Numerical investigation of the coupled heat transfer of liquefied gas storage tanks

It is a common situation that the liquefied gas tanks are always heated by the outer hot environments, which affecting the safety of the tanks. In this paper, numerical studies were conducted to reveal the heat transfer characteristics of this circumstance. The coupled heat transfer process among the thermal environment, the tank wall and the fluid in the tank was thoroughly investigated by simultaneously solving the temperature fields of both the solid region and the fluid region as well as the flow fields of both the liquid phase and the vapor phase inner the tank. The results showed that affected by the near wall flow and the wall boiling, the heat transfer presented different patterns in the stable thermal stratification stage and the de-stratification stage. In the stable stratification stage, the heat flux from the liquid phase wall to the medium distributed uniformly along the axial direction of the tank, while in the de-stratification stage, it differed a lot at the different positions.     

Numerical investigations on the fast filling of hydrogen tanks

High-pressure storage of hydrogen in tanks is a promising option to provide the necessary fuel for transportation purposes. The fill process of a high-pressure tank should be reasonably short but must be designed to avoid too high temperatures in the tank. The shorter the fill should be the higher the maximum temperature in the tank climbs. For safety reasons an upper temperature limit is included in the requirements for refillable hydrogen tanks (ISO 15869) which sets the limit for any fill optimization. It is crucial to understand the phenomena during a tank fill to stay within the safety margins.The paper describes the fast filling process of hydrogen tanks by simulations based on the Computational Fluid Dynamics (CFD) code CFX. The major result of the simulations is the local temperature distribution in the tank depending on the materials of liner and outer thermal insulation. Different material combinations (type III and IV) are investigated.Some measurements from literature are available and are used to validate the approach followed in CFX to simulate the fast filling of tanks. Validation has to be continued in the future to further improve the predictability of the calculations for arbitrary geometries and material combinations.     

Thermodynamic analysis of the emptying process of compressed hydrogen tanks

During the driving of fuel cell vehicles, the fast depressurization of compressed hydrogen tanks plus the high storage pressure and the low thermal conductivity of carbon fiber reinforced plastic (CFRP) can lead to significant cooling of the tank. This can result in a temperature below −40 °C inside the compressed hydrogen tanks and cause safety problems. In this paper, a thermodynamic model that incorporates the nature of external natural convection was developed to describe the emptying process of compressed hydrogen tanks and was validated by experiments. Thermodynamic analyses of the emptying process were performed to study the global heat transfer characteristics and the effects of ambient temperature, defueling rate, defueling pattern, initial and final density of hydrogen gas, liner and CFRP thickness and the crosswind velocity on the final temperature decreases of hydrogen gas, the inner wall and the outer wall.     

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.     

A fast response heat pump water heater using thermostat made from shape memory alloy

The heat pump water heater produces hot water so slow at low ambient temperature that it frequently could not meet the hot water load demand in winter. The present study develops a fast response heat pump water heater (FRHP) designed with two separate tanks (supply and holding tank) which are connected by a thermostat made from shape memory alloy (SMAV). The SMAV is a mechanical heat-sensitive device made from shape memory alloy which keeps the valve closed when the water temperature is not high enough. This will isolate the tanks and let the vapor compression cycle heat up the supply tank only. The speed of temperature rise thus is increased. The SMAV will open and induce a natural circulation between two tanks to transfer the heat from the supply tank to the holding tank, when water is heated to a designated temperature. A 100 l FRHP was built and tested in the present study. The experimental results showed that the temperature response speed of the supply tank, before SMAV is opened, reaches 1.056 °C/min and the holding tank, after SMAV is opened, reaches 0.828 °C/min at ambient temperature 20 °C. The FRHP will heat up 50 l water in the supply tank with 30 °C temperature rise within 40 min in winter which is acceptable in domestic application. The energy consumption is in the range 0.008–0.016 kWh/l of hot water at about 55 °C.     

