CONVECTIVE HEAT-TRANSFER FROM FIRE GASES

Measurements of heat transfer and gas flow have been made for fires in model compartments. The results imply that, for most of the ceiling above a large scale fire, the heat transfer coefficient from the hot ceiling flow is equal to that expected in free convection, i.e. independent of the flow velocity. Close to the point of plume impingement heat transfer coefficients may exceed those in natural convection; in which case heat transfer is strain limited and Nu is-proportional-to Re1/2. If flaming extends to a significant fraction of the ceiling height the kinetic energy of the mean plume flow is greatly reduced on impingement. This leads to a reduction in strain rate and heat transfer, an increase in the initial depth of the ceiling flow and a decrease in its velocity. These findings are important in assessing the size of fire that will trigger an automatic sprinkler system.