High mixing effectiveness lobed nozzles and mixing mechanisms

Abstract

For a circular lobed nozzle with the exit plane displaced from the center body, adding a central plug at exit or replacing the nozzle with an alternating-lobe nozzle can improve the mixing effectiveness. In this study, numerical investigations of jet mixing in the lobed nozzles with a central plug and alternating-lobe nozzles in pumping operation were conducted. The effects of the central plugs with the wake ranging from attached to separated flow on the mixing were analyzed, along with the mechanism of improving the mixing performance in a “sword” alternating-lobe nozzle. The simulation results reveal that the large-scale mixing rate, which is dominated by streamwise vortices, is related to the intensity of the attainable heat and mass transfer in the streamwise vortices. The effects of the streamwise vortices on the normal vortex ring are virtually a manifestation of the heat and mass transfer/mixing process of the streamwise vortices. The simulation results also show that the central plug with the attached rear-flow performs better in improving the mixing effectiveness and pumping performance; on the contrary, if the rear-flow is separated, more pressure loss will be induced. In particular, a completely separated flow over the rear of the central plug will severely degrade the attainable heat and mass transfer in the streamwise vortices. For the sword alternating-lobe nozzle, wider sword deep troughs help to increase the flux of the secondary stream around the core region and delay the confluence of the primary stream in the region between the deep and shallow troughs. Thus, the mixing is improved in the middle and posterior segments. Compared to the lobed nozzle with a central plug, the improved sword alternating-lobe nozzle can achieve a higher mixing effectiveness with much less pressure loss, which is preferred in situations when the power loss of the engine is restricted.