Search Results (115)

Ammonia, a zero carbon energy vector, is under consideration for decarbonising marine and energy storage applications due to its high mass-based energy density compared to many alternatives. In addition, there is widespread existing supply and transportation infrastructure due to ammonia’s use as a fertiliser. When injected in its liquid form, however, ammonia behaves quite differently to traditional fuels due to its high saturation pressure and enthalpy of vaporisation, amongst other things. This means that fundamental data on ammonia sprays need to be collected in order to understand ammonia spray behaviour and calibrate models of ammonia sprays needed for design in the virtual world. Previous work on ammonia sprays has mostly focused on spray morphology at a macroscopic level (such as liquid penetration length). However, there are fewer studies of ammonia sprays at a microscopic level. In this study, liquid ammonia was injected into a constant-volume chamber using a direct injector at two injection pressures (100 bar and 150 bar) and a range of ambient pressures from 3–13 bar. This range of ambient conditions spans regimes from flash-boiling to non-flash-boiling, thereby enabling systematic investigation of the transition between these regimes. A laser diffraction technique was used for measuring the droplet sizes of the spray at different locations (in a cylindrical volume with a diameter of 10 mm) within the spray plume at 10 kHz, and the nominal droplet sizes were quantified by the Sauter Mean Diameter (SMD). These SMD values provided, at a microscopic level, an insight of the atomisation of the spray as it left the nozzle and penetrated into an environment with different densities. It was found that the tested injector leads to a breakup dominant spray behaviour with liquid ammonia and hence the SMD values decrease as ambient pressure increases. In addition, the droplets are generally smaller at the outer edge of the spray plume compared to the inner part and both the injection pressure and injection duration have a strong effect on the droplet sizes. Full article

Superhydrophilic micro/nano-porous media have extensive applications in electronic thermal management and energy storage systems. Predicting two-phase pressure drop in complex porous structures is of great importance for system design and optimization while remaining highly challenging. This study systematically investigates the two-phase flow resistance characteristics of bi-porous microstructures with multiple particle sizes and porosities under varying boiling regimes. Experimentally, porous samples were fabricated via vacuum sintering using nickel powders and pore-forming agents (CaCl₂), which exhibit superhydrophilicity and enhanced wicking characteristics. A visualized experimental platform was constructed to investigate the impact of pore size combinations, flow velocities, and boiling states on pressure drop. The dataset obtained through multi-factor saturated boiling experiments was further used to derive a semi-empirical model for the two-phase flow pressure drop based on the classic Kozeny-Carman (K-C) and Akagi-Chisholm (A-C) correlations. Results show that the pore size combinations and boiling states have a significant impact on the resistance performance. The proposed model achieves an average prediction deviation below 15.7%, confirming its reliability and its effectiveness as a design framework for optimizing high-capillary-force porous wicks in advanced thermal management systems. Full article

Cachichín (Oecopetalum mexicanum) is a tropical fruit tree native to Mexico and Central America, whose fruit contains an edible seed with potential nutraceutical properties. Empirical toasting of the cachichín seed often compromises the quality of its bioactive compounds. In a first experiment, this study evaluated the effects of time (25, 35, and 45 min) and temperature (115, 134, and 148 °C) to establish a controlled toasting process. The colorimetric properties were evaluated using a HunterLab colorimeter. The stability and structural integrity of fatty acids were assessed through the iodine value and Fourier Transform Infrared Spectroscopy (FTIR) in oils extracted by two methods: extrusion and Soxhlet. The most intense thermal treatments caused progressive darkening and significant lipid degradation. Although chemical variability was observed among treatments, the main functional groups of both saturated and unsaturated fatty acids remained structurally stable. The treatment at 134 °C for 25 min mitigated excessive degradation, achieving a better balance among color preservation, physicochemical properties, and lipid stability. Furthermore, Soxhlet extraction resulted in better preservation of unsaturated fatty acids under these controlled toasting conditions. In a second experiment, this controlled method outperformed traditional treatments (boiling and commercial toasting), preserving a desirable color and low water activity (a_w). Fatty acid analysis confirmed that this treatment maintained lipid stability, notably preserving unsaturated fatty acids (oleic, linoleic and linolenic acids) regardless of the extraction method. These results demonstrate that optimizing thermal processing is fundamental for maximizing the nutritional value of the cachichín seed, enhancing its potential in the food industry. Full article

