Effect of recess length on flow dynamics in gas-centered liquid-swirl coaxial injectors under supercritical conditions

The flow dynamics in gas-centered, liquid-swirl coaxial injectors at supercritical conditions are comprehensively investigated using the large eddy simulation technique. This class of injectors has been used in many high-performance propulsion and power-generation systems. Gaseous oxygen is axially injected into the center post of the injector at a temperature of 687.7 K, while liquid kerosene is tangentially introduced into the coaxial annulus at a temperature of 492.2 K. The operating pressure of 25.3 MPa substantially exceeds the thermodynamic critical pressures of oxygen and kerosene. Detailed flow structures and mixing layer characteristics are examined, with special attention to the effect of the distance between the end of the center post and the entrance of the taper region, that is, the recess length. Six different cases ranging from no recess to fully recessed are investigated. Various controlling mechanisms of the injector flow dynamics, including recirculating motion near the fuel injection slit, shear-layer instabilities in the recess region, and vortex expansion and merging in the taper region are considered. Dominant frequencies of the axial and azimuthal instability modes are characterized and compared to empirical correlations. Dynamic analyses are conducted for the overall computational domain, as well as individual regions. Among the six cases, only the fully recessed Case 1 exhibits strong pressure oscillations over the entire injector, which are found to be closely related to the longitudinal acoustic oscillations in the center post. In cases with partial or no recess, Case 2 with a long recess achieves the best mixing, and Case 6 with zero recess leads to the worst mixing.