Fluid thermal stratification in a non-isothermal liquid hydrogen tank under sloshing excitation

To the safe space operation of cryogenic storage tank, it is significant to study fluid thermal stratification under external heat leaks. In the present paper, a numerical model is established to investigate the thermal performance in a cryogenic liquid hydrogen tank under sloshing excitation. The interface phase change and the external convection heat transfer are considered. To realize fluid sloshing, the dynamic mesh coupled the volume of fluid (VOF) method is used to predict the interface fluctuations. A sinusoidal excitation is implemented via customized user-defined function (UDF) and applied on tank wall. The grid sensitivity study and the experimental validation of the numerical mode are made. It turns out that the present numerical model can be used to simulate the unsteady process in a non-isothermal sloshing tank. Variations of tank pressure, liquid and vapor mass, fluid temperature and thermal stratification are numerically investigated respectively. The results show that the sinusoidal excitation has caused large influence on thermal performance in liquid hydrogen tank. Some valuable conclusions are arrived, which is important to the depth understanding of the non-isothermal performance in a sloshing liquid hydrogen tank and may supply some technique reference for the methods of sloshing suppression.     

Experimental analysis on the effect of pipe and orifice diameter in inter tank hydrogen transfer

Hydrogen is an energy carrier which can be utilized in many sectors like stationary and transportation energy with nearly zero emission. Hydrogen energy is more efficient when compared to other energy sources. Hydrogen can be a replacement for fossil fuels in future as it emits only water when it is burned. In this work a mathematical model of transfer of hydrogen between two tanks has been developed using MATLAB simulink software version 21. Flow of hydrogen inside the pipe is controlled by orifice and diameter of this orifice and pipe diameter itself has some impact on outcome parameters such as inlet temperature of pipe, outlet temperature of pipe, heat transfer from one tank to other tank and hydrogen gas flow rate. The influence of orifice diameter as well as initial pressures on outcome parameters of hydrogen gas transfer model has analyzed in this work. From the simulation results it is inferred that when one initial pressure kept constant and other initial pressure keep on varying, no change in inlet temperature, decrease in outlet temperature, increase in heat transfer and increase in gas flow rate were observed when orifice diameter increase in size from 2 cm then 4 cm and then 6 cm. The research work has significant guidance for safety and efficient way of transporting hydrogen through pipeline from one tank to other tank.     

Fluid sloshing dynamic performance in a liquid hydrogen tank

In the present study, a numerical model is built to investigate the hydrodynamic performance in a sloshing liquid hydrogen tank under a sinusoidal excitation. The motion mesh coupled the volume of fluid method is adopted to capture the fluctuation of the free surface during sloshing. The sloshing dynamic response of the free surface is specially evaluated. Meanwhile, the sloshing force and moment, and pressure variation are numerically studied. The results show that the free surface has stable interface shapes with “Z” or “S” type profiles in the initial period. As time elapses, the sinusoidal wave propagates, some disturbances occur at the interface with different wave amplitudes. For fluid close to the tank wall, it suffers much more from external excitation with large amplitude fluctuations. For the symmetrically distributed measuring points, opposite fluctuating profiles form with almost the same amplitude. Influenced by fluid motion, the point of the maximum liquid pressure makes fluctuations as well. The measuring points far from the symmetry axis of the tank have severe fluctuating variations. With some valuable conclusions arrived, the present study is significant to the in-depth comprehension on fluid sloshing dynamical behavior in non-isothermal cryogenic tanks.     

Thermal design analysis of a 1 L cryogenic liquid hydrogen tank for an unmanned aerial vehicle

Thermal design analysis of a 1-L cryogenic liquid hydrogen storage tank without vacuum insulation for a small unmanned aerial vehicle was carried out in the present study. To prevent excess boil-off of cryogenic liquid hydrogen, the storage tank consisted of a 1-L inner vessel, an outer vessel, insulation layers and a vapor-cooled shield. For a cryogenic storage tank considered in this study, the appropriate heat inleak was allowed to supply the boil-off gas hydrogen to a proton electrolyte membrane fuel cell as fuel. In an effort to accommodate the hydrogen mass flow rate required by the fuel cell and to minimize the storage tank volume, a thermal analysis for various insulation materials was implemented here and their insulation performances were compared. The present thermal analysis showed that the Aerogel thermal insulations provided outstanding performance at the non-vacuum atmospheric pressure condition. With the Aerogel insulation, the tank volume for storing 1-L liquid hydrogen at 20 K could be designed within a storage tank volume of 7.2 L. In addition, it was noted that the exhaust temperature of boil-off hydrogen gas was mainly affected by the location of a vapor-cooled shield as well as thermal conductivity of insulation materials.     

