Screen channel LAD bubble point tests in liquid hydrogen

This paper presents experimental results for the liquid hydrogen bubble point tests for liquid acquisition devices (LADs) operating in low gravity cryogenic propulsion systems. The purpose of the test was to investigate parameters that affect screen channel LAD performance in a low pressure liquid hydrogen (LH₂) propellant tank and to demonstrate several ways to increase the LH₂ bubble point pressure. Three fine mesh screen channel LAD samples were tested in LH₂ over the range of 16.7 K < T < 21.1 K and 31.5 kPa < P < 155 kPa using gaseous helium and hydrogen as pressurant gases. Results show that bubble point pressure is affected by screen mesh type, liquid temperature and pressure, and type of pressurization gas. Higher bubble points are achieved by using a finer mesh screen and pressurizing and subcooling the liquid with gaseous helium. In addition, there is evidence that the screen pore is itself temperature dependent.     

Investigation on no-vent filling process of liquid hydrogen tank under microgravity condition

In order to investigate the no-vent filling performance under microgravity, the computational fluid dynamic (CFD) method is introduced to the study, where a model aiming at filling a liquid hydrogen (LH₂) receiver tank is especially established. In this model, the solid and fluid regions are considered together to predict the coupled heat transfer process. The phase change effect during the filling process is also taken into account by embedding a pair of mass and heat transfer models into the CFD software FLUENT, one of which involves liquid flash driven by pressure difference between the fluid saturated pressure and the tank pressure, and the other one indicates and calculates the evaporation–condensation process driven by temperature difference between fluid and its saturated state. This CFD model, verified by experimental data, could accurately simulate the no-vent filling process with good flexibility. Moreover, no-vent filling processes under different gravities are comparatively analyzed and the effects of four factors including inlet configuration, inlet liquid temperature, initial wall temperature and inlet flow rate, are discussed, respectively. Main conclusions could be made as follows: 1) Compared to the situations in normal gravity, the no-vent filling in microgravity experiences a more adequate liquid–vapor mix, which results in a more steady pressure response and better filling performance. 2) Inlet configuration seems to have negligible effect on the no-vent filling performance under microgravity since liquid could easily reach the tank wall and then cause a sufficient fluid-wall contact under any inlet condition. 3) Higher initial tank wall temperature may directly cause a higher pressure rise in the beginning, while this effect on the final pressure is not significant. Sufficient precooling and reasonable inlet liquid subcooled degree are suggested to guarantee the reliability and efficiency of the no-vent fill under microgravity.     

Parametric optimization and analysis of thermodynamic venting system in liquid hydrogen tank under microgravity

This paper used the lumped vapor model to simulate the self-pressurization and thermodynamic venting process in the cryogenic liquid hydrogen tank with 90% filling rate under microgravity. Based on the orthogonal experiment, twenty-five cases with different parameters were designed. Daily evaporation rate and cooling capacity utilization efficiency were proposed to evaluate the exhaust loss during the jet process and depressurization efficiency during depressurization process. The four parameters of spray bar including exhaust rate, diameter of inner tube of heat exchanger, nozzle length and number of nozzles were optimized and the weight of influence on the daily evaporation rate and cooling capacity utilization efficiency was analyzed. Results showed that nozzle length and number of nozzles played a key role in the daily evaporation rate and cooling capacity utilization efficiency, while diameter of inner tube of heat exchanger had little effect. Under the same mass flow, smaller nozzle length, lower exhaust rate and fewer nozzles could increase the inlet velocity, thereby reducing the daily evaporation rate and improving the cooling capacity utilization efficiency. The thermal stratification could be eliminated well. In addition, arranging two nozzles up and down was more advantageous than arranging a single nozzle in the middle. The research results were useful for improving the efficiency of the thermodynamic venting system (TVS) and extending the propellant storage time in orbit.     

