Multi-scalar mixing in turbulent coaxial jets

Abstract

Although natural and industrial flows often transport and mix multiple scalars, relatively few studies of turbulent multi-scalar mixing have been undertaken. In the present work, a novel three-wire thermal-anemometry-based probe – capable of simultaneously measuring velocity, helium concentration and temperature – is used to investigate the evolution of multiple scalars ( $\phi _1$ , $\phi _2$ , $\phi _3$ ) and velocity in turbulent coaxial jets. The jets consist of (i) a centre jet containing a mixture of helium and air ( $\phi _1$ ), (ii) an annular jet containing pure (unheated) air ( $\phi _2$ ) and (iii) a coflow of (pure) heated air ( $\phi _3$ ). Axial measurements are made at three different momentum flux ratios ( $M=0.77, 2.1, 4.2$ ). Increasing $M$ was observed to result in complex, competing effects. Larger momentum flux ratios cause the potential core of the centre jet to decrease in size, greater scalar fluctuations and more rapid correlation of $\phi _1$ and $\phi _2$ . However, at the same time, certain statistics, including those describing the velocity field, evolve more slowly. Moreover, the flow near the beginning of the fully merged region appears to be less mixed at higher values of $M$ . The present work finally demonstrates that differences can be observed in the evolution and mixing of coaxial jets between those in which $M<1$ and those in which $M>1$ , thereby presenting an opportunity by which the mixing process in coaxial jets may be controlled.