Study On Flow And Heat Transfer Characteristics Of Latent Functional Thermal Fluid In Heat Storage System

Latent functional thermal fluids have been widely applied in heat storage systems,but the complexity of heat transfer process of latent functional thermal fluids has brought obstacles to its industrial application.Therefore,it is an important prerequisite for improving the efficiency of phase-change energy storage to find out the solid-liquid variable heat transfer and flow characteristics of latent functional thermal fluids in energy storage systems.However,there is still a lack of accurate numerical model to study the convective effect in the heat transfer process.Therefore,this thesis proposes a numerical model of latent functional thermal fluid based on Boltzmann transport equation and enthalpy transformation method,and uses this model to study the flow and heat transfer characteristics of latent functional thermal fluid in the heat storage system.The main contents of this thesis are as follows:(1)In view of the need of numerical simulation of fluid dynamic heat transfer of latent functional thermal fluid in heat storage system,the lattice Boltzmann numerical model of heat transfer and flow of single fluid latent heat type functional heat fluid was derived,and the macroscopic equation describing the flow heat transfer of latent heat type functional heat fluid was proposed.The continuity equation,momentum equation and energy equation were obtained by multiscale transformation of the lattice Boltzmann equation by Chapman-Enskog expansion.The results obtained from the existing model were compared with the analytical solutions and previous works,which verified that the proposed model can be used to simulate the flow heat transfer of latent functional thermal fluid.(2)The natural convection of latent functional thermal fluid in the heat storage system was studied,and it was concluded that the phase transition temperature was the key parameter to enhance the flow heat transfer in the square cavity when the natural convection of latent functional thermal fluid was used to enhance heat transfer under a constant heat source.When the transformation temperature is close to the hot wall temperature or cold wall temperature,the enhanced heat transfer effect is low.The temperature difference between phase transition temperature and wall surface temperature enhances the heat transfer enhancement effect of capsule particles during melting.The best heat transfer performance can be obtained in the range of 0.25～0.75.Under the time period temperature boundary condition,the average Nusselt number of the wall changes with the wall temperature,and the change frequency is the same as that of the wall temperature.The temperature difference between hot and cold walls will increase the heat transfer effect in the cavity.The increase of the temperature amplitude of the hot wall will increase the mean Nusselt number of the wall,but the mean value remains the same.The increase of the volume fraction of phase change capsules will greatly enhance the heat transfer efficiency of the fluid in the cavity.When the volume fraction is 5%,the effect of heat transfer enhancement can be increased to56%.(3)The mixed convection characteristics of latent functional thermal fluid under the rotation of the circular tube were studied,and it was concluded that the mixed convection heat transfer could significantly enhance the heat transfer efficiency of latent functional thermal fluid.When the Rayleigh number is small,the counterclockwise rotation of the circular tube can effectively increase the convective effect in the square cavity and enhance the convective heat transfer of the fluid.However,when the Rayleigh number increases,the circular tube rotates clockwise and the natural convective heat transfer caused by buoyancy in the square cavity can effectively improve the heat transfer efficiency.Heat transfer in the system decreases with the increase of Stefan number,but the effect becomes less and less after Stefan number is greater than 0.3.Under the same Richardson number,the rotation direction of the circular tube has a great influence on the internal heat transfer of the other cavity,so corresponding strengthening methods should be selected for different systems.(4)Firstly,a three-dimensional lattice Boltzmann model was constructed.The accuracy of the model was verified by the convection problem of the latent functional thermal fluid under the temperature difference of the wall,and the calculation results were compared with those of the two-dimensional model.Finally,the mixed convection of latent functional thermal fluid under the influence of the top drive is simulated.It is found that the driving direction of the top has a great influence on the heat transfer inside the system.When the Richardson number is 0.1,the top move in the opposite direction of the heat transfer direction is more conducive to strengthen the heat transfer.With the increase of Richardson number,the heat transfer inside the square cavity will be more intense as the top cap moves along the direction of heat transfer.This thesis has 41 figures,3 tables and 111 references.