Effect of microgravity on flow boiling heat transfer of liquid hydrogen in transportation pipes

The flow boiling phenomenon of liquid hydrogen (LH2) during transportation in microgravity is very different from that under terrestrial condition. In this study, a saturated flow boiling of LH2 in a horizontal tube has been simulated under microgravity condition using coupled level-set and volume of fluid method. The validation of the developed model shows good agreement with the experimental data from the literature. The changes of heat fluxes and pressure drops under different gravitational accelerations were analyzed. And, the variation of heat fluxes with different wall superheat and contact angle were compared between microgravity (10⁻⁴g) and normal gravity (1g) condition. Also, the influence of surface tension were studied under microgravity. The numerical results indicate that the heat flux decrease with the decrement of gravitational acceleration. And the heat transfer ratio decrease with the increment of wall superheat in the nucleate boiling regime. The heat transfer slightly reduce when considering surface tension. In addition, the changes of contact angle have a more significant impact on heat transfer under microgravity condition.     

Efficiency analysis of depressurization process and pressure control strategies for liquid hydrogen storage system in microgravity

In order to investigate dynamic characteristics of pressure fluctuation and thermal efficiency of a liquid hydrogen (LH₂) storage system during depressurization process under microgravity condition, a transient CFD model of LH₂ tank is established. Based on the assumption of lumped vapor, a UDF code is developed to solve phase change and heat transfer between liquid phase and vapor one. The thermal efficiency is provided for assessing the performance of different pressure control methods. Results show that raising the injection velocity and decreasing the temperature of the injection liquid can enhance the effect of fluid mixing and shorten the depressurization time. Increasing the pressure lower limit can also improve the efficiency of depressurization process. The model can predict the tendency of pressure changes in the tank, and provide theoretical guide to design LH₂ tank and optimize its parameters for space application.     

Diffusion of liquid hydrogen and oxygen in nonlinear mixed convection nanofluid flow over vertical cone

The phenomenon of triple diffusive nonlinear mixed convection over a vertical cone is completely a new concept of rheological study and behaviours of such flows are not reported in the open literature as yet. The study of such flows has relevance to various industrial applications. In the current study, nanoparticles are considered in the working fluid that comprises of water as base fluid with three diffusive components, namely, heat, liquid hydrogen and oxygen. This innovative physical problem has mathematical formulation in the form of nonlinear partial differential equations (PDE's). In order to simplify the mathematical analysis, these equations are non-dimensionalised and then linearized, by utilizing non-similar Mangler's transformations and technique of Quasilinearization, respectively. The implicit finite difference scheme is used to transform the linear PDE's into block tri-diagonal system, which is then solved by utilizing Varga's algorithm. In this work, interesting results have been obtained, for example, the presence of nonlinear convection parameter for temperature leads to increase in the velocity profile, local skin-friction coefficient as well as the local wall heat transfer rate, while it causes reduction in the temperature profile. The wall suction reduces the concentration profiles, while it increases the corresponding gradients. The local Nusselt number is low for the mixture of nanoparticles, liquid hydrogen and oxygen, and water as compared to that for the corresponding ordinary mixture (i.e. not containing the nanoparticles). The surface roughness effects on transport rates are observed in terms of their sinusoidal variations which are prominent away from the apex of the cone. Further, the impacts of nanoparticles remains same for the present flow regime as in case of regular water based nanofluid flow systems.     

Performance analysis of vapor-cooled shield insulation integrated with para-ortho hydrogen conversion for liquid hydrogen tanks

A composite thermal insulation system consisting of variable-density multi-layer insulation (VDMLI) and vapor-cooled shields (VCS) integrated with para-ortho hydrogen (P-O) conversion is proposed for long-term storage of liquid hydrogen. High-performance thermal insulation is realized by minimizing the thermal losses via the VDMLI design and fully recovering the cold energy released from the sensible heat and P-O conversion of the vented gas. Effects of different design considerations on the thermal insulation performance are studied. The results show that the maximum reduction of the heat leak with multiple VCSs can reach 79.9% compared to that without VCS. The heat leak with one VCS is reduced by 61.1%, and further reduced by 11.6% after adding catalysts. It is found that the deterioration of the insulation performance has an almost linear relationship with catalytic efficiency. A unified criterion with relative optimization efficiency is finally proposed to evaluate the improvement of the VCS number.     

