Measurements and modeling of non-premixed tubular flames: structure, extinction and instability

Abstract

Flame curvature is important in turbulent combustion simulation where curvature induced variations of the scalar dissipation, local flame extinction and flame instabilities may dominate the overall flame behavior. On the other hand, constant efforts are being made to improve the accuracy of detailed chemical mechanisms for better simulation of flames. In this work, opposed tubular flames are established for the first time where the uniformly-curved flame surface allows detailed structural study to investigate the interac-tions of curved and stretched flow field with chemical reactions. The laser-induced spontaneous Raman scattering technique has been proven invalu-able in flame diagnostics. In this work, a Raman diagnostic system is applied to non-premixed tubular flames produced by N2-diluted H2, CH4 or C3H8 burning against air at various stretch rates. The temperature and major species concentrations in these flames are measured. Numerical simulations of the corresponding tubular flame are also carried out using models that include detailed chemistry and transport formulation. The experi-mental data are compared with the simulation results, which show good agreement be-tween them. Cross-comparison between the simulated tubular flame data with those of the opposed-jet flat flame help to reveal the effects of curvature on flame temperature and extinction. Near-extinction, non-premixed, tubular flames using different types of fuel-diluents combinations are also studied. Attention is directed towards the role of curvature on flame extinction and cellular instability. Extinction stretch rates of various types of fuel-diluents mixture burning against air are measured. Both concave and convex curved flames are investigated whenever the experimental condition permits. The onset conditions of cellular instability of flames produced by certain fuel-diluent mixture are recorded. The role of curvature on the formation of cellular flames is discussed by comparing extinction and cellular instability data of the non-premixed tubular flames subject to different degrees of curvature.