Transient Numerical Study Of The Swirl Flow Characteristic In Ranque-Hilsch Vortex Tube

Vortex tube, a simple thermal device without any moving part, can separate compressed gas into two gas flow with a significant temperature difference. However, due to the swirl flow through the tangential nozzle with a high rotation speed which contributes to the high turbulence intensity of the flow field in vortex tube, a consistent view of temperature separation (or energy separation) mechanism has not been gotten since the vortex tube had been invented in the 1930s.Obviously, the swirl flow dominates the flow field in the vortex tube, but the study about the swirl characters is not enough. While in other devices where the swirl flow also occurs, the vortex breakdown in the high intensity swirl flow can inspire the precessing vortex core (PVC) located around the central reverse flow zone and the PVC is regarded as a coherent structure in swirl flow, which deviates the tube center, precesses the tube axis and affects the swirl flow itself bitterly. In the history of vortex tube flow field study, the investigation on the swirl flow characteristics was not enough presented especially on the unsteady flow field development aspectsand the study on the large scale coherent structure of precessing vortex core can contribute to a better understanding about the flow structure in vortex tube, which is hoped to get a more reasonable theory about the energy separation.In this paper, an unsteady three-dimensional model was adopted with Reynold Stress Model to close the Navier-Stokes transport equation and the numerical simulation was performed on FLUENT 15.0 to study the swirl flow in the vortex tube. Time-averaged typical quantities in the vortex tube swirl flow field were derived, such as time-averaged total temperature field and energy separation results. Besides, the time-averaged axial and tangential velocity field were obtained to present the basic vortex structure type. When coming to the unsteady characters in vortex tube, the swirl number was calculated for the first time to confirm the high intensity swirl flow in the vortex tube. Several methods were performed to show the periodic oscillation in the transient velocity field and an iso-vorticity surface was presented to show the spiral flow structure in the vortex tube. A series development of axial velocity flied on a meridian plane from the very beginning at transient calculation were analyzed to show the vortex breakdown formation. Some evidences has shown that the vortex core deviates the tube center and precesses around the tube axis at a certain frequency. Based on the Fast Fourier Transformation frequency domain analytical approach, the precessing vortex core can affect the flow field around it, even the flow field at the lower reaches. Besides, the Strouhal number was calculated with the peak frequency. At last based on the swirl characters in the vortex tube, an explanation of energy separation was proposed.