Review on studies of the emptying process of compressed hydrogen tanks

Compressed hydrogen tanks are now widely used for onboard hydrogen storage in fuel cell vehicles (FCVs). However, because of the high storage pressure and the low thermal conductivity of carbon fibre reinforced polymer (CFRP), the emptying of such tanks during driving or emergency release can cause a significant temperature decrease and result in an in-tank gas temperature below the low safety temperature limit of ?40 °C even in warm weather. Once the gas temperature within the tank is lower than ?40 °C, the sealing elements at the boss of the tank may fail, and glass transition of the polymer liner of the type IV tank may occur; both can cause hydrogen leakage and severe safety problems. In this paper, the heat transfer correlations, thermodynamic analyses, computational fluid dynamics (CFD) simulations, experimental studies, and thermal management methods associated with the emptying process of compressed hydrogen tanks are comprehensively reviewed. Future research directions on this topic are suggested.     

Physical model of onboard hydrogen storage tank thermal behaviour during fuelling

A physical model to simulate thermal behaviour of an onboard storage tank and parameters of hydrogen inside the tank during fuelling is described. The energy conservation equation, Abel-Noble real gas equation of state, and the entrainment theory are applied to calculate the dynamics of hydrogen temperature inside the tank and distribution of temperature through the wall to satisfy requirements of the regulation. Convective heat transfer between hydrogen, tank wall and the atmosphere are modelled using Nusselt number correlations. An original methodology, based on the entrainment theory, is developed to calculate changing velocity of the gas inside the tank during the fuelling. Conductive heat transfer through the tank wall, composed of a load-bearing carbon fibre reinforced polymer and a liner, is modelled by employing one-dimensional unsteady heat transfer equation. The model is validated against experiments on fuelling of Type III and Type IV tanks for hydrogen onboard storage. Hydrogen temperature dynamics inside a tank is simulated by the model within the experimental non-uniformity of 5 °C. The calculation procedure is time efficient and can be used for the development of automated hydrogen fuelling protocols and systems.     

基于太阳能利用的相变蓄热水箱结构优化

为解决因太阳能的不稳定性等因素导致的太阳能蓄热水箱储热/放热能力的不保证性问题,提出采用中低温有机相变材料58号石蜡作为相变蓄热材料的圆台式太阳能相变蓄热水箱。采用计算流体力学(computational fluid dynamics,CFD)数值模拟的计算方法,在保证总蓄水体积(以100 L为例)不变的情况下,对水箱中不同内胆倾斜角度分别为75°、80°、85°、90°、95°、100°、105°的放热过程进行数值模拟,综合对比和分析水箱放热性能模拟结果,得到当倾斜角度为105°时的相变蓄热构件放热性能最佳,可为太阳能相变蓄热水箱的结构优化设计提供理论依据。     

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).     

Thermal–fluid behavior of the hydriding and dehydriding processes in a metal hydride hydrogen storage canister

A study of the hydrogen absorption and desorption processes using LaNi₅ metal hydride is presented for investigation on the influences of expansion volume and heat convection. The hydrogen storage canister comprises a cylindrical metal bed and a void of expansion volume atop the metal. The expansion volume is considered as a domain of pure hydrogen gas. The gas motion in the metal hydride bed is treated as porous medium flow. Concepts of mass and energy conservation are incorporated in the model to depict the thermally coupled hydrogen absorption and desorption reactions. Simulation results show the expansion volume reduces the reaction rates by increasing thermal resistance to the heat transfer from the outside cooling/heating bath. The assumption usually adopted in simulating heat transfer in a metal hydride tank that heat convection in the reaction bed may be ignored is not valid when expansion volume is used because heat convection dominates the heat transfer through the expansion volume as well as the metal bed. The details of the thermal flow pattern are demonstrated. It is found that, due to the action of thermal buoyancy, circulations are likely to happen in the expansion volume. The hydrogen gas accordingly, instead of going directly between the inlet/outlet and the metal bed, tends to move with the circulation along the boundary of the expansion volume.     

