Numerical investigation of passive scalar transport and mixing in a turbulent unconfined coaxial swirling jet

Abstract

This paper presents the numerical results of a study on passive scalar mixing in coaxial jets under the influence of swirl. Two cases with different swirling strengths are considered and compared to a non-swirling case. Each jet is injected with an individual scalar to better understand the mixing between jets. The case with intermediate swirling strength exhibits faster centerline decay and radial spread in the mean streamwise velocity in the downstream region when compared to the non-swirling case. The case with strong swirling exhibits the formation of an internal recirculation zone. The mean scalar distributions indicate a better spreading rate of scalars in the intermediate swirling case in the far downstream region, whereas the strong swirling case exhibits maximum spread far upstream. These results are confirmed based on entropy, which is a measure of the diffusion of scalars. Furthermore, the ambient fluid reaches the centerline earlier with the introduction of swirl as a result of an increase in the entrainment rate caused by swirl. The enhanced presence of ambient fluid is observed to influence scalar fluctuations. Otherwise negligible turbulent azimuthal fluxes of both scalars exhibit higher values in the swirling cases. The positive correlation between scalar fluctuations is more evident farther upstream in the intermediate swirling case compared to the non-swirling case. This phenomenon is a result of the enhanced presence of ambient fluid. However, in the strong swirling case, this process takes place far upstream based on additional effects of the internal recirculation zone. The joint probability density function of scalar fluctuations at the leading centerline stagnation point of the internal recirculation zone contains two distinct peaks, indicating the presence of flapping between inner and outer jet scalars caused by the oscillating stagnation point.