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.     

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.     

Heat transfer characteristics of microencapsulated phase change material slurry in laminar flow under constant heat flux

Due to its large apparent specific heat during the phase change period, microencapsulated phase change material slurry (MPCMS) has been suggested as a medium for heat transfer. In this paper, the convective heat transfer characteristics of MPCMS flowing in a circular tube were experimentally and numerically investigated. The enhanced convective heat transfer mechanism of MPCMS, especially in the thermal fully developed range, was analyzed by using the enthalpy model. Three kinds of fluid–pure water, micro-particle slurry and MPCMS were numerically investigated. The results show that in the phase change heat transfer region the Ste number and the Mr number are the most important parameters influencing the Nusselt number fluctuation profile and the dimensionless wall temperature. Re_b, d_p and c also influence the Nusselt number profile and the dimensionless wall temperature, but they are independent of phase change process.     

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.     

Modeling heat and moisture transfer through fibrous insulation with phase change and mobile condensates

This paper reports on a transient model of coupled heat and moisture transfer through fibrous insulation, which for the first time takes into account of evaporation and mobile condensates. The model successfully explained the experimental observations of Farnworth [Tex. Res. J. 56 (1986) 653], and the numerical results of the model were found to be in good agreement with the experimental results of a drying test. Based on this model, numerical simulation was carried out to better understand the effect of various material and environmental parameters on the heat and moisture transfer. It was found that the initial water content and thickness of the fibrous insulation together with the environmental temperature are the three most important factors influencing the heat flux.     

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.     

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.     

CFD simulation of hydrodynamics and heat transfer in gas phase ethylene polymerization reactors

The hydrodynamics and temperature of a two-dimensional gas–solid fluidized bed of gas phase olefin polymerization reactor had been studied. A two-fluid Eularian Computational Fluid Dynamics (CFD) model with closure relationships according to the kinetic theory of granular flow has been applied in order to simulate the gas–solid flow. Fluidization regime and gas–solid flow pattern were investigated using three different drag models. Model predictions of bed pressure drop were compared with corresponding experimental data reported in the literature to validate the model. The predicted values were in reasonable agreement with the experimental data. The temperature behavior of fluidized bed with various drag models was investigated. The temperature gradient in the primary section of the bed was much larger than the gradient in other sections and the effect of all drag models on temperature gradient along the bed was approximately similar.     

Effect of heat transfer through the release pipe on simulations of cryogenic hydrogen jet fires and hazard distances

Jet flames originated by cryo-compressed ignited hydrogen releases can cause life-threatening conditions in their surroundings. Validated models are needed to accurately predict thermal hazards from a jet fire. Numerical simulations of cryogenic hydrogen flow in the release pipe are performed to assess the effect of heat transfer through the pipe walls on jet parameters. Notional nozzle exit diameter is calculated based on the simulated real nozzle parameters and used in CFD simulations as a boundary condition to model jet fires. The CFD model was previously validated against experiments with vertical cryogenic hydrogen jet fires with release pressures up to 0.5 MPa (abs), release diameter 1.25 mm and temperatures as low as 50 K. This study validates the CFD model in a wider domain of experimental release conditions - horizontal cryogenic jets at exhaust pipe temperature 80 K, pressure up to 2 MPa ab and release diameters up to 4 mm. Simulation results are compared against such experimentally measured parameters as hydrogen mass flow rate, flame length and radiative heat flux at different locations from the jet fire. The CFD model reproduces experiments with reasonable for engineering applications accuracy. Jet fire hazard distances established using three different criteria - temperature, thermal radiation and thermal dose - are compared and discussed based on CFD simulation results.     

A numerical investigation of various phase change models on simulation of saturated film boiling heat transfer

There is a great variety of two‐phase models in numerical simulations. The performance of each model complicates the numerical simulation of boiling. The challenge of the right choice of heat and mass transfer models makes this type of problem more complicated. In this research work, the volume of the fluid two‐phase model has been used to simulate the film boiling of saturated liquid. The geo‐reconstruction method also reconstructs the interface of two phases. The models of the sharp interface, Lee and Tanasawa have been employed among the available models for calculating the phase change rate and the source terms of the equations. The Numerical solver of the phase‐change is verified through the Stefan one‐dimensional vaporizing problem. Correct empirical coefficients used in both Lee and Tanasawa models are presented. Bubble detachment time, flow pattern, the periodic Nusselt number, and the bubble form have been investigated in all three phase change models. Two Berenson and Klimenko experimental correlations have been used for verification of Nusselt number derived from simulations. The Nusselt number shows a proper fit with the Klimenko's Nusselt number. Obtained Nusselt number demonstrates the Lee model is more precise than other phase change models in simulating of film boiling on the flat plate.     