Two-dimensional thermal analysis of liquid hydrogen tank insulation

Liquid hydrogen (LH₂) storage has the advantage of high volumetric energy density, while boil-off losses constitute a major disadvantage. To minimize the losses, complicated insulation techniques are necessary. In general, Multi Layer Insulation (MLI) and a Vapor-Cooled Shield (VCS) are used together in LH₂ tanks. In the design of an LH₂ tank with VCS, the main goal is to find the optimum location for the VCS in order to minimize heat leakage. In this study, a 2D thermal model is developed by considering the temperature dependencies of the thermal conductivity and heat capacity of hydrogen gas. The developed model is used to analyze the effects of model considerations on heat leakage predictions. Furthermore, heat leakage in insulation of LH₂ tanks with single and double VCS is analyzed for an automobile application, and the optimum locations of the VCS for minimization of heat leakage are determined for both cases.     

烧结余热回收竖式炉内气体流动和传热的数值模拟

运用数值模拟的方法,通过求解非线性联立的质量、动量、能量及组分守恒偏微分方程组,借助多物理场祸合软件Comsol和模拟软件FLUENT模拟出竖式炉内气体流动,换热特性以及料层阻力特性。模拟结果表明:冷却段高度是影响竖式炉内气固流动换热的因素之一,高度增加,烧结矿温度降低,冷却风的温度升高,空气出口携带火用值先增加后趋于平缓,气固比也影响着竖式炉内气体流动和换热,气固比增加,烧结矿的出口温度和循环空气的平均出口温度逐渐降低,热空气携带的火用值先增加后减小;竖式炉直径也是影响因素之一,但相比于冷却高度和气固比影响不是很大。由模拟的结果给出的冷却段高度为7 m,气固比1 400 m~3/t,竖式炉直径12 m时,竖式炉炉内的气固换热最强,与现场实际的结构参数和操作参数比较吻合。     

Experimental investigation of a molten salt thermocline storage tank

Thermal energy storage is considered as an important subsystem for solar thermal power stations. Investigations into thermocline storage tanks have mainly focused on numerical simulations because conducting high-temperature experiments is difficult. In this paper, an experimental study of the heat transfer characteristics of a molten salt thermocline storage tank was conducted by using high-temperature molten salt as the heat transfer fluid and ceramic particle as the filler material. This experimental study can verify the effectiveness of numerical simulation results and provide reference for engineering design. Temperature distribution and thermal storage capacity during the charging process were obtained. A temperature gradient was observed during the charging process. The temperature change tendency showed that thermocline thickness increased continuously with charging time. The slope of the thermal storage capacity decreased gradually with the increase in time. The low-cost filler material can replace the expensive molten salt to achieve thermal storage purposes and help to maintain the ideal gravity flow or piston flow of molten salt fluid.     

Simultaneous Estimation of Properties in a Combined Mode Conduction–Radiation Heat Transfer in a Porous Medium

Simultaneous estimation of thermophysical and optical properties such as the thermal conductivity, the scattering albedo, and the emissivity of a 1‐D planar porous matrix involving combined mode conduction and radiation heat transfer with heat generation is reported. Coupled energy equations for the gas and solid phase account for the nonlocal thermal equilibrium between the two phases. Performances of the genetic algorithm (GA) and the global search algorithm (GSA) in simultaneous estimation of three properties are analyzed. Both the GA and the GSA utilize a priori knowledge of the axial gas temperature distribution, and the magnitudes of the convective and the radiative heat fluxes at the outer surface of the porous matrix. With volumetric radiative information needed in the solid‐phase energy equation computed using the discrete transfer method, the two energy equations are simultaneously solved using the finite volume method. GSA provides better estimation, and computationally, it is much faster than the GA.     

Mixed convective flow and thermal stratification in hot water storage tanks during discharging mode

An analysis of transient, two dimensional, mixed convection and thermal stratification in cylindrical hot water storage tanks is presented. The governing equations together with inflow and outflow boundary conditions are written for laminar mixed convection flow using a finite volume based computational code in the dynamic discharging mode based on Boussinesq approximations and conjugate heat transfer. The equations are solved numerically and the results are obtained for aspect ratios of the tanks ranging from 1 to 4 in the Richardson number range of 10⁵ to 10⁸ using a finite volume based computational code. The dynamic discharging mode is considered using a conjugate heat transfer model. The transient temperature profiles in the bulk fluid reveal reduced mixing at higher Richardson numbers during discharging process. The system performance in the dynamic mode of operation is defined by a Mix Number and discharging efficiency parameter. Mixing at the bottom of the tank due to inflow of low temperature water from the load is found to have significant influence on the storage efficiency. The discharging efficiency decreases with Fourier number due to increased thermal degradation with time.     

