Research On Heat And Mass Transfer With Local Thermal Non-equilibrium In Porous Media

Local thermal non-equilibrium effects within porous media widely exist in many fields of science and technology.Both coal fire extinguishing and preventing technology using liquid nitrogen and transpiration cooling technology involve them,but there is a great difference between the two technologies.The thermal conductivity of solid matrix for the former is very small,but for the latter is relatively large.The obvious difference of thermal conductivity may have important influence on local thermal non-equilibrium effects in the process of heat and mass transfer in porous media.In this dissertation,the local thermal non-equilibrium effects in porous media with different thermal conductivities were investigated by using experimental and numerical methods based on the theory of local thermal non-equilibrium.Main works include:(1)We built a roadway model,and experimentally investigated the heat and mass transfer characteristics with liquid nitrogen injection.The results indicate that the injection location and amount of liquid nitrogen,together with air leakage,significantly affect the temperature distribution in coal porous media.The secondary temperature drop happens in the middle and bottom layer close to the lowest temperature region.(2)We proposed two multi-scale models for regular spheres and irregular coal particles,respectively.The simulation results are consistent with the experiments.The local thermal non-equilibrium effects in porous media with small thermal conductivity were numerically investigated by using the multi-scale models.The results show that the local thermal non-equilibrium effects at the inlet are very obvious.The particle diameter,mass flow rate of nitrogen gas,inlet fluid temperature and initial temperature of porous media have important influence on the effects.(3)Transpiration cooling processes with liquid water phase change were numerical explored by the modified separated flow model.The results show that the local thermal non-equilibrium effects at the cold and hot surface and the boundary of two phase regions are very obvious.