MIXING AND REACTION IN CURVED LIQUID SHEAR LAYERS

The concentration field of mixing layers subject to stabilizing and de stabilizing streamwise curvature was investigated at post-mixing-trans ition conditions. A set of operating conditions was implemented, ident ical to those at which straight layers were previously investigated in the same facility, in order to compare the effects of hydrodynamic in stabilities upon scalar mixing. Quantitative imaging of planar laser-i nduced fluorescence was used for (i) passive scalar measurements, and (ii) chemical product measurements. Similar to the straight mixing lay er, the results for the curved layers show that beyond the mixing tran sition the layer continues to evolve, and undergoes a small change in its scalar structure. At conditions just past the mixing transition bo th stable and unstable layers have average mixed-fluid compositions wh ich are uniform across the layer, and average chemical product concent ration profiles which are symmetric. At more fully developed condition s, the scalar field evolved: the average mixed-fluid concentration dev eloped a small lateral variation, while the chemical product concentra tion profiles became asymmetric. Similar to the straight layer, the mi xture-fraction PDF is believed to be of the tilted type for the most f ully developed layer examined, with the marching PDF being a poor repr esentation. Consistent with previous investigations, the growth rate o f the unstable layer was found to be higher than that of straight or s table layers. The most important result is that the measured mixing ef ficiency of all the layers (curved and straight) was found to be the s ame: both the total mixed-fluid composition, and the volume fraction o f mixed fluid were the same for all unstable, stable, and straight lay ers. The amount of mixed fluid (and of chemical product formed) was la rger for the unstable layer, but always in a fixed proportion to the l ayer's thickness. The lack of increase in the mixing efficiency for th e unstable layer is surprising, given that previous hydrodynamic measu rements had shown enhanced turbulent transport for the unstable case. Thus, for all liquid shear layers studied, the rate of scalar mixing a ppears to be directly proportional to the entrainment rate (which esse ntially determines the layer's growth rate), and not to any hydrodynam ic measures.