Numerical analysis of convective flow and thermal stratification in a cryogenic storage tank

In this study, the liquid–vapor mixture model was used for a numerical study of natural convective flow in a cryogenic tank with a capacity of 4.9?m³ under various conditions of heat flux and filling level to understand the early stages of convective flow phenomena and the consequent thermal stratification of cryogenic liquid. Two cryogens—liquefied natural gas (LNG) and liquefied nitrogen (LN₂)—were compared to observe their effects. LN₂ exhibited faster vaporization owing to its lower heat of vaporization. It was observed that higher heat flux and lower filling level led to faster vaporization and relatively vigorous heat transfer, showing early thermal stratification.     

Analysis and assessment of partial re-liquefaction system for liquefied hydrogen tankers using liquefied natural gas (LNG) and H2 hybrid propulsion

Boil-off gas (BOG) is inevitable on board liquefied hydrogen tankers and must be managed effectively, by using it as fuel, re-liquefying it or burning it, to avoid cargo tank pressure issues. This study aims to develop a BOG re-liquefaction system optimized for l60,000 m³ liquefied hydrogen tankers with an LNG and hydrogen hybrid propulsion system. The proposed system comprises hydrogen compression and helium refrigerant sections with 2 J–Brayton cascade cycles. Cold energy recovery from the fuels and feed BOG exiting the cargo tanks was used. The system exhibits a coefficient of performance (COP) of 0.07, a specific energy consumption (SEC) of 3.30 kWh/kg_LH2, and exergy efficiency of 74.9%, with the hydrogen BOG entering the re-liquefaction system at a feed temperature of −220 °C. The theoretical COP and SEC values at ideal conditions were 0.09 and 2.47 kWh/kg_LH2, respectively. The effects of varying the hydrogen compression pressure, inlet temperature of the hydrogen expander, feed hydrogen temperature and helium compression pressure were investigated. Additionally, the LNG-to-hydrogen fuel ratio was adjusted to satisfy the Energy Efficiency Design Index (EEDI) Phase 2 and 3 emission requirements.     

Effect of gas injection mass flow rates on the thermal behavior in a cryogenic fuel storage tank

Large scale using of liquid hydrogen and liquid oxygen on energy engineering, chemical engineering and petrochemical industries, bring a series of non-equilibrium thermal behaviors within fuel storage tanks. Accurate simulation on the thermal behavior in cryogenic fuel storage tanks is therefore a critical issue to improve the operation safety. In the present study, a 2-dimensional numerical model is developed to predict the active pressurization process and fluid thermal stratification in an aerospace fuel storage tank. Both external heat penetration and heat exchange occurring at the interface are accounted for in detail. The volume of fluid method is adopted to predict the thermal physical process with high-temperature gas injected into the tank. The effect of the gas injection mass flow rate on the tank pressure, the interface phase change, and the fluid temperature distribution are investigated respectively. Finally, some valuable conclusions are obtained. The present study may supply some technique references for the design of the pressurization system.     

Estimation of indirect CO2 emissions of a hydrogen‐powered medium size vehicle for the NEDC cycle

Reduction in greenhouse effect gases emission is a major source of concern nowadays. Internal combustion engines, as the most widely used power generation mean for transportation, represent a large share of such gases, which motivates active research efforts for alternative solutions. In this regard, PEM fuel cells represent a promising prospect and are thoroughly investigated, whether experimentally or through numerical simulation. The present work presents a simulation of the power potential of a PEM fuel cell, which is integrated to the full power electric traction chain of a medium size car. The cell potential is modelled by taking into account the different types of polarization. The driving performances of the vehicle and its hydrogen consumption are evaluated through a simple mathematical model and an application is performed for the New European Driving Cycle (NEDC) standard driving cycle. A preliminary sizing of the proton exchange membrane fuel cell (PEMFC) membrane area for the chosen vehicle is presented, along with that of a hydrogen storage tank for a typical autonomy. The main goal of the simulation is to estimate CO₂ indirect emissions due to the production of the needed hydrogen for the cycle via an electrolyser, compared with the case of a gasoline fueled vehicle. This is performed solely on a ‘fuel tank to wheel’ basis in order to have comparable figures. The results indicate that the environmental advantage of hydrogen cars is quite questionable if hydrogen is produced using carbon‐based energy sources. Copyright © 2009 John Wiley & Sons, Ltd.     

