A review of jet mixing enhancement for aircraft propulsion applications

Abstract

This article reviews techniques applicable to enhancing the mixing of jets, with particular emphasis on infrared (IR) signature reduction of high-speed jets. Following a brief introduction to the IR signature of jet plumes and the fundamentals of jet mixing, this paper discusses rapid mixing technologies under the categories of: geometric modifications (to the nozzle); high shear stress mixing; normal stress mixing; self-acoustic excitation; external acoustic excitation; mechanically oscillated; self-oscillated. It is shown that mixing enhancements of the order of 100 per cent are possible with some techniques and that by combining techniques this can be increased by at least as much again. Simple geometric calculations are presented which demonstrate that with rectangular nozzles such high levels of mixing enhancement may be necessary in order to reduce IR signature. Some apparent rapid mixing technologies, however, have been shown to increase jet spreading without increasing entrainment, whereas other techniques can reduce entrainment as easily as they can increase it.