Research on systems for producing liquid hydrogen and LNG from hydrogen-methane mixtures with hydrogen expansion refrigeration

In industrial production processes, gas mixtures with hydrogen and methane as main useful components are often obtained as by-products, which are often not well utilized. In this paper, an innovative approach is proposed to produce both liquid hydrogen and LNG from industrial by-products with H₂ and CH₄ as main components. Taking the purified hydrogen-methane mixtures as the research object, four different separation-liquefaction processes (namely Open Loop-N₂, Open Loop-H₂, Closed Loop-N₂, Closed Loop-H₂) are constructed and optimized, with refrigeration supplied with hydrogen expansion at the cryogenic section and nitrogen or hydrogen expansion at the precooling section. A distillation column is set up before the mixture enters the cryogenic section to facilitate the production of high purity methane and hydrogen products. Every system achieves excellent energy integration, and the load of condenser and reboiler in the column is borne by the hydrogen expansion cycle in the cryogenic section. For each process, the influence of hydrogen mole proportion in feed gas between 10% and 90% on the process performance is analyzed. The results show that the purities of LNG and liquid hydrogen products obtained by the system are higher than 99.99%, and the specific energy consumption of the systems is within 18.01–41.72 kWh·kmol⁻¹ for different situations. At the same time, an open loop and a closed loop are constructed, respectively, to investigate the necessity of recovering cold energy of boil-off gas. The results suggest recommendation of open loop system with nitrogen precooling.     

Technical and cost analysis of imported hydrogen based on MCH-TOL hydrogen storage technology

The international hydrogen supply chain has been commercialized and promoted hydrogen trade. With the global energy transition, the two are expected to play a more important role and make hydrogen become a major international energy trade category similar to natural gas and LNG. This paper considers importing two hydrogen sources to Huizhou of China through MCH-TOL hydrogen storage technology from Saudi Arabia, which are produced from Natural gas + CCS and from renewable energy sources. It is estimated that the costs of dehydrogenation and purification after landing are 27.6 CNY/kgH₂ and 32.7 CNY/kgH₂ respectively, which are difficult to be competitive. Therefore, the strategy and goal of cost reduction are proposed. It is expected to control the costs of dehydrogenation and purification to less than 25 CNY/kgH₂, and explore the feasibility of developing large-scale and economically competitive hydrogen import business in China.     

Prediction of liquid hydrogen flow boiling critical heat flux condition under microgravity based on the wall heat flux partition model

Critical heat flux (CHF) of liquid hydrogen (LH2) flow boiling under microgravity is vital for designing space cryogenic propellant conveying pipe since the excursion of wall temperature may cause system failure. In this study, a two-dimensional axisymmetric model based on the wall heat flux partition (WHFP) model was proposed to predict the CHF condition under microgravity including the wall temperature and the CHF location. The proposed numerical model was validated to demonstrate a good agreement between the simulated and experimentally reported results. Then, the wall temperature distribution and the CHF location under different gravity conditions were compared. In addition, the WHFP and vapor-liquid distribution along the wall under microgravity were predicted and its difference with terrestrial gravity condition was also analysed and reported. Finally, the effects of flow velocity and inlet sub-cooling on the wall temperature distributions were analysed under microgravity and terrestrial gravity conditions, respectively. The results indicate that the CHF location moves upstream about 5.25 m from 1g to 10⁻⁴g since the void fraction near the wall reaches the breakpoint of CHF condition much earlier under the microgravity condition. Furthermore, the increase of the velocity and decrease of the sub-cooling have smaller effects on the CHF location during LH2 flow boiling under microgravity.     

