Flame size and heat flux correlations are obtained by experiments for circular flames beneath an unconfined inert ceiling, and are compared against those for one-dimensional flames in a downward channel. The both correlations have resulted in the proportionality of the area covered by a ceiling flame to heat release rate and the sole dependence of flame heat transfer on the distance from the source normalized by the flame length. Heat release rate per unit flame area is found to be significantly smaller with circular flames than with one-dimensional flames. It suggests a weaker entrainment of ambient air to circular flames than to one-dimensional flames. The weak heat flux from a ceiling flame suggests the importance of the preheating of combustible ceiling by hot gas layer for the fast fire spread generally observed in real and experimental room fires. Dimensional analysis offers a partial explanation for the weaker entrainment in circular ceiling flams than in one-dimensional ones. It also suggests the proportionality of horizontal velocity to the distance from the upstream end of the burning surface, and faster velocity with one-dimensional flames than with circular ones.
