Experimental investigation and analysis of flame height transition and air entrainment of near-wall rectangular-source fires at various distances

Abstract

This work provides a comprehensive experimental investigation of the flame transition behavior of near-wall fires at various distances, employing five rectangular-sources (aspect ratios (L/W) varied from 1:1 to 71:1) with the longer side of the source facing the wall. The aspect ratio range covered the axi-symmetrical, rectangular- and linear sources. The flame transition behavior from completely-attaching wall (wall fire), to partly-touching wall and finally non-touching wall (free fire) with increasing wall-fire distance (D) was investigated for various heat release rates (HRR) (Q˙) with total 262 test conditions. Firstly, for completely-attaching wall fire, the flame height (ℓf, w) correlated well with a defined non-dimensional HRR (Q˙2*=Q˙/ρ0CpT0gL5/2) into two regimes, using the longer side (L) attaching to the wall as the characteristic length scale. For Q˙2*≤1.08, it followed a 2/3 power dependence; meanwhile for Q˙2*>1.08, it followed a 2/5 power dependence. The flame heights in the two regimes can be well described globally by extending Quintiere and Grove's model for free rectangular-source fires with “mirror” approach. Then, with increasing of wall-fire distance (D), the flame lowest touching-wall point height (Zf, t) increased, meanwhile the flame height (ℓf) decreased. The critical wall-fire distance (Dc) for flame turning from partly-touching wall to non-touching wall were further quantified. This flame transition behavior was found to be represented by two derived non-dimensional quantities of L/ℓf, 0 and D/ℓf, 0, based on analysis of balance between buoyancy force of the flame (upward) and inertial force of side-entrained air flow (horizontal) as well as air entrainment change at wall side. The experimental results, revealed transition and proposed formulas provide a basis to quantify the flame behavior of near-wall fires.