table 2, the change in inlet pressure has the most significant impact on the outlet state. This is mainly due to the intense Joule-Thomson effect and turbulent dissipation during the throttling process of high-pressure hydrogen gas. This quantitative summary not only reveals the sensitivity levels of different parameters, but also points out the key direction for robust optimization in marine environments—for example, when facing continuously changing working conditions and external stimuli, a multi-stage throttling strategy should be prioritized to distribute the pressure drop, thereby keeping the single-stage temperature drop and pressure fluctuation within safe thresholds.

Future parameter studies should also incorporate insights from new materials and extreme conditions. Research on the hydrogen adsorption properties of advanced alloys and hydrogen storage reactor engineering [77,79] points to the next frontier: the collaborative optimization of structure and materials to address hydrogen embrittlement and corrosion. Similarly, research on the safety and emission strategies of hydrogen explosions under fire conditions warns us that the parameters optimized for normal conditions must also be evaluated for safety under fault conditions, which is of crucial importance for ships.

In conclusion, the future direction of optimizing the parameters of marine relief valves will shift from a deterministic design paradigm to a robustness and reliability-oriented design. This requires integrating the marine environmental loads directly into the CFD and FSI simulations used for optimization, and applying the UQ and GSA frameworks to ensure that the final design not only performs well at a single design point, but also maintains performance and safety throughout the entire expected operating range of the vessel, thus truly addressing the challenges and decision complexity emphasized in the literature.

4.4. Valve Core Geometry Effects in a Corrosive and Erosive Marine Setting

The valve core, as the primary actuating element, dictates the flow path and energy dissipation mode during throttling. Its geometric optimization is therefore paramount for achieving stable pressure reduction and minimal flow resistance. In marine applications, however, the discussion of valve core geometry cannot be divorced from the long-term integrity of its precise geometrical features under the synergistic assault of hydrogen embrittlement and salt spray corrosion. The performance benefits of an optimized shape are contingent upon the maintenance of that geometry throughout the valve’s service life, a period characterized by a highly aggressive operational environment.

Land-based studies have effectively demonstrated that subtle geometric changes can yield significant performance benefits. Yu et al. [64] convincingly showed that the cone angle of a throttling valve significantly affects hydrogen flow patterns and temperature distribution, with smaller angles inducing strong turbulence. Ye et al. [78] conducted a comparative analysis of three common valve core shapes (straight-edge type, arc-edge type, and flat-bottom type), as shown in Figure 6. They concluded that the arc-edge conical valve core produced the smallest pressure gradient and the weakest vortex, highlighting the crucial importance of micro-rounding treatment. Wang et al. [44] further extended this by optimizing a spring-loaded valve core with a convex cylindrical feature, which effectively improved dynamic response and pressure stability. These studies collectively underscore the profound impact of valve core geometry on flow field quality and transient behavior.

However, the critical, unaddressed question for marine deployment is durability. The performance advantages of a specific cone angle or a meticulously rounded edge are contingent on the maintenance of that geometry. The marine salt spray environment is inherently corrosive and erosive. Over time, the continuous impingement of high-velocity hydrogen, potentially carrying microscopic salt aerosols, can lead to erosion-corrosion of the valve core and seat surfaces. This wear will inevitably alter the carefully optimized profiles, blunting sharp edges, increasing surface roughness, and modifying clearances. The consequence is a gradual but inevitable degradation of performance: increased leakage past the valve core, loss of pressure regulation accuracy, and a potential shift in flow-induced vibration characteristics. The literature’s promising optimizations, while valid for initial performance, present a potentially optimistic picture of long-term operational stability because they do not account for this geometric degradation.

This challenge is compounded by the risk of hydrogen embrittlement. The synergistic effect of tensile stresses (from pressure and vibration) and hydrogen atom diffusion can lead to sub-critical crack growth in susceptible materials. A micro-crack initiated at a geometrically sensitive stress concentration point (e.g., an imperfectly rounded edge) can propagate, leading to sudden brittle fracture rather than gradual wear. The studies reviewed, while highlighting geometric performance, typically do not couple their flow-stress analyses with hydrogen diffusion and material degradation models. Research on materials, such as the hydrogen absorption behavior of titanium aluminides [80] and the plasma nitriding of stainless steel for valve cores, points to the material-level solutions, but does not integrate them with the geometric design. Therefore, a geometry that appears optimal from a pure fluid dynamics perspective might inadvertently create a stress concentration that becomes a preferred site for HE-induced failure in the marine environment.

