CFD Study of Merging Turbulent Plane Jets

Abstract

A series of two-dimensional computational fluid dynamics (CFD) simulations of merging turbulent plane jets were conducted to establish the point at which the jets can be regarded as having merged to form a single jet. Simulations were conducted to quantify the influence of the nozzle width, jet axis separation, Reynolds number, and the relative jet strength on the distance from the source to the merger point. The simulation results were then compared with experimental results published in literature. As the jet fluid exited the two nozzles, the shear layer between the jet and the ambient fluid drove the ambient fluid entrainment into the jets. However, a finite volume of fluid trapped was between the two jets, and a recirculation zone with two counterrotating vortices was established. This vortex pair reduced the pressure between the jets, drawing them together more rapidly than theoretical models predicted. The simulation results indicated that equal-strength jets merged at a distance downstream of 1.4 times the initial nozzle centerline separation provided that the nozzle-separation-to-nozzle-width ratio was greater than 18. As the source momentum flux of one jet decreased relative to the other, the relative distance to merger also decreased. However, for source momentum flux ratios between 0.2 and 0.9, the merger distance was only a very weak function of the momentum flux ratio. The CFD-measured merging distance was significantly closer to the jet nozzles than current models predict. Therefore, the mixing of pollutants by using parallel plane jets is less efficient than currently thought.