Mixing and Mass Exchange for Cavities in Supersonic Flows

Abstract

The ability of wall-mounted cavities to facilitate flameholding is largely dependent on local conditions within the recirculation region, that are, generally, vastly different from the adjacent supersonic flow. The local stoichiometry depends on the mixing within the flameholder and the mass exchange with the core flow. These processes are integrally linked to fuel injection location and its coupling with the recirculation region flow field. In the present study planar laser-induced fluorescence (PLIF) was used to obtain a twodimensional image of the streamwise fuel distribution in a non-reacting, directly-fueled cavity in a Mach 2.2 flow. Fuel was injected from two different locations within the cavity chosen to result in parallel and counterflow mixing with respect to the primary cavity circulation. Injection from both locations resulted in a largely fuel rich recirculation region. Parallel injection from the cavity step was characterized by pooling of fuel within the cavity's trapped vortices with the majority of mixing occurring in the shear layer and along the cavity walls. Counterflow injection from the cavity floor resulted in a leaner flow field attributed to enhanced mixing with the impinging shear layer, larger quantities of fuel escaping the recirculation region and limited mass exchange between the trapped vortices. The local concentration fields generated by both fueling schemes suggested that under analogous reacting conditions the region around the shear layer would serve as the most probable flame anchoring location. Nomenclature D= cavity depth, mm L = cavity length, mm M = Mach number P inj = injection pressure, atm P 0 = stagnation pressure, atm q r = fuel-to-air dynamic pressure ratio T 0 = stagnation temperature, K V = velocity, m/s X = mole fraction in air, % θ = cavity aft ramp angle, deg. = floor injection angle, deg. = global equivalence ratio

Cavity-based flameholding for chemically-reacting supersonic flows

“…Liu et al [48] performed a RANS computation to examine the effect of fueling rate on the fuel distribution in a cavity with rear wall fueling and exposed to a Mach 2 airflow. Increased fueling progressively flooded the cavity volume and pushed the stoichiometric mixture fraction contour into the shear layer, similar to the experimental observations of Barnes et al [2] for the case of front wall fueling.…”

“…Yet as was reported by Barnes et al [2] in the study mentioned above, recirculation zone equivalence ratios may be up to an order of magnitude higher than indicated by global values. In this study [2] acetone-PLIF images for the case of parallel injection from the front cavity wall, as shown in Fig. 11, revealed that even for very small global equivalence ratios the majority of the cavity recirculation zone may be too rich for flameholding.…”

“…A 2-D RANS computation by Mishra and Sridhar [57] showed significant entrainment of fuel into the primary vortex and a leaner upstream cavity volume for several angled floor injection configurations at a location 0.3 L from the front wall. Barnes et al [2] utilized acetone-PLIF to study fuel-air mixing in a directly-fueled cavity exposed to a Mach 2.2 airflow. Acetoneseeded nitrogen was injected as a surrogate for ethylene.…”

“…The mixture bounded by the upper flammability limit contour is too rich for combustion. Whereas the region between the two flammability limit contours, within the shear layer and near the fuel injectors, is indicative of where combustion is likely to occur [2]. effects and increased kinematic viscosity also play a role in the observed reduction in shear layer growth rate and entrainment [16].…”

Nitric Oxide PLIF Visualization of Simulated Fuel-Air Mixing in a Dual-Mode Scramjet (Invited)

“…Gruber et al 12 investigated a supersonic cavity design using NO and OH PLIF, injecting reacting and non-reacting ethylene. Barnes et al 13 used acetone PLIF to determine fuel distribution in a directly injected cavity. Thakur et al 14 determined concentration distribution behind a cavity step.…”