Validation of LES Predictions of Scalar Mixing in High-swirl Fuel Injector Flows

Abstract

A combined experimental and computational study is described, aimed at validation of LES predictions of the flow and mixing fields created by a fuel injector which incorporates a single, radially-fed annular swirl stream surrounding a central axial jet flow. The focus of this work was on turbulent mixing processes, so an isothermal (water flow) experiment was carried out, using a representative injector geometry operated at appropriate Reynolds and Swirl Numbers. Combined SPIV and PLIF instrumentation allowed simultaneous instantaneous measurements of velocity vector and a conserved scalar introduced into the fuel stream. For the first time in such a flow, all 3 components of the turbulent scalar flux were available for validation of LES-based CFD predictions. A careful assessment of experimental errors was carried out. Very close to the injector exit plane (x/DS < 0.1), the measured data were prone to larger errors (~20%). Further downstream (x/DS = 0.2 − 1.5), spatial filtering errors were much smaller (~5 − 10%). Comparison of LES predictions with experimental data showed very good agreement for both 1st and 2nd moment statistics, as well as spectra and scalar pdfs. It is particularly noteworthy that comparison between LES computed and measured scalar fluxes was very good; this represents successful validation of the simple (constant Schmidt No.) SGS model used for this complex and practically important flow.