| The solid-liquid phase change material(PCM)possesses the high latent heat and little temperature variation during phase change,and has been widely used in the thermal energy storage of new energy resource and temperature regulation of power devices.However,the low conductivity of PCM leads to the slow heat transfer rate of system.In this dissertation,in order to reveal the heat transfer mechanism and enhancement,the heat transfer enhanced methods in solid-liquid phase change such as implement of non-uniform heat flux boundary,adding of fin and high-thermalconductivity particles are proposed,and the lattice Boltzmann method is applied to investigate the effects of heat flux distribution,location of fin and volume fraction of high-thermal-conductivity particles.Furthermore,in order to improve the accuracy of lattice Boltzmann method,the phase change lattice Boltzmann model in cylindrical coordinate system,quasi-enthalpy based phase change lattice Boltzmann model,enthalpy-transforming based phase change lattice Boltzmann model and phase change discrete unified gas kinetic scheme(DUGKS)have been proposed and verified.The major body of this dissertation is as follows:(1)For solid-liquid phase change in a cavity,the phase change lattice Boltzmann model has been established.The non-uniform heat flux boundary condition is employed to enhance the heat transfer of PCM.The uniform heat flux distribution,linear heat flux distribution and parabolic symmetry heat flux distribution are applied to the left wall of cavity respectively.The effects of heat flux distributions on the temperature of PCM and location of solid-liquid interface are investigated.Furthermore,the natural convection route is changed by inclining the cavity to enhance the heat transfer.The results show that when the control parameter of linear distribution is 0.006,inclining the cavity 15o can increase the liquid fraction by 0.4%,compared with the case without inclination.(2)The lattice Boltzmann model for battery thermal management based on PCM at cold temperature is developed to investigate the effects of thermal conductivity and latent heat of PCM on the heat dissipation rate of battery.Based on this,the PCM is applied to the cavity heated by a fin.The impacts of fin’s location and size,and the inclination angle of cavity are studied.The results show that,increasing the latent heat is more effective than decreasing the thermal conductivity in temperature maintaining.Besides,increasing the angle from 30o to 45o is able to weaken the heat transfer through right wall of fin,leading to the slower melting rate.(3)In order to enhance the heat transfer performance,the high-thermalconductivity nano-particles are added to form the nano-fluid.The nano-fluids are used in active battery active thermal management and latent thermal energy storage respectively.The impacts of nanoparticle’s volume fraction on the performances of temperature regulation and energy storage are considered.Based on this,the separate plate is applied in this dissertation to weaken the heat accumulation at the top of the enclosure resulted from the natural convection.The results demonstrate that with 0.01% volume fraction nanoparticle,the melting time with separate plate placed in the middle is reduced by 5.5% compared with that without separate plate.(4)For the phase change process in cylindrical coordinate system,the solid-liquid phase change lattice Boltzmann model in cylindrical coordinate system is developed and verified by solving the problems of one-region phase change and melting by convection in cylindrical coordinate system.Then,the solid-liquid phase change model in a spherical capsule is constructed and the effects of non-uniform wall temperature is considered.The results present that increasing the slope of wall temperature distribution is able to raise the melting rate of PCM at the top of capusle,leading to the downwards sold-liquid interface and high-temperature region.(5)The enthalpy model is modified to remove the errors introduced into the macroscopic transportation equations during the Chapman-Enskog expansion.As a result,the time partial differential term of latent heat to accelerate the melting process is eliminated.Based on this,the quasi-enthalpy based lattice Boltzmann model is proposed,in which the iterations to obtain the temperature and liquid fraction are no longer necessary.(6)In order to obtain the continuously differentiable relation between temperature and enthalpy,the polynomial scheme used in enthalpy-transforming lattice Boltzmann model has been proposed.The comparisons of linear scheme,hyperbolic tangent scheme and polynomial scheme in the problem of phase change by convection have been conducted.The results show that the linear scheme’s variation of liquid fraction is more smooth other two schemes’ variations.However,the polynomial scheme is more suitable for differential scanning calorimeter curve.(7)The discrete unified gas kinetic scheme for phase change is developed to overcome the obstacles for conventional lattice Boltzmann model dealing with complicated curved boundary condition.The model is verified by solving the problems of flow and heat transfer of porous plate,two-region phase change and melting by convection.The non-uniform mesh is available in this model and the fine mesh is able to reveal the information of flow and temperature more accurately.Besides,since the collision term is replaced by the update of new distribution function,the phase interface effect can be avoided.In summary,the heat transfer enhancement mechanism in the process of solidliquid phase change has been revealed and it has been tried to extend the application of Boltzmann transportation equation in solving solid-liquid phase change problem,which are of importance to improving the efficiencies of thermal energy storage and temperautre regulation. |
