Molecular-mixing measurements and turbulent-structure visualizations in a round jet with tabs

An experimental study of the use of tabs for mixing enhancement in the near field of round water jets at Reynolds numbers between $5.2\times10\sp3$ and $3.2\times10\sp5$ is presented. A non-intrusive experimental technique has been successfully developed to quantify the time-average of concentration distributions in natural jets, and the integrated chemical product as a function of the streamwise distance in jets with and without tabs. The effects of the tab, the Reynolds number, and the chemical equivalence ratio on molecular mixing have been studied. The chemical product present six diameters downstream of the jet exit increases 35 percent by the addition of two tabs and 45 percent by that of four tabs at a Reynolds number of $1.49\times10\sp5.$ Hence, four tabs enhance molecular mixing more than two tabs do, although two tabs have more effect on the jet-velocity decay than four tabs do. The product formation rate is fairly constant with respect to streamwise distance for turbulent jets with zero, two, and four tabs. The amount of mixed product is determined by both the molecular mixing and the equivalence ratio for the chemical reaction. Molecular mixing and the effects of the tabs are found to be independent of Reynolds number, once the jet is turbulent, although they are dependent on Reynolds number when the jet is transitional. The effects of the tabs are less in a transitional jet than in a turbulent jet. Laser-induced fluorescence was used to visualize detailed structures in the near field of the jets with zero, one, two, and four tabs. Instability of the separation region that exists behind a tab could be the source of the streamwise vortices