Effects of low air pressure on radiation-controlled rectangular ethanol and n-heptane pool fires

Abstract

Experiments on rectangular ethanol and n-heptane pool fires were conducted at different altitudes in Hefei (99.8kPa) and Lhasa (66.5kPa). The burners tested had the same fuel area of 900cm2, but with aspect ratio of long side to short side (n=l/w) varied from 1 and 8. The individual and combined influences of low pressure and aspect ratio on burning rate, temperature, puffing frequency, flame height and radiation for the two fuels were interpreted and formulated. First, burning rate was found to be proportional to ambient air pressure under radiation control, the main reason is that radiative heat flux decreased with pressure due to the pressure affecting the soot absorption coefficient. Flame temperature slightly increased, leading to higher flame puffing frequency at low pressure. Flame height was almost insensitive to pressure as H∝p0. Second, for aspect ratio n, flame temperature was constant and independent of fuel type and burner shape. With increasing n, burner wall temperature increased at the long side, and decreased drastically at the short side, especially n=8. This was attributed to the change of flame tilt and heating of the burner side, caused by variation of entrainment motion. Flame puffing frequency was found to increase with n as a function of f∼(ΔT/T∞)(n+1)/2n. The flame was observed to split into small clusters by enhanced asymmetric entrainment, and H decreased with increasing n as H∼(1/n)2/5Q˙4/15. Considering fuel differences, with increasing n, the burning rate of the ethanol pool fire decreased, and n-heptane showed the opposite trend.