Flow Control of Jet Mixing Using a Pulsed Fluid Tab Nozzle

Abstract

Flow control techniques for increasing the rate of jet mixing in axisymmetric nozzle flows have been investigated experimentally. The near-field jet development of a clean axisymmetric nozzle is used as a datum to judge increased mixing effectiveness. A combination of water tunnel and high-speed airflow facilities are used to assess the nearfield jet behaviour. Classical solid tab, fluid tab (i.e. discrete radially discharging control jets located close to the core jet nozzle exit), and pulsed fluid tab nozzles are compared. The effect of the fluid tab flow rate, pulse rate and pulse phase are studied. The measurements indicate that nozzles equipped with fluid tabs and pulsed fluid tabs generate a similar streamwise vortex formation process (and hence display increased mixing) as occurs in a solidtabbed nozzle. The performance of pulsed fluid tabs is strongly influenced by the tab flow rate, pulse frequency and phase. In low speed tests the mixing effectiveness with a pair of pulsed fluid tabs working 180 o out of phase was as good as a twin solid tab nozzle for a total control jet flow rate of only 0.5% of the core jet flow. In high speed testing similar benefits of fluid tab control jets over solid tab designs were observed, although evidence was obtained that the interaction mechanisms between the streamwise vortices which produce the increased mixing across the core jet shear layer may be subtly different under low speed and compressible conditions. Further study of pulsed fluid tabs is recommended, since they have attractive performance benefits over other rapid jet mixing control mechanisms (can be easily switched off when not needed), and offer attractive flexibility as the basis for an optimum active control mixing device.