Numerical investigation of a swirl diffuser with a novel design using large eddy simulations

In the present study, a swirl diffuser with a novel design is proposed, in which three geometrical parameters, the angle of the guide vanes φ, the height of the convergent chamber h, and the radius of the diffuser outlet r, can be varied independently. Large eddy simulations were conducted to examine the airflow characteristics downstream of the swirl diffuser. Grid independence was obtained for the grid number 4000,000, and this result was validated by comparison with an experiment. The vorticities, the instantaneous and averaged velocities, and the evolution of the age of air were investigated. The turbulent kinetic energy budget and the momentum balance in the time-averaged Navier-Stokes equations were then studied in an attempt to clarify the mechanism of the swirl diffuser's high diffusing performance. With the increasing swirling strength, the pressure gradient force could not balance the centrifugal force in the radial direction. As a result, the vortex core expanded, generating turbulence. A non-dimensional swirl number parameter was suggested which can be used for determining the structure of the swirl diffuser airflow. To validate this non-dimensional parameter, different combinations of φ, h, and r were examined. It was found that only if the swirl number is the same, will the airflow structure be similar. The results of our study are likely to be useful for obtaining deeper insights about the swirl diffuser airflow, and for better understanding of the effect of the controlling parameter on the swirl diffuser's performance.