Numerical modeling of self-pressurization and pressure control by a thermodynamic vent system in a cryogenic tank

This paper presents a numerical model of a system-level test bed—the multipurpose hydrogen test bed (MHTB) using the Generalized Fluid System Simulation Program (GFSSP). MHTB is representative in size and shape of a space transportation vehicle liquid hydrogen propellant tank, and ground-based testing was performed at NASA Marshall Space Flight Center (MSFC) to generate data for cryogenic storage. GFSSP is a finite volume-based network flow analysis software developed at MSFC and used for thermofluid analysis of propulsion systems. GFSSP has been used to model the self-pressurization and ullage pressure control by the Thermodynamic Vent System (TVS). A TVS typically includes a Joule–Thompson (J–T) expansion device, a two-phase heat exchanger (HEX), and a mixing pump and liquid injector to extract thermal energy from the tank without significant loss of liquid propellant. For the MHTB tank, the HEX and liquid injector are combined into a vertical spray bar assembly. Two GFSSP models (Self-Pressurization and TVS) were separately developed and tested and then integrated to simulate the entire system. The Self-Pressurization model consists of multiple ullage nodes, a propellant node, and solid nodes; it computes the heat transfer through multilayer insulation blankets and calculates heat and mass transfer between the ullage and liquid propellant and the ullage and tank wall. A TVS model calculates the flow through a J–T valve, HEX, and spray and vent systems. Two models are integrated by exchanging data through User Subroutines of both models. Results of the integrated models have been compared with MHTB test data at a 50% fill level. Satisfactory comparison was observed between tests and numerical predictions.     

Experimental investigation on pressurization performance of cryogenic tank during high-temperature helium pressurization process

Sufficient knowledge of thermal performance and pressurization behaviors in cryogenic tanks during rocket launching period is of importance to the design and optimization of a pressurization system. In this paper, ground experiments with liquid oxygen (LO₂) as the cryogenic propellant, high-temperature helium exceeding 600 K as the pressurant gas, and radial diffuser and anti-cone diffuser respectively at the tank inlet were performed. The pressurant gas requirements, axial and radial temperature distributions, and energy distributions inside the propellant tank were obtained and analyzed to evaluate the comprehensive performance of the pressurization system. It was found that the pressurization system with high-temperature helium as the pressurant gas could work well that the tank pressure was controlled within a specified range and a stable discharging liquid rate was achieved. For the radial diffuser case, the injected gas had a direct impact on the tank inner wall. The severe gas-wall heat transfer resulted in about 59% of the total input energy absorbed by the tank wall. For the pressurization case with anti-cone diffuser, the direct impact of high-temperature gas flowing toward the liquid surface resulted in a greater deal of energy transferred to the liquid propellant, and the percentage even reached up to 38%. Moreover, both of the two cases showed that the proportion of energy left in ullage to the total input energy was quite small, and the percentage was only about 22–24%. This may indicate that a more efficient diffuser should be developed to improve the pressurization effect. Generally, the present experimental results are beneficial to the design and optimization of the pressurization system with high-temperature gas supplying the pressurization effect.     

高压、小气枕低温贮箱智能增压技术

介绍了一种针对航天地面试验的高压、小气枕低温贮箱智能增压技术.采用多路电磁阀加装合适的节流元件,仿人工智能开关控制技术,具有简明、确切、迅速等人工智能的特点,在高压系统的贮箱压力控制中取得成功,避免了贮箱压力因阀门动作过度而产生超调振荡,从而有效地提高了调节的品质,提高箱压的控制精度与响应速度.     

