Effects of turbulence on the mechanism of Ranque–Hilsch effect at low Reynolds numbers

A Ranque–Hilsch vortex tube (RHVT) is a device in which high-pressure compressed air is separated into high- and low-energy streams solely through the high-speed swirling flow field formed by the gas within a circular tube. The mechanism of energy separation in RHVT has been studied using the data obtained by direct numerical simulation at low Reynolds numbers, which are typically 1∕100 of that of the experiments. The variation of the total temperature was tracked along the time-averaged streamlines. It has been shown that the energy separation occurs mostly in the counter-flow region, where the total temperature of the gas flowing to the cold outlet decreases. We found that the correlation between the turbulent fluctuations of the velocity and the total enthalpy gradient is responsible for the energy separation. In other words, it is the positive correlation between the turbulent fluctuations of the velocity and the total temperature gradient that makes total temperature drop. Energy separation occurs also when the flow is steady, although the temperature difference is small. In this case, the work by the axial viscous shear stress makes a major contribution to the energy separation.