CARS Temperature Measurements in Sooting, Laminar Diffusion Flames

Abstract

Abstract Temperature distributions have been measured in axisymmetric ethylene-air diffusion flames using high spatial resolution coherent anti-Stokes Raman spectroscopy. As ethylene flow increased and the flame approached a smoke-point condition, the temperatures attained in the upper part of the flame were reduced by about 300 K below maximum radial temperatures low in the flame. Addition of diluent Na to ethylene caused a reduction in temperature low in the flame but increased temperature higher in the flame. Maximum temperatures attained in all ethylene flames were between 0.84 and 0.89 of respective adiabatic flame temperature (AFT) The upper temperature of the near-smoke-point flame was only 0.76 of AFT. Results are compared with the generalized flame front model of Mitchell. Mie scattering measurements are also discussed. Brief studies with propane and a nonsooting CO flame are reported; maximum axial and radial temperatures were between 0.84 and 0.87 of AFT. Results indicate the importance of thermal loss from soot radiation, radial transport processes and fuel pyrolysis. Nonluminous radiation and finite reaction rates are other possible factors. Overall results highlight how the upper luminous part of the sooting ethylene flames is likely above the primary flame front and is a soot burnout zone. For such cases peak axial temperature tended to occur below the luminous tips, an indication also of the location of the true end of the flame. A broad axial and radial zone near peak axial temperature was established in the upper part of all flames