Influence of surface evaporation on stratification in liquid hydrogen tanks of different aspect ratios

The effect of evaporation on stratification in large liquid hydrogen storage tanks of different aspect ratios is computed. A homogeneous two-phase model is used and the continuity, momentum and energy equations for the two phases are solved. Evaporation at the liquid–vapor interface is incorporated through a source term for mass transfer. The amount of stratification is seen to progressively increase as the aspect ratio of the tanks increases. However, the surface evaporation brings down the differences in the amount of stratification with changes of aspect ratio. The predictions for stratification in the pre-evaporation and evaporation phases are discussed.     

Influence of liquid hydrogen and nitrogen on MHD triple diffusive mixed convection nanoliquid flow in presence of surface roughness

An innovative study of influence of surface roughness and nanoparticles on mixed convection flow is considered in presence of liquid hydrogen and liquid nitrogen. In fact, in order to understand the effects of surface roughness and nanoparticles on the flow characteristics of MHD triple diffusive mixed convection nanoliquid flow along an exponentially stretching rough surface, the flow problem is modelled in terms of highly nonlinear partial differential equations subject to the appropriate boundary conditions. Then, those equations are made non-dimensional with the application of non-similar transformations. The resultant nonlinear dimensionless coupled partial differential equations with boundary constraints are solved by using the Quasilinearization technique in combination with the implicit finite difference scheme. The liquid hydrogen and liquid nitrogen are considered as species concentration components. The surface roughness is modelled by a sine wave representation and hence the sinusoidal variations have been observed in gradients such as skin-friction coefficient, heat and mass transfer rates. It is observed that the effects of surface roughness on the skin-friction coefficient are more prominent near the origin than that in downstream. The addition of nanoparticles into the ambient ordinary fluid enhances the skin-friction coefficient and reduces the magnitude of wall heat transfer rate for both cases of smooth and rough surfaces. The rapid variations have been observed in the wall mass transfer rate due to the surface roughness in comparison to that of skin-friction coefficient and wall heat transfer rate. Further, the magnitude of wall mass transfer rate of liquid nitrogen is higher than that of liquid hydrogen.     

Theoretical investigation on heat leakage distribution between vapor and liquid in liquid hydrogen tanks

As a key factor affecting thermal behaviors of liquid hydrogen (LH₂) tanks, heat leakage plays an important role in accurate prediction of pressure build-up for safe storage and transportation of LH₂. Uniform heat flux between vapor and liquid in LH₂ tanks is widely adopted as thermal boundary condition in predicting pressure build-up process. However, a distribution of heat flux between vapor and liquid was observed during the self-pressurization process in the experimental test. In light of this, an analytically theoretical model of revealing the energy exchange process among the vapor, liquid and inner wall is proposed to investigate the heat leakage distribution ratio (HDR) between vapor and liquid in LH₂ tanks. The feasibility of the model is validated by the experimental results from NASA. In the whole self-pressurization process of 25,000 s, HDR reduces from 0.803 to 0.235 under a liquid fill ratio of 90% and a total heat leakage of 71.3 W. The results show that the existence of inner wall and different thermal properties between the vapor and liquid make the heat leakage flux non-uniformly distributed into the vapor and liquid. And the geometric structure of tank, thermal properties and initial states of the vapor and liquid have a significant effect on HDR. When coupling the model with thermal multi-zone model, the relative error in pressure prediction is reduced by 61.8% against experimental results. Benefiting from the coupled model, the relative error in pressure prediction caused by the uniform heat flux boundary condition reduces from 90.16% to 8.15%. The present work establishes theoretical foundation on analyzing heat leakage distribution between the vapor and liquid for LH₂ tanks, and provides useful guidance on modifying boundary conditions in accurately predicting thermal behaviors of LH₂ tanks.     

