Experiments on turbulent mixing and chemical reactions in a liquid mixing layer

Abstract

The processes of entrainmentt and mixing are investigated in reacting and non-reacting, uniform density, liquid mixing layers over a wide range of Reynolds numbers. In non-reacting cases, a passive scalar technique is used to measure the probability density function (pdf) of the composition field. Chemically reacting experiments employ a diffusion-limited acid-base reaction to directly measure the extent of mixing. The diagnostics are based entirely on the laser induced fluorescence technique. The fluorescence signal is measured by self-scanning linear photodiode arrays using high speed, real-time computer data acquisition. The system is capable of yielding species concentration data with a spatial resolution of 100 µm and a temporal resolution of 0.8 msec. Results show that the vortical structures in the mixing layer initially roll up with a large excess of high speed fluid in the cores. During the mixing transition, not only does the amount of mixed fluid increase, but the composition also changes. It is found that the pdf of the mixed fluid, above the mixing transition, is quite uniform across the entire transverse extent of the layer. Furthermore, it is asymmetric and biased toward the high speed fluid. Experimental evidence indicates that the turbulent transport, in the cases studied, is dominated by large scale structures and is not adequately described by standard gradient-diffusion models. The fluid composition in the mixing layer, suggested by the present results, is in qualitative agreement with many aspects of the recent theoretical model of Broadwell and Breidenthal. The amount of product formed in the layer is compared to Mungal's measurements in gas, and, it is observed that the liquid layer has about 50% less product. The mean concentration of the mixed fluid, for a mixing layer at a velocity ratio of 0.38, becomes constant at 0.57 above the mixing transition. This corresponds to an entrainment ratio of 1.32, in agreement with the gaseous result of Konrad at the same velocity ratio.