Fluid Thermal Stratification Research Articles

Experimental study on sloshing mechanical characteristics in a partially filled storage tank

AbstractDue to excellent performance, hydrogen is treated as the most promising energy carrier. However, during the storage of liquid hydrogen, there are still some thorny issues that need to be addressed urgently, such as fluid thermal stratification and sloshing phenomenon. To efficiently grasp fluid sloshing mechanical characteristics, a simple visual sloshing experiment rig was established by using a rectangle transparent water vessel. The variations of the interface shape and the impact force during sloshing were monitored and analyzed. The effects of sloshing frequency, horizontal acceleration, and initial liquid height on fluid sloshing mechanical mechanism were investigated. The results show that when the external sloshing excitation is close to the first order natural frequency, obvious interface fluctuation and large amplitude sloshing force variation are observed. The present work is of significance to strengthen the understanding of fluid sloshing mechanical performance and may lay a solid foundation for fluid sloshing suppression and long‐term storage of cryogenic fuels.

1G and microgravity tank self-pressurization: Research on cryogenic fluid thermal stratification

A correlation numerical model was constructed, and relevant experiments were simulated to investigate the influence of gravity level on the self-pressurization process of the cryogenic fluid of liquid nitrogen. The calculated results agreed well with the experimental data. On this basis, the influences of a normal gravity environment and a microgravity environment on the self-pressurization process of liquid nitrogen in the normal-temperature fluid Perfluoro-n-Pentane (PnP, or C5F12) tank of National Aeronautics and Space Administration (NASA) were analyzed. According to the calculation results, an explicit horizontal interface is formed between the gas and liquid phases in the tank during the 1G (G = 9.81 m/s2) self-pressurization process. The ullage pressure increases as time progresses, and obvious thermal stratification phenomena are observed in the ullage along the axial and radial directions. However, the thermal stratification of the liquid zone is not obvious. The ullage near the gas‒liquid interface shifts from counterclockwise movement to clockwise movement, while the liquid changes from clockwise movement to counterclockwise movement. In the process of microgravity self-pressurization, the gas‒liquid interface in the tank presents an irregular shape, and the ullage pressure increases continuously before 400 s and then tends to be stable. The axial thermal stratification of fluid in the tank tends to disappear as time progresses, but obvious radial thermal stratification is observed. The ullage in the tank moves up and down as a response to the Marangoni migration effect. The temperature of the liquid zone in the tank during 1G self-pressurization tends to be consistent more quickly than that during microgravity self-pressurization, and the average temperature is higher. However, the temperature reduction rate of the ullage in the tank during microgravity self-pressurization is higher than that during 1G self-pressurization. Numerical simulation results can provide references to further study the on-orbit pressure control technique of cryogenic liquid tanks.

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. • Tank pressurization and fluid temperature distribution are investigated. • The pressure rise rate increases with the gas injection mass flow rate. • Serious liquid evaporation occurs in the initial pressurization period. • Vapor condensation increases with time and gas injection mass rate. • The penetration depth of thermal capacity experiences increases with time.

Influence of thermal stratification on vertical natural convection—Experimental investigations on the example of thermal energy storage systems

Stratified thermal energy storages (TESs) are a promising solution for the large-scale energy storage problem of surplus renewable energy. Recent studies have shown parasitic convection occurring in near-wall regions inside such storage tanks, decreasing the working fluid's thermal stratification and reducing their exergy efficiency. This paper presents an experimental investigation of vertical convective flows in thermally stratified environments to complement the theoretical studies in this field. Specifically, we consider natural convection within a stratified laminar flow driven not by active heating but by the temperature gradient along a vertical wall, as is the case in real TES systems. The insights gained into the fundamental physical mechanisms of stratified vertical convection can promote efficiency improvements in TES systems. Therefore, we combine multiple particle image velocimetry and temperature measurements at different heights and thus obtain high-resolution vector fields of the entire wall jet flow and vertical temperature profiles for a TES model experiment. We appropriately modify scaling arguments found in the literature to develop a theory specifically suited to the experimental setup. The experimental data agree well with the modified theory. The results show two laminar counter-directed jets next to the vertical sidewall. In regions with high temperature gradients, the wall jets slow down, and flow reversals occur next to them. Moreover, the wall jets are asymmetric due to temperature-dependent fluid properties in conjunction with the ambient fluid stratification. In the stratification's upper, hot part, the wall jet is thinner and faster than the bottom jet in the cold region.

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.

Thermal performance of liquid hydrogen tank in reduced gravity

Fluid temperature stratification is significant to the safe operation of space storage tanks. A calculation model, accounting for the liquid–vapor phase change, is developed to investigate the thermal physical process in a liquid hydrogen (LH2) tank in reduced gravity. Viscous flow is considered in calculation model with Ra ranging from 0.1 to 105 to ensure the continuity of natural convection. The stratified layer parameters, the tank pressure rise, and the interface phase change are studied respectively. Influences of the initial liquid height, the initial ullage temperature and the tank wall heat flux on the development of thermal stratification are estimated. The results show that the stratified layer thickness rises with the initial liquid height. While the initial liquid height is large, it costs more time for the wholly development of fluid thermal stratification. Largely influenced by the temperature of the stratified layer, both tank pressure and phase change capacity increase with the initial liquid height. It seems that the initial ullage temperature has a weak effect on the development of fluid thermal stratification. Both the tank pressure and the phase change quality increase with the initial ullage temperature. The external tank wall heat flux promotes the development of thermal stratification. The stratified layer has a larger thickness and develops faster for the larger heat flux. Both tank pressure and phase change capacity increase with the external heat flux. Meanwhile, the intersection point exists between any two profiles.