Looking beyond static geometry, the principles of dynamic control and system integration explored in other contexts are highly relevant. The decentralized control strategies for pressure swing adsorption [81] and the dynamic behavior analysis of control valves in fluid systems [82] underscore the potential for intelligent, adaptive valve cores. A static, optimized geometry may not be optimal across all ship operating conditions (e.g., varying load due to weather). The integration of real-time sensors and control algorithms, hinted at in systems for carbon deposit removal [83], could enable dynamic adjustment of the effective flow area, compensating for performance drift due to wear. Furthermore, the performance of system components like valves can have cascading effects, as seen in studies on valve-regulated batteries in renewable systems [84] and the analysis of atomizer geometries [85], reinforcing that valve core performance is not isolated but critical to overall system efficiency and safety. Studies on spontaneous ignition from diaphragm rupture [86] also serve as a stark reminder that geometric details at the sealing interface are paramount for safety, a concern that is magnified by corrosive wear in marine environments. Complementing safety considerations, recent investigations into J-T characteristics and thermal management [87,88,89] provide critical insights into the temperature evolution and flow dynamics during hydrogen decompression, which are essential for predicting cryogenic risks and ensuring material compatibility in marine PRVs.

In light of this, future research on valve core geometry must adopt a co-design philosophy, integrating materials science, control engineering, and fluid dynamics. The focus should shift from identifying a single, statically optimal geometry to developing geometries and systems that are:

(1)

Robust to Performance Degradation: Shapes that are less sensitive to minor geometric changes caused by wear.

(2)

Resistant to Failure Initiation: Designs that minimize stress concentrations and are conceived from the outset for compatibility with hydrogen embrittlement-resistant alloys and advanced surface coatings.

(3)

Adaptive and Intelligent: Exploring designs that can integrate with control systems to maintain optimal performance despite geometric changes or varying operational demands.

Only through such a holistic approach can the initial performance benefits of geometric optimization be sustained throughout the demanding service life of a marine hydrogen PRV.

However, the critical, unaddressed question for marine deployment is durability under synergistic corrosion and erosion. The following Section 4.4.1 provides a dedicated, in-depth analysis of these degradation mechanisms and a reassessment of the long-term feasibility of the geometric optimizations discussed herein.

4.4.1. Degradation Mechanisms of Precision Engineered Surfaces in Marine Environments and Long-Term Feasibility of Geometric Optimization

In order to accurately emphasize the “marine priority” perspective, this section clearly elaborates on how the synergistic effect of salt fog corrosion and high-speed hydrogen erosion degrades the surface of the precise engineering on the valve core, and critically assesses the long-term feasibility of the geometric optimization proposed in the literature.

In the marine environment, chloride ions can penetrate the passivated oxide layer and cause pitting corrosion. These pit corrosion areas, under the cyclic loading caused by vibration and pressure fluctuations, act as stress concentration points and may expand into stress corrosion cracking, thereby sharply reducing the fatigue life. At the same time, the high-speed flowing hydrogen gas (which may carry entrained salt spray aerosols) mechanically wears away the protective oxide layer, constantly exposing fresh metal to corrosive attacks. This synergistic effect of erosion and corrosion accelerates material loss and surface roughening. Moreover, the marine environment exacerbates the hydrogen embrittlement problem. The corrosion reaction on the surface generates active hydrogen atoms, which are more likely to diffuse into the metal lattice at the plastic deformation sites caused by the erosion impact.

The aforementioned surface degradation mechanisms directly affect the performance of geometric optimization. Optimization schemes such as the arc-edge design proposed by Ye et al. [78] to reduce eddies, or the convex cylindrical features proposed by Wang et al. [44] to improve dynamic response, their performance advantages essentially rely on the preservation of their precise geometric features. The marine degradation mechanisms lead to: the gradual blunting of key edges, which are crucial for controlling flow separation and eddy generation; an increase in surface roughness, which significantly increases turbulent surface friction and may prematurely trigger boundary layer transition, thereby changing the flow coefficient and noise characteristics of the valve; and erosion and wear at the interface between the valve core and the valve seat will increase leakage flow, reduce pressure regulation accuracy and damage the closing ability.

By re-evaluating the previously reviewed geometric optimization from the perspective of marine durability, significant concerns can be identified. For instance, the micro-rounded edge design is optimal in its initial state, but its thin and smooth lip is prone to rapid wear. A design optimized for a 50-micron sharp radius may lose its performance advantage when the radius erodes to 200 microns, making it less feasible and requiring the use of materials with excellent corrosion resistance. Complex profiles, such as convex cylindrical features, in the areas of new valves that create favorable pressure gradients, may become locations for local pitting and crack initiation. Their feasibility is conditional and depends on whether they are combined with anti-hydrocracking substrates and a robust protective coating. Multi-stage labyrinth flow channels, due to their numerous contractions and flow changes, become hotspots for erosion. Their performance degradation may not be gradual but occur rapidly upon the failure of a key edge, making their feasibility moderate to low, and highly dependent on manufacturing quality and material selection.

