Air Staging and Swirl-Assisted Mixing and Combustion of Methane and Air

Abstract Two-stage combustors offer an effective method for the suppression of combustion-generated pollutants. The imposition of swirl on the fuel and/or oxidizer flows causes recirculation which enhances mixing that in turn reduces NOx (i.e., total nitrogen oxide) formation. We investigate the characteristics of a burner that allows for both good mixing through swirl, and staging. Temperature measurements establish the presence of separate rich and lean zones during staging. Relative uniformity of the temperature profiles, and small magnitudes of temperature fluctuations are characteristic of the burner exit. In the burner product can be either dispersed (with no radial swirl-type structure) or concentrated into separate lean and rich regions. High speed video images confirm the swirling of the inner rich flame. The swirl allows interaction of the primary region with the secondary zone, and shears the secondary fuel jets causing further turbulence and vorticity that enhances mixing and prevents hot spots. Under conditions favoring unmixedness, product distribution occurs on relatively thin flamelet sheets that curve and wrinkle due to vorticity. In general, lower fractions of secondary air are beneficial from the perspective of mixing and homogeneity. The emission index of NOx, i.e., EINOx values correlate with the relative fractions of primary air. Fluid dynamical effects supersede those related to stoichiometry in the burner. The maximum EINOx value is lower than typical EINOx values for jet-type nonpremixed flames. Therefore, we conclude that we have successfully designed and characterized a low-NOx burner.