Performance characteristics of a divergent vortex tube

Abstract

In this paper performance characteristics for a Ranque-Hilsch vortex tube are presented. The vortex tube was of divergent design and optimised for both size and performance, to be used to supply cold air to turbomachinery experiments with feed air in the supply pressure range 3.0–7.0 bar, and with overall feed mass flow rates in the range 0.05–0.14 kg/s. Optimum fixed parameters selected from the literature were: the ratio of cold-exit diameter to tube inlet internal diameter (0.50); the ratio of combined inlet-nozzle throat area to the tube inlet internal cross-sectional area (0.22); the vortex tube half-angle (3°); the number of inlet nozzles (6); the half-angle of the hot exhaust conical splitter (45°) and the ratio of tube length to tube inlet internal diameter (20). Free parameters that were investigated were: the overall total-to-static pressure ratio between inlet and cold-exit conditions (3.0, 5.0, and 7.0) and the cold-gas mass fraction (0.0–1.0). Comprehensive experiments were performed to characterise the vortex tube performance in terms of the following parameters: non-dimensional cold and hot-outlet temperatures; non-dimensional cold and hot-gas energy separation; isentropic efficiency referenced to cold-gas expansion; and coefficient of performance (COP). A transient heat flux correction technique was implemented to correct for unsteady heat flux terms from the working gas to the vortex chamber walls. Joule-Thomson effects were accounted for by introducing a corrected inlet total temperature. Cold-exit and hot-exit temperature separations were found to increase with pressure ratio. Non-dimensional cold-exit temperature had a minimum at cold-gas mass fraction of approximately 0.40. Non-dimensional hot-exit temperature increased approximately linearly with cold-gas mass fraction. COP decreased with increasing pressure ratio and had a peak value at cold-gas mass fraction approximately 0.60. Isentropic efficiency was relatively insensitive to pressure ratio and had a peak value at cold-gas mass fraction approximately 0.23. In the range investigated the lowest non-dimensional cold-exit temperature (normalised by inlet temperature) was 0.91, the highest coefficient of performance was 0.097, and the highest isentropic efficiency was 0.23. Full details of the design are given, allowing implementation by other researchers.