Therefore, future geometric optimization shoud shift from seeking a single, static optimal solution to developing degradable and adaptive geometric shapes. This includes: adopting robust geometric design, prioritizing shapes that are insensitive to minor geometric variations in performance, such as wider and more gradual contours may be more robust than sharp and complex features; promoting the collaborative design of geometry with materials and coatings, the geometric shape must consider compatibility with marine-grade materials and advanced coatings applied through technologies such as supersonic flame spraying or physical vapor deposition; and exploring designs with in-service adaptability, capable of integrating with real-time control systems to adjust the effective flow area and compensate for performance drift caused by wear.

In conclusion, although the research based on the terrestrial environment has provided valuable insights into the fluid dynamics benefits of optimizing the core geometry of valves, directly applying this to marine conditions poses risks. The “marine priority” assessment conducted here indicates that unless many of the optimized geometries are explicitly designed in coordination with material selection and surface engineering strategies to withstand the relentless coupled degradation of the marine environment, their long-term feasibility is questionable. This shift in perspective is crucial for the development of pressure regulating valves that not only perform well initially but also remain reliable throughout the entire service life of the vessel.

4.5. Multi-Stage Throttling Strategies: Weighing Performance Against Marine Reliability

Multi-stage throttling is a cornerstone strategy for managing the substantial enthalpy and achieving controlled decompression of high-pressure hydrogen. For marine applications, the selection of an appropriate strategy involves a critical trade-off between depressurization performance and inherent reliability under persistent vibration, corrosion, and the unique thermodynamic behavior of hydrogen in marine conditions. The shipboard environment places a premium on simplicity, robustness, and minimal maintenance, as component failure or performance decay can have severe operational consequences far from support facilities. Furthermore, the accurate prediction of temperature evolution, governed by the J-T effect and flow dynamics, is critical to avoid risks ranging from material embrittlement to ice formation. The following are analyses of several mainstream multi-stage throttling structures:

4.5.1. Porous Plate Structures

The porous plate structure, due to its adjustable flow area and relatively simple manufacturing process, has become a common choice in multi-stage pressure reduction systems. The research by Hou et al. [66] indicates that using a single-layer porous plate and optimizing the pore arrangement (such as 180° relative angle) can effectively reduce turbulent dissipation and stabilize the flow field. This type of structure performs well under medium pressure differences and flow rates but has limited adaptability to extreme conditions and is prone to fatigue failure of the porous plate due to stress concentration under high pressure differences.

4.5.2. Maze Channel-Type Throttling

Maze channel-type throttling (such as the π-HPRV proposed by Li et al. [68]) achieves stepwise pressure reduction by extending the flow path and increasing the flow resistance. This structure performs exceptionally well in suppressing vortices and homogenizing the flow field and is particularly suitable for applications with high pressure differences and low flow rates. However, its flow channels are complex, require high machining accuracy, and have a large flow resistance, which may lead to an increase in system pressure loss and pose challenges to the response capability under frequent operating condition variations in ships.

4.5.3. Tesla Valve Structures

The Tesla valve structure (Qian et al. [90], Zhang et al. [91]) demonstrates outstanding energy dissipation control capabilities during depressurization, thanks to its feature of no moving parts and high reverse flow resistance. It is particularly suitable for ship scenarios that are sensitive to vibration and require high reliability. However, its depressurization efficiency is highly dependent on the number of stages and the geometric configuration of the flow channels. In large flow conditions, shock waves within the flow channels are prone to generate significant noise and vibration.

4.5.4. Combined Multi-Stage Structures

The combined multi-stage throttling structure (such as the porous sleeve and valve core combination proposed by Chen et al. [67,92]) integrates the advantages of various throttling elements and can achieve stable pressure regulation within a wide range of operating conditions. This type of structure demonstrates outstanding performance in terms of design flexibility and robustness, but it also brings about issues such as complex structure, high integration difficulty, and increased cost.

The following

Table 3. Comparative Analysis of Multi-level Throttling Strategies.