Ignition and heat radiation of cryogenic hydrogen jets

In the present work release and ignition experiments with horizontal cryogenic hydrogen jets at temperatures of 35–65 K and pressures from 0.7 to 3.5 MPa were performed in the ICESAFE facility at KIT. This facility is specially designed for experiments under steady-state sonic release conditions with constant temperature and pressure in the hydrogen reservoir. In distribution experiments the temperature, velocity, turbulence and concentration distribution of hydrogen with different circular nozzle diameters and reservoir conditions was investigated for releases into stagnant ambient air. Subsequent combustion experiments of hydrogen jets included investigations on the stability of the flame and its propagation behaviour as function of the ignition position. Furthermore combustion pressures and heat radiation from the sonic jet flame during the combustion process were measured. Safety distances were evaluated and an extrapolation model to other jet conditions was proposed. The results of this work provide novel data on cryogenic sonic hydrogen jets and give information on the hazard potential arising from leaks in liquid hydrogen reservoirs.     

Experimental study of natural convection heat transfer in a microencapsulated phase change material slurry

A new microencapsulated PCM (Phase Change Material) slurry (MEPCS) at high concentration (45% w/w) was developed based on microencapsulated Rubitherm RT6. Its heat storage and heat transfer characteristics have been experimentally investigated in order to assess its suitability for integration into a low temperature heat storage system for solar air conditioning applications. Differential scanning calorimetry tests have been conducted to evaluate the cold storage capacity and phase change temperature range. An experimental setup was built in order to quantify the natural convection heat transfer occurring from a vertical helically coiled tube immersed in the MEPCS. First, tests were carried out using water in order to obtain natural convection heat transfer correlations and then a comparison was made with the results obtained for the MEPCS. It was found that inside the phase change interval the values of the heat transfer coefficient for the MEPCS were significantly higher than for water, under identical temperature conditions.     

CFD modeling of hydrogen dispersion under cryogenic release conditions

The use of hydrogen as a fuel should always be accompanied by a safety assessment concerning the case of an accidental release. To evaluate the potential hazards in a spill accident both experiments and simulations are performed. In the present work, the CFD code, ADREA-HF, is used to simulate the liquefied hydrogen (LH2) spill experiments (test 5, 6, 7) conducted by the Health Safety Laboratory (HSL). Two horizontal releases, the one along the ground and the other one at a distance above the ground, and one vertical release are examined with spill rate 60 lt/min. The main focus of this study is on the presence of humidity in the atmosphere and its effect on the vapor dispersion. When humidity is present is cooled, condenses and freezes due to the low prevailing temperature (∼20 K near the release), and releases heat. In addition, during the release hydrogen droplets are formed due to mechanical and flashing break up, and water droplets and ice crystals due to humidity phase change. Therefore, two models are tested: the hydrodynamic equilibrium model, which assumes that the phases are in thermodynamic and kinematic equilibrium and the non hydrodynamic equilibrium model (slip model), which assumed that the phases are in thermodynamic equilibrium but they can obtain different velocities. The fluctuating wind direction was also taken into account, since it greatly affects the hydrogen dispersion. The computational results are compared with the experimental measurements, and it is concluded that humidity along with the slip effect influences the buoyancy of the cloud to a great extent. The best simulation case (humidity and slip effect) is consistent with the experiment for all three tests for the majority of the sensors.     

Numerical study on particle distribution characteristics of slush hydrogen in a cryogenic tank

A numerical model considering phase change and heat transfer was established by the Euler-Euler two-fluid method to investigate the storage characteristics and two-phase flow field of slush hydrogen. Numerous numerical simulations were performed to discuss the effect of particle diameter (d_p = 0.02–0.5 mm), content of solid hydrogen (α_s = 10%–50%), and heat leakage (q = 50–200W·m⁻²) on the flow field. It was found that particle deposition could occur during the storage process, and there exist moving vortices with contrary directions under specific conditions. The sedimentation characteristics and vortex size are influenced by many factors including particle size, solid hydrogen content, and heat leakage. An increase in particle size could lead to the strengthening of precipitation and the expansion of the counterclockwise vortex region on the right side of the tank. And the increase in solid hydrogen content could result in more deposition and more collisions and friction between particles. Moreover, the increase in heat leakage could increase the area of the counterclockwise vortex. Numerical results of the deposition and flow field characteristics in the storage tank could clearly show the physical law of the slush hydrogen so that the uniform distribution of slush hydrogen could be promoted for efficient storage and application.     

