Calibration and validation of a thirtieth scale model for a study of the circulation of gas in the waste of French longwall retreat coalfaces

This report describes part of a programme of work to develop physical, aerodynamically scaled models for the investigation of the flow and dispersion of gas in French longwall coalmines. The main aim of this part of the work was to adapt an existing l/30th scale model of the coalface and waste to represent conditions in French mines, and then to calibrate and validate the model. The model was originally constructed as part of a project jointly funded by British Coal, the CEC, and the French Mining Industry. Its original details were, however, designed to represent British mining conditions. Hence the changes in some of the detailed structure.The aerodynamic scaling of the model has been derived previously. This report summarises the main requirements for achieving aerodynamically scaled flow in the l/30th scale model. These include: a minimum value of the Reynolds number for the flow on the face to ensure fully turbulent flow; and for the dispersion of gas layers, a constant Richardson number (the ratio of energy required to disperse the layer to the energy available from the turbulent flow); pressure gradients due to ventilation, resistance of the face, resistance of the waste, and density differences of gas layers, all to be in proportion; proportionate seepage flow through a model waste; and dispersion in the model waste. These requirements lead to the use of a surrogate gas which maximises the density difference between air and the gas layer in order to enable the greatest viable reduction in scale. This led to the selection of a heavier-than-air gas, sulphur hexafluoride SF6, as a surrogate for methane. With the heavier-than-air surrogate gas, the model is used in an inverted position. The model is 5 m long (representing ISO m of face length) and 3 m wide (representing up to 90 m of waste) and can be tilted to the required seam slopes by a system of hydraulic rams.The modifications to the model for it to be operated as a representation of a part of a French coal face include a new gas injection system and a system for injecting flows around the periphery of the model, so that the model can be used to represent a part of longer faces. The gas injection system is designed to provide a distribution of gas emission similar to that in computer simulations but with flexibility to change by altering connections exterior to the model. Sampling devices were attached in an array across the model to measure gas concentrations in the waste.The data available from the underground face to be simulated (DORA 1 SUD at the mine Reumaux) are described, showing how the values are scaled to specify the values for the model. Where data are not available from DORA 1 SUD for pressure gradients, extrapolations are made from data from another similar face.The physical modelling is complementary to computational modelling (by INERIS), and the computer simulation is used to define the detailed specification for the model. The computer simulations gave estimates for: (i) the target specifications for the flows to be injected around the periphery of the model (representing flows coming from the further parts of the waste outside the rectangular zone in the model); (ii) the distribution of gas emission within the modelled zone; and (iii) the distribution of permeability within the modelled zone. These target specifications are described, together with the approximations made in reaching the practical values suitable for the model.After two initial simulations, improvements to the realism of the model were introduced, these included: installation of an armoured face conveyor; alteration of the face width to be closer to that of the real mine; and alteration of the ventilation by adding a supplementary fan to balance pressures between face and waste at the equivalent of the centre of the face.