Thermal radiation hazards from LPG-fireballs

Turbulently burning fuel-rich fireballs cool both by thermal radiation and by mixing with ambient cold air. Both mechanisms have been considered important. In this work we first study their relative effectiveness By re-analyzing published, small scale experimental results and by two simple analytical models, which take into account only one of the cooling mechanisms at a time. The models indicate that mixing dominates over radiation. Detailed analytical calculations of thermal radiation are prohibitively complicated due to the fact that the temperature of the fireball, including its effective surface temperature, can not be determined by averaging in spite of the strong turbulence present. However, using some plausible assumptions we are able to extrapolate the empirical, thermally radiating fraction of the energy from experimentally available firehall sizes to such larger masses that are relevant in industrial accidents. This is done by scaling the relevant parameters (fireball radius, its burning and cooling time) with the released fuel mass Mf. The radiating fraction turns out to be a weakly decreasing function of the mass (~Mf-1/6). In the end we estimate the health hazards from thermal radiation to people at different distances as a function of Mf.