The Effect of Quench Air on the Upstream Swirling Flow in a Rich-Quench-Lean Combustor With a Moderate Swirl Intensity Under Isothermal Conditions

Abstract The present work examines the effect of secondary/quench air on the upstream swirling flow in a model Rich-Quench-Lean (RQL) combustor. The measurements are performed under non-reacting conditions at atmospheric pressure. The RQL rig consists of three flow streams, namely primaryair through the swirler, fuel stream air through a centrally placed radial jet injector, and downstream quench air. A high shear nozzle/swirler with a swirl number of 0.51 is used. The quench air is issued through five staggered single-row holes at a downstream location. In the present isothermal study, the air is issued through the fuel injector, which is referred to as the fuel stream air. The measurements are performed at the swirler exit Reynolds number (Re) of 12000. Re is defined based on the hydraulic diameter of the swirler and bulk velocity for the imposed primary air flow rate. The quench air effect is studied by measuring the velocity field and fuel stream concentration. For velocity measurements, the particle image velocimetry (PIV) technique is used. To measure the fuel stream concentration, the planar laser-inducedfluorescence (PLIF) technique is used by seeding the acetone tracer into the fuel stream. The following three flow test cases are investigated. First, the flow field with only primary air flow through swirler is examined. The following flow features are noted — an annular high-velocity swirling jet (SwJ), inner recirculation zone (IRZ) owing to the vortex breakdown, and outer recirculation zone (ORZ) near the dump plane due to confinement. Next, the fuel stream air effect is examined. The fuel stream air flow rate is determined based on the imposed equivalence ratio of 1.6 for the butane fuel equivalent volumetric flow rate. With the fuel stream flow, swirl strength is observed to drop, as inferred from the decrease in the radial velocity and increase in the axial velocity. Due to reduced swirl intensity, IRZ length increases with the fuel stream flow. From the air-fuel mixing data, an annular fuel-rich region is observed in the inner shear layer (ISL) which lies between high-velocity SwJ region and IRZ. The air-fuel mixing primarily takes place in ISL due to high velocity fluctuations. Finally, the influence of the quench air is examined by varying the mass ratio (MR) which is defined as the ratio of quench air to the primary air mass flow rate. MR values of 1 and 1.7 are used. With the quench air injection, the IRZ size is significantly reduced, and it is contained up to the quench jet location owing to aerodynamic blockage. A fuel-lean region is observed downstream of the quench jet, as expected. The quench air is observed to entrain into the upstream swirling flow, resulting in a decrease in swirl strength and dilution of the fuel-rich mixture.