Emergence of small-scale mixing in a T-mixer

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The efficient mixing of fluids is key in many applications in chemical, mechanical, environmental and aerospace engineering. Typical examples range from chemical reactors to combustion chambers, but also the dispersion of contaminants in the ocean or atmosphere. The mixing rate is given by the surface of contact between the two fluids and the mass-diffusion coefficient. In most applications, turbulence sets the mixing rate, because eddies occurring at multiple scales strongly deform and convolute the surface between the fluids. In turbulence, the largest eddies are prescribed by the geometry, whereas the smallest possible eddies are found at the Kolmogorov scale. By contrast, the scalar concentration exhibits fluctuations down to the Batchelor scale, which in liquids is over an order of magnitude smaller than the Kolmogorov scale. As a consequence, in liquids, the small-scale properties of turbulent mixing remain poorly understood at a fundamental level, which prevents the scale-up of chemical reactions in many systems. In this proposal, mixing is experimentally investigated in a T-mixer setup with long inlets, which allow fully developed inflows. The mixing channel has a hydraulic diameter of four centimeters which results in a Batchelor scale of tenths of micrometers in the regime of interest. This will be resolved in the experiments with high-resolution laser-induced fluorescence. The research in this proposal will yield a better understanding of small-scale mixing in turbulence, and more specifically of how this emerges and is influenced by flow transitions as the Reynolds number increases.