Throttling Structure Type	Main Advantages	Applicable Scenarios/Conditions	Potential Limitations
Porous plate structure	Simple structure, easy to process, low cost	Medium pressure difference and stable flow rate	Fatigue under high pressure differences, poor adaptability to extreme working conditions
Maze passage style	Flow field uniform, suppression effect good, pressure reduction stable	High pressure difference and low flow rate environment	Flow channel complex, processing requirements high, and the flow resistance large
Tesla valve structure	No moving parts, strong anti-vibration, high reliability	Vibration-sensitive scenarios requiring high reliability	Pressure reduction efficiency under the number of stages and the geometric shape, and noise prone under large flow conditions
Combined structure	Robust performance, strong adaptability, high control accuracy	Complex and variable working conditions	Complex structure, high integration difficulty and high manufacturing cost

systematically compares the key characteristics of the above four mainstream multi-level throttling strategies:

The comparative analysis in Table 3 outlines the inherent trade-offs associated with each throttling strategy. When evaluating these strategies for ship-borne applications, additional marine-specific factors must be carefully considered. Structures featuring moving parts or complex internal geometries—such as combined throttling assemblies—may be more prone to performance degradation under sustained vibration and highly corrosive maritime conditions. In contrast, robust designs without moving parts, like the Tesla valve, are often favored for their reliability, though they may entail greater size and weight—a significant drawback in the context of ship lightweighting objectives. Consequently, there is a pressing need to develop a systematic evaluation framework that integrates marine environmental factors to effectively guide the selection and optimization of multi-stage throttling strategies for pressure relief valves in ship-borne systems.

In other aspects of the hydrogen energy system, multi-stage strategies and flow channel designs have also been widely investigated. Filina et al. [93] studied on-site hydrogen production for internal combustion engine vehicles, a process that may involve multiple reaction or separation stages. Qian et al. [90] analyzed hydrogen decompression using multi-stage Tesla valves for hydrogen fuel cell applications. Zhang et al. [91] conducted a numerical analysis on the behavior and design of a novel multi-stage hydrogen PRV. Chen and Jin [92] examined the influence of porous plates on hydrogen flow within an L-shaped high-pressure relief valve. Chen and Jiang [94] analyzed the Mach number of hydrogen flow in labyrinth channels under high-pressure gradients. Li et al. [95] investigated the pressure flow characteristics and temperature variations in a new type of hydrogen pressure regulator. These studies contribute to optimizing flow uniformity and enhancing overall system performance.

Although multi-level throttling strategies have achieved remarkable results in improving the depressurization performance, there is still a lack of systematic comparative and optimization studies for the special working conditions of ships (such as continuous vibration, salt spray environment, wide fluctuations in flow rate, etc.). Therefore, the future development direction should not be limited to the optimization of a single strategy but should focus on building a systematic comprehensive evaluation system for marine application scenarios. This system needs to establish a multi-criteria decision analysis framework, while paying attention to depressurization efficiency and energy loss, and assigning weight scores to key marine attributes such as vibration tolerance, corrosion protection capability, multiphase flow adaptability, maintenance convenience, and weight and volume. Through the in-depth insights from fluid and thermal characteristic research, this evaluation system can provide scientific basis for the selection of multi-level throttling schemes for ship hydrogen systems, thereby ensuring the depressurization performance while meeting the strict requirements of the marine environment for the robust reliability of the system, achieving the optimal balance between depressurization efficiency, structural reliability, environmental adaptability and cost, and further exploring the innovative application potential of intelligent materials and adaptive structures in multi-level throttling.

4.5.5. Critical Assessment of Simplifications in Multi-Stage Throttling Studies

While multi-stage throttling strategies show significant potential for improving pressure reduction uniformity and energy dissipation, their numerical and experimental analyses are often limited by computational and practical constraints. The high computational cost of fine-grid multiphysics simulations has led to the frequent use of simplified assumptions in previous work. This section critically examines these assumptions, identifies their specific limitations, and discusses their impact on the prediction of real-world phenomena—such as vibration, freezing, and multiphase transitions—with support from relevant literature.

A common simplification in many CFD studies (e.g., Chen et al. [56], Hou et al. [66]) is the use of steady-state simulations to reduce computational cost. This approach neglects transient effects such as pressure surges during valve actuation, flow instabilities under rapidly varying loads, and dynamic FSI. As a result, it fails to capture evolving flow structures and risks of flow-induced vibration, leading to underestimation of pressure fluctuation amplitudes and omission of critical low-frequency vibration modes essential for fatigue life prediction in marine applications. For instance, although Qian et al. [38] reported a ∼50% reduction in displacement oscillation amplitude via valve core optimization using transient analysis, their validation was conducted under controlled land-based conditions. The effectiveness of such designs under the broad-spectrum, random excitations typical of ship propulsion systems remains unverified, potentially overestimating operational stability at sea.