Simulation of Jet-Induced Geysers in Reduced Gravity

Control of cryogenic propellant tank pressure during tank refueling and expulsion in low gravity is an important technical challenge to overcome for future long duration missions in space. One method proposed to control tank pressurization involves the use of jet-induced geysers. Two-dimensional computational models have been developed and used with limited success in previous efforts to predict geyser heights in microgravity. A three-dimensional flow simulation is used to model jet-induced geysers in reduced gravity. Geyser flows are commonly characterized by the presence of turbulent jets, transient flow, deforming free surfaces, and surface tension effects. As is the case for many turbulent flow applications, accuracy in simulating complex turbulent flows is critically dependent on the selection of a suitable turbulence model. The sensitivity of the simulation geyser predictions to a suite of popular turbulence models is assessed. Simulation results are compared to available experiment results. By expanding upon the work already completed, the model is used to simulate a broad range of cases within the experiment test matrix. Simulation results suggest the two dimensional simulation using the k-ε turbulence model provides the most accurate results for jet-induced geysers in reduced gravity when compared to available experiment data.     

B型LNG模拟舱内低温流体两相流动与相变传热数值研究

本文以内容积为40 m3，绝热层厚度为400 mm的新型独立B型液化天然气船模拟舱为研究对象，对模拟舱内低温流体的两相流动及相变传热问题进行了非稳态三维CFD模拟。采用流体体积函数（VOF）模型追踪气液相界面，利用Lee模型作为相变模型，在相变模型中考虑了静压的影响，对模拟舱的温度分布及静态BOG生成速率进行了计算。研究了在不同液位以及绝热层存在破损的情况下模拟舱的温度分布及静态BOG生成速率的变化，同时研究了当模拟舱密闭时的增压特性。对比模拟结果与实验结果，偏差在10%以内，模型对模拟舱内的低温液体的蒸发过程模拟较好，可为新型独立B型液化天然气实船的设计和改进提供参考。     

EDU liquid acquisition device outflow tests in liquid hydrogen: Experiments and analytical modeling

This paper presents experiments and modeling of the most recent set of liquid acquisition device (LAD) vertical outflow tests conducted in liquid hydrogen. The Engineering Development Unit (EDU) was a relatively large tank (4.25 m³) used to mimic a storage tank for a cryogenic storage and transfer flight demonstration test. Six 1-g propellant tank outflow tests were conducted with a standard 325 × 2300 rectangular cross-section curved LAD channel conformal to the tank walls over a range of tank pressure (158–221 kPa), ullage temperature (22–39 K), and mass flow rate (0.0103–0.0187 kg/s) per arm. An analytical LAD channel solver, an exact solution to the Navier-Stokes equations, is used to model propellant outflow for the LAD channel. Results shows that the breakdown height of the LAD is dominated by liquid and ullage gas temperatures, with a secondary effect of flow rate. The best performance is always obtained by exposing the channel to cold pressurant gas and low flow rates, consistent with the cryogenic bubble point model. The model tracks the trends in the data and shows that the contribution of flow-through-screen pressure drop is minimized for bottom outflow in 1-g, versus the standard inverted outflow.     

Numerical and experimental comparisons of the self-pressurization behavior of an LH2 tank in normal gravity

In optimizing the design of cryogenic storage facilities for future in-orbit or on-surface applications the boil-off and the self-pressurization rates must be accurately predicted for different g-levels and for a variety of heat loads and distributions. In this paper, a two-phase CFD model is presented that describes the self-pressurization behavior of a flightweight partially full LH2 tank in normal gravity. Existing experimental data at different fill levels are used to assess the predictive capability of the model. The model’s predictions indicate favorable agreement with the experimentally measured pressure histories. Small deviations are observed for the median fill level cases where it is suggested that a non-uniform heat load may be the source of this discrepancy.     

液氧贮箱增压过程中气枕空间温度场的数值模拟

液氧贮箱增压气体输送过程中,气枕空间的温度场会发生较大的变化,并且会引起相界面附近低温液体的温升.基于双膜阻理论建立热质交换模型,通过模拟贮箱内非稳态热流过程,分析气枕空间在增压气体输送过程中温度场的变化及其对于贮箱内低温液氧的影响.     

