Mechanism of Ranque–Hilsch effect at low Reynolds numbers

The mechanism of energy separation in the Ranque–Hilsch vortex tube 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. The rate of change of the total enthalpy, which is proportional to the total temperature, was decomposed into four terms. It has been shown that the term due to 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. • The mechanism of energy separation in the Ranque–Hilsch vortex tube was revealed. • The analysis is based on the data obtained by direct numerical simulation. • Energy separation occurs in the counter-flow region, where total temperature drops. • Turbulence in the counter-flow region acts as a heat pump. • The fluctuations of velocity and total temperature gradient cause energy separation.