At the moment, there are no available data or studies exploring the production of naphtha boiling range hydrocarbons via hydroprocessing of pretreated residual lipids. To that aim, this study targets the production of naphtha, jet and diesel boiling range hydrocarbons via hydroprocessing of refined waste cooking oils utilizing solar hydrogen. The technology was first optimized in a TRL-3 plant. A heteroatom removal catalyst and a saturation catalyst were combined with an isomerization and hydrocracking catalyst to upgrade lipids. The results show that the severity of the process plays an important role in the yields of the fuels. Higher naphtha yields were observed at 663 K, 13.78 MPa and a liquid hourly space velocity of 0.33 h⁻¹, leading to the production of a fuel consisting of 34 wt% naphtha, 23 wt% jet and 42 wt% diesel boiling range hydrocarbons. Subsequently, the technology was validated and demonstrated in an industrially relevant unit (TRL-5). The results from the fuel characterization show that the diesel fraction can be used as a high-quality road transport drop-in fuel, as it is characterized by a high cetane index (~96) and a high flash point (414 K). Although jet and naphtha meet most commercial fuel specifications, further optimization of the process is necessary to meet fuel standards. In conclusion, the current work provides novel data relevant to industrial applications for road, aviation and maritime fuel production via hydroprocessing of refined waste cooking oil. Full article

Enhancing the evaporator configuration of plate heat exchangers is essential for improving the overall efficiency of organic Rankine cycle (ORC) systems. To investigate the evaporator’s heat transfer characteristics, an experimental ORC test rig was developed. The experiments were conducted at saturation temperatures of 62.8–86.2 °C, mass fluxes of 5.0–16.6 kg/(m²·s), and heat fluxes of 3.1–9.2 kW/m², spanning subcooled boiling, saturated two-phase, and superheating regions. The heat flux showed minimal variation with heat source temperature, whereas higher mass fluxes resulted in substantial increases in generator power and thermal efficiency due to enhanced convection and vaporization. The overall and refrigerant heat transfer coefficients rise with heat source temperature and mass flux, peaking under moderate conditions and declining as the superheating region becomes constrained. Comparison with existing correlations reveals pronounced deviations, indicating their limited applicability under the present operating conditions. A nondimensional correlation was established using dimensional analysis and multivariate regression to predict heat transfer across the subcooled boiling, saturated two-phase, and superheating regions. The proposed correlation yielded a mean absolute percentage error of 15.9%, demonstrating good predictive accuracy and providing a reliable theoretical basis for performance evaluation and design optimization of plate evaporators in ORC systems. Full article

This study experimentally investigated boiling phenomena and heat transfer enhancement on sintered Cu micro/nanoporous surfaces under saturated pool boiling conditions. To evaluate the effects of the combined micro/nanostructures, microporous Cu layers and pillar-integrated surfaces were fabricated using micro-sized (diameter <75 mm) metal powder sintering, while nanostructures were formed through thermal oxidation. Boiling experiments revealed that the boiling heat transfer coefficient (BHTC) and critical heat flux (CHF) of the microporous Cu surfaces surpassed those of the reference surface SiO₂. The microporous pillar surface exhibited the best performance, demonstrating enhancements of approximately 2.7-fold and 7.3-fold in CHF and BHTC, respectively. High-speed imaging attributed this improvement to increased nucleation site density, rapid detachment and generation of small bubbles, efficient surface rewetting by capillary wicking, and liquid–vapor pathway separation enabled by the pillar geometry. Distinct transient temperature peaks and recoveries were observed on the oxidized pillar surfaces. Despite temporary overheating, strong capillary wicking from the superhydrophilic nanostructures recovered to the nucleate-boiling regime, which suppressed irreversible dryout and extended the boiling performance beyond the smooth surface CHF by 2.1 times. The results revealed that increasing the nucleation site density, enhancing the capillary-driven liquid supply, and ensuring effective separation of the vapor and liquid pathways improved the boiling heat transfer in multiscale porous structures. The sintered Cu micro/nanoporous surfaces demonstrated stable and efficient heat transfer across a wide range of heat fluxes, highlighting their potential for advanced thermal management applications and realizing optimally designed high-performance boiling surfaces. Full article