火箭燃料贮箱热力学排气系统控压性能仿真研究

本文介绍了低温推进剂在轨管理中热力学排气系统(thermodynamic venting system,TVS)的构成及工作原理。基于VOF模型,编写用户自定义程序考虑箱内存在的热质传递现象,采用CFD模拟贮箱内部的压力变化和温度场分布。构建的CFD模型能够较好地模拟TVS的工作特性,证明CFD研究的有效性。针对不同液体初始充灌率、节流条件等影响因素进行变工况模拟,揭示TVS的工作规律。计算TVS在微重力下的控压性能,验证其在微重力下的可行性并获得运行规律。     

低温推进剂贮箱增压过程的传热传质数学模拟

针对火箭发动机地面试验中低温液氧贮箱的预增压和增压过程建立了气相空间的传热、传质数学模型.运用实际气体的状态方程、连续性方程、能量守恒方程以及推进剂与气相空间的传热、传质方程等组成了关于气相空间参数的微分方程组,并运用四阶Runge-Kutta算法对其进行求解.获得了气相空间的压力、温度、增压气体流量、液氧挥发速率以及贮箱壁温等参数的变化规律.结果表明,在发动机启动前的预增压过程中,气相空间的温度和压力急剧增加,液氧的挥发速率也增加很快;发动机启动后的保持增压阶段,由于气相空间的体积不断发生变化,气相空间参数的变化趋于平缓,液氧表面向气相空间的传质速率也趋于稳定.     

4种不同加注结构在无排放加注中的性能比较

以175 L立式低温绝热气瓶为载体,对L型、U型、侧喷淋和直管4种不同的加注结构进行了实验研究,根据各结构随着供液压力变化对气瓶压力、最终充满率和加注时间的性能影响,得出直管加注结构更适用于小型立式低温容器的加注结构。通过分析各加注结构的结构特性及效果,验证了MMCAP(Martin Marietta Crgogenic Analysis Program)分析模型关于液面扰动比气相空间气液混合更容易提高冷凝率的预测是合理的。     

Simulation Research of Vaporization and Pressure Variation in a Cryogenic Propellant Tank at the Launch Site

In order to improve depiction of pressure variation and investigate the interrelation among the physical processes in propellant tanks, a 2D axial symmetry Volume-of-Fluid (VOF) CFD model is established to simulate a large-sized liquid propellant tank when the rocket is preparing for launch with propellant loaded at the launch site. The numerical model is considered with propellant free convection, heat transfer between the tank and the external environment, thermal exchange between propellant and inner tank wall surfaces, gas compressibility, and phase change modeled under the assumption of thermodynamic equilibrium. Vaporization rate of the vented LH2 tank and prediction of pressure change in the tank pressurized with GHe are obtained through simulation. We analysis the distributions of phase, temperature, and velocity vectors to reveal interactions among the propellant’s own convection motion, heat transfer and phase change. The results show that the vaporization rate is mainly affected by heat leaks though the tank wall when the tank is vented, but it does not completely accord with the trend of the leakage because of convection motion and temperature nonuniformity of the liquid propellant in the tank. We also find that the main factors on pressure variation in the pressurized tank are the heat transfer on the tank wall surface bonding the ullage and propellant vaporization which has comparatively less influence.     

Three-dimensional analysis for liquid hydrogen in a cryogenic storage tank with heat pipe–pump system

This paper presents a study on fluid flow and heat transfer of liquid hydrogen in a zero boil-off cryogenic storage tank in a microgravity environment. The storage tank is equipped with an active cooling system consisting of a heat pipe and a pump–nozzle unit. The pump collects cryogen at its inlet and discharges it through its nozzle onto the evaporator section of the heat pipe in order to prevent the cryogen from boiling off due to the heat leaking through the tank wall from the surroundings. A three-dimensional (3-D) finite element model is employed in a set of numerical simulations to solve for velocity and temperature fields of liquid hydrogen in steady state. Complex structures of 3-D velocity and temperature distributions determined from the model are presented. Simulations with an axisymmetric model were also performed for comparison. Parametric study results from both models predict that as the speed of the cryogenic fluid discharged from the nozzle increases, the mean or bulk cryogenic fluid speed increases linearly and the maximum temperature within the cryogenic fluid decreases.     

