Swirl effects on shock structure in free under-expanded supersonic-nozzle airflow

The effects of convective Mach number and air-fuel density ratio have been investigated experimentally under both non-swirling and swirling conditions in supersonic-nozzle flow comprising diamond shock structure with coaxial fuel injection. A convergent nozzle of inlet-to-exit area ratio of 25 was used to generate a free under-expanded supersonic airflow with maximum near-field Mach number of 2.0. Non-reacting conditions were considered, wherein fuel was simulated by helium and argon gases. Schlieren diagnostic technique with 6 ns exposure was implemented to allow for accurate visualization of shock structure by preventing any fluctuations of flowfield from showing up on the captured image. Two distinct diamond shock sub-structures were identified, namely a primary one generated off nozzle-rim and a secondary structure generated off the coaxial injection system and air-fuel shear layer. The primary shock sub-structure is affected mainly by the properties of airflow, whereas the secondary structure strongly depends on the properties of injected fuel, primarily convective Mach number. Imparting swirl to airflow results in significant reduction in the strengths of both primary and secondary structures, as compared to non-swirling conditions. Changing convective Mach number with swirl does not affect primary structure significantly; however, the secondary structure weakens with introduction of fuel injection and gradually diminishes with decrease in convective Mach number. No significant differences were observed with change in air-fuel density ratio.