Analysis of the flow mixing and energy separation in a Ranque–Hilsch vortex tube based on an area ratio study

Abstract

Ranque-Hilsch vortex tube is a pressure-driven thermal device that is an essential component of waste pressure utilization, especially in gas industry. However, the lack of an effective optimization design method and relative low efficiency restrict its development. In this study, the flow field and energy separation of five area ratios (AR) from 0.02–0.32 with the same inlet areas were analyzed based on the AR study of a vortex tube through experiments and numerical simulation. To determine the correlation of area ratio and energy separation, the basic flow field distributions are obtained and the flow structure is analyzed. The results show that for the fifth tube, denoted by AR5, the inlet mass flowrate remains largely constant (min = 9.27–9.24 g/s) with extremely small changes in the back pressure at hot exit (Ph = 11.2–18 kPa); in contrast, for AR1, the back pressure at hot exit (Ph = 10–148 kPa) and inlet mass flowrate (min = 6.8–4.26 g/s) vary significantly with increasing cold mass fraction. Moreover, despite using the same moderate cold orifice ratio of 0.48 for all the five tubes, back flow can occur at the cold exit even at an extremely large cold mass fraction of 0.82 for AR5. In addition, various flow structures and mixing and their corresponding causes inside the vortex tube are summarized. The comparison of the flow and thermal behaviors under the five ARs reveals opposite working patterns of reverse flow boundary and off-optimization under the non-flow mixing condition. These conflicting working patterns of reverse flow boundary can be attributed to the multi-parameter interaction of vortex tube. Thus, the criteria for tube optimization may unilaterally fail unless the precession frequency is studied. Finally, a performance evaluation index that quantifies the energy separation performance is proposed. The index serves to improve the tube optimization criteria and further develop the optimization of vortex tubes.