Investigation of stretch and curvature effects on flames

Abstract

Flame response on curvature is very important for understanding and predicting of both laminar and turbulent combustion. In this work, curvature effects on flames are studied analytically, numerically, and experimentally by comparing the opposed jet flame and tubular flame. The cold flow field of the tubular burner and the opposed tubular burner is solved analytically and the appropriate choice of stretch rates is given for comparison of different flames. The analytical solutions are compared to numerical solutions to verify the results. A physical analysis using basic conservation equations to study stretch and curvature effects on premixed flames is carried out. It shows that the curvature effects are coupled with the stretch effects for the first time. The positive curvature strengthens the preferential diffusion and the negative curvature weakens the preferential diffusion; the strengthening or weakening effect is proportional to the ratio of flame thickness to flame radius. Based on the analysis, correlations on flame speed and flame temperature for the stretched and curved flames are given. With these correlations, the flame speed and flame temperature for the curved stretched flames can be predicted from the information of stretched planar flames. The asymptotic analysis for the flame speed and flame temperature of three specific stretched and curved premixed flames: the opposed jet flame, the tubular flame, and the spherical flame is resolved. The comparison between the asymptotic solutions and the numerical solutions is satisfied. With the correlations derived from the physical analysis, the asymptotic solutions can be extended to flame with any values of stretch rate, curvature and Lewis number. The opposed jet and opposed tubular diffusion flames are compared numerically. The curvature influence on diffusion flame temperature and extinction is revealed precisely for the first time. The result shows that the curvature theory on premixed flames can be extended to diffusion flames with excellent results. The curvature affects the diffusion flames by the same mechanism found in premixed flames (i.e. by strengthening or weakening the preferential diffusion). Resulting from the preferential diffusion, curvature has important influence on flame temperature and extinction stretch rate if the Lewis numbers are far away from unity and the ratio of flame thickness to flame radius is on the order of unity. The measured flame properties and flame structure with laser-induced Raman scattering are compared between the opposed jet premixed flame and the tubular premixed flame (Lean H2, CH4 and C3H8 premixed flames). The multicomponent transport model is necessary for good prediction of the lean H2 flames. The experimental results have good agreement with numerical simulation and are consistent with the theoretical analysis.