Study on reversible hydrogen uptake and release of 1,2-dimethylindole as a new liquid organic hydrogen carrier

Indole derivatives have been considered as promising liquid organic hydrogen carriers (LOHCs) for onboard hydrogen storage applications. Here a new member of indole family, 1,2-dimethylindole (1,2-DMID), was reported as a potential liquid organic hydrogen carrier with a hydrogen storage content of 5.23 wt%, a meting point of 55 °C and a boiling point of 260 °C. Full hydrogenation and dehydrogenation of 1,2-DMID can be achieved with fast kinetics under mild conditions. The hydrogenation of 1,2-DMID followed the first order kinetics with an apparent activation energy of 85.1 kJ/mol. Dehydrogenation of fully hydrogenated product, octahydro-1,2-DMID was conducted over 5 wt% Pd/Al₂O₃ at 170–200 °C. The stored hydrogen can be completely released at 180 °C in 3 h and at 200 °C in 1 h. The energy barrier of dehydrogenation of octahydro-1,2-DMID was calculated to be 111.9 kJ/mol 3 times cycles of hydrogenation and dehydrogenation were employed to test the recycle ability of 1,2-DMID. The structures of intermediates were also discussed by means of Material Studio calculations.     

Numerical investigation of the leakage and explosion scenarios in China's first liquid hydrogen refueling station

Promoting fuel cells has been one of China's ambitious hydrogen policies in the past few years. Currently, several hydrogen fueling stations (HRSs) are under construction in China to fuel hydrogen-driven vehicles. In this regard, it is necessary to assess the risks of hydrogen leakage in HRSs. Aiming at conducting a comprehensive consequence assessment of liquid hydrogen (LH2) leakage on China's first liquid hydrogen refueling station (LHRS) in Pinghu, a pseudo-source model is established in the present study to simulate the LH2 leakage using a commercial CFD tool, FLACS. The effects of the layout of the LHRS, leakage parameters, and local meteorological conditions on the LH2 leakage consequence has been assessed from the perspectives of low-temperature hazards and explosion hazards. The obtained results reveal that considering the prevailing southeast wind in Pinghu city, the farthest low-temperature hazard distance and lower flammable limit (LFL) -distance occurs in the leakage scenario along the north direction. It is found that the trailer parking location in the current layout of the LHRS will worsen the explosion consequences of the LH2 leakage. Moreover, the explosion will completely destroy the control room and endanger people on the adjacent road when the leakage equivalent diameter is 25.4 mm. The performed analyses reveal that as the wind speed increases, the explosion hazard decreases.     

Numerical investigation on subcooled pool film boiling of liquid hydrogen in different gravities

A clear understanding of bubble dynamics and heat transfer characteristics of hydrogen boiling in microgravity is significant for achieving safe and high-efficiency utilization of liquid hydrogen in space. In the present paper, a numerical simulation model is developed to predict the subcooled pool film boiling for liquid hydrogen in different gravities. The computations are based on the volume of fluid method combined with Lee's phase change model. The results show that the bubble released from the wavy gas-liquid interface might grow to a larger size before departure with the decrease of gravity, and poor heat transfer performance is observed in reduced gravity. However, once the gravity level is low enough or the subcooling of liquid is sufficiently large, instead of bubble formation and release at the vapor-liquid interface, a thin gas film layer is almost observed and maintained in the surface of horizontal flat or wire heater.     

Effect of gravity conditions on heat transfer and flow fluctuations of liquid hydrogen in a non-isothermal horizontal circular tube

Liquid hydrogen phase transition is a common phenomenon in space missions for space vehicles using low temperature liquid hydrogen as propellant. In this study, a numerical model with coupled RANS solver and VOF/Level-set method was used to simulate the liquid hydrogen phase transition in a non-isothermal horizontal circular tube under different gravity conditions (1g-10^?4 g). The gas phase hydrogen produced by evaporation of liquid hydrogen was calculated by Lee model. The statistics of the overall volume, heat flux, mass flow rate, mean velocity of gas phase hydrogen was carried out. The data results shown that the flow fluctuations were strongest under the gravity acceleration of 10^?1 g relative to other gravity conditions. The average bubble volume at 10^?1 g was the smallest, which was 11.58% smaller than that at 10^?3 g condition. The intermittent contact with the tube wall, which leaded to intermittent long bubble and flow resistance, was the main reason.     