Validation of two-phase CFD models for propellant tank self-pressurization: Crossing fluid types,scales, and gravity levels

This paper examines our computational ability to capture the transport and phase change phenomena that govern cryogenic storage tank pressurization and underscores our strengths and weaknesses in this area in terms of three computational-experimental validation case studies. In the first study, 1g pressurization of a simulant low-boiling point fluid in a small scale transparent tank is considered in the context of the Zero-Boil-Off Tank (ZBOT) Experiment to showcase the relatively strong capability that we have developed in modelling the coupling between the convective transport and stratification in the bulk phases with the interfacial evaporative and condensing heat and mass transfer that ultimately control self-pressurization in the storage tank. Here, we show that computational predictions exhibit excellent temporal and spatial fidelity under the moderate Ra number – high Bo number convective-phase distribution regimes. In the second example, we focus on 1g pressurization and pressure control of the large-scale K-site liquid hydrogen tank experiment where we show that by crossing fluid types and physical scales, we enter into high Bo number – high Ra number flow regimes that challenge our ability to predict turbulent heat and mass transfer and their impact on the tank pressurization correctly, especially, in the vapor domain. In the final example, we examine pressurization results from the small scale simulant fluid Tank Pressure Control Experiment (TCPE) performed in microgravity to underscore the fact that in crossing into a low Ra number – low Bo number regime in microgravity, the temporal evolution of the phase front as affected by the time-dependent residual gravity and impulse accelerations becomes an important consideration. In this case detailed acceleration data are needed to predict the correct rate of tank self-pressurization.     

Performance analysis of no-vent fill process for liquid hydrogen tank in terrestrial and on-orbit environments

Two finite difference computer models, aiming at the process predictions of no-vent fill in normal gravity and microgravity environments respectively, are developed to investigate the filling performance in a liquid hydrogen (LH₂) tank. In the normal gravity case model, the tank/fluid system is divided into five control volume including ullage, bulk liquid, gas–liquid interface, ullage-adjacent wall, and liquid-adjacent wall. In the microgravity case model, vapor–liquid thermal equilibrium state is maintained throughout the process, and only two nodes representing fluid and wall regions are applied. To capture the liquid–wall heat transfer accurately, a series of heat transfer mechanisms are considered and modeled successively, including film boiling, transition boiling, nucleate boiling and liquid natural convection. The two models are validated by comparing their prediction with experimental data, which shows good agreement. Then the two models are used to investigate the performance of no-vent fill in different conditions and several conclusions are obtained. It shows that in the normal gravity environment the no-vent fill experiences a continuous pressure rise during the whole process and the maximum pressure occurs at the end of the operation, while the maximum pressure of the microgravity case occurs at the beginning stage of the process. Moreover, it seems that increasing inlet mass flux has an apparent influence on the pressure evolution of no-vent fill process in normal gravity but a little influence in microgravity. The larger initial wall temperature brings about more significant liquid evaporation during the filling operation, and then causes higher pressure evolution, no matter the filling process occurs under normal gravity or microgravity conditions. Reducing inlet liquid temperature can improve the filling performance in normal gravity, but cannot significantly reduce the maximum pressure in microgravity. The presented work benefits the understanding of the no-vent fill performance and may guide the design of on-orbit no-vent fill system.     

