Experimental investigation on the jet mixing flows of lobed nozzles using laser induced fluorescence

Abstract

An experimental investigation of the vortical and turbulent structures in the jet mixing flows of lobed nozzles had been conducted. The technique of the laser induced fluorescence (LIF) was used in the water channel to accomplish the flow visualization and instantaneous quantitative concentration field measurements of the jet mixing flows. The results revealed that, compared with a circular jet flow, the lobed jet flow had shorter laminar region. The scale of the spanwise Kelvin-Helmholtz (normal) vortical structure became smaller, and small scale turbulent vortices appeared earlier in the lobed jet flow. The decay rates of the average concentration along streamwise distance were larger in the lobed jet flow than that in the circular jet flow. Based on the flow visualization and instantaneous quantitative concentration field measurements, two aspects of mechanism of the mixing enhancement of a lobed nozzle are suggested: One is that a lobed nozzle can accelerate the cut-and-connect which is responsible for the avalanche of three dimensional and smaller-scale motions and the generation of high turbulence. Another is that the stretch effect of streamwise vortices on Kelvin-Helmholtz (normal) vortices also enhances the energy cascade process, which results in the creation of much small-scale intense turbulence and enhances the mixing of the jet flow with ambient flow. * Ph. D. student ** Professor Copyright (£) 1996 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. INTRODUCTION Lobed nozzles/mixers are extraordinary fluid mechanic devices for efficient mixing of two co-flow streams with different velocity, temperature and/or spices. Such devices have been known since the earliest days of jet engines and received considerable attention for reducing jet noise during the 1960's. More recently, they have emerged as attractive approaches for mixing the core and bypass streams of turbofan engines to improve propulsion efficiency, reduce the specific fuel consumption (sfc) and suppress the infrared radiation emission (Power et. al.. 1994, Presz et. al., 1994; Huhui et. al., 1996). Lobed nozzles/mixers have also been received attention for using in supersonic ejectors for jet noise reduction at aircraft take-off and landing as well as in combustors for enhancing mixing between fuel and air (Tillman et. al. 1993). The lobed nozzle/mixer consists of a splitter plate with a convoluted trailing edge (Fig. 1.). Paterson (1982) measured the velocity and turbulent characteristics downstream of a lobed mixer using Laser Doppler Velocimeter (LDV) and concluded that the lobed mixer can cause large scale streamwise vortices to be shed at the trailing edge of the lobes, so that the downstream of flow field is embedded with an array of large scale streamwise vortices of alternating sign, which are believed to be primarily responsible for the enhanced mixing. Much of the later work on the lobed mixers/no/zles concentrated on discovering the underlying physics of the lobed mixing process. The work of Werle et. al. (1987) and Eckerle et. al. (1990) all suggested that the formation process of the large scale streamwise vortex was an inviscid one, which was proposed to take in three basic steps: vortices form, intensify and rapidly break down into small scale turbulent structure.