The Effect of Tube Curvature on Temperature Separation Efficiency of Ranque-Hilsch Vortex Tube

Abstract

Vortex tube that splits a single compressed gas stream into two separate hot and cold streams had been successfully used for spot cooling, and refrigeration. Significant temperature gradient exists between hot and cold stream ends that could be utilized for power generation using thermo-electric generators. Distance between hot and cold ends could be vital for small inaccessible down-hole well locations which may require the use of curved vortex tubes. Efficiency of vortex tube depends on temperature difference between hot and cold ends. In this work, effects of tube curvature on temperature separation efficiency are investigated through numerical simulations. Numerical models of straight and curved vortex tubes are developed in a commercial computational fluid dynamics package Ansys-fluent®. For the curved tube, multiple curvature angles are used to analyze the effects of curvature on velocity and temperature fields inside the vortex tube. The standard κ − ε turbulence model is used to model three-dimensional turbulence. The cold stream mass fraction is varied by controlling hot exit pressure. The numerical results for 110° curved vortex tube are validated through published experimental data and are found to be in good agreement. It is found that the curvature has affirmative results on temperature separation efficiency as compared to straight tube. This is mainly due to the energy separation phenomenon governed by the multi-circulation loop extension and multiple vortex formation in curved vortex tubes. Curvature angles of 180° and 270° have similar effects on the vortex tube where the maximum ΔTc obtained is 15.7 K which is about 5.3% higher than the straight vortex tube. The temperature separation ΔThc values for curved tubes are comparable with straight tube, the maximum being 25.2 K for the 150° curved vortex tube which is about 0.8 per higher than the straight tube. The temperature separation efficiency for curved vortex tubes with curvature angles larger than 150° is found to be higher than straigt tube, the maximum value being 8.7% for the 270° curved tube. A profound investigation of the effects of curvature on energy separation phenomenon in a vortex tube had been lacking and this research attempts to fill that gap. This novel work is expected to provide insight into the energy separation mechanisms in vortex tubes and lead the way to their use in thermo-electric power generation.