Search Results (29)

The deflagration-to-detonation transition (DDT) is a critical process for achieving reliable ignition in detonation-based propulsion systems, such as Rotating Detonation Engines (RDEs). This study experimentally investigates the effect of spatial variations in blockage ratio (BR) on flame acceleration and detonation onset within a modular pre-detonator. Three DDT device configurations (converging, constant, and diverging) were designed to have an identical average BR of 0.5 and were tested over equivalence ratios ranging from 0.64 to 1.6. High-speed imaging, pressure transducers, and schlieren visualization were employed to characterize flame propagation velocity, pressure evolution, and exit wave structures. The converging configuration consistently promoted earlier detonation onset and higher success rates, especially under fuel-rich conditions (ϕ = 1.6), while the diverging configuration failed to initiate detonation in all cases. Enhanced flame compression in the converging layout led to strong coupling between the shock and reaction fronts, facilitating robust detonation formation. These findings indicate that the spatial distribution of BR, rather than average BR alone, plays a decisive role in DDT performance. This work offers validated design insights for optimizing pre-detonator in RDE applications. Full article

Combustion caused by hydrogen-dominated combustible gas mixtures presents critical threats to nuclear safety during severe accidents in nuclear power plants, primarily due to their propensity for flame acceleration, deflagration, and subsequent detonation. Although the direct initiation of detonation from localized hydrogen accumulation at critical concentrations remains challenging, flame acceleration can induce rapid pressure escalation and lead to deflagration-to-detonation transition under suitable conditions. The ultra-high-pressure loads generated almost instantaneously will pose serious risks to containment integrity and equipment or instrument functionality within nuclear facilities. This paper investigates the flame acceleration mechanism and criterion, which is crucial for precise hydrogen risk assessment. A hydrogen combustion flame acceleration model is developed, integrating both laminar and turbulent flame propagation across multiple control volumes. Validated against the RUT test, the model demonstrates high fidelity with a maximum 3.17% deviation in flame propagation velocity and successfully captures the pressure discontinuity. The developed model enables comprehensive simulation with improved predictive accuracy of the flame acceleration process, making it an essential tool for advancing fundamental understanding of hydrogen behavior and severe accident analysis. This model’s development marks a paradigm in nuclear safety research by providing an analytical instrument for integrated severe accident programs in nuclear power plants, contributing to improving the potential hydrogen risks assessment and management in next-generation reactor safety. Full article

Battery electric buses (BEBs) are widely regarded as a safe and sustainable alternative to internal combustion vehicles. However, the lithium-ion batteries that power them present safety risks. This paper provides a comprehensive overview of the safety of battery electric buses, highlighting current challenges, relevant regulations and proposed solutions to enhance safety. There are significant shortcomings in the fire safety regulations for buses, especially concerning qualification methods for bus interiors. Enclosed spaces and structures represent the most critical risks for these transport systems. The presence of large vehicles, such as BEBs, in tunnels could increase the risk of transitioning from deflagration to detonation. Fires involving such vehicles produce more soot than fires from internal combustion engine buses (ICEBs) and have slightly higher toxicity levels. High-pressure water spraying systems are not yet an effective solution, as not all the heat is removed if the thermal runaway has already been triggered for several minutes, and their action remains largely limited to the outside of the battery pack. Another critical issue is cybersecurity. Managing and protecting BEBs from cyber threats is complex and requires robust strategies. Full article