Cryogenic thermal analysis of the HANARO cold neutron moderator cell

Accurate prediction of a temperature distribution is very important for investigating the heat transfer characteristics in a cryogenic system such as a cold neutron moderator cell. The cold neutron moderator cell is a 13.2-cm-diameter cylinder with the liquid hydrogen in it contained by an aluminum alloy cell. It will be placed in a cold neutron hole that is inside the heavy water reflector tank of HANARO. The liquid hydrogen serves both as a neutron moderator and a single-phase cryogenic coolant for the cold neutron moderator cell. For the cold neutron moderator cell, its interactions with neutrons and gamma-rays produce a heat load of 629 W on the aluminum alloy cell and liquid hydrogen itself. A half of the cold neutron moderator cell was modeled with a three-dimensional axisymmetric cylinder geometry by using the HEATING 7.3 code. A numerical analysis was performed to determine the temperature distribution over the entire inner surface of the moderator cell, whose moderator and cell walls are heated differently, under a steady-state operating condition. This paper presents the steady-state temperature distribution for providing the cryogenic thermal analysis and for supporting the thermal stress analysis of the moderator cell walls.     

CFD simulation of heat transfer and phase change characteristics of the cryogenic liquid hydrogen tank under microgravity conditions

The heat transfer and phase change processes of cryogenic liquid hydrogen (LH₂) in the tank have an important influence on the working performance of the liquid hydrogen-liquid oxygen storage and supply system of rockets and spacecrafts. In this study, we use the RANS method coupled with Lee model and VOF (volume of fraction) method to solve Navier-stokes equations. The Lee model is adopted to describe the phase change process of liquid hydrogen, and the VOF method is utilized to calculate free surface by solving the advection equation of volume fraction. The model is used to simulate the heat transfer and phase change processes of the cryogenic liquid hydrogen in the storage tank with the different gravitational accelerations, initial temperature, and liquid fill ratios of liquid hydrogen. Numerical results indicate greater gravitational acceleration enhances buoyancy and convection, enhancing convective heat transfer and evaporation processes in the tank. When the acceleration of gravity increases from 10^?2 g0 to 10^?5 g0, gaseous hydrogen mass increases from 0.0157 kg to 0.0244 kg at 200s. With the increase of initial liquid hydrogen temperature, the heat required to raise the liquid hydrogen to saturation temperature decreases and causes more liquid hydrogen to evaporate and cools the gas hydrogen temperature. More cryogenic liquid hydrogen (i.e., larger the fill ratio) makes the average fluid temperature in the tank lower. A 12.5% reduction in the fill ratio resulted in a decrease in fluid temperature from 20.35 K to 20.15 K (a reduction of about 0.1%, at 200s).     

660 MW发电机漏氢原因分析与处理对策

鸳鸯湖电厂1号发电机为QFSN6602型汽轮发电机,采用水氢氢冷却方式,正常运行时发电机内氢压高于定子冷却水压力;当定子线棒存在裂纹并发生泄露时,将会导致定子冷却水含氢量急剧升高,从而使定子冷却水进入发电机造成发电机烧损.定子冷却水箱安装氢气泄露检测仪,在线检测定子冷却水箱内氢气含量,当氢气浓度达2%时就会报警.讨论了发电机定子冷却水箱内检测仪报警后的原因分析及处理,为同类机组类似故障处理提供参考.     

Significance of ortho-para hydrogen conversion in the performance of hydrogen liquefaction process

Hydrogen is an energy vector and is produced just like electricity. In order to overcome the shortcomings associated with its low molecular weight and energy density per unit volume, hydrogen is liquefied for storage and transportation purposes. The liquefaction of hydrogen differs from that of other substances as it involves the reactive transformation of its isomeric states. At 25 °C, molecular hydrogen consists of 75% orthohydrogen and 25% of parahydrogen. As the normal boiling point, hydrogen essentially exists in the para-state, which is preferred because of its lower boil-off gas rate. However, the conversion of ortho-to-para hydrogen is an exothermic reaction, and this enthalpy of conversion enhances the total reversible work by about 15%. Little work has been done regarding ortho-to-para hydrogen conversion from the process systems point of view. Therefore, parametric analysis of this vital conversion reaction was studied with potential impact on the performance of cryogenic heat exchangers, reactors configuration and mode of operation, and probable impact on the energy efficiency of the liquefaction process. An alternate approach to simulate the reaction is also proposed. The results show that the current approaches to process design need to be changed. The study opens avenues for more in-depth analysis and optimization approaches to present a holistic framework for future integrated energy systems.     

