Experimental Studies of Cavity and Core Flow Interactions With Application to Ultra-Compact Combustors

Abstract

Reducing the weight and decreasing pressure losses of aviation gas turbine engines improves the thrust-to-weight ratio and improves efficiency. In ultra-compact combustors (UCC), engine length is reduced and pressure losses are decreased by merging a combustor with adjacent components using a systems engineering approach. High-pressure turbine inlet vanes can be placed in a combustor to form a UCC. In this work, experiments were performed to understand the performance and associated physics within a UCC. Experiments were performed using a combustor operating at pressures in the range of 520–1030 kPa (75–150 psia) and inlet temperature equal to 480–620 K (865 R–1120 R). The primary reaction zone is in a single trapped-vortex cavity where the equivalence ratio was varied from 0.7 to 1.8. Combustion efficiencies and NOx emissions were measured and exit temperature profiles were obtained for various air loadings, cavity equivalence ratios, and configurations with and without representative turbine inlet vanes. A combined diffuser-flameholder (CDF) was used to study the interaction of cavity and core flows. Discrete jets of air immediately above the cavity result in the highest combustion efficiencies. The air jets reinforce the vortex structure within the cavity, as confirmed through coherent structure velocimetry of high-speed images. The combustor exit temperature profile is peaked away from the cavity when a CDF is used. Testing of a CDF with vanes showed that combustion efficiencies greater than 99.5% are possible for 0.8 ≤ Φcavity ≤ 1.8. Temperature profiles at the exit of the UCC with vanes agreed within 10% of the average value. Exit-averaged emission indices of NOx ranged from 3.5 to 6.5 g/kgfuel for all test conditions. Increasing the air loading enabled greater mass flow rates of fuel with equivalent combustion efficiencies. This corresponds to increased vortex strength within the cavity due to the greater momentum of the air driver jets.

Experimental Characterization of the Reaction Zone in an Ultra-Compact Combustor

“…3(a)], where inlet air is divided into discrete air passages to allow sufficient core air diffusion and distribution while providing a transport mechanism for cavity products flowing into the core region. Further details of this front-end design can be found in Blunck et al 6 A variant of this design [CDF-2, shown in Fig. 3(b)] was examined in the first study.…”

“…A similar UCC sector has been reported previously. 6,33 A brief description of the UCC sector is provided below. Figure 2 shows a top-down view of the planar section of a UCC that displays flow elements and directions.…”

“…Water cooling protected the rakes from the combustion environment and assisted in quenching the products. The layout and location of the probes is the same as detailed in Blunck et al 6 Total unburned hydrocarbons (THC) data were collected using a Beckman Model 402 flame-ionization detector; carbon dioxide (CO 2 ) and carbon monoxide (CO) data were collected using Beckman non-dispersive infrared analyzers; and NO x data were collected using an EcoPhysics Model 700 chemiluminescent analyzer. Manufacturer specifications state that the uncertainty of the emissions measurements is ± 1% of the data range on the analyzer.…”

“…[1][2][3][4][5] These improvements can increase thrust-to-weight ratio and/or decrease thrust specific fuel consumption. 6,7 In the current design the high-pressure turbine inlet guide vanes (IGVs) are integrated into the combustor, as proposed by Zelina et al 8 Therefore, the flow-turning and acceleration effected by the IGVs are integrated with the combustion process to achieve engine length reduction. A study has shown that the potential for further reduction exists through the removal of the compressor outlet guide vanes (OGVs).…”

“…Sturgess and Hsu 14 showed that entrainment of the core flow into the trapped-vortex cavity resulting from cavity vortex motion varies with the fuel and air jet momentum. Blunck et al 6 studied the effect of varying the trapped-vortex cavity air driver forward-to-aft momentum ratio, cavity air loading, and front-end air passage location on combustion efficiency, exit temperature profile, and cavity vortex strength. High combustion efficiency over a wide range of equivalence ratios was observed for elevated air loadings, forward-to-aft momentum ratios near 0.9, and the lowest air passages open in the front-end.…”