This study proposes an analytical model for the long-term prediction of boil-off gas (BOG) generation in cryogenic storage tanks. The model assumes a saturated liquid and a superheated vapor under open-vent conditions. Heat ingress is estimated using steady-state thermal conduction analysis, and evaporation is then computed from thermodynamic equilibrium. In the first stage, a thermal resistance network quantifies the heat flux transferred to the liquid and vapor regions inside the tank. The network represents external convection, insulation conduction, and internal convection as thermal resistances. In particular, natural convection on the external and internal tank walls, as well as heat transfer at the liquid–vapor interface, are incorporated through appropriate convective heat-transfer correlations. In the second stage, the temporal variations in temperature and phase change of the vapor and liquid are computed. Each phase is modeled as a lumped mass at equilibrium, and the heat ingress obtained from the thermal resistance network is used to simulate the temperature evolution and evaporation process. A numerical model is also developed to capture the time-dependent variations in liquid and vapor heights and the corresponding BOG generation. The proposed model is applied to a 1.0 m³ liquid nitrogen storage tank and validated through comparison with the BoilFAST and SINDA/FLUINT models. The results confirm the validity of the model in terms of heat ingress, vapor temperature evolution, and BOG history. This study provides a practical framework for predicting long-term evaporation phenomena in cryogenic storage tanks and is expected to contribute to the thermal design and performance evaluation of cryogenic storage systems. Full article

The present study aims to experimentally investigate pool boiling heat transfer characteristics, such as critical heat flux (CHF) and boiling heat transfer coefficient (BHTC), of pure distilled water (d-H₂O) and functionalised graphene nanoplatelet (f-GnPs)–d-H₂O nanofluids using a nichrome (Ni-Cr) test wire as the heating element. The distilled water (dH₂O) and GnP (5–10 nm and 15 µm, Cheap Tubes, USA) were chosen as the base fluid and nanomaterial, respectively. The GnP was chemically functionalized and dispersed in dH₂O using a probe sonicator. The nanofluids were characterized by measuring the zeta potential distribution and pH to ensure stability on day 1 and day 10 following preparation. The results show that the zeta potential values range from −31.6 mV to −30.6 mV, while the pH values range from 7.076 to 7.021 on day 1 and day 10, respectively. The novelty of the present study lies in the use of f-GnPs with a controlled size and stable nanofluid, confirmed through zeta potential and pH analysis, to determine the heat transfer behaviour of a Ni-Cr test wire under pool boiling conditions. The pool boiling heat transfer characteristics, such as CHF and BHTC, were observed using the fabricated pool boiling heat transfer test facility. Initially, the dH₂O and f-GnP–dH₂O nanofluids were separately placed in a glass container and heated using a pre-heater to reach their saturation point of 100 °C. The electrical energy was gradually increased until it reached the critical point of the Ni-Cr test wire, i.e., the burnout point, at which it became reddish-yellow hot. The CHF and BHTC were predicted from the experimental outputs of voltage and current. The results showed an enhancement of ~15% in the CHF at 0.1 vol% of f-GnPs. The present study offers a method for enhancing two-phase flow characteristics for heat pipe applications. Full article

This study numerically investigates the growth and dynamics of a single vapor bubble in a rectangular microchannel under subcooled and saturated inlet conditions using the phase-field method coupled with the Lee phase-change model. Results demonstrate that subcooled flow induces early bubble nucleation, pronounced lateral expansion along the heated wall, and prolonged bubble-wall contact due to stronger condensation at the interface and thinner microlayer formation. Enhanced recirculating vortices and steeper thermal gradients promote vigorous evaporation and increased local heat flux, resulting in faster downstream bubble propagation driven by significant axial pressure gradients. Analysis of temperature gradient and heat flux profiles confirms that subcooled conditions produce higher wall heat flux and more frequent peaks in evaporative flux compared to the saturated case, indicating intensified phase-change activity and thermal transport. Conversely, saturated conditions produce more spherical bubbles with dominant vertical growth, weaker condensation, and symmetrical thermal and pressure fields, leading to slower growth and delayed detachment near the nucleation site. These findings highlight the critical influence of inlet subcooling on bubble morphology, flow structures, heat transfer, and pressure distribution, underscoring the thermal management advantages of subcooled boiling in microchannel applications. Full article

