Investigations of the feasibility of using aerodynamic models for studying the local build – up of methane gas and risk of frictional ignition in mines. Final report on CEC Contract 7258 – 03/107/08

Studies have been carried out to investigate airflows in mine models, especially with regard to the transport of airborne scalar material (notably methane gas, but also dust produced during mining operations), and to examine how they relate to what happens at full-scale in an actual underground mine. The rationale of the work is that, if models can be shown to provide data representative of actual mine ventilation engineering, then they can provide cost-effective alternatives to full-scale investigations.The work set out in the first instance to identify the properties of the mine airflow most relevant to the problem in question. These were deemed to be: a) the bulk airflow, and associated transport of airborne scalar material (i.e., methane gas), along the longwall face itself; and b) the transport of material out of regions of partially-enclosed, or poorly-ventilated, flow (e.g., in the cutting zone, in headings). For the former, the quantity most appropriate to the general transport of gas and to the dispersion of ‘bursts’ released onto the face is the bulk diffusivity of the coalface airflow. Here the greater the diffusivity, the more rapidly such bursts are smoothed out and so the more rapidly instantaneous gas concentrations decay to acceptable levels. For the latter, the quantity most appropriate to the ventilation of partially-enclosed regions like those described is the characteristic retention time. Here, the shorter the retention time, the more rapidly the material is removed and so – in the steady state – the lower the concentration of accumulated gas. For the purposes of scaling, both quantities need to be non-dimensionalised with respect to the system characteristic dimension and air velocity. Thus we arrive at the important parameters to be scaled, K* and H respectively. A third dimensionless quantity is also important; namely, the Reynolds’ number (Re) which characterises the nature of the various fluid mechanical forces acting and so is a governing physical parameter in all considerations of aerodynamic transport.Experiments were carried out to investigate these properties and to examine how they scale between small-scale and full-scale systems. The gas was simulated using smoke or dust tracers (neglecting for the purpose of this feasibility exercise the question of gas buoyancy). Such tracers were monitored using detectors operating on the principles of optical scattering. The basis of most of the experiments was the ‘tracer decay’ method, in which the transport properties of the aerodynamic system under investigation were determined from observations of the changes in tracer concentration with time immediately following the removal of the tracer source.For the bulk flow along the face itself, the experiments were conducted in a 1/10-scaIe laboratory model, a full-scale surface model, and an actual underground mine respectively. These experiments showed that dispersion is controlled not only by turbulent diffusion but also by the entrapment of material into coherent vortex structures in the wakes of roof supports and other blockage elements. This means that the flow cannot be regarded as equivalent to that through a ‘rough pipe’ (as earlier workers had suggested for flow in mine roadways). Thus scaling between large and small systems depends both on the characteristic Re for the coalface as a duct and on the corresponding Re for flow about the roof supports, etc..For the ventilation of the cut and of headings, experiments were conducted in the 1/10-scale model and in the full-scale surface system. For the flow cavity (or eddy) in the lee of the cut, ventilation was found to be influenced by ‘boundary-layer’ effects introduced by the presence of the shearer itself. For the headings, the removal of entrapped material was found to be faster at the return end, due to the increased mixing associated with the “”jet-effect”” of the air exiting from the coalface. In addition, removal was faster for empty headings than for ones containing equipment, etc..During these experiments, the complex nature of the coalmine airflow became apparent. Nevertheless, despite the limited scope of the study, the feasibility of using small-scale models to investigate ventilation problems was clearly indicated and a set of scaling relationships relevant to the transport of methane (and other airborne materials) was drawn up. These may be summarised as follows:-* For the bulk airflow along the coalface, then scaling can be achieved for systems from 1/10-scale up to full-scale provided that the mean air velocityexceeds 2 m/s.* For the flow in the lee of the cut, direct scaling applies if the value of Re (for the cut) exceeds 50,000. Otherwise a correction factor must be applied.* For non-ventilated headings, direct scaling applies for all practical values of Re (for the heading) for systems from 1/10-scale up to full scale.At this stage, however, these should be regarded as preliminary working guidelines. Further work is required to establish firm criteria for more complex, more realistic situations (including the presence of moving machinery, falling coal and rock, water sprays, auxiliary air moving apparatus, etc.). “”