The Impact of Variable Inlet Mixture Stratification on Flame Topology and Emissions Performance of a Premixer/Swirl Burner Configuration

Abstract

The work presents the assessment of a low emissions premixer/swirl burner configuration utilizing lean stratified fuel preparation. An axisymmetric, single-or double-cavity premixer, formed along one, two, or three concentric disks promotes propane-air premixing and supplies the combustion zone at the afterbody disk recirculation with a radial equivalence ratio gradient. The burner assemblies are operated with a swirl co-flow to study the interaction of the recirculating stratified flame with the surrounding swirl. A number of lean and ultra-lean flames operated either with a plane disk stabilizer or with one or two premixing cavity arrangements were evaluated over a range of inlet mixture conditions. The influence of the variation of the imposed swirl was studied for constant fuel injections. Measurements of turbulent velocities, temperatures, OH * chemiluminescence and gas analysis provided information on the performance of each burner set up. Comparisons with Large Eddy Simulations, performed with an 11-step global chemistry, illustrated the flame front interaction with the vortex formation region under the influence of the variable inlet mixture stratifications. The combined effort contributed to the identification of optimum configurations in terms of fuel consumption and pollutants emissions and to the delineation of important controlling parameters and limiting fuel-air mixing conditions.

A Comparison of the Characteristics of Planar and Axisymmetric Bluff-Body Combustors Operated under Stratified Inlet Mixture Conditions

“…A description of the isothermal flow and the stable flame characteristics under operation away from the blow off limit region has been presented in references [11,13,16,19] for the two burner configurations, respectively. The present work focuses on the comparative examination of the topologies obtained in the disk flame operated under a high swirl level ( = 1) and the square burner counterinjected with a low fuel-to-approach-air velocity ratio at ultra-lean conditions located near the LBO margin.…”

“…The burners have been previously investigated under isothermal operation to establish the mixing topologies that sustain the reacting fields presented below [11,13,16,19]. Mean and turbulent velocities, temperatures, and OH * and CH * chemiluminescence (CL) images were obtained using a two-component LDV system, thin digitally compensated thermocouples, and a chemiluminescence imaging system (LaVision Flame Master consisting of a CCD camera, an image intensifier IRO unit, camera optical filter at 307±10 nm and achromat lens for OH imaging).…”

“…These flame front structures form a wide angle relative to the incoming flow due to the effect of the high swirl intensity and seem to occasionally penetrate from the periphery toward the base of the flame within the recirculation region. The overall disposition of the flame front under stratification seems to differ both in terms of length, placement, and anchoring behaviour from both nonpremixed [16,19] and fully premixed axisymmetric [4,7,8] counterpart topologies. The transient OH * chemiluminescent images for the square verify the two-tongued flame front placement previously implied by the time-mean experimental and simulation results ( reaction within the formation region, which is in stark contrast to the reported behavior [6,12] of ultralean and limiting slender body stabilization of fully premixed flames.…”

“…In the first configuration stratified flames, established by staged fuel-air premixing in a double-cavity arrangement (Figure 1(c)) formed along three concentric disks and stabilized in the vortex region of the afterbody A ( = 0.025) under the additional action of a coflowing swirl stream (Figure 1(b)) are investigated. The combustion tunnel and the studied burner are similar to those reported in [13] and have been optimized through a series of systematic tests [16]. Fuel is supplied through the central shaft ( = 0.010) that is running along the central air supply tube ( = 0.052) into the internal hollow of the active bluff-body disk B (Figure 1(c)).…”

“…This formulation was coupled to a nine-step reduced scheme for propane (i.e., . This represents an extension of the mechanism described in [11,13,16], now including the radical OH to facilitate more direct comparisons with the measured 6 Journal of Combustion chemiluminescent OH * and has been tuned to reproduce the flame speed over a section of the lean Φ range as described in [11]. All the species are explicitly solved on the computational grid, and it is likely that intermediate radicals with shorter time scales might not be properly resolved within the context of the employed refinement.…”