Numerical investigation of local thermal non-equilibrium effects in coal porous media with cryogenic nitrogen injection

The technology of fire prevention and extinguishing using liquid nitrogen or cryogenic nitrogen has been widely applied in underground coal fire. In this paper, a multi-scale model has been proposed to investigate the complex heat transfer characteristics in coal porous media (CPM) with cryogenic nitrogen injection. In the model, the fluid energy equation at representative elementary volume (REV) scale is coupled with the heat conduction equation of a solid particle at pore scale considering the temperature variation in the coal particle, and the equivalent diameter is employed to simplify the heat conduction process in the solid particle. The experiment has been carried out and the result is consistent with the simulation obtained by the multi-scale model. The influences of equivalent diameter, mass flow rate, inlet fluid temperature and initial temperature on the local thermal non-equilibrium (LTNE) effects have been investigated numerically by using the multi-scale model. The results show that the time to reach local thermal equilibrium (LTE) between fluid and solid center is longer due to the small thermal conductivity of coal matrix, and it is affected dramatically by equivalent diameter and mass flow rate. The results also indicate that the inlet fluid temperature and initial temperature of CPM have important influence on the temperature difference between solid center and fluid.