Pulse detonation engines (PDEs) have become a transformative technology in the field of aerospace propulsion due to the high thermal efficiency of detonation combustion. However, initiating detonation waves within a limited space and time is key to their engineering application. Direct initiation, though theoretically feasible, requires very high critical energy, making it almost impossible to achieve in engineering applications. Therefore, indirect initiation methods are more practical for triggering detonation waves that produce a deflagration wave through a low-energy ignition source and realizing deflagration to detonation transition (DDT) through flame acceleration and the interaction between flames and shock waves. This review systematically summarizes recent advancements in DDT methods in pulse detonation engines, focusing on the basic principles, influencing factors, technical bottlenecks, and optimization paths of the following: hot jet ignition initiation, obstacle-induced detonation, shock wave focusing initiation, and plasma ignition initiation. The results indicate that hot jet ignition enhances turbulent mixing and energy deposition by injecting energy through high-energy jets using high temperature and high pressure; this can reduce the DDT distance of hydrocarbon fuels by 30–50%. However, this approach faces challenges such as significant jet energy dissipation, flow field instability, and the complexity of the energy supply system. Solid obstacle-induced detonation passively generates turbulence and shock wave reflection through geometric structures to accelerate flame propagation, which has the advantages of having a simple structure and high reliability. However, the problem of large pressure loss and thermal fatigue restricts its long-term application. Fluidic obstacle-induced detonation enhances mixing uniformity through dynamic disturbance to reduce pressure loss. However, its engineering application is constrained by high energy consumption requirements and jet–mainstream coupling instability. Shock wave focusing utilizes concave cavities or annular structures to concentrate shock wave energy, which directly triggers detonation under high ignition efficiency and controllability. However, it is extremely sensitive to geometric parameters and incident shock wave conditions, and the structural thermal load issue is prominent. Plasma ignition generates active particles and instantaneous high temperatures through high-energy discharge, which chemically activates fuel and precisely controls the initiation sequence, especially for low-reactivity fuels. However, critical challenges, such as high energy consumption, electrode ablation, and decreased discharge efficiency under high-pressure environments, need to be addressed urgently. In order to overcome the bottlenecks in energy efficiency, thermal management, and dynamic stability, future research should focus on multi-modal synergistic initiation strategies, the development of high-temperature-resistant materials, and intelligent dynamic control technologies. Additionally, establishing a standardized testing system to quantify DDT distance, energy thresholds, and dynamic stability indicators is essential to promote its transition to engineering applications. Furthermore, exploring the DDT mechanisms of low-carbon fuels is imperative to advance carbon neutrality goals. By summarizing the existing DDT methods and technical bottlenecks, this paper provides theoretical support for the engineering design and application of PDEs, contributing to breakthroughs in the fields of hypersonic propulsion, airspace shuttle systems, and other fields. Full article

In the maritime industry, hydrogen fuel cell ships demonstrate significant potential for development due to their environmental friendliness and high efficiency. However, the risks of fire and explosion caused by hydrogen leakage pose severe challenges to their safety. To enhance the safety of hydrogen fuel cell ships and mitigate the explosion hazards caused by leakage, this study employs the XiFoam solver in the OpenFOAM v9 to establish an explosion model for a full-scale hydrogen fuel cell compartment within a hydrogen fuel cell ship. The model simulates the transient explosion process following high-pressure hydrogen leakage under varying initial hydrogen concentrations and premixed fuel conditions. By analyzing the temporary evolution of temperature distribution, flame front propagation, and explosion pressure, the study provides a comprehensive understanding of the safety implications of hydrogen leakage at different locations within the fuel cell. Specifically, increasing the hydrogen concentration from ΦH₂ = 0.10 to ΦH₂ = 0.18 and ΦH₂ = 0.20 significantly elevates the overpressure peak and accelerates the flame speed from 250 m/s to 370 m/s, with local pressure gradients approaching the deflagration to detonation transition threshold. The simulation results contribute valuable insights into optimizing hydrogen fuel cell design, formulating effective fire safety strategies, and improving overall ship safety. Full article

This study numerically explores the initiation of detonation through shock wave reflection and focusing on a 90-degree wedge in varying mixtures of hydrogen–air. The simulations were conducted using the ddtFoam code, an integral part of the OpenFOAM open-source Computational Fluid Dynamics (CFD) package of density-based code for solving the unsteady, compressible Navier–Stokes equations. The simulation results unveil three potential outcomes in the corner post-reflection: deflagrative ignition in the corner, deflagrative ignition with intermediate transient phases leading to a delayed transition to detonation in the trailing combustion zone close to the apex of the wedge, and ignition with an immediate transition to detonation, resulting in the formation of a detonation wave in the corner tip. In the experimental investigation, the transition velocity for the stoichiometric mixture stood at approximately 719 m/s. In contrast, the numerical simulation indicated a transition velocity of 664 m/s for the same stoichiometric mixture, reflecting a 5.5% decrease in velocity. Such an underestimation level of 5–8% by the simulation results was observed for mixtures of 25–45% H₂ in air. Full article

Methanol can be synthesized using green electricity and carbon dioxide, making it a green, carbon-neutral fuel with significant potential for widespread application in engines. However, due to its low ignition energy and high laminar flame speed, methanol is susceptible to hotspot-induced pre-ignition and even knocking under high-temperature, high-load engine conditions, posing challenges to engine performance and reliability. This paper systematically reviews the manifestations and mechanisms of pre-ignition and knocking in methanol engines. Pre-ignition can be sustained or sporadic. Sustained pre-ignition is caused by overheating of structural components, while sporadic pre-ignition is often linked to oil droplets entering the combustion chamber from the piston crevice. Residual exhaust gas trapped within the spark plug can also initiate pre-ignition. Knocking, characterized by pressure oscillations, arises from the auto-ignition of hotspots in the end-gas or, potentially, from deflagration-to-detonation transition, although the latter requires further experimental validation. Factors influencing pre-ignition and knocking, including engine oil, in-cylinder deposits, structural hotspots, and the reactivity of the air–fuel mixture, are also analyzed. Based on these factors, the paper concludes that the primary approach to suppressing pre-ignition and knocking in methanol engines is controlling the formation of pre-ignition sources and reducing the reactivity of the air–fuel mixture. Furthermore, it addresses existing issues and limitations in current research, such as combustion testing techniques, numerical simulation accuracy, and the mechanisms of methanol–oil interaction, and offers related recommendations. Full article