Melting heat transfer characteristics of a horizontal ice cylinder immersed in an immiscible liquid

The melting characteristics of a horizontal ice cylinder immersed in an immiscible liquid were investigated both experimentally and analytically. A clear cylindrical ice layer formed around a horizontal cooling tube with a coaxial outer heated tube was melted in oil as the test immiscible liquid in the annulus between the ice and outer tube. Both the melting behavior of ice and the flow patterns of the immiscible liquid were observed under a variety of outer wall temperature conditions. In the analysis, two boundary layers were introduced for both the melt water film and ambient liquid, respectively. It was found from the experiments that the tendency of the mean Nusselt number changes clearly at Ra = 10⁶, which corresponds to the temperature condition T0 = 12.0 °C. This analysis might be used to estimate the melting characteristics of such a system during the initial stage of low temperature conditions. © 1998 Scripta Technica, Heat Trans Jpn Res, 27(5): 336–352, 1998     

Numerical simulation of heat and mass transfer during the absorption of hydrogen in a magnesium hydride

This paper presents a numerical work aiming at the prediction of the characteristics of an industrial tank filled with hydrides for hydrogen storage. A validation of the method is given and is followed by the resolution of an example which shows the importance of achieving a three-dimensional modelling for the design of an industrial tank. Finally, recent results obtained on a magnesium hydride laboratory tank are given.     

Review on heat transfer analysis in thermal energy storage using latent heat storage systems and phase change materials

Thermal energy storage (TES) is a technology that stocks thermal energy by heating or cooling a storage medium so that the stored energy can be used later for heating and cooling applications and for power generation. TES has recently attracted increasing interest to thermal applications such as space and water heating, waste heat utilisation, cooling, and air conditioning. Phase change materials (PCMs) used for the storage of thermal energy as latent heat are special types of advanced materials that substantially contribute to the efficient use and conservation of waste heat and solar energy. This paper provides a comprehensive review on the development of latent heat storage (LHS) systems focused on heat transfer and enhancement techniques employed in PCMs to effectively charge and discharge latent heat energy, and the formulation of the phase change problem. The main categories of PCMs are classified and briefly described, and heat transfer enhancement technologies, namely dispersion of low‐density materials, use of porous materials, metal matrices and encapsulation, incorporation of extended surfaces and fins, utilisation of heat pipes, cascaded storage, and direct heat transfer techniques, are also discussed in detail. Additionally, a two‐dimensional heat transfer simulation model of an LHS system is developed using the control volume technique to solve the phase change problem. Furthermore, a three‐dimensional numerical simulation model of an LHS is built to investigate the quasi‐steady state and transient heat transfer in PCMs. Finally, several future research directions are provided.     

Effect of micro-pin-fin arrays on the heat transfer and combustion characteristics in the micro-combustor

Micro-combustor is an important component elements of the micro-thermophotovoltaic (MTPV) conversion device. The combustion stability is critical to improve its thermal performance, and thus three kinds of combustors are compared by computational fluid dynamics (CFD), which includes single – channel combustor, alternate permutation combustor and in-line combustor. The influences of micro-pin-fin arrays on the performance of the micro-combustor are discussed. Results indicate that the maximum surface temperature of combustor with fins is about 100 K higher than that without fins and the mean temperature and heat flux of in-line combustor are always higher in magnitude than those of the alternate permutation combustor. Analysis in this paper reveals that comparing with single-channel combustor, the micro-combustor with fins greatly enhances the heat transfer process through the wall. There are low velocity zones in the tail of fins, which can gather the reactants and prolong the residence time which make the combustion more sufficient and improve the effect of stable combustion. Meanwhile, under calculated conditions, the influence of micro-pin-fin arrays on the combustion reaction is stronger as the flow rate increase. The fin array in micro-combustor does not only improve the wall temperature but also minimize the wall temperature difference along the axial direction. Moreover, when the inlet velocity is larger than 4 m/s, the hydrogen conversion ratios of micro-combustors with fins was not strengthened obviously with the further increase of inlet velocity.