Experimental study on the Ranque-Hilsch vortex tube

Abstract

The Ranque-Hilsch vortex tube cooler (RHVT) has been investigated in the Low Temperature Group at Eindhoven University of Technology. The research was focussed on a thorough experimental investigation of the flow inside the system at a large variety of experimental conditions. Three measurement techniques (cylinder type Pitot Tube (CPT), single-probe hot-wire anemometry (SPHWA), thermocouple (THC)) have been designed and calibrated. The CPT is used to measure the pressure and two velocity components. The SPHWA is used to measure the mean-three dimensional ?V , the average fluctuation dav, the intensity of the fluctuation, the flatness and skewness and the spectral analysis. The THC is used to measure the recovery temperature. Two set-ups have been designed for the investigation: a small set-up with one inlet nozzle, and a larger set-up with 8 inlet nozzles distributed around the interference. For both set-ups, there are several access ports in the wall of the vortex tube along its length. We proved that the three measurement techniques can be used for the investigation of the flow properties inside the RHVT. The measured velocity distribution inside the RHVT shows a profile that, in the center region, is a solid-body rotation, and in the periphery region, tends to a potential vortex motion. The root-mean-squared (RMS) value of the radial velocity component has a maximum at the center and decreases with the radius. In the periphery region the RMS of the radial velocity is almost zero. The maximum RMS value of the radial velocity has the same order of magnitude as the average fluctuation, which indicates that it is due to the wobbling of the flow pattern and the fluctuation. The relative intensities of the fluctuation in the center and near the wall are more than 20%. The flatness and skewness of the turbulence indicate that, in the center and near the wall, the turbulence inside the RHVT is random and non-Gaussian, while in the intermediate region, the turbulence is ordered and Gaussian. Near the vortex chamber in the center region, the recovery temperature is constant. Furthermore, we proved the existence of a recirculating flow inside the RHVT and suggested the existence of acoustic phenomena. The parameters of both these phenomena depend on the inlet flow, the cold fraction, and the geometry. The acoustic phenomena near the entrance are very strong, and damped towards the hot end. With a thermodynamic analysis, relationships between the three temperatures (Tin, Th, Tc) and the pressures (pin, ph, pc) are derived using an irreversibility factor £ir. It is found that the maximum irreversibility factor depends on the ratio pin/pc of the inlet pressure pin and the cold exit pressure pc. A model, introduced by Ahlborn et al. is analyzed. The assumptions, made by Ahlborn et al., have been discussed. A modification of Ahlborn’s model (MAM) has been described. The comparison between the results of the two models shows that the predicted temperature differences with the modified model are smaller than those predicted with the original Ahlborn model. The results of the modified model agree better with the results of the experiments than the original model. The modified model is still an empirical model. So we expect that turbulence and acoustics play more important roles in the mechanism of the energy separation inside the RHVT.