An experimental study on burning rate and flame tilt of optical-thin heptane pool fires in cross flows

Abstract

This paper reports a comprehensive experimental study on evolutions of burning rate and flame tilting of medium-to-large size pool fires with cross flows. Square pool fires with length of 25, 35, 45, 60 and 70cm were employed with heptane as fuel, whose burning behaviors are in radiation-controlled optical-thin regime in still air. The evolutions of their burning rates and flame tilt angles were measured with cross flow air speed from 0 to 4.5m/s, in which range the data is still lacking in the literature which mostly concerned relative smaller pools (up to 25–30cm) in laboratory or relative larger pools (> 1.0m) by outdoor burning where the wind is hard to maintain. It was found that there were several transitions for the burning rate with increase in cross flow air speed in this range for different size pool fires. For relative smaller pools (25and 35cm), the burning rate increased and then decreased; meanwhile for the medium pool (45cm), the burning rate first increased then decreased, and further increased then finally decreased. However, for the relative larger pools (60and 70cm), the burning rate first increased then decreased and finally increased with flow speed up to 4.5m/s. These transitions were discussed based on physically the change of the controlling mechanisms due to heat feedback or the aerodynamic extinction at the flame leading edge (Damköhler number). The corresponding cross flow air speeds for these transitions correlated well with buoyancy induced velocity scaled by [(gD)0.5] based on Froude number. The flame tilt angles measured were compared with previous correlations to validate their applicability in this range. This work provided a comprehensive data base revealing the transitions of burning behaviors of medium-to-large pool fires with increase in cross flows air speed.