Pool fire flame base drag behavior with cross flow in a sub-atmospheric pressure

Abstract

This paper investigates the effect of cross flow of air on flame base drag in pool fires in sub-atmospheric pressure, where no data is available, by combining the effects of these two factors: sub-atmospheric pressure and cross flow. Experiments were carried out using heptane and acetone as fuels for 10–25cm square pool fires where the cross flow was provided by a wind tunnel built in Lhasa city-Tibet (64kPa) as well as correspondingly by a wind tunnel in Hefei city (100kPa). Additional larger scale experiments were also conducted with 45cm, 60cm and 70cm square pool fires in Hefei city. The evolution of flame base drag length as a function of cross flow air speed was quantified. The flame base drag length was found to be larger in the sub-atmospheric pressure than that in normal pressure, indicating higher risk of causing fire spread to nearby fuel at the ground level. Previous correlations based on a wind Froude number are not in agreement with each other. In addition, they do not include all the physics and they do not account for the effect of the pressure change on flame base drag length. A new theoretical approach is proposed accounting for the relation of wind force with the buoyancy of the flame and the interaction of wind with the total heat release rate from the fire. It follows that three independent dimensionless quantities are introduced: the (inverse) fire source Froude number owing to flame buoyancy, the total buoyancy–wind interaction number and the ratio of fuel vapor density near the fuel surface to the air density. This theory allows us to develop and validate a correlation for flame base drag length to account for pressure effect and for generalizing the existing relations for flame base drag.