Experimental and Numerical Investigations of Annular Swirling Primary Flow in a Short Subsonic Air-Air Ejector with Entraining Diffuser
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In the current research, the annular swirling flow in a short subsonic air-air ejector that can be found in helicopters, fixed wing aircrafts, ships and other applications was investigated experimentally and numerically under different inlet flow conditions and geometric parameters. These ejectors are attached to the exhaust systems of gas turbine engines. A key phenomenon studied was the large wake that appears downstream of the primary nozzle of the ejector system. This phenomenon is typically called core separation in the industry. The effects of the core separation on the ejector performance parameters were also studied. It was found experimentally that severe core separation took place for swirl angles greater than 20 degrees. This core separation leads to severe distortion in the exit velocity and temperature profile. The core separation also changed the pressure recovery along the device axis and this leads to a rebalancing of the pumping at the mixing tube inlet and entraining diffuser. This affected overall ejector pumping, back pressure and film cooling effectiveness. Numerical studies were carried out using two-equation turbulence models that were available in ANSYS Fluent. In a series of CFD simulations, the measured flow parameters in the annulus were used as CFD inlet boundary conditions (AIBC). In another series of CFD simulations, the measured flow parameters at the nozzle exit were applied as CFD inlet boundary conditions (NEBC). With applying both the AIBC and NEBC using cold and hot inlet flow conditions with swirl angles < 20o, the RNG k-ε turbulence model showed superiority over the other turbulence models and a useful tool for effectively designing such devices particularly with applying the NEBC. At higher swirl angles the CFD did not accurately predict the strong changes in pressure recovery of such devices. In the current study, detailed performance data of air-air ejectors including the core separation phenomenon were generated. The understanding of the impact of core separation on such devices was improved. Deficiencies of the two-equation turbulence models in predicting the core separation in the ejector devices were identified. The effects of the swirl strength and other parameters on the ejector performance criteria were evaluated.