Assessment of CFD models for hydrogen fast filling simulations

High injection pressures are used during the re-fueling process of vehicle tanks with compressed hydrogen, and consequently high temperatures are generated in the tank, potentially jeopardizing the system safety. Computational Fluid Dynamics (CFD) tools can help in predicting the temperature rise within vehicle tanks, providing complete and detailed 3D information on flow features and temperature distribution. In this framework, CFD simulations of hydrogen fast filling at different working conditions are performed and the accuracy of the numerical models is assessed against experimental data for a type 4 tank up to 70 MPa.     

Three dimensional numerical computations on the fast filling of a hydrogen tank under different conditions

Hydrogen-fueled vehicles offer a clean and efficient alternative for transportation. Compressed gas in high pressure tanks is a popular storage mode for hydrogen fuel. Time required for filling a hydrogen tank for vehicular applications should be short. But quick filling of hydrogen tanks at high pressures can result in high gas temperatures which can damage the tank and lead to its rupture. Hence the real time monitoring of gas temperature is essential during filling. This paper reports the findings of numerical simulation of filling process of hydrogen tanks. Real gas effects are considered. Local temperature distribution in the tank is obtained at different durations of the fill. Effect of changes in ambient temperature and initial and inlet gas temperatures is studied. Results of the study can aid in optimizing the filling time and in identifying the most suitable locations for the feedback devices within on-board hydrogen tanks.     

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow

The influences of thermophysical properties of porous media on superadiabatic combustion with reciprocating flow is numerically studied in order to improve the understanding of the complex heat transfer and optimum design of the combustor. The heat transfer performance of a porous media combustor strongly depends on the thermophysical properties of the porous material. In order to explore how the material properties influence reciprocating superadiabatic combustion of premixed gases in porous media (short for RSCP), a two‐dimensional mathematical model of a simplified RSCP combustor is developed based on the hypothesis of local thermal non‐equilibrium between the solid and the gas phases by solving separate energy equations for these two phases. The porous media is assumed to emit, absorb, and isotropically scatter radiation. The finite‐volume method is used for computing radiation heat transfer processes. The flow and temperature fields are calculated by solving the mass, moment, gas and solid energy, and species conservation equations with a finite difference/control volume approach. Since the mass fraction conservation equations are stiff, an operator splitting method is used to solve them. The results show that the volumetric convective heat transfer coefficient and extinction coefficient of the porous media obviously affect the temperature distributions of the combustion chamber and burning speed of the gases, but thermal conductivity does not have an obvious effect. It indicates that convective heat transfer and heat radiation are the dominating ways of heat transfer, while heat conduction is a little less important. The specific heat of the porous media also has a remarkable impact on temperature distribution of gases and heat release rate. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 35(5): 336–350, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20120     

徐深气田深层气井温度压力分布计算

徐深气田井深3500～4000m,地层压力30～40MPa,井底温度可达150℃,各生产井不同程度含有CO2,给井下管柱安全带来挑战。气井井筒中流体的温度和压力分布是影响井下管柱受力、变形的重要因素,准确进行温度、压力分布的计算,对气井的生产动态分析和管柱优化具有重要意义。针对徐深气田深层气井的流体性质和井身结构,根据传热学理论和质量、能量守恒原理,将井筒分成若干个节点,考虑油管、套管、水泥环及地层之间的传热性质,建立温度、压力耦合模型,并运用迭代法对模型进行求解。分析计算显示,气井产量、CO2含量、油管直径等参数对井筒温度、压力分布具有重要影响:产量越高,井筒温度下降越慢,而压力下降越快;CO2含量越高,温度下降越慢,而压力下降越快;油管直径越大,温度下降越快,压力下降越慢。实例计算表明,所建立的耦合模型简单方便,具有较高的精度,适合深层气井的温度、压力分布计算,计算结果可为气井的生产动态分析和管柱优化提供依据。