Thermodynamic simplifications are also prevalent. Many studies assume either isothermal flow, ignoring temperature changes, or adiabatic expansion, neglecting heat exchange with the valve body and environment. These assumptions misrepresent the J-T effect, which is critical for hydrogen—especially near or from cryogenic states. Isothermal models ignore cooling entirely, while adiabatic models tend to overpredict temperature drops by neglecting environmental heat ingress. Both lead to inaccurate predictions of local cryogenic conditions, masking risks such as ice formation, material embrittlement, and thermal stress. For example, Jin et al. [51] used superheated steam as an analog for hydrogen, citing similar pressure–velocity trends. However, this analogy fails for thermal effects due to hydrogen’s distinct thermodynamic properties (e.g., specific heat ratio, J-T inversion curve), likely resulting in non-conservative estimates of minimum wall temperatures and overlooked seal failure risks.

Moreover, most models treat hydrogen as a single-phase ideal or real gas, ignoring possible phase changes or condensation/desublimation during rapid expansion. This assumption is invalid in cases of moisture ingress or with liquid hydrogen (LH₂) boil-off gas containing droplets. Without multiphase modeling, studies such as Li et al. [68] on labyrinth channels cannot capture ice particle formation, flow blockage, altered erosion patterns, or shifts in flow coefficients—limiting predictive reliability in humid marine environments or during LH₂ transients.

In FSI analyses, structural components such as valve cores and perforated plates are often modeled as rigid or linearly elastic, ignoring plastic deformation, wear, fatigue damage accumulation, and hydrogen embrittlement. For instance, Wang et al. [44] proposed an optimized convex cylindrical valve core with improved dynamic response in a simulated FSI study, but its long-term integrity under combined cyclic stress, hydrogen-assisted cracking, and salt spray corrosion was not assessed. Such models cannot predict failure modes like fatigue crack growth or HE-induced brittle fracture.

Furthermore, most numerical and experimental studies are conducted under idealized laboratory conditions, excluding external excitations such as ship engine vibrations, salt spray corrosion, or thermal cycling. This decouples valve performance from its intended marine environment. For example, the turbulent dissipation rates, noise levels, and optimal geometries reported by Chen et al. [56] for multi-stage perforated plates may not apply under ship-borne vibrations, which can alter boundary layers, vortex shedding, and cause fretting wear—leading to unpredicted performance degradation.

Specific cases further illustrate these limitations. Qian et al. [90] used a steady-state compressible flow model with the RNG k–ε turbulence model to analyze hydrogen flow in a Tesla valve. While effective in flow resistance and energy dissipation, the steady-state assumption and Reynolds-averaging underestimated transient vortex shedding and aerodynamic noise under off-design or fluctuating conditions. The lack of pulsating flow validation leaves acoustic performance and dynamic structural loads poorly characterized. In another example, Chen et al. [67] employed a coupled CFD–FSI approach for a multi-stage perforated sleeve valve but modeled vibration input as single-frequency sinusoidal excitation. This does not represent the broad-spectrum random vibrations typical of ship engines and waves, so the predicted fatigue life and dynamic response may be non-conservative. Similarly, Li et al. [68], in studying a π-HPRV labyrinth channel, assumed constant wall temperature, decoupling fluid thermodynamics from structural thermal mass. This neglects convective heat transfer and thermal inertia, leading to non-conservative estimates of minimum gas and wall temperatures—overlooking risks of seal embrittlement or ice plugs.

In summary, although simplified models enable rapid prototyping and parametric screening of multi-stage throttling structures, they introduce significant and often unquantified uncertainties in predicting real-world performance—especially under demanding marine conditions. Assumptions of steadiness, simplified thermodynamics, single-phase flow, linear material behavior, and decoupled environments collectively yield optimistic predictions of performance and durability. For instance, the use of steady-state RANS models in multi-stage throttling studies may lead to an underestimation of peak Mach numbers by 15–20% and turbulent dissipation rates by an order of magnitude, thereby masking potential risks of high-frequency vibration and structural fatigue under marine operating conditions. Future work should prioritize developing and experimentally validating high-fidelity, transient, multiphysics models that incorporate non-equilibrium thermodynamics, multiphase flows, advanced material degradation models, and realistic environmental load spectra. Experimental campaigns should explicitly replicate ship-like vibration and corrosion cycles to provide reliable data for model calibration and ensure that optimized designs remain effective and durable throughout their service life at sea.

