Numerical analysis of flame heating on arbitrarily oriented condensed fuel surfaces

Heat transfer from the flame to a surface located downstream of the fuel, in the flame spread direction, is likely to influence the spread rate and flame extents, as the coupled thermal and flow fields get altered. Additionally, if the fuel surface is inclined to the normal gravity direction, it can induce complex natural convection induced flow-fields that can further affect the flame characteristics. The primary focus of the present paper is on investigating the combined effects of downstream surface (over fire region) temperature conditions and angular orientation of condensed fuel surface, on the flame characteristics. Inclination angles in the range of - 60°<θ<60°with respect to the vertical axis, under normal gravity conditions are considered. A validated numerical model, which solves transient, two-dimensional, gas-phase governing conservation equations with appropriate interfacial coupling conditions, is employed. Methanol is employed as the condensed fuel. Global single-step reaction chemistry for methanol-air oxidation is used to model the finite rate chemical kinetics. An optically thin approximation based formulation for radiation heat transfer is used to account for losses by absorbing species in a non-luminous flame produced by methanol. The model is first validated against the experimental result for flame on a vertically oriented surface, in terms of a factor called local heat flux parameter (HFP). Heat transfer characteristics of the flame to the condensed fuel surface and to the isothermal wall is then presented and compared in terms of local HFP for all the orientation cases. The differences in the structure of the laminar diffusion flames are discussed in detail. Further, a comparison of the flame structure obtained in the present case against a case having an adiabatic surface in the over fire region is also presented. © 2011 Elsevier Ltd. All rights reserved.