Comparison of Hypermixing Injectors Using a Mixture-fraction-sensitive Imaging Techinique

13th Australasian Fluid Mechanics Conference Monash University, Melbourne, Australia 13-18 December 1998 Experiments which visualise the fuel-mixture-fraction for different types of fuel injectors considered for use in supersonic combustors are described. The flowfields of two different fuel injectors are compared in a qualitative manner. INTRODUCTION Successful development of a supersonic combustion ramjet (SCRAMJET) engine depends critically on finding ways to mix fuel and air efficiently at supersonic flow conditions (Gutmark et al, 1995.). A planar laser-induced fluorescence (PLIF) method for visualising fuel mixture fraction was developed by Fox et al (1998). This paper describes an application of that method to two different mixing flowfields. EXPERIMENTAL METHOD Flow conditions and model The injectors used in this experiment are shown in Figs. 1 and 2. The injectors were mounted on the base of a strut which spanned the nozzle of the shock tunnel. The first of these (Fig. 1) has a segmented blunt trailing-edge and is termed the castellated injector. The other (Fig. 2) is referred to as the swept compression-expansion ramp (SCER) because of its trailing-edge configuration. These injectors are termed hypermixers and are designed to generate streamwise vorticity to enhance mixing by increasing the interfacial surface area and the magnitude of gradients between the fuel-air interfaces (Gaston et al, 1998). The experiments were performed in the Australian National University’s T3 free-piston shock tunnel (Stalker, 1972). Freestream conditions at a pressure of 40 kPa, a temperature of 700 K and Mach number of 4.8 were produced with a chemical composition of 1.6% NO, 1.2% O2, 0.06% O, with a balance of N2 in the test section. The fuel was injected at a pressure of 40 kPa and a temperature of 190 K, giving a Mach number of 1.7; its composition was 0.3% CO2 with a balance of hydrogen. x/h = 1.0 x/h = 3.0 x/h = 5.0 x/h = 10.0 Jet centre plane FLOW Figure 1: Schematic of the castellated injector showing the positions of the laser sheet used in the experiment. PLIF Excitation and Detection PLIF involves illuminating the flow with a thin sheet of laser light tuned to excite electronic transitions in a chemical species in the flow, in our case nitric oxide (NO). The fluorescence induced by this illumination is focussed onto an intensified charge-coupled device (ICCD) camera to produce an image of fluorescence intensity in that region (See Fig. 3). By exciting two or more appropriate transitions simultaneously and satisfying certain constraints on the gas composition and the flow environment, the technique can yield signals that depend monotonically on fuel-mixture fraction (Fox et al, 1998). The laser sheet sampled differx/h = 1.0 x/h = 3.0 x/h = 5.0 x/h = 10.0 Jet centre plane h FLOW Figure 2: Schematic of the swept compressionexpansion ramp (SCER) injector showing the positions of the laser sheet used in the experiment.