Computational study of the performance of different gases in convergent-divergent type vortex tube

A vortex tube is a well-known thermofluidic device used for the cooling and heating purposes. The thermal separation performance of vortex tube is based on its geometrical configuration. It has already been observed in the previous literature of CDVT that the performance of vortex tube can be improved up to 56% by replacing the straight tube geometry with convergent-divergent type tube and the working gas of air with helium. However, no investigation has been done previously for other working gases in CDVT. Therefore, exploring the influence of other working gases on the thermal separation performance of CDVT is also very important. In order to extend the previous work and comprehend the impact of various gas properties on the flow phenomena and thermal separation performance of Convergent-Divergent vortex tubes (CDVT), this paper presents an extensive 3D CFD study using six working gases: helium, neon, argon, nitrogen, carbon dioxide, and air. This CFD study is conducted for a broad range of cold mass fractions. CFD simulation is done for the thermal separation analysis and the investigation of variation in fluid flow six different working gases in CDVT. Simulation is done in ANSYS Fluent with 3D geometry of fluid domain considering the structured hexahedral mesh and standard κ–ε turbulence model. While the nature of the flow is found unaltered for various gases, it is found that the values of velocity and temperature distributions inside CDVT vary for different gases because of the variations in the molecular weights and specific heat ratios of the gases. While heating and cooling capacities are greater for larger values of specific heat and higher temperature separations, both hot and cold temperature separations found to increase with increasing specific heat ratios. The COP_ref. and COP_H.P. of CDVT is found considerably dependent on gas type and cold mass fraction.

An exergy study shows that physical exergy at the CDVT outlet is found to be less than the kinetic exergy. Kinetic exergy at the hot outlet is found to be lesser than the cold outlet kinetic exergy. However, kinetic exergies at both outlets of CDVT get lost to the environment. Actual exergy efficiency at the hot outlet is found to be more than at the actual exergy efficiency at the cold outlet. The maximum actual exergy efficiencies are obtained in CDVT when operating with Ar gas, which is 2.25% at the cold outlet and 18% at the hot outlet of CDVT.