Swirling flow in thrust nozzles

This paper investigates the effects of adding swirl to a dump combustor- nozzle propulsion system. The results of cold-flow testing are summarized and compared to a numerical analysis of the nozzle flowfield. The cold- flow testing included thrust, mass flow rate, and pressure measurements for two different conical nozzle in combination with four swirlers, one of which was a blank. Five-port probe measurements were made across the nozzle entrance. The measurements were used to determine the stagnation pressure and swirl distributions and to provide initial conditions for a nozzle analysis program. The measured results show that swirl has a strong effect on the stagnation pressure distribution. The largest losses occur near the swirl axis. Swirl also significantly reduces the system discharge coefficients. In contrast, the effect of swirl on the nozzle stream thrust efficiency was small, but measurable. Swirl reduced the nozzle stream thrust efficiency by about 0.5 percent for the highest swirl tested. Computed nozzle flowfields that do not account for the stagnation pressure distribution in the swirling flow can produce highly distorted and unrealistic results. Comparison of measured and computed results (which use measured stagnation pressure and swirl distributions) shows reasonable agreement. (Author)