Traditional empirical correlations for predicting saturated flow boiling heat transfer coefficients (HTC) often struggle with accuracy and generalizability, particularly across different refrigerants, heat exchanger geometries, and operating conditions. To address these limitations, this study investigates the application of machine learning for more robust HTC prediction. A comprehensive dataset was compiled, consisting of 22,608 data points from over 140 published studies, covering 18 pure refrigerants under diverse experimental setups. The primary goal was to evaluate the performance of different machine learning approaches—Wide Neural Network (WNN), Linear Regression (LR), and Support Vector Machine (SVM)—in predicting HTCs across varying tube types and heat exchanger configurations. The results indicate that the WNN model achieved the highest predictive accuracy, with a Root Mean Square Error (RMSE) of 1.97 and a coefficient of determination (R²) of 0.91, corresponding to less than 5% prediction error for all refrigerants. These outcomes confirm that machine learning models can effectively capture the complex thermofluid interactions involved in boiling heat transfer. This work demonstrates that data-driven methods provide a reliable and generalizable alternative to empirical correlations. Full article

Background: Accurately detecting the onset of saturated boiling in herbal medicine extraction processes is critical for improving production efficiency and reducing energy consumption. However, the traditional monitoring methods based on temperature suffer from time delays. To address the challenge, acoustic emission (AE) signals were used in this study owing to its sensitivity to bubble behavior. Methods: An AE signal acquisition system was constructed for herbal extraction monitoring. Characteristics of AE signals at different boiling stages were analyzed in pure water systems with and without herbs. The performance of AE-based and temperature-based recognition of boiling stages was compared. To enhance applicability in different herb extraction systems, multivariate statistical analysis was adopted to compress spectral–frequency information into Hotelling’s T² and SPE statistics. For real-time monitoring, a self-adaptive decision-making boiling judgment method (BoilStart) was proposed. To evaluate the robustness, the performance of BoilStart under different conditions was investigated, including extraction system mass and heating medium temperature. Furthermore, BoilStart was applied to a lab-scale extraction process of Dabuyin Wan, which is a practical formulation, to assess its performance in energy conservation and efficiency improvement. Results: AE signal in the 75–100 kHz frequency band could reflect the boiling states of herbal medicine extraction. It was more sensitive to the onset of saturated boiling than the temperature signal. Compared with SPE, Hotelling’s T² was identified as the optimal indicator with higher accuracy. BoilStart could adaptively monitor saturated boiling across diverse herbal systems. The absolute error of BoilStart’s boiling determination ranged from 1.5 min to 2.0 min. The increasing-temperature time was reduced by about 22–36%. For the extraction process of Dabuyin Wan, after adopting BoilStart, the increasing-temperature time was reduced by about 29%, and the corresponding energy consumption was lowered by about 26%. Conclusions: The first AE-based method for precise boiling state detection in herbal extraction was established. BoilStart’s model-free adaptability met industrial demands for multi-herb compatibility. This offered a practical solution to shorten ineffective heating phases and reduce energy consumption. Full article

This study experimentally investigates the boiling heat transfer characteristics of R32 and R410A refrigerants in heat exchangers, systematically analyzing the effects of tube thickness, saturation temperature, latent heat, liquid-phase density, and viscosity. The average boiling heat transfer coefficients (HTCs) of R32 and R410A were compared across varying mass flow rates and saturation temperatures. The results reveal that, independent of tube thickness, the boiling HTC of R32 exhibits a non-monotonic increase followed by a decrease with rising mass flow rate. Additionally, elevated saturation temperatures reduced vaporization latent heat, liquid-phase density, and gas-phase viscosity, while the flow pattern may also change. Meanwhile, R32 demonstrated superior boiling heat transfer performance compared to R410A under equivalent conditions. Furthermore, the correlation is proposed to predict the HTCs, indicating ±10% prediction error. This study provides critical insights for optimizing refrigeration systems and advancing heat exchanger modeling frameworks. Full article

