Flame attachment and downstream heating effect of inclined line fires

Abstract

Experiments were conducted to quantify downstream heating from inclined fires generated by a 25 cm wide, 5 cm deep gaseous line burner. A variety of orientation angles (θ) and fire heat-release rates (HRR) were employed simulating, at reduced scale, the dynamics of two-dimensional (2-D) inclined flame spread as found in wildland or building fires. The total heat flux q˙f″(x) to a nearly-adiabatic surface ahead of the burner was measured by a water-cooled total heat flux gage at different downstream locations (x). It was found that, with an increase in fire HRR or θ, q˙f″(x) increases. The same is true for both the flame projection length on the inclined surface (Lf,p) and the flame attachment length (La). The location of flame attachment divides a flame into an attachment region (0≤x≤La) and a liftoff region (La<x≤Lf,p). A scaling analysis was performed to correlate the modified non-dimensional heat flux q˙f*(x) and the normalized downstream distance x/La in these two regions as a piecewise power-law function. Lf,p is scaled by a non-dimensional HRR, Q˙θ*=Q˙/(ρ∞cpT∞(gcosθ)1/2D3/2L), together with the angle between a flame and an inclined surface (α). La is well represented by two derived non-dimensional quantities, L/Lf,0 and Lf,p/Lf,0 (Lf,0 is free flame height), based on the analysis of the balance between the buoyancy force of a flame (upward) and the inertial force due to air entrainment (horizontal). The quantitative comparison with previous flame spread tests over inclined PMMA samples demonstrates the effectiveness of the proposed correlations in predicting downstream heating of 2-D flame spread over inclined terrains.