Radiation Heat Flux from Pool Fires

The radiation from the hot gases and incandescent soot deep within the flame passes through the visible "surface" to external objects. The amount of heat emitted varies with the distance over which emission occurs and the concentration and type of emitting species within that path. As the thermal radiation passes through the atmosphere outside the flame it is attenuated by absorption of energy in the infrared wavelengths corresponding to the absorption bands of the atmospheric gases (principally carbon dioxide and water vapour). The attenuating effect on radiation is significant even over path lengths of a few tens of metres. Water sprays, mists and smoke can also strongly attenuate radiation.

The radiative heat emission process is modelled by assuming that the radiation comes from the visible surface. The surface emissive power, SEP, of a flame is the heat radiated outwards per unit surface area of the flame. Thus the use of SEPs is a two-dimensional simplification of a very complex three-dimensional heat radiation problem. These models can be used for reliable prediction of external radiative heat fluxes to within about a flame length of the fire. They cannot be used for near impingement conditions however.

The model average SEP depends on the fuel type and on the pool diameter. A uniform SEP is used over the whole of the sides of the model flame shape. This results in underprediction of the radiative heat flux near the base of large pool fires because the model fails to take account of the small bright, highly emissive region at the base of the flame. The following formulae are used in Shell FRED. For land-based LNG pool fires (Shell FRED 2004):

The physical effects modelled by this equation are the increase in SEP with increasing emitting path length, given by the first exponential term:

And the appearance and increase in the amount of dark obscuring smoke on the outside of the flame, given by the second exponential term:

For LPG and propane the SEP is given by:

The second term is only included if the pool diameter exceeds 18m. For butane, gasoline and kerosene, the SEP is given by:

Above figure shows the variation of SEP with pool diameter. There is a peak SEP for all three classes of fuels, representing a fire when the emission from bright areas of flame is at a maximum, but before significant amounts of smoke have appeared outside the flame. The peak occurs earlier for heavier hydrocarbons because the flames contain higher soot concentrations.

Above figure shows variation of model average SEP with pool diameter for different fuels!