Hydrogen is regarded as a promising energy source for the future due to its clean combustion products, remarkable efficiency and renewability. However, its characteristics of low-ignition energy, a wide flammable range from 4% to 75%, and a rapid flame speed may bring significant explosion risks. Typically, accidental release of hydrogen into confined enclosures can result in a flammable hydrogen–air mixture with concentration gradients, possibly leading to flame acceleration (FA) and deflagration-to-detonation transition (DDT). The current study focused on the evolutions of the FA and DDT of homogenous/inhomogeneous hydrogen–air mixtures, based on the open-source computational fluid dynamics (CFD) platform OpenFOAM and the modified Weller et al.’s combustion model, taking into account the Darrieus–Landau (DL) and Rayleigh–Taylor (RT) instabilities, the turbulence and the non-unity Lewis number. Numerical simulations were carried out for both homogeneous and inhomogeneous mixtures in an enclosed channel 5.4 m in length and 0.06 m in height. The predictions demonstrate good quantitative agreement with the experimental measurements in flame-tip position, speed and pressure profiles by Boeck et al. The characteristics of flame structure, wave evolution and vortex were also discussed. Full article

Combustion through detonation marks an important leap in efficiency over standard deflagration methods. This research introduces a Pulsed Detonation Combustor (PDC) model that uses Hydrogen as fuel and Oxygen as an oxidizer, specifically targeting carbon-free combustion efforts. The PDC aerodynamic features boost operating cycle frequency and facilitate Deflagration-to-Detonation Transition (DDT) within distances less than 200 mm by means of Hartmann–Sprenger resonators and cross-flow fuel/oxidizer injection. The achievement of quality mixing in a short-time filling process represents not only higher cycle operation but also enhanced performances. The scope of this paper is to assess the impact of different fuel injectors with different opening areas on the performances of the PDC. This assessment, expressed as a function of the Equivalence Ratio (ER), is conducted using two primary methods. Instantaneous static pressures are recorded and processed to extract the maximum and average cycle pressure and characterize the pressure augmentation. Thrust measurements obtained using a load cell are averaged over the detonation cycle to calculate the time-averaged thrust. The specific impulse is subsequently determined based on these thrust measurements and the corresponding mass flow data. Full article

System failure in large-scale electrolyzer and fuel cell installations may cause the formation of explosive H₂–air–steam mixtures. Detonation properties (e.g., detonation cell size) and flame dynamics features (e.g., flame acceleration, runup distance, and deflagration-to-detonation transition “DDT”) of these mixtures were investigated experimentally and numerically to gain a more in-depth understanding of the hazards of H₂–air–steam under conditions pertinent to PEM-based electrolyzers and fuel cells (temperatures between 50 °C and 80 °C and pressures between 20 and 40 bar). While our results confirm the findings of previous studies in terms of the cooling effects of steam on detonation, we found that operating pressures between 20 and 40 bar counteract the effect of steam, making the H₂–air–steam mixture more detonable. This is particularly evident from the experimental data on detonation cell size and runup distance at pressures greater than 20 bar. Full article

Traditional exhaust-gas turbocharging exhibits hysteresis under variable working conditions. To achieve rapid-intake supercharging, this study investigates the synergistic coupling process between the detonation and diesel cycles using gasoline as fuel. A numerical simulation model is constructed to analyze the detonation characteristics of a pulse-detonation combustor (PDC), followed by experimental verification. The comprehensive process of the flame’s deflagration-to-detonation transition (DDT) and the formation of the detonation wave are discussed in detail. The airflow velocity, DDT time, and peak pressure of detonation tubes with five different blockage ratios (BR) are analyzed, with the results imported into a one-dimensional GT-POWER engine model. The results indicate that the generation of detonation waves is influenced by flame and compression wave interactions. Increasing the airflow does not shorten the DDT time, whereas increasing the BR causes the DDT time to decrease and then increase. Large BRs affect the initiation speed of detonation in the tube, while small BRs impact the DDT distance and peak pressure. Upon connection to the PDC, the transient response rate of the engine is slightly improved. These results can provide useful guidance for improving the transient response characteristics of engines. Full article

