Effect of nozzle spacing on flow mixing and development of triple rectangular free jets arrayed in a line

Abstract

The effect of nozzle spacing on the flow characteristics of triple rectangular free jets was studied experimentally and numerically. The nozzle spacing ratio S/d_e, where d_e is the equivalent diameter of a rectangular nozzle with an aspect ratio of 2, was varied from 2.06 to 4.13. The Reynolds number based on the nozzle exit velocity and the equivalent diameter was 1.5 × 10⁴. The mean and fluctuating velocities were measured using a constant-temperature hot-wire anemometer with an X-type probe. Three-dimensional numerical simulations of triple rectangular free jets were also performed using Open FOAM 5.0, and the numerical results depicted the vortical structures in the jets. Triple rectangular free jets with a small nozzle spacing ratio, specifically S/d_e = 2.06, merge in the vicinity of the nozzle. The mean velocity and the turbulent kinetic energy near the jet centerline become higher than those with a large nozzle spacing ratio. An axis-switching phenomenon occurs after the jets merge, and the whole cross-sectional shape of the triple rectangular free jets changes to vertically long elliptical free jet. Farther downstream, the elliptical free jet becomes round. The flow characteristics of each of the triple rectangular free jets with a large nozzle spacing ratio, specifically S/d_e = 4.13, are similar to those of a single rectangular free jet. Downstream, the streamwise mean velocities of triple rectangular free jets become identical for the dimensionless coordinates of x/S and y/S.