4.6. Summary of Research Status and Methodologies

To provide a clear and concise overview of the current research landscape,

Table 4. A summary of research status, methodologies, and gaps for marine hydrogen pressure reducing valves.

Research Focus Area	Current Research Status & Core Methodologies	Main Findings/Advancements	Identified Gaps & Lack of Marine Adaptability
Transient Flow Dynamics	Status/Methods: Transient CFD and Fluid–Structure Interaction (FSI) simulations; Dynamic mesh techniques; Optimization of spring stiffness and valve core geometry.	Findings: Valve core optimization can suppress displacement oscillations and improve dynamic response. The depressurization process can be characterized into distinct stages (rapid response, closing, equilibrium).	Gaps: Simulations and validations are predominantly conducted under static, land-based conditions. The performance of optimized designs under broad-spectrum, random marine vibrations remains unproven, risking resonance and instability.
Turbulence & Compressible Flow	Status/Methods: Steady-state and transient CFD (RANS, LES); Analysis of Mach number distribution and turbulent dissipation rate; Optimization of multi-stage perforated plates.	Findings: Multi-stage throttling promotes subsonic flow and reduces energy consumption. Structural parameters (e.g., number of stages, hole diameter) significantly influence flow stability and energy loss.	Gaps: Models assume smooth walls and ideal gases, neglecting the impact of marine-induced surface roughening (corrosion) on flow resistance and acoustics. RANS models may underestimate peak Mach numbers and turbulent dissipation, affecting noise and fatigue predictions.
Parameter Optimization	Status/Methods: Parametric CFD studies; Steady-state experiments; Determining optimal parameters (orifice size, spring stiffness, valve core angle) for efficiency.	Findings: Quantitative influence ranges of key parameters on outlet pressure and temperature have been established (see Table 2). Optimal parameter sets for peak performance under deterministic conditions are identified.	Gaps: Optimization is performed without considering marine environment as a key input. Lack of Uncertainty Quantification (UQ) and Global Sensitivity Analysis (GSA) to ensure robustness against stochastic marine perturbations (vibration, thermal cycles).
Valve Core Geometry	Status/Methods: CFD comparison of different spool shapes (e.g., straight-edge, arc-edge, flat-bottom); FSI-based shape optimization (e.g., convex cylinder).	Findings: Micro-rounding (arc-edge) and specific convex features can reduce vortices, pressure gradients, and improve dynamic stability.	Gaps: The long-term durability of precision-engineered geometries under synergistic erosion-corrosion is unaddressed. Performance advantages may be eroded by geometric degradation. Lack of co-design with HE-resistant materials and coatings.
Multi-stage Throttling Strategies	Status/Methods: CFD analysis of various structures (Porous plates, Labyrinth channels, Tesla valves, Combined structures); Comparative performance evaluation.	Findings: Different strategies offer trade-offs: Porous plates (simplicity vs. fatigue), Labyrinths (flow uniformity vs. complexity), Tesla valves (reliability vs. size). Combined structures offer high performance and adaptability.	Gaps: A systematic evaluation framework for marine applications is lacking. Simplifications in studies (steady-state, single-phase, rigid structure) lead to over-optimistic predictions. The reliability of complex structures under vibration and corrosion is questionable.
Multi-physics Coupling	Status/Methods: Recognized as critical, but high-fidelity coupled simulations are challenging. Initial attempts with FSI and thermal-stress analysis.	Findings: Highlights the importance of coupled phenomena like transient shock-boundary layer interaction and thermo-structural response.	Gaps: Fully integrated and experimentally validated models are scarce. Key couplings (e.g., vibration-shock interaction, synergistic corrosion-fatigue) are not quantitatively predictable. Models lack real ship vibration data and material degradation models.

summarizes the key research areas, methodologies, main findings, and critically, the identified gaps and marine adaptability challenges for marine hydrogen PRVs. This synthesis underscores that while significant progress has been made in understanding fundamental flow dynamics and structural optimization, a pronounced gap remains in validating these findings under realistic, coupled marine environmental conditions.

This summary underscores a critical consensus across all research domains: while the foundational understanding of hydrogen flow dynamics and structural mechanics in PRVs is maturing, a significant and systematic gap exists in addressing the unique, coupled challenges of the marine environment. The current state of the art, largely developed and validated under benign, land-based conditions, offers valuable design principles and performance trends. However, the direct transferability of these findings to marine service is hampered by the lack of integrated models and experimental data that account for the synergistic effects of persistent vibration, salt spray corrosion, and operational transients. Consequently, the transition from laboratory-optimized components to sea-worthy, reliable valves necessitates a paradigm shift in research focus towards marine-adapted, multi-physics validation and the development of robust designs that prioritize long-term performance stability over peak efficiency under ideal conditions. This synthesis directly sets the stage for the specific conclusions and future research directions outlined in the following section.

