Experimental and numerical investigation of premixed tubular flames

Fundamental flame response in a stretched and curved flow field is investigated in a unique optically accessible tubular burner. Time-averaged, one-dimensional spatially resolved temperature and major species measurements are obtained in a set of stretched, phi = 0.175 premixed H-2/air tubular flames using visible laser-induced Raman spectroscopy. The very lean H-2/air flames are formed under relatively high stretch rates, 90 less than or equal to kappa less than or equal to 215 s(-1), with 227 s(-1) being the extinction condition. In tubular flames, both stretch and thermal-diffusive effects are dependent on both aerodynamic straining and flame curvature. Thermal-diffusive effects in highly curved (similar to2 mm flame radius) tubular flames significantly influence the flame structure, leading to flame temperature increases of similar to120 K over the planar unstretched flame temperature (similar to1180 K). The standard program for modeling stretched planar flames (Oppdif) is modified for the cylindrical geometry of the tubular flame. Comparisons of the Raman measurements with numerical simulations for tubular premixed flames, using complex chemistry and detailed transport properties, show excellent agreement at low rates of stretch (i.e., kappa less than or equal to 127 s(-1)). At higher flame stretch, hence increased curvature, numerical simulations using the currently available transport data and chemical kinetic mechanisms incorrectly predict the flame structure. The experimental observations show extinction occurring (similar to227 s(-1)), while numerical simulations overpredict the extinction limit (similar to750 s(-1)). Simulations using four different H-2/air chemical kinetic mechanisms show that the flame structure is very sensitive to the particular mechanism and the molecular diffusion coefficients. Evaluation of molecular diffusion coefficients indicates that the thermodiffusive properties of the deficient reactant species, H-2, strongly affect the tubular flame structure. Thus, the products in the flame zone (phi = 0.34) are enriched from the initial reactant mixture (phi = 0.175) by flame curvature and the rapid diffusion of H-2.