Effects of turbulence on flame radiation from diffusion flames

Abstract

Enhancement of heat transfer by flame radiation in furnaces can increase the efficiency and decrease the volume of a furnace. Flame radiation from turbulent diffusion flames, which prevail in furnaces, depends both on the chemistry of soot formation and the turbulent mixing processes of the flow. Previous work has shown that thermochemical effects (i.e. fuel or air preheating, oxygen enhancement, etc.) related to soot formation chemistry and flame radiation can be correlated in turbulent diffusion flames by using smoke-point properties of laminar diffusion flames. This work addresses the other part of the turbulent flame problem which involves the effects of the turbulent flow field on soot formation and radiation for given fuel and thermochemicla conditions. Radiation measurements in turbulent momentum, controleld jet flames were made by using five nozzles (1 mm, 1.5 mm, 2 mm, 3 mm, 4 mm) having a large contraction ratio and burning three fuels: acetylene (C 2 H 2 ), propylene (C 3 H 6 ) and methane (CH 4 ). By employing the relatively well understood flow of a turbulent jet flames, the following fundamental issue was addressed: it soot formation in turbulent diffusion flames determined by the overall residence (macroscale) time or by the straining rate of the small (Kolmogorov) scales wherein combustion occurs? Correlations of the present data obtained by using a simple soot formation and radiation model support the small scale (Kolmogorov) straining rate hypothesis.