Kuang-Yu Hsu

Mixing and Combustion Studies Using Cavity-Based Flameholders in a Supersonic Flow

An experimental investigation of the mixing and combustion processes that occur in and around a cavity-based flameholder in a supersonic flow is reported. Cavity-based flameholders are commonly found in hydrocarbon-fueledscramjet combustors; however, detailed information concerning the behavior of these devices, their optimal shape and fueling strategies, combustion stability, and interactions with disturbances in the main airflow (i.e., shock trains or shock-boundary layer interactions) is largely unavailable in the existing literature. This work is part of an ongoing research program aimed at providing information to help fill these voids and improve the overall understanding of cavities for use as scramjet flameholders.

In-Stream Hypermixer Fueling Pylons in Supersonic Flow

This paper presents results from both computational fluid dynamic and wind-tunnel experiments of in-stream fueling pylons injecting air, ethylene, and methane gas into Mach number 2.0 cold airflow. Three fuel-injection pylons studied include a basic pylon, a ramp pylon, and an alternating-wedge pylon. The latter two pylons introduce streamwise vorticity into the flow to increase mixing action. The computational fluid dynamic solution was accomplished using the commercial code FLUENT®. Three wind-tunnel experimental techniques were used: aerothermal probing, Raman spectroscopy, and nitric-oxide planar laser-induced fluorescence. Four measures reported include streamwise vorticity, total-pressure-loss, mixing efficiency, and flammable plume extent. The ramp and alternating-wedge pylons show decisive increases in mixing capability compared with the basic pylon for a finite distance downstream of the injector. The alternating-wedge pylon exhibits a measurable increase in total pressure loss compared with the basic pylon, and the ramp pylon exhibits a negligible increase in total pressure loss compared with the basic pylon. For comparison, the downstream mixing effectiveness of the three pylons is compared with the downstream mixing effectiveness of a transverse circular wall injector studied in past research. In addition, a qualitative comparison between the computational fluid dynamic and wind-tunnel experimental results is made.

Particle Image Velocimetry in a Nonreacting and Reacting High-Speed Cavity

Particle image velocimetry measurements were taken at the center plane of a high-speed cavity combustor in nonreacting and reacting conditions at fuel flows corresponding to medium, medium-high, and high fuel-loading conditions with supersonic core flow velocities. Calculation of the instantaneous and averaged pathlines, vorticity, swirling strength, and divergence of the velocity field revealed a highly unsteady three-dimensional flow with coherent eddy structures formed at the stagnation zone of the shear layer against the downstream ramp of the cavity, which appear to be convected upstream in the cavity. Comparison of the shear layer location, thickness, and impingement stagnation zone revealed a number of changes in the mean and unsteady velocity behavior that were dependent on the heat release in the cavity and shear layer. As combustion shifted from the cavity at medium fuel loading into the shear layer at high fuel loading, the volumetric expansion compressed the primary recirculation zone and thic...

Fuel-Air Injection Effects on Combustion in Cavity-Based Flameholders in a Supersonic Flow

Abstract : The Air Force Research Lab, Propulsion Directorate, Wright-Patterson Air Force Base, Ohio has studied several designs regarding cavity flameholding for supersonic RAMJET (SCRAMJET) applications. The most recent of these studies have concluded that direct injection of ethylene fuel into the aft cavity ramp produced an efficient, robust flameholder given specific freestream condition and fuel flow rate. The main goals of this experiment are: 1) study the effect on combustion of direct fuel and air injection in the main flameholding cavity and 2) characterization of the operational limits (i.e., sustained combustion limits) over a variety of fuel and air flow rates. Direct injection of both fuel and air provided additional capability to tune the cavity such that a more stable decentralized flame results. The addition of air injection provided the most improvement over the baseline case (fuel only) near the upstream portion of the cavity close to the cavity step. Direct air injection provided a second source of oxygen to be consumed during the combustion process thereby expanding the operational limits drastically for each selected fuel flow. This experimental investigation was limited by the size of the flow controllers available and by the maximum allowable material temperature given cavity flow parameters. Lean blowout was not observed to be a function of injected air flow.

Flow Field Studies of Diamond Shaped Fuel Injector in a Supersonic Flow

Studies were performed to characterize the flow structure and the mixing characteristics of a diamond-port flush-wall fuel injector at Mach 2.0 airflow. The goals were to examine the flow structure in the near-field region of the diamond- and circular-port injectors and to quantify the effects of igniter torch flow on the near-field flow structure with and without torch gas. Planar laser-induced fluorescence was used to document the flow trajectories and the injector-barrel shock shape for the injectors. For the inert mixing studies, the flow was seeded with a nitric-oxide-trace probe molecule, whereas for the reacting experiments, naturally occurring hydroxyl was used. The present experiments confirmed recent computation results that implied that under certain conditions, a diamond-shaped injector port could be tailored to improve mixing and produce an additional secondary flow structure with gasdynamic flame-holding potential. It was also shown that an igniter torch system could be incorporated without adversely affecting the tailored flow structure.

