Emergence of small-scale mixing in a T-mixer

流体的有效混合在化学、机械、环境和航空航天工程中的许多应用中都是关键。典型的例子包括从化学反应器到燃烧室，但也包括污染物在海洋或大气中的扩散。混合速率由两种流体的接触面和质量扩散系数确定。在大多数应用中，湍流设置了混合速率，因为在多个尺度上发生的涡会强烈变形并使流体之间的表面卷曲。在湍流中，最大的涡流是由几何学规定的，而最小的涡流则是在科尔莫戈洛夫尺度上找到的。相比之下，标量浓度表现出向下波动的Batchelor标度，在液体中，它比Kolmogorov标度小一个数量级以上。因此，在液体中，湍流混合的小尺度特性在基本水平上仍然知之甚少，这阻碍了许多系统中化学反应的放大。在该方案中，混合是在T型混合器装置中进行的实验研究，该装置具有长入口，允许充分发展的入口。混合通道的水力直径为4厘米，这导致在感兴趣的区域中的巴谢勒刻度为十分之一微米。这个问题将在高分辨率激光诱导荧光实验中得到解决。这一方案的研究将更好地理解湍流中的小尺度混合，更具体地说，随着雷诺数的增加，这种混合是如何发生的，并受到流动转变的影响。

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.