The initial convective flow in fire

Abstract

This study concerns physical modeling of the initial fire environment generated by fire in an enclosure, which persists up to that time in a fire when recirculation of combustion products begins to influence the further yield of products. It was the primary purpose to investigate experimentally the validity of modeling relations, proposed previously, for the convective flow generated by “power-law” fires, i.e., fires growing in heat-release rate with a specific power of time from ignition. Three wood-crib fires of different fire-growth rates were combined with three different ceiling heights under large flat ceilings for a total of nine experimental configurations. The experimental fires were power-law fires growing with the second power of time. Temperatures and velocities were measured in the hottest gas layer under the ceiling. The data were shown to be well correlated in nondimensional variables of the modeling theory. It was possible to establish analytical expressions for the nondimensional temperature and velocity fields. A useful finding, which appears to be valid also for other kinds of fire growth than that investigated (including steady fires), is that the local gas velocity in the hottest layer can be related directly to the local temperature rise and ceiling clearance, regardless of fire-growth rate and time from ignition.