PVT gauging with liquid nitrogen

Experimental results are presented for pressure–volume–temperature (PVT) liquid quantity gauging of a 0.17 m³ liquid nitrogen tank pressured with ambient temperature helium in the normal gravity environment. A previously reported PVT measurement procedure has been improved to include helium solubility in liquid nitrogen. Gauging data was collected at nominal tank fill levels of 80%, 50% and 20% and at nominal tank pressures of 0.3, 1.0, and 1.7 MPa. The test tank was equipped with a liquid pump and spray manifold to circulate and mix the fluid contents and therefore create near-isothermal conditions throughout the tank. Silicon diode sensors were distributed throughout the tank to monitor temperatures. Close-spaced arrays of silicon diode point sensors were utilized to precisely detect the liquid level at the nominal 80%, 50%, and 20% fill levels. The tests simulated the cryogenic tank-side conditions only; helium mass added to the tank was measured by gas flowmeters rather than using pressure and temperature measurements from a dedicated helium supply bottle. Equilibrium data for cryogenic nitrogen and helium mixtures from numerous sources was correlated to predict soluble helium mole fractions. Results show that solubility should be accounted for in the PVT gauging calculations. Mole fractions predicted by Dalton’s Law were found to be in good agreement with the compiled equilibrium data within the temperature–pressure range of interest. Therefore, Dalton’s Law was deemed suitable for calculating ullage composition. Gauging results from the PVT method agreed with the reference liquid level measurements to within 3%.     

用于低温存储系统的多层绝热性能分析

真空多层绝热的性能好坏直接影响到低温贮箱的安全性。根据修正的Lockheed模型,计算冷边界温度、热边界温度、层密度等对均匀层密度多层绝热性能影响,并对三区域变密度的多层绝热性能进行分析,最后针对在轨、地面状态时对低温贮箱漏热方面的要求提出采用复合多层绝热的概念,得出复合多层绝热具有优良的隔热性能。     

Assessment of the effects of release variables on the consequences of LNG spillage onto water using FERC models

Liquefied natural gas (LNG) release, spread, evaporation, and dispersion processes are illustrated using the Federal Energy Regulatory Commission models in this paper. The spillage consequences are dependent upon the tank conditions, release scenarios, and the environmental conditions. The effects of the contributing variables, including the tank configuration, breach hole size, ullage pressure, wind speed and stability class, and surface roughness, on the consequence of LNG spillage onto water are evaluated using the models. The sensitivities of the consequences to those variables are discussed.     

Study on the heat transfer of high-vacuum-multilayer-insulation tank after sudden, catastrophic loss of insulating vacuum

One of the worst accidents that may occur in a high-vacuum-multilayer-insulation (HVMLI) cryogenic tank is a sudden, catastrophic loss of insulating vacuum (SCLIV). There is no doubt that the gases leaking into the insulation jacket have some influence on the heat transfer process of it. However, this issue has not been thoroughly studied so far. In this paper, a test rig was built up and experiments were conducted using a SCLIV cryogenic tank and with nitrogen, helium and air as the working medium, respectively. The venting rates of the tank and temperature in the insulation jacket were measured respectively after the three different gases leaking into the jacket. A heat transfer model describing the heat transfer process of a SCLIV tank was also presented. The calculated results using this model were compared against the experimental data. It is found that the heat transfer performance of the HVMLI cryogenic tank after SCLIV is strong relevant to the type of gas leaking into the insulation jacket.     

大型低温液体贮存站贮罐设计选型论证(续)

运用热力学第一定律对低温液体在贮存、运输转注过程中的热力学特性进行了分析,定量分析、计算了低温液体节流过程的节流汽化率值,提出了减少节流过程所形成的汽化率的有效途径;论证了在大型低温液体贮存站贮罐设计选型时,应根据不同的低温液体液源、液体品质和操作工况等条件进行设计选型;论证了除合理地选择正确的低温液体贮罐结构形式外,还应高度重视低温液体贮罐的安全泄放设计,以确保低温液体贮存站的安全性能、使用性能和技术经济性能均处于最佳状态。