Research On The Local Thermal Non-equilibrium Effect For Heat Transfer In Porous Media

Traditional method to investigate the flow and heat transfer phenomenon in porous media is mainly based on the local thermal equilibrium(LTE) assumption which assumes the temperatures of the solid and fluid phases equal the same. The LTE model is used to describe the heat transfer in porous media. However, when the internal heat transfer in porous media is not sufficiently large, the temperature difference of the two phases in porous media cannot be neglected, and consequently the LTE assumption is not applicable. In this situation, the heat transfer process must be described by the local thermal non-equilibrium(LTNE) model, and the deviation from the LTE condition is called the of LTNE effect. The LTNE effect is important for many engineering regions including porous media heat sink, transpiration cooling and geothermal exploitation.However, due to the complication of the double temperature coupling, the previous investigations on the model and effect of LTNE are not sufficient, leaving many scientific problems to be solved. The objective of the present work is to improve the framework of LTNE model, to reveal the influence of LTNE effect on the heat transfer in porous media, and to increase the prediction accuracy of heat transfer in porous media. First, based on the volume averaging method, an original LTNE model of an impermeable wall bounded by a porous medium is developed. It is found that the local tangential thermal resistance in the wall has a thermal bridge effect on the heat flux splitting on the boundary, which can explain the mechanism of heat flux splitting on the boundary of a porous medium. From this model, a general thermal boundary condition model is also developed for porous media. Second, the analytical solution of thermally developing flow in a porous medium channel is presented. The influences of the local non-equilibrium effect on the thermal entrance length and other heat transfer characteristics are analyzed, showing the quantitative relation between the local non-equilibrium effect and the Biot number of porous media and solid-to-liquid effective thermal conductivity ratio. Third, in consideration of the importance of effective stagnant thermal conductivity on the LTNE model, the Simulated Annealing Algorithm and the Discrete Element Method are creatively combined to model and predict the effective stagnant thermal conductivity of sintering porous media consisting of random particles. The reason why the predicted effective thermal conductivity is much less than that predicted by the traditional LTNE model is presented, and the sensitivity of effective thermal conductivity on the LTNE effect is analyzed. Then, the above three models are validated by the comparison of macro-scale model and pore-scale model, the comparison of simulation results and analytical solutions, and the comparison of simulation results and experimental results. Fourth, CO2 geological sequestration and enhanced geothermal system are taken as two engineering applications to analyze the LTNE effect in the transient process and two-phase flow in porous media. For enhanced geothermal system, the rock size is much larger than the fracture size, so the classic internal heat transfer coefficient needs amendment. In this situation, the thermal resistance of local heat conduct is large, leading to a strong local non-equilibrium effect, which is dominant in the heat transfer process of enhanced geothermal system. It is found that in the condition of large particles, high flow rate and two phases, the LTNE effect is dramatic in the initial phase of CO2 geological storage, and has a large influence on the thermal stress analysis on the near injection well region.