Radiative energy transfer from turbulent diffusion flames

Abstract

Transmittance and radiance measurements have been performed on turbulent ethane and propane flames by a flame array method that had been applied previously to arrays of laminar flames. The results showed that turbulent flames cannot be regarded simply as laminar flames of increased optical depth. Graphs of absorptance vs. radiance for the turbulent flames were shifted towards increased absorptance in comparison with the corresponding laminar-flame data. As one consequence of this shift, radiance values extrapolated to infinite number of flames, N x, were reduced by a factor of 0.71 for ethane and 0.79 for propane with respect to those for laminar flames. Moreover, at fuel flow rates of 120 cm 3/s for each turbulent flame and of 4.5 cm 3/s for each laminar flames, the transmittance of an array of n turbulent ethane flames equaled that of about 2.6 n laminar ethane flames, while their radiance equaled that of only about 1.4 n laminar flames. The corresponding ratios were 2.1 and 1.2, respectively, with propane flames. Measurements of total radiative power of single turbulent propane flames showed that within the fuel flow range of 70 to 316 cm 3/s a constant fraction χ of the total rate of heat release is emitted as radiation. The value of χ was about .21 for several burner nozzle geometries and about .25 for a 20 mm i.d. straight-tube burner. Effects of fuel flow rate and of flame spacing on radiative properties have been explored, and power-law relationships linking fuel flow rate, flame dimensions and radiative properties are proposed.