Study on thermal stratification in liquid hydrogen tank under different gravity levels

In the present study, one CFD model is selected to research the effect of gravity scale on the thermal performance in liquid hydrogen tank. Four gravity levels (1g0, 10−1g0, 10−2g0 and 10−3g0) are compared to recognize the influence of the reduced gravity on fluid thermal stratification. The results show that with the increasing of the gravity level, the vapor temperature distribution becomes more uniform, and the liquid stratum layer develops faster. Compared the CFD results with the results of two stratification theoretical models, the stratum thickness calculated by CFD model is close to the values of Tellep model. While the stratum temperature of CFD model is much closer to that of Reynolds model. With vortex occurring among two slosh baffles, the streamline in the liquid stratum likes a plume. Influenced by the surface tension in reduced gravity, liquid close to the tank wall moves up with the interface becoming curved. The interface area rises with the decrease of gravity. The gravity of 10−1g0 still plays the dominant role with the interface area of 10−1g0 being almost the same as that of 1g0. While for others, the effect of the surface tension shows up gradually.

Experimental study on refrigeration performance and fluid thermal stratification of thermodynamic vent

Long term storage of cryogens is significant to the future space exploration. Thermodynamic vent is treated as an efficient method to realize the tank pressure control and the efficient storage of cryogenic fuels. In the present study, an experimental rig is built to research the pressurization and refrigeration performance of thermodynamic vent with R123, under the influence factors of initial liquid height, external heat load, pressure control band and circulation flow. Meanwhile, the fluid thermal stratification is investigated for the best experiment condition. The results show that the opening of the throttling orifice and the total circulation flow pipe size should be matched well to achieve the best operation of TVS. Moreover, the circulation flow rate has a direct effect on the tank pressure control and the development of the fluid thermal stratification. Some valuable conclusions drawn from the present study could be applied to cryogenic propellants of space vehicle.

Thermal physical performance in liquid hydrogen tank under constant wall temperature

Abstract A calculation model is developed to investigate the pressurization performance and thermal stratification in a liquid hydrogen (LH2) tank. Viscous flow is considered in the stratification model to ensure the continuity of fluid flow and heat exchange. The tank pressure rise, energy distribution and thermal stratification are studied respectively. Compared to the results without considering the phase change, the tank pressure, the stratified layer temperature and the ullage temperature calculated with phase change considered, have increased about 69.98%, 70.90% and 15.53%. Moreover, influences of the gravity level, initial wall temperature and initial liquid height on the development of thermal stratification are analyzed. It turns out that the larger the gravity level is, the faster the liquid thermal stratification develops. The effect of the initial wall temperature on the growth of thermal stratification is the same. Both the ullage pressure and the stratified temperature increase with time. While the gravity level is larger than a certain value and the initial wall temperature is less than a certain value, the ullage pressure and the stratified temperature decrease firstly, and then increase. Meanwhile, it costs much more time for fluid thermal stratification development fully for a larger initial liquid height.

Ground experimental investigation of thermodynamic vent system with HCFC123

Thermodynamic vent system (TVS) is of significance to long term on-orbit storage of cryogenic fuel. In the present study, one ground experiment is established to investigate the pressure control performance of TVS with the working fluid of HCFC123. The tank pressure variation and fluid temperature change are respectively studied in self-pressurization, injection mixing and active refrigeration phases. The refrigeration capacity supplied by TVS and fluid thermal stratification during different phases, are specially illustrated. The effectiveness of the pressure control performance by means of TVS has been proven in the present experiment. The total venting gas loss is about 17.3 kg for 16 throttling cycles. The corresponding refrigeration capacity of TVS is 1216–1415 W, with circulation volume flow of 95 L/h and throttling ratio of 16.8–18.9%. Moreover, it costs approximately 6 h for the liquid thermal stratification developing fully. While in active refrigeration phase, the vapor has the maximum temperature reduction of 9.2 °C, and the variation of the liquid temperature is limited in 2.0 °C. When compared to direct venting method, TVS has recovered more than 23.22% venting loss for 4.75 h' operation. On the whole, the effective pressure control and heat load elimination of TVS are greatly confirmed.

Measurement of thermal conductivity of fluids using 3-ω method in a suspended micro wire

A thermally controlled, compact device employing the 3-ω technique, used to measure the thermal conductivity of fluids, is designed, developed, and presented in this paper. The 3-ω method, which analyzes temperature oscillations data in the frequency domain, requires a microscopic sample and extremely low heating power. The functionality is derived from the approximate solutions of temperature oscillations of a line heater based on the infinite line-heater model over an empirically and analytically chosen range of frequencies. The method is devoid of errors related to transient measurements, fluid thermal stratification and mobility errors, which pose difficulties in other methods. A platinum (99.99% pure) wire of 50 µm diameter and a length of 30 mm, suspended in a sample volume of 25 µl of the test fluid, serves simultaneously as the heater and thermometer. Structure-wise, the device is designed to support measurements over a range of temperatures and fluid pressures providing modularity and flexibility to the instrument. The device is successfully employed to measure the thermal conductivity of de-ionized water for temperatures between 15 and 35°C with an accuracy of ±1.2% inmeasurement.