5. Conclusions and Future Development Directions

5.1. Main Research Findings and Conclusions

This review has systematically examined recent advancements in the design and analysis of the PRVs for ship-borne hydrogen storage systems. The following conclusions are drawn from a synthesis of the existing literature:

Theoretical Modeling: Research has established foundational models based on thermodynamics and fluid mechanics to describe energy dissipation and turbulence generation during high-pressure hydrogen throttling. These models provide a basic framework for understanding complex internal flows. However, they predominantly rely on idealized assumptions and fail to adequately depict the tight coupling between multiple physical fields (fluid–structure-thermal-chemical) and the material-hydrogen interaction mechanisms under extreme conditions. Crucially, the synergistic effects between transient turbulent flow, Joule–Thomson cooling, structural vibration, and hydrogen embrittlement are seldom modeled in an integrated manner. This constitutes a significant knowledge gap, as valve performance degradation often stems from these interactions rather than isolated physical effects. Consequently, moving beyond idealized assumptions is essential for developing accurate predictive models that can more faithfully represent the complex multi-physics environment within pressure reducing valves under real-world operating conditions.

Numerical Simulation: CFD has become the dominant tool for studying internal valve flows. Simulations have successfully elucidated the significant influence of valve core geometry (e.g., cone angle) and porous plate structural parameters (e.g., number of stages, hole diameter, arrangement) on flow characteristics, Mach number distribution, energy consumption, and noise. Studies consistently show that structural optimization (e.g., multi-stage throttling, convex cylinder valve cores) effectively enhances pressure stability and reduces turbulent dissipation. Nonetheless, simulations face persistent challenges in accurately capturing compressible, high-Mach-number turbulence and in the computational efficiency of multi-field coupling. Traditional turbulence models (e.g., RANS) exhibit inherent limitations in predicting complex phenomena such as shock-wave/boundary-layer interactions. Quantitatively, these models may underestimate peak Mach numbers by 15–20% and turbulent dissipation rates by up to an order of magnitude, leading to non-conservative designs if not complemented by high-fidelity methods such as LES or experimental validation.

Experimental Research: Experimental studies, utilizing high-pressure test platforms coupled with advanced measurement techniques like Particle Image Velocimetry (PIV), have validated the transient response characteristics of multi-stage PRVs (e.g., HMSPRV) and the beneficial effects of valve core optimization. These results confirm the potential of optimized designs to improve dynamic response and suppress pressure fluctuations. However, most experiments are conducted under land-based conditions, creating a severe lack of systematic validation for performance in specific marine environments characterized by continuous vibration, salt spray corrosion, and temperature variations.

Structural Optimization and Application: Research has clearly demonstrated the advantages of multi-stage throttling strategies (e.g., Tesla valves, labyrinth channels, combined porous structures) in distributing pressure drop, mitigating temperature decrease, and minimizing turbulence and vibration. Valve core geometric optimization (e.g., specific cone angles, edge rounding) profoundly impacts flow field uniformity and stability. However, many optimized structures struggle to meet the stringent lightweight and integration requirements of marine applications due to complex manufacturing, high cost, excessive size, or weight. Furthermore, the robustness of pressure regulation under variable ship operating conditions requires improvement. Therefore, this paper systematically summarizes the quantitative influence range of key parameters on the outlet pressure and temperature through Table 2, revealing the uncertainty characteristics of the performance of the pressure reducing valve under multi-physics field coupling. This provides data support for the subsequent design and optimization of highly reliable marine pressure reducing valves.

A key conclusion and the central thesis of this review is that the performance and reliability of marine hydrogen PRVs are governed by tightly coupled multi-physics interactions. The primary novelty of this work lies in its systematic identification of these specific couplings—such as vibration-shock interaction and synergistic corrosion-fatigue—and its structured road-map for tackling them through integrated models, advanced simulations, and marine-focused experimentation.