Transverse Supersonic Controlled Swirling Jet in a Supersonic Cross Stream

The spreading rate and mixing of a transverse jet in high-speed crossflow were modified using a swirling injector with a central control jet. The controlled supersonic swirling injector (CSSI) could be used to affect mixing both in the core and the shear layer of the jet. Rayleigh/Mie scattering from flowfield ice crystals and planar laser-induced fluorescence of the NO molecules were used to characterize penetration and mixing of the CSSI for six different cases. Instantaneous images were used to study the dynamical structures in the jet, whereas ensemble images provided information regarding the jet trajectory. Standard deviation images revealed information about the large-scale mixing/entrainment. Probability density functions were used to evaluate the probability and location of freestream, mixed, and jet fluid. They were also used to track the centerline and jet boundary on a dynamic scale. Side- (streamwise)-view images showed that the injector was capable of providing high penetration when compared to circular and swirling baseline injectors. An increase of 16% in mixing area was observed with the optimal case as compared with the other control cases. End- (spanwise)-view images show a maximum of 78 % increase in total area contained within the jet boundary for the optimal case when compared to the circular injector. Higher spanwise extent of the jet boundary was also observed with controlled cases, which could provide higher interfacial area for better mixing between the jet and the cross stream when compared to their baseline counterparts.

Upstream Mixing Cavity Coupled with a Downstream Flameholding Cavity Behavior in Supersonic Flow (Postprint)

Experimental investigations of the flow fiel d associated with three upstream direct - injection acoustic resonance cavities coupled with a previously designed downstream combustion cavity in a non -reacting flow are described. All of the upstr eam mixing cavities were acoustically open (shear layer r ea ttachment on the downstream wall of the cavity) with the length -to -depth ratio (L/D) on the order of 1. The previously established downstream combustion cavity had an L/D of 4.7 and an aft ramp angle ( �) of 22.5 degrees. The three upstream mixing cavities we re characterized in Mach 2 free stream flow with injection at three locations (upstream wall, center, downstream wall) within each cavity. Injection at the upstream wall of the cavity provided great er penetration height into the free stream as well as faster mixing with the free stream compared with injection at the center or downstream wall of the cavity. Injection at the center of the cavity resulted in the injectant diffusing laterally in the cav ity bef ore being ejected into the free stream. Injection at the downstream wall of the cavity displayed characteristics of both injection at the upstream wall and center of the cavity . The flow over the downstream cavity was similar for all injection loca tions. Pressure taps along the ceiling and floor of the test section showed no distinct change in pressure readings due to either cavity length or injection location within the cavity.

Particle Image Velocimetry in an Isothermal and Exothermic High-Speed Cavity

Particle image velocimetry measurements were taken at the center plane of a high-speed cavity combustor in isothermal and reacting conditions at fuel flows corresponding to medium, medium-high, and high power conditions with supersonic bulk flow velocities. Calculation of the instantaneous and time-averaged particle traces, vorticity, principal stresses, and divergence of the velocity field revealed a highly unsteady, three-dimensional flow with coherent eddy structures formed at the stagnation zone of the shear layer against the downstream ramp of the cavity that appear to be convected upstream in the cavity. Comparison of the shear layer location, thickness, and reattachment stagnation point revealed a number of changes in the mean and unsteady velocity behavior that were dependent on the heat release in the cavity and shear layer. As combustion shifted from the cavity at medium power into the shear layer at high power, the volumetric expansion compressed the primary recirculation zone and thickened the downstream boundary layer at the cavity exit. Combustion in the cavity tended to attenuate cavity and shear layer unsteadiness. When the combustion shifted to the shear layer, velocity unsteadiness increased, though not to the amplitudes measured without combustion.

Mixing Study of Strut Injectors in Supersonic Flows

Mixing characteristics of various strut injectors in supersonic flows were examined using Planar Laser-Induced Fluorescence technique. CFD studies were used to further the understanding of the aerodynamics and mixing characteristics of the strut injectors. Effects of wedge angle, strut root length, and shape of strut base on fuel distributions were evaluated. Struts with a shorter root length create a stronger flow expansion at the base that enhances the fuel penetration in the transverse direction because of the strong upward transport behind the strut. However, the strong flow expansion in the strut base could also creates the strong flowfield that causes the fuel to be distributed closer to the symmetric plane and potentially reduces the mixing performance.

Flow Distortion: Computational Investigation of a Shocked Cavity Flameholder

Cavity flameholder experiments with incident shocks have recently been performed at the Air Force Research Laboratories (AFRL) Aerospace Systems Directorate (RQ) in Research Cell 19 (RC19). The incident shocks are intended to replicate flow distortion from an inlet. A Computational Fluid Dynamics (CFD) effort, described here, was performed for evaluation and assessment. The computations used the Reynolds-averaged-Navier Stokes (RANS) approach with a 2-equation turbulence model and a 22-species finite-rate kinetics model. In general, CFD results are in reasonably good agreement with the experiment. The analysis indicates that flow distortion has a significant impact on the cavity flowfield, which can lead to ignition failure as observed by the experiment for one of the configurations. Insights from the CFD were used to shed light into the ignition problems, revealing that conditions for the case that did not light were not favorable for flameholding.