The growth, structure and detectability of fires in mines and tunnels

Abstract

The growth, structure, and detectability of two fires in a long, ventilated passageway (2 meters high ×5-1/2 meters wide) were studied in full-scale tests within the Bureau's Experimental Mine at Bruceton, Pennsylvania. Temperatures, carbon monoxide concentrations, and submicrometer particulate densities were measured with vertical arrays of sensors at various distances downstream of the fire zone. Test I involved the slow heating of a 10,000 kg pile of coal which smoldered for a day before the onset of flaming combustion. In test II, a 3,600 kg pile of oil shale rubble was ignited rapidly with burning diesel fuel. Marked stratification characterizes the steady-state CO concentrations for both smoldering and flaming even to distances beyond 30 meters. However, the transition to flamming strongly perturbs the steeply stratified profiles for steady-state smoldering. The thermal profiles dissipated much more rapidly than CO profiles in both the near and the far fields. With current state-of-the-art, the thermal sensor spacing requirements are very severe. Only continuous thermal sensors in the near field immediately above the fire zone are of any practical value. The spacing requirements for CO sensors are in the more realistic range of hundreds of meters. A model is presented that quantifies the detection problem in terms of a characteristic growth time to reach a critical fire size. Detector spacing requirements are then specified in terms of the ventilation velocity, growth rate, and alarm threshold. The model is applied to a realistic scenario for fire development in a conveyor-belt haulageway in an underground coal mine. Product-of-combustion sensing is shown to be more realistic than thermal sensing, with spacing requirements that are sensitive to the ventilation velocity and the fire growth rate.