Mixing and solvent exchange near the turbulent/non-turbulent interface in a quasi-2D jet

Abstract

We inject with jet mixtures of ethanol and dissolved anise oil upward into quiescent water with jet Reynolds numbers, $500<R{e}_0<810$. Nucleation of oil droplets, also known as the ouzo effect, follows from the entrainment and mixing with the ambient water, in which the oil has much lower oil solubility than the initial jet fluid. We experimentally investigate the local concentration during solvent exchange near the turbulent/non-turbulent interface (TNTI) in the quasi-two-dimensional turbulent jet. Using a light attenuation technique, we measure the concentration fields for the solvent exchange case (the nucleated oil droplets) and the reference dye case (passive scalar).

Combining conventional and conditional mean profiles, we show that the nucleation of oil droplets is initiated near the TNTI upstream, and penetrates into the turbulent region as the jet travels downstream. Persistent nucleation not only sustains the scalar dissipation rate, but also leads to enhanced temporal fluctuations of the concentrations till the downstream (high) position. The probability density functions (PDFs) of the concentration exhibit pronounced bimodal shapes near the TNTI and positively-skewed curves toward the centerline, which also suggest that the oil droplets nucleate across the jet upon mixing.

In addition to the qualitative investigation, we also describe the process more quantitatively. Based on the idea that the concentration field of the nucleated oil micro-droplets is linked to the ethanol concentration field running in the background, we estimate the nucleation rate near the TNTI using a control volume approach. We model the concentration field of the nucleated oil using that of the passive scalar and the ternary phase diagram of a water, ethanol, and anise oil mixture.

This study extends our previous work on the mean field, revealing more details of the turbulent statistics induced by solvent exchange. The findings shed new light on the interplay between mixing and nucleation in a quasi-2D turbulent jet. We also provide the first modeling of solvent exchange in turbulent flows with a simple model based on ethanol concentration field and the phase diagram.