Turbulent flow characteristics for enhanced thermal mixing in square and triangular jets

In this study, turbulent flows and the associated mixing characteristics are numerically investigated for noncircular jets. The selected nozzle shapes are round, square, and equilateral triangular, and a passive scalar is adopted to quantify the flow mixing. To achieve a constant mass flow rate, the same cross-sectional area is maintained, and the Reynolds stress model (RSM) is applied to show the turbulence-driven secondary flows. For Re = 21,000, the flow fields of noncircular jets are compared to those of the round jet. After 5 < x/D_e < 10, the half-widths of the round, square, and triangular jets increase linearly. This self-similar structure of the jet flow is rapidly developed for the noncircular jets. Compared to that for the round jet, the spreading rate for square jet increases by 20% and 27% for the triangular jet. From the mixedness values, the mixing is estimated to be enhanced by 16.7% for the square jet and 22.9% for the triangular jet. To elucidate the flow characteristics related to this feature, the scatter plots and streamlines of the secondary flows, vorticity transport equation, and anisotropy-invariant map are discussed. From the results, a balanced relationship between the entrained flow of ambient fluid and the outward spreading flow is observed, and the related turbulence structure is confirmed.