Numerical Simulation Of Airflow And Temperature Distribution In Mine Gobs

Spontaneous combustion and thermal hazard in mining regions have long been risks for coal mine safety, although extensive investigations have been conducted by lots of researchers. Most spontaneous fires occur in gobs, which are in fact porous media composed by uncollected coal and collapsed rock fragments after underground excavation. Inevitable air leakage from working faces towards gob regions is a vital reason for coal oxidation and resulted spontaneous combustion. Therefore, investigation on airflow and temperature distribution amid underground mining, and corresponding ventilation strategies, can greatly aid preventing coal fires and promoting safe production.With new advances of thermo-fluid theories in porous media, this paper numerically investigates the airflow patterns and temperature distributions in both working-faces and gobs. Firstly, based on the volumetric average theory in porous media, this paper develops detailed numerical models for flow, heat transfer and oxygen concentration, by applying a vorticity-stream function approach. The Brinkman-Forchheimer-extended Darcy model is employed to describe the airflow patterns inside gobs, which are characterized by layer, transitional and turbulent flow patterns. A single-domain approach is employed to make properties continuously across the fluid/porous interface, thus working faces and gobs are incorporated into a set of unified models. The heat transfer model can take into account heat generation by equipments, heat exchange between rock stratum and airflow and heat generation by coal oxidation, etc. In addition, expressions for porosity and permeability as continuous and non-uniform distribution inside gobs are deducted according to the subsidence theory of overlying stratum. Infinite volume approach is applied for dispersion of governing equations, which are compiled and solved with FORTRAN language. Secondly, two-stage model validations with both experimental data from test rigs and in-situ measurement are accomplished. Satisfying results prove the accuracy of foregoing set of models. Finally, temperature distributions inside the gob are visualized and compared under different air flowrates. Different porosity distribution models are used as input to a simulator for thermo-fluid dynamics in a gob that models mine airflow, temperature and oxygen content distributions.Numerical predictions show an asymmetric airflow distribution in along with asymmetric temperature distribution in the gob. This results in larger airflow velocity in areas near the intake comparing with those near the return airway, but less obvious differences are observed deeper into the gob. In the gob, temperature distribution contour lines look like arcs centered by the intake airway. If the oxygen content in the air is not enough to sustain coal oxidation, temperature increase stops, therefore a region with constant temperature distribution is formed. Significant temperature changing magnitudes are revealed in the back part of gob along the airflow direction, which is prone to spontaneous combustion. Greater air flowrate brings much more intense oxidation inside gobs and higher temperature conditions. What's more, the highest temperature throughout the gob moves towards the return airway with increasing air flowrate. Results show that two dimensional variable porosity models possess obvious advantages comparing with field data.