Experimental and numerical investigation of non-premixed tubular flames

Abstract

Non-premixed tubular flames are established using the opposed tubular burner for the first time, which enables the quantitative study of curvature effects on flame behavior. A detailed structural investigation using the spontaneous Raman scattering technique is conducted on flames with constant curvature using 15% H 2 diluted by N 2 against air at various stretch rates. The measured temperature and major species concentrations agree well with the numerical prediction, and this validates the numerical model. Comparing with the numerical results of the opposed-jet flat flame, the tubular flame data show that the curvature weakens the effects of preferential diffusion for the diluted hydrogen flames where the curvature is concave towards the fuel stream. To further prove this discovery, near-extinction non-premixed tubular flames using different types of fuels are also studied. Hydrogen flames ( Le < 1) automatically change their orientation of curvature at high stretch rates and cellular structures are observed. Methane flames ( Le ≈ 1) do not show any preference on the direction of curvature before extinction is reached and no cellular structures are observed. Propane flames ( Le > 1) prefer concave curvature, which is opposite to the hydrogen flames, and cellular structures appear throughout the burner’s operational range. These phenomena confirm that for flames with Le < 1, convex curvature promotes combustion, enhances the flame temperature and therefore retards extinction; vise versa for flames with Le > 1. Curvature has minimal effects on tubular non-premixed flames with Lewis number close to unity. The comparison between experimental and calculated extinction conditions also supports the above conclusions.