Evolution of a Jet-in-Coflow

Abstract

The jet-in-coflow is a two-stream configuration having engineering applications in combustors and gas turbine engine exhausts. In practical systems, the coflow generates a boundary layer of the outer wall of the jet pipe and may also have a certain level of turbulence. In the current work, the evolution of this flow configuration is studied using an air-air turbulent jet in a low turbulence coflow (turbulence intensity < 6%), and the 2D velocity field is measured by planar particle image velocimetry. Cases of varying coflow ratio (ratio of coflow velocity to jet velocity) of 0 (turbulent free jet), 0.09, 0.15, and 0.33 are generated by keeping a constant velocity jet (Re = 14000) and varying the coflow velocity. The trends of jet centerline properties such as velocity decay, jet spread, and jet momentum of jet-in-coflow cases, scaled to represent an equivalent free jet, show deviations from that of the turbulent free jet. The radial profile of mean velocity shows a region of velocity deficit, compared to a turbulent free jet, on the coflow side in the jet-in-coflow cases. In contrast, the turbulence intensity and Reynolds shear stress profiles show an enhanced peak near the interface for the jet-in-coflow cases. Further, conditional statistics were extracted by detecting the interface between the jet and the surroundings, wherein the same trends are observed. The low turbulence levels of the coflow have little effect on the jet/coflow interface, as seen by the conditional enstrophy diffusion and tortuosity compared to a turbulent free jet. The differences at the jet/coflow interface of a jet-in-coflow with respect to a turbulent free jet are attributed to the boundary layer initially developed by the turbulent coflow over the pipe generating the jet, and these are seen throughout the near-to-intermediate field (0≤x/D≤40).