Rotating detonation engines (RDEs) are considered to be promising thrusters for aerospace propulsion. Detonation initiation in RDEs can be accompanied by a destructive explosion of an excess volume of the fuel mixture in the combustor. To exclude this phenomenon, a “mild” rather than “strong” initiation of detonation is required. For the mild initiation of detonation in RDEs, it is necessary to ignite a mixture of a certain minimum volume sufficient for deflagration-to-detonation transition (DDT). In this study, the critical conditions for detonation initiation through DDT in a semiconfined slit combustor simulating the RDE combustor with a separate supply of ethylene and oxygen diluted with nitrogen (from 0 to 40%) were obtained experimentally. It turned out that for the mild initiation of detonation, it is necessary to ignite the mixture upon reaching the critical (minimum) height of the combustible mixture layer. Thus, for the mild initiation of detonation in the undiluted C₂H₄ + 3O₂ mixture filling such a slit combustor, the height of the mixture layer must exceed the slit width by approximately a factor of 12. In terms of the transverse size of the detonation cell λ the minimum layer height of such mixtures in experiments is ~150λ. Compared to the experiments with the premixed composition, the critical height of the layer is 20% larger, which is explained by the finite rate of mixing. As the degree of oxygen dilution with nitrogen increases, the critical height of the layer increases, and the role of finite rate mixing decreases: the results no longer depend on the method of combustible mixture formation. Full article

The current study primarily aimed to simulate detonation initiation via turbulent jet flame acceleration in partial-premixed H₂-air mixtures. Different vertical concentration gradients were generated by varying the duration of hydrogen injection (diffusion time) within an enclosed channel filled with air. H₂-air mixtures with average hydrogen concentrations of 22.5% (lean mixture) and 30% (near stoichiometric mixture) were investigated at diffusion times of 3, 5, and 60 s. Numerical results show that the vertical concentration gradient significantly influences the early stage of flame acceleration (FA). In the stratified lean mixture, detonation began at all the diffusion times, and comparing the flame-speed graphs showed that a decrease in the diffusion time and an increase in the mixture inhomogeneity speeded up the flame propagation and the jet flame-to-detonation transition occurrence in the channel. In the stratified H₂-air mixture with an average hydrogen concentration of 30%, the transition from a turbulent jet flame to detonation occurred in all the cases, and the mixture inhomogeneity weakened the FA and delayed the detonation initiation. Full article

Systematic three-dimensional numerical simulations of flame acceleration and deflagration-to-detonation transition (DDT) in a semi-confined flat slit combustor are performed. The combustor is assumed to be partly filled with the stoichiometric ethylene–oxygen mixture at normal pressure and temperature conditions. The objective of the study is to reveal the conditions for DDT in terms of the minimum height of the combustible mixture layer in the slit, the maximum dilution of the mixture with nitrogen and the maximum slit width. The results of the calculations are compared with the available experimental data. The calculation results are shown to agree satisfactorily with the experimental data on the slit-filling dynamics, flame structure, the occurrence of the preflame self-ignition center, DDT, and detonation propagation. DDT occurs in the layer at a time instant when the flame accelerates to a velocity close to 750 m/s. DDT occurs near the slit bottom due to the formation of the self-ignition center ahead of the leading edge of the flame as a result of shock wave reflections from the walls of injector holes at the slit bottom and from the corners of the conjugation of the slit bottom and side walls. The decrease in the height of the mixture layer, the dilution of the mixture with nitrogen, and the increase in the slit width are shown to slow down flame acceleration in the slit and increase the DDT run-up distance and time until DDT failure. The obtained results are important for determining the conditions for mild initiation of detonation via DDT in semi-confined annular RDE combustors. Full article

Hydrogen and ammonia are primary carbon-free fuels that have massive production potential. In regard to their flame properties, these two fuels largely represent the two extremes among all fuels. The extremely fast flame speed of hydrogen can lead to an easy deflagration-to-detonation transition and cause detonation-type engine knock that limits the global equivalence ratio, and consequently the engine power. The very low flame speed and reactivity of ammonia can lead to a low heat release rate and cause difficulty in ignition and ammonia slip. Adding ammonia into hydrogen can effectively modulate flame speed and hence the heat release rate, which in turn mitigates engine knock and retains the zero-carbon nature of the system. However, a key issue that remains unclear is the blending ratio of NH₃ that provides the desired heat release rate, emission level, and engine power. In the present work, a 3D computational combustion study is conducted to search for the optimal hydrogen/ammonia mixture that is knock-free and meanwhile allows sufficient power in a typical spark-ignition engine configuration. Parametric studies with varying global equivalence ratios and hydrogen/ammonia blends are conducted. The results show that with added ammonia, engine knock can be avoided, even under stoichiometric operating conditions. Due to the increased global equivalence ratio and added ammonia, the energy content of trapped charge as well as work output per cycle is increased. About 90% of the work output of a pure gasoline engine under the same conditions can be reached by hydrogen/ammonia blends. The work shows great potential of blended fuel or hydrogen/ammonia dual fuel in high-speed SI engines. Full article