The transient development of a methane-air diffusion flame is studied numerically in the present work in a confined geometry. An explicit finite difference based numerical model is developed for the solution of transient reacting flow with varying thermodynamic and transport properties. The effect of air preheat on the flow and temperature fields at different time planes are studied by comparing two cases with air temperatures of 300 and 400 K, respectively. The preheating of air is found to complicate the flow situation with the formation of additional vortices. Thermal stratification in the flow causes a radial stretching of the flame, which is more evident with higher preheat temperature. In the absence of any preheating a stable recirculation of ambient air is observed to extend from the exit plane into the domain adjacent to the wall. The mixing of ambient air results in gradual drooping in the bulk mean temperature of the flow. Such drooping nature in the bulk temperature distribution is not found with air preheat due to the absence of any stable recirculation. The complication in the initial flow transients delays the attainment of steady situation for the flame with preheat. Copyright (C) 2005 John Wiley Sons, Ltd.