Effect of Swirl on Mixing in Underexpanded Supersonic Airflow

摘要

This study examines the effect of imparting swirl to underexpanded nozzle airflow on supersonic mixing undernmatchedmass flow conditions. A convergent nozzle with swirling capabilities is used to generate the underexpandednairflow. Fuel is injected coaxially at the nozzle throat. Nonreacting conditions are considered, wherein fuel isnsimulated by mixtures of helium, argon, and krypton inert gases. It was found that the positive effect of swirl onnsupersonic mixing is noticeable, even at the low degree of swirl examined here. Better mixing was observed betweennthe supersonic and subsonic parts of the diamond-shock air flowfield at no fuel injection as well as between thensupersonic air and the subsonic injected fuel. The compressiblemixing-layer thickness was found to increase slightlynwith swirl.However,when normalized to its incompressible counterpart, the compressible thicknesswas observed tondecrease with swirl, because the former increases more with swirl than the latter. Decreasing the air–fuel relativenMach number and/or density ratio was found to deteriorate mixing. On the other hand, decreasing thencompressibility of the injected fuel (i.e., injecting fuel at lower subsonic Mach numbers) improves mixing.