Effects of anodic gas conditions on performance and resistance of a PBI/H3PO4 proton exchange membrane fuel cell with metallic bipolar plates

In this study, polybenzimidazole (PBI) is used as membrane material of the high-temperature membrane electrode assembly which has the features of high-performance stability and high CO tolerance. Moreover, compared to graphite bipolar plates, metallic bipolar plates have better mechanical properties and seismic capacity, as well as lighter weight. We thus use metallic bipolar plates and a PBI-based membrane electrode assembly to setup a single cell and examine its performance. The experimental results show that the cell temperature has a significant effect on the cell performance. When the temperature increases from 120 °C to 180 °C, the performance is significantly enhanced. Moreover, the CO tolerance of the fuel cell increases along with the temperature. At the same time, methane is fed in the anode stream to assess the performance of the cell under different simulated methane reformate gases. The test of various CH₄/H₂ mixtures reveals the residual methane in the reformate gases only decreases fuel cell performance slightly due to the dilution effect. We also examined H₂/CO/N₂/CH₄ mixtures in this study, and these had only a small effect on the fuel cell performance at cell temperatures higher than 160 °C. As such, it is recommended that the cell temperature should be kept higher than 160 °C.     

Numerical study on unsteady heat transfer and fluid flow in a closed cylinder of reciprocating liquid hydrogen pumps

Modeling and optimization of liquid hydrogen (LH₂) pumps require accurate in-cylinder heat transfer correlations. However, the applicability of existing correlations based on gas mediums to LH₂ remains to be verified. In this paper, the unsteady heat transfer and fluid flow in a closed LH₂ pump cylinder are numerically studied by adopting the gas spring model. The phase shifts and temperature distribution in the closed pump cylinder are investigated. LH₂ is less affected by in-cylinder heat transfer and has a more uniform temperature distribution compared to nitrogen gas, while a low-temperature zone appears near the piston face at 120 rpm. Finally, the validity of Lekic's correlation in predicting the heat flux of the LH₂ compression process in the closed pump cylinder is verified, and the efficiency decrement versus rotational speed is analyzed based on the correlation. This work would be useful for selecting a proper in-cylinder heat transfer model for predicting the thermodynamic process in reciprocating LH₂ pumps.     

Modeling and thermodynamic analysis of thermal performance in self-pressurized liquid hydrogen tanks

Liquid hydrogen (LH₂) is one of the most economic methods for large-scaled utilization of hydrogen energy. However, safe operation and storage of LH₂ relies on accurate prediction of the pressure rise and adequate investigation on thermal behaviors inside LH₂ tank. In light of this, a modified thermal multi-zone model (TMZM) considering heat and mass transfer between vapor and liquid is developed in this paper. The model has a maximum relative error of 4.67% in predicting pressure rise against the experimental results from NASA. A thermodynamic analysis method is proposed to clarify the influences of key parameters including the temperature, compressibility factor and density of vapor, and working conditions including heat leakage and initial superheated degree on the pressurization rate. The results indicate that temperature of vapor in the ullage and vapor-liquid interfacial mass transfer rate are two main parameters determining the pressurization rate, and the effects of the two parameters are different between different stages. The distinction of stages depends on heat leakage and initial superheated degree. For the working condition with an initial filling rate of 50% and a heat leakage of 10 W, temperature of vapor is the parameter dominates pressurization rate during 96.8% of the whole self-pressurization process. Heat leakage also has a vital impact on the distinction of stages, when heat leakage increases to 80 W, the temperature of vapor dominating stage will reduce to 46.4%. Furthermore, pressurization rate is sensitive to initial superheated degree in the ullage. An increase of 4 K of the initial superheated degree leads to a 53.3% decrease of the pressurization rate. This study provides a useful method for the reliable design and quick optimization of high performance LH₂ tanks.