Effect of Chamber Backpressure on Swirl Injector Fluid Mechanics

Fluid mechanics of a liquid swirl injector element at various chamber backpressures were investigated. The center-jet swirling element was designed using typical liquid propellant rocket engine parameters, then manufactured and tested in a high-pressure, optically accessible, cold flow facility. Water was injected into a chamber pressurized with gaseous nitrogen at a constant swirl injector flow rate of 0.09 kg/s. The chamber backpressure ranged from 0.10 to 4.81 MPa. The film thickness and spray angle near the nozzle exit were measured by shadowgraphy. The film thickness was also measured within the injector upstream of the exit through a transparent nozzle tube section. Increasing the chamber backpressure for this fixed mass flow rate increased the film thickness from predicted design values. Measured discharge coefficient values increased with increasing chamber backpressure, reflecting the observed increase in internal nozzle film thickness. The spray angle decreased for increasing chamber backpressure.