Safety investigation of hydrogen charging platform package with CFD simulation

Hydrogen has been expected as one of the most promising green energy sources, especially in transportation section. Despite its great potential as a new source of energy, it is reluctant to build hydrogen charging stations for the fear of accidents such as hydrogen leakage, fire, and following explosion. To reduce those problems and promote the acceptance of hydrogen charging station, this study focuses on the hydrogen charging platform package (HCPP) which is a new type of the mobile hydrogen station. Hydrogen leakage cases are investigated using CFD (computational fluid dynamics) simulation. The simulation is performed with the whole configuration of the HCPP including main components, storage, compressor, and dispenser. Based on the risk assessment, hydrogen leak scenarios with high possibilities of accidents are simulated. The simulation results show the leak length of hydrogen gas, its dispersion, and the various ranges of volume ratios of leaked hydrogen gas. Based on the simulation results, it is clearly confirmed that the leaked hydrogen gas with high concentration stays inside the HCPP. Therefore, the effects of ventilation to reduce the possibility of the explosion are continuously considered to investigate the safety of the HCPP in the case of the leakage accident.     

Hydrogen and LNG production from coke oven gas with multi-stage helium expansion refrigeration

Here we propose a novel cryogenic system to simultaneously produce liquid hydrogen (LH₂) and liquefied natural gas (LNG) from coke oven gas. The coke oven gas, simplified as a mixture of methane and hydrogen, directly enters the cryogenic system. Due to the very low temperature of liquid hydrogen, helium is selected as the refrigerant, and the energy needed for the liquefaction is supplied by a multi-stage helium expansion refrigeration system. The high-purity liquid hydrogen and LNG products are obtained with the help of a cryogenic distillation column. The whole cryogenic process is simulated with the Aspen HYSYS software to determine the parameters of each process point and key component. We found that the process is able to produce LH₂ and LNG of very high purity. Using the power consumption of the product liquefaction as the major performance parameter for the analysis, optimum parameters of the multi-stage helium expansion liquefaction process could be found. The results show that the proposed system can achieve a methane recovery rate of 97.9% and a hydrogen recovery rate of 99.7% with acceptable energy consumption.     

Comparison of the expenses required for the on-board fuel storage systems of hydrogen powered vehicles

The on-board storage of hydrogen in vehicles requires sophisticated tank systems and appreciable energy expenditure. The energy requirement for the loading of liquid hydrogen in a cryogenic storage tank is substantially higher than the amount needed to charge a corresponding hydride vehicle tank. However, the hydride tank must provide a considerable part of its hydrogen fuel energy content in order to meet the energy requirements due to its own weight. This amount increases rapidly with increasing design range. Considering the total primary energy flow for both cases, it turns out that a clear break-even point exists. For design ranges higher than about 200 km the liquid hydrogen storage requires less energy expense than the hydride storage tank, the difference being very substantial (more than 100%) in the usual 400–500 km range. Similar results are found in the costs of the on-board storage of hydrogen for different design ranges. A comparison of these results with the storage costs studied earlier for large-scale stationary hydrogen storage facilities indicates that, from the specific capacity point of view, the break-even conditions are very similar in both cases, although due to different reasons.     

A homogeneous non-equilibrium two-phase critical flow model

A non-equilibrium two-phase single-component critical (choked) flow model for cryogenic fluids is developed from first principle thermodynamics. Modern equations-of-state (EOS) based upon the Helmholtz free energy concepts are incorporated into the methodology. Extensive validation of the model is provided with the NASA cryogenic data tabulated for hydrogen, methane, nitrogen, and oxygen critical flow experiments performed with four different nozzles. The model is used to develop a hydrogen critical flow map for stagnation states in the liquid and supercritical regions.     

