Mixing Enhancement in Subsonic Jet Flow Using the Air-Tab Technique

Abstract

M IXING enhancement in jet flow has long been the topic of extensive research due to its important applications in aircraft propulsion and combustion. It also has military importance when mixing enhancement is used to suppress the infrared red signature for the fighter jet exhausts. Many passive or active approaches or techniques to enhance jet mixing have been developed and explored, and some of these techniques had also been reviewed and summarized by Gutmark and Grinstein [1]. In passive control strategies, the tabs or vortex generators in particular are simple but effective techniques in flow control. Bradbury and Khadem [2] were the pioneers to study the effect of solid tabs on jet flow. They found that the nozzle boundary-layer thickness, turbulence level, and convergent ratio did not have very strong influence on the jet development. On the other hand, to insert small rectangular tabs into the jet flow on the nozzle perimeter would induce some profound effects on the jet development. Since then, many researchers have been working on this technique. Notable examples include Samimy et al. [3], Reeder and Samimy [4], Zaman et al. [5], Yu and Koh [6], Paoli et al. [7] and many others. Zaman [8] conducted a systematic test on a series of nozzles with various shapes or orifices and found that the spreading of most asymmetric jets was not much different from that of a round jet, but the biggest increase in jet spreading was observed with the tabs. However, the penalty of introducing the tabs is also found to be significant. The thrust lost due to the tabs varies from 4.1 to 23.7%when theflowblockage (proportional to the facing area of tabs) increases from 1.1 to 14.1%. Tominimize the thrust lost caused by tabs but yet maintain its effectiveness in mixing enhancement is important to practical applications. Researchers have been searching for new techniques with better effects on mixing but minimum penalty on thrust. This Note presents results using the socalled air-tab technique. The air-tab technique is achieved by injecting a small amount of air (less than 1% of the volume flow rate of the primary jet) into the plume of the primary jet at choked speed and at certain attacking angles (45 and 90 deg with respect to the primary jet direction). The air-tab technique shows minimum or no impact on thrust, but it would be able to provide significant effect as the solid tab on the mixing enhancement. A similar idea had been used in air and fuel mixing: for example, the experiments conducted byMilanovic and Zaman [9,10]. The description of the experimental setup is presented subsequently. It will be followed by the results and discussion. The Note ends with brief concluding remarks.