This article presents the results of a study on the possibility of using a single-speed multiphase model with free surface allowance for simulating boiling and condensation processes. The simulation is based on the VOF method, which allows the position of the interphase boundary to be tracked. To increase the stability of the iterative procedure for numerically solving volume fraction transfer equations using a finite volume discretization method on arbitrary unstructured grids, the basic VOF method is been modified by writing these equations in a semi-divergent form. The models of Tanasawa, Lee, and Rohsenow are considered models of interphase mass transfer, in which the evaporated or condensed mass linearly depends on the difference between the local temperature and the saturation temperature with accuracy in empirical parameters. This paper calibrates these empirical parameters for each mass transfer model. The results of our study of the influence of the values of the empirical parameters of models on the intensity of boiling and evaporation, as well as on the dynamics of the interphase boundary, are presented. This research is based on Stefan’s problem of the movement of the interphase boundary due to the evaporation of a liquid and the problem of condensation of vapor bubbles water columns. As a result of a series of numerical experiments, it is shown that the average error in the position of the interfacial boundary for the Tanasawa and Lee models does not exceed 3–6%. For the Rohsenow model, the result is somewhat worse, since the interfacial boundary moves faster than it should move according to calculations based on analytical formulas. To investigate the possibility of condensation modeling, the results of a numerical solution of the problem of an emerging condensing vapor bubble are considered. A numerical assessment of its position in space and the shape and dynamics of changes in its diameter over time is carried out using the VOF method, taking into account the free surface. It is shown herein that the Tanasawa model has the highest accuracy for modeling the condensation process using a VOF method taking into account the free surface, while the Rohsenow model is most unstable and prone to deformation of the bubble shape. At the same time, the dynamics of bubble ascent are modeled by all three models. The results obtained confirm the fundamental possibility of using a VOF method to simulate the processes of boiling and condensation and taking into account the dynamics of the free surface. At the same time, the problem of the studied models of phase transitions is revealed, which consists of the need for individual selection of optimal values of empirical parameters for each specific task. Full article

A rigorous framework for determining actual and minimum boil-up ratios in binary distillation combining analytical mass and energy balances with an extended Ponchon–Savarit graphical approach was implemented. First, global balances across the enriching and stripping sections yield a closed-form expression of the boil-up ratio (V_B) based on enthalpy differences. Second, the V_B was directly determined from an enthalpy–composition diagram by measuring the enthalpy segments between the saturated liquid, vapor, and heat-duty points. Applying this method to high-stage columns confirms that the two methods converge on identical V_B values. Based on these findings, a unified graphical methodology was developed to determine the minimum boil-up ratio (V_Bmin). V_Bmin can be determined on the same diagram by locating the intersections of the extremal tie lines in both the enriching and exhausting sections, analogous to the reflux-pinch points. This procedure was systematically validated across the five canonical feed thermal states. The implemented method is a graphical approach based on the Ponchon–Savarit technique, developed for binary systems. Full article

This study presents a comprehensive experimental investigation into the frictional pressure drop of two-phase flows—boiling and condensation—in horizontal minichannels, emphasizing its impact on the energy efficiency of vapor compression systems. A total of 3553 data points were obtained using six low-GWP refrigerants (R32, R134a, R290, R410A, R513A, and R1234yf) across a wide range of operating conditions in multiport aluminum tubes with hydraulic diameters of 0.715 mm and 1.16 mm. The dataset covers mass fluxes from 200 to 1230 $\mathrm{kg}{\mathrm{m}}^{-2}{\mathrm{s}}^{-1}$, saturation temperatures between 5 °C and 55 °C, and vapor qualities from 0.05 to 0.95. Results showed a strong dependence of frictional pressure gradient on vapor quality, mass flux, and channel size. Boiling flows generated higher frictional losses than condensation, and high-density refrigerants such as R32 exhibited the largest pressure penalties, which can directly translate into increased compressor power demand. Conversely, higher saturation temperatures were associated with lower frictional losses, highlighting the role of thermophysical properties in improving energy performance. Additionally, an inverse correlation between saturation temperature and frictional pressure gradient was observed, attributed to variations in thermophysical properties such as viscosity and surface tension. Existing correlations from the literature were assessed against the experimental dataset, with notable deviations observed in several cases, particularly for R134a under high-quality conditions. Consequently, a new empirical correlation was developed for predicting the frictional pressure drop in two-phase flow through minichannels. The proposed model, formulated using a power-law regression approach and incorporating dimensionless parameters, achieved better agreement with the experimental data, reducing prediction error to within ±20%, improving the accuracy for the majority of cases. This work provides a robust and validated dataset for the development and benchmarking of predictive models in compact heat exchanger design. By enabling the more precise estimation of two-phase pressure drops in compact heat exchangers, the findings support the design of refrigeration, air-conditioning, and heat pump systems with minimized flow resistance and reduced auxiliary energy consumption. This contributes to lowering compressor workload, improving coefficient of performance (COP), and it ultimately advances the development of next-generation cooling technologies with enhanced energy efficiency. Full article