Numerical investigation of cavitating flow in liquid hydrogen

The objective of this paper is to investigate the cavitating flow in liquid hydrogen. The aims are to (1) study the physical aspects of cavitation dynamics in cryogenic environment and validate a thermodynamic cavitation model based on bubble dynamic equation, (2) conduct a global sensitivity analysis to assess the sensitivity of the response to temperature dependent material properties and model parameters, and to calibrate the parameters of the cavitation model for suitable flow conditions, (3) assess the thermodynamic cavitation model over a wide range of conditions. Numerical computations are performed on the 2D quarter caliber hydrofoil experimentally investigated by Hord [Cavitation in liquid cryogens II-hydrofoils. NASA CR-2156; 1973a; Cavitation in liquid cryogensIII-Ogives. NASA CR-2242; 1973b]. The numerical simulations are performed by solving the multiphase Unsteady Reynolds-averaged Navier–Stokes (URANS) Equations via the commercial code CFX using a thermodynamic cavitation model, the k–ω SST turbulence model is used as the closure model. The results showed that the thermodynamic effect has significantly affects the cavitation dynamics, including the vapor pressure and cavity structures. The isothermal case yields a substantially larger cavity attached on the hydrofoil due to the thermodynamic effect under thermal conditions. The predicted pressure and temperature inside the cavity is steeper under the cryogenic condition than that under the isothermal condition, which shows better agreement with the experimental measurements. Based on the surrogate model, the global sensitivity analysis is conducted to assess the role of model parameters regulating the condensation and evaporation rates, and uncertainties in material properties. It is indicated that the material properties are more critical than the model parameter controlling the condensation and evaporation rates. Based on the recommended model parameter values, better prediction of the cryogenic cavitation could be attained.     

CFD simulations of filling and emptying of hydrogen tanks

During the filling of hydrogen tanks high temperatures can be generated inside the vessel because of the gas compression while during the emptying low temperatures can be reached because of the gas expansion. The design temperature range goes from ?40 °C to 85 °C. Temperatures outside that range could affect the mechanical properties of the tank materials. CFD analyses of the filling and emptying processes have been performed in the HyTransfer project. To assess the accuracy of the CFD model the simulation results have been compared with new experimental data for different filling and emptying strategies. The comparison between experiments and simulations is shown for the temperatures of the gas inside the tank, for the temperatures at the interface between the liner and the composite material, and for the temperatures on the external surface of the vessel.     

The design and optimization of a cryogenic compressed hydrogen refueling process

Cryo-compressed hydrogen storage has excellent volume and mass hydrogen storage density, which is the most likely way to meet the storage requirements proposed by United States Department of Energy（DOE）. This paper contributes to propose and analyze a new cryogenic compressed hydrogen refueling station. The new type of low temperature and high-pressure hydrogenation station system can effectively reduce the problems such as too high liquefaction work when using liquid hydrogen as the gas source, the need to heat and regenerate to release hydrogen, and the damage of thermal stress on the storage tank during the filling process, so as to reduce the release of hydrogen and ensure the non-destructive filling of hydrogen. This paper focuses on the study of precooling process in filling. By establishing a heat transfer model, the dynamic trend of tank temperature with time in the precooling process of low-temperature and high-pressure hydrogen storage tank under constant pressure is studied. Two analysis methods are used to provide theoretical basis for the selection of inlet diameter of hydrogen storage tank. Through comparative analysis of the advantages and disadvantages of the two analysis methods, it is concluded that the analysis method of constant mass flow is more suitable for the selection in practical applications. According to it, the recommended diameter of the storage tank at the initial temperature of 300 K, 200 K and 100 K is selected, which are all 15 mm. It is further proved that the calculation method can meet the different storage tank states of hydrogen fuel cell vehicles when selecting the pipe diameter.     

Thermodynamic characteristic in a cryogenic storage tank under an intermittent sloshing excitation

A numerical calculation model is developed to study the coupled thermal dynamic performance in a cryogenic fuel tank under an intermittent sloshing excitation. Both external heat inputs and the intermittent excitation are realized by User-defined functions. The volume of fluid method is adopted to simulate fluid sloshing, coupled with the mesh motion treatment. Validated against related fluid sloshing experiments, the numerical model was turned out to be acceptable on fluid sloshing prediction. Cooled by subcooled liquid, vapor is always in condensation. The middle vapor pressure test point suffers less from the intermittent excitation and has a linear pressure decrease profile, while the middle liquid pressure test point has fluctuating variations. For vapor and interface temperature monitors, obvious temperature fluctuations appear in the second holding period. While for liquid test points, the temperature profiles experience intensive fluctuations during sloshing periods and stable temperature variation during holding periods. Due to the holding period of external excitation, the tank pressure reduction in intermittent sloshing case is less than that in continuous sloshing case. That is to say the tank pressure decrease rate could be adjusted by proper intermittent excitation. This work is significant to deeply understand fluid sloshing phenomenon under some irregular external excitations in fuel storage tanks.