A physical scale model of flows in the waste of a retreat longwall coalface.

A physical model has been developed to simulate the flows and mixing of gases in the waste of a longwall retreat coalface. The model is used in conjunction with computational modelling, and the distributions of permeability and gas emission in the model waste follow those in Computer simulations (Pokryszka et al 1996). The physical model represents at 1/70th scale a 200 m long, 3 m high coalface with a waste extending up to 280 m to the face start line. The model is tilted to the same seam slopes as the real face, e.g. 22° ascensional Ventilation along the face and 7° slope to the dip. The purpose is to investigate the flow of three components: Ventilation seepage flow through the waste; methane; and nitrogen injected to quell spontaneous combustion. A heavier-than-airSurrogate gas (SFg) is used in place of methane, and therefore the model is inverted so that gas layers form along the “roof.” The main forces driving the flows in the waste (pressure gradients due to Ventilation and the density difference of gas layers) are kept in proportion by appropriate scaling of the permeability. Calibration and validation used data from a coalface at a mine in the Lorraine coalfield. The model is now in operational use.