OFFICIAL Air filled hole: Non-isothermal Flow and the Boussinesq Approximation With the absence of forced air flow in mining blastholes, the assumption was made that fluid flow and heat transfer was governed by natural, rather than forced, convection – arising due to temperature differences in the heated coal seam, ground and air. In COMSOL, this coupling was managed by the ‘Non-isothermal Flow’ module, as temperature was constantly changing in the transient system. This fluid behaviour is governed by a simplified form of the Navier-Stokes relation. As the air was near ambient temperature, there was a moderate temperature difference at the seam (70oC – 20oC), the hole was not enclosed, and to keep a feasible simulation time, the Boussinesq approximation was applied. This simplified the Navier-Stokes equation for incompressible flow, making the driving force buoyancy induced, arising from slight density differences (Carasi, 2016). Turbulent flow using a k-ε model was chosen as it provided good convergence at low velocities (COMSOL, 2018). Loaded Hole The interaction of AN-based bulk explosives with hot ground were modelled, in this case the self-heating coal seam, through conductive heat transfer. It involved testing two distinct products, being a traditional pumped emulsion blend behaving as a solid, and ANFO, defined as porous media. The unstemmed section just above the bulk was air, open to the atmosphere, defined by the same physics as in the open blasthole scenario in the previous section. Mesh The mesh refers to the size and refinement of 3D elements which create nodes throughout the model, it is at each node where calculations occur based on the conditions at neighbouring nodes. It is thought that a finer mesh produces more physically real results. However, too fine of a mesh can induce convergence issues and significantly increase computation time, so a balance must be struck between result validity and runtime feasibility (Cadiou S, 2020). For the blasthole model, a ‘finer’ tetrahedral mesh was employed at the wall boundary between the ground, coal seam and air, as well as inside the blasthole itself, where heat transfer and fluid flow interactions are most significant. The mesh grows outward to a normal size by the edge of the ground boundary, as heat transfer interactions become insignificant, except for the top of the blasthole at the open boundary. A vertical crosssection of the mesh used across all models is shown in Figure 9 (a) and (b), a wireframe shows the mesh at boundaries rather than throughout the 3D model. (a)
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