No abstract

Partial premixing can be induced by design in combustors, occurs inadvertently during turbulent nonpremixed combustion, or arises through inadequate fuel-air mixing. Therefore, it is of interest to investigate the effect of partial premixing in a burner that mimics conditions that might occur under practice. In this investigation, we report on similitude of partially premixed flames encountered in practical complex and multi-dimensional burners with simpler, less complex flames, such as counterflow flamelets. A burner is designed to simulate the more complex multi-dimensional flows that might be encountered in practice, and includes the effects of staging, swirl, and possible quenching by introduction of secondary air. The measurements indicate that the structure of partially premixed flames in complex, practical devices can be analyzed in a manner similar to that of flamelets, even if substantial heat transfer occurs. In particular, the flame structure can be characterized in terms of a modified mixture fraction that differentiates the lean and rich zones, and identifies the spatial location of the flame.

An experimental analysis was performed on a rapid-mixing burner in which the mixing of methane and air occurs in discrete vortex structures prior to burning. Air is distributed by large vortex structures, and burning of fuel-air are mixtures occurs in smaller eddies. The burner design was evaluated by utilizing various visualization and diagnostic techniques. The advantages and disadvantages of each are discussed. High speed video is employed to evaluate fickering phenomena. Mixing induced by vortex structures is visualized by means of scattering across a laser light sheet. Velocity measurements made with a laser Doppler velocimeter confirm flow divergence and high kinetic energy at the flow edges through which the vortex structures transit. Temperature measurements made with a fast response thermocouple reveal a well-mixed combustion zone. The flame flickers in a manner similar to that occuring in isothermal helium plumes. Therefore, the results present sufficient evidence that a rapid-mixing burner has been successfully devised using radial fuel injection and a noncircular duct cross-section.

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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.