Evolution of heat feedback in medium pool fires with cross air flow and scaling of mass burning flux by a stagnant layer theory solution

This paper quantifies experimentally the evolution of heat feedbacks through conduction, convection and radiation in medium square pool fires (10-25 cm) with horizontal cross air flows ranged in 0-3.0 m/s. Ethanol and heptane are used as representative fuels producing typically less-sooty and sooty flames. Results show that the overall conduction heat feedback flux through the four sides increases with cross air flow speed, being more prominent for smaller pools than for larger ones. And the rate of overall conduction heat feedback increment is more prominent for heptane, of which the rate of increase with flow speed nearly 2 times that of ethanol. Meanwhile, the radiation heat feedback declines with cross air flow speed, with its contribution fraction nearly negligible at larger flow speeds for both fuels. The convection feedback fraction increases with cross air flow speed. Its increment is more prominent (a) for heptane (increasing fast with flow speed to be even the dominate one among the three heat feedbacks) than ethanol (increasing slowly with flow speed; being nearly constant and maintaining to be the dominant one); and (b) for larger pools than for smaller ones. The changing of dominant heat feedback mechanism with cross air flows, results in the change of the scaling behavior of the mass burning flux with pool size. A stagnant layer solution theory is then proposed, by including fuel mass transfer Spalding number B, to describe the change of mass burning flux of different size pool fires with cross air flow speed for different fuels in relative strong cross air flows [as indicated by Froude number (Fr = u/[gd]1/2) larger than about unity]. Experimental data are shown to be well correlated by the proposed theory.