Wall-fire radiant emission. Part 1: Slot-burner flames, comparison with jet flames

Wide-angle radiometer measurements of total radiant emission Q ˙ r and scanning-slit-radiometer measurements of the vertical distribution of radiant emission, q ˙ r ′ ( z ) were performed on 0.38-m-wide slot-burner flames adjacent to a 2.2-m-high water-cooled wall. Comparison measurements were performed on free-burning slot-burner flames and on jet flames. Methane, ethane, ethylene and propylene were used as fuels, with total heat-release rates, Q ˙ t o t , ranging from about 10 to 60 kW. Except for methane at low heat-release rates, the radiant fractions x r = Q ˙ r / Q ˙ t o t were nearly equal for free-burning slot burner flames and for jet flames. However, χr was reduced by 18 to 36 percent when the slot burner was placed adjacent to the wall. Despite the reductions, trends of χr with sooting tendency were not altered by the presence of the wall. Placing the slot burner adjacent to the wall reduced the rate of rise of q ˙ r ′ ( z ) and the peak values to roughly 2/5th of the values for free-burning slot-burner flames, and caused an even larger reduction of the rate of decay, to roughly 1/6th of the free-burning case. The wall effect left the heights of peak emission unchanged, but nearly doubled the flame heights. Dimensionless correlations, q ˙ r ′ ( z ) L r / Q ˙ r vs z / L r with several alternative choices of the radiation flame length Lr were examined. Good correlations were obtained only by setting Lr proportional to the standard deviation σ of distributions q ˙ r ′ ( z ) , while other choices of Lr were unsatisfactory. Defined in this manner, Lr was found to vary with the 1/3 power of Q ˙ t o t for jet flames and with the 1/2 power for slot-burner flames. The observed exponents suggest that buoyant flame heights are controlled by a proposed radiant cooling mechanism rather than the conventionally assumed turbulent mixing mechanism. They are also consistent with a constant mean volumetric heat release rate. For the wall flames, the peak radiant fluxes scaled with about the 2/3 power of heat-release rate.