5.2. Future Development Direction

Driven by the global clean energy transition and carbon neutrality goals, hydrogen is playing an increasingly prominent role as a zero-carbon energy carrier. Maritime transportation, with its large cargo capacity, has emerged as a central mode for intercontinental hydrogen trade. Within ship-borne hydrogen storage systems, the hydrogen pressure reducing valve is a critical pressure regulation device whose performance directly determines the safety and efficiency of the transportation process. However, the combination of the unique marine environment and the inherent properties of hydrogen have led to current research facing bottlenecks, including unstable performance, insufficient reliability, and poor adaptability to diverse operational scenarios. Based on the limitations identified in existing research, this review highlights the following specific research gaps and proposes corresponding future directions:

5.2.1. Theoretical Modeling

Research gap: Currently, there is a lack of a comprehensive theoretical model that can accurately describe the multi-field coupling effects such as ship vibration, salt fog corrosion, and temperature fluctuations. In particular, the interaction mechanism between the hydrogen-material interface under extreme conditions remains unclear.

Development direction: A multi-field coupling model of fluid-solid-heat-chemistry needs to be established that moves beyond idealized assumptions. The focus should be on studying the penetration and diffusion behavior of hydrogen atoms in materials under the coupled influence of mechanical stress and thermal gradients, and their impact on the mechanical properties of the materials. The goal is to enhance predictive capability concerning unsteady flow, structural vibration, thermal effects, and material failure mechanisms.

5.2.2. Numerical Simulation

Research gap: The existing turbulence models have limited accuracy when dealing with high Mach number and compressible hydrogen flow. Multi-physics coupling calculation efficiency is low, and there is a lack of an efficient simulation framework applicable to the ship environment.

Development direction: Develop an adaptive turbulence model based on hydrogen properties and introduce machine learning methods to optimize model parameters. Improve adaptive grids and parallel computing technologies to enhance the efficiency of multi-field coupling simulations and achieve precise prediction of material failure risks during long-term operation. Further research should also focus on improving predictive capabilities under realistic marine environmental and operational conditions. Furthermore, future simulations must mandatorily adopt high-fidelity real-gas models (e.g., leveraging the NIST REFPROP database) to accurately capture hydrogen’s thermophysical properties across wide temperature and pressure ranges. This is particularly critical for correctly predicting the sign change in the Joule–Thomson effect, which is fundamental for assessing cryogenic risks and material compatibility.

5.2.3. Experimental Innovation

Research gap: The existing experimental platforms are unable to replicate the complex environments of ships (such as continuous vibration, salt fog corrosion, and temperature cycling). There is also a lack of high spatiotemporal resolution methods for measuring flow fields and hydrogen concentrations. Consequently, the vast majority of experimental results lack validation for marine operational conditions, representing a critical barrier to the reliable deployment of PRVs in ship-borne systems.

Development Direction: Future experimental research must shift from land-based validation to marine-environment-centric verification. This requires the development of integrated, multi-functional test rigs capable of superimposing programmable mechanical vibration (simulating ship engine and wave-induced vibrations), controlled salt spray corrosion cycles, and rapid thermal cycling onto high-pressure hydrogen flow tests. Concurrently, advanced in situ diagnostic techniques, such as Raman spectroscopy for quantitative hydrogen concentration measurement and terahertz imaging for non-destructive evaluation of material degradation under coatings, should be adopted to achieve high-fidelity data acquisition under these coupled stresses.

5.2.4. Structural Design and Material Challenges

Research gap: The pressure stability of the valve core structure under multiple working conditions is insufficient. There is a lack of systematic research on anti-vibration and anti-corrosion design, as well as a contradiction between lightweighting and high performance.

Development Direction: Conduct bionic and topological optimization design of valve cores; integrate intelligent sensing and adaptive control algorithms. Develop composite materials and coating systems resistant to hydrogen embrittlement and corrosion. Establish unified testing standards and reliability assessment norms for marine relief valves to enhance performance under harsh operating conditions.

5.2.5. Standardization and Regulatory Framework

Research gap: Currently, there is a lack of international standards and certification systems for pressure reducing valves under special conditions such as ship vibration, salt fog, and inclination.

Development Direction: Promote the formulation of dedicated standards for marine hydrogen pressure reducing valves by ISO, IEC, and major classification societies (such as Class NK, DNV), specifying requirements for materials, design, testing, and certification. Actively follow and participate in relevant international projects regarding the safety protocols of marine hydrogen systems. Develop a compliance traceability system based on digital twins and blockchain to achieve transparent management of full life cycle data.

By systematically addressing these research gaps, the core challenges related to pressure regulation in hydrogen ocean transportation can be resolved. This will provide safe and efficient technical support for intercontinental hydrogen trade, significantly improve performance stability, environmental adaptability, and operational reliability of pressure reducing valves, and accelerate the industrialization of marine hydrogen transportation. These advances will contribute strongly to global energy structure transformation and the achievement of carbon neutrality goals.