Experimental study of the thermal separation in a vortex tube Yunpeng Xue ⇑ , Maziar Arjomandi, Richard Kelso School of Mechanical Engineering, The University of Adelaide, South Australia 5005, Australia article info Article history: Received 17 June 2012 Received in revised form 9 October 2012 Accepted 16 December 2012 Available online 27 December 2012 Keywords: Ranque effect Ranque-Hilsch vortex tube Forced and free vortex Thermal separation Vortex flow abstract A vortex tube, a simple mechanical device capable of generating separated cold and hot fluid streams from a single injection, has been used in many applications, such as heating, cooling, and mixture separation. To explain its working principle, both experimental and numerical investigations have been undertaken and several explanations for the temperature separation in have been proposed. However, due to the complexity of the physical process in the vortex tube, these explanations do not agree with each other well and there has not been a consensus. This paper presents an experimental study of the flow properties in a vortex tube focusing on the thermal separation and energy transfer inside the tube. A better understanding of the flow structure inside the tube was achieved, based on the observed three-dimensional velocity, turbulence intensity, temperature and pressure distributions. The gradual transformation of a forced vortex near the inlet to a free vortex at the hot end is reported in this work. The calculated exergy distribution inside the vortex tube indicates that kinetic energy transformation outwards from the central flow contributes to the tem- perature separation. Experimental results found in this research show a direct relationship between the formation of hot and cold streams and the vortex transformation along the tube. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction From a single injection of compressed air, a Ranque-Hilsch vor- tex tube generates instant cold and hot streams at the opposite ends of the tube. Fig. 1 shows the structure of a counter-flow vor- tex tube, which consists of a straight tube with a port for tangential injection and exits at each end. With the tangential injection of compressed gas, the cold stream is exhausted from the central exit near the inlet, and the hot stream is exhausted from the peripheral exit at the other end of the tube. Xue et al. [1] summarised differ- ent explanations for the thermal separation in a vortex tube. The critical analysis of these explanations reveals that there has not been a well-accepted explanation for the temperature separation in a vortex tube so far. To identify the mechanism of thermal separation in a vortex tube, understanding of the physical process inside the tube is essential. Xue et al. [2] conducted a qualitative analysis of the flow behaviour in a vortex tube using flow visualization techniques, in which a flow recirculation, named the multi-circulation, was iden- tified, whereby part of the central flow moved outwards and returned to the hot end. Hence, they suggested that flow streams separate with different temperatures because of the sudden expan- sion near the inlet to generate the cold flow, and partial stagnation of the multi-circulation near the hot end to generate the hot flow. The flow properties inside the vortex tube have been studied by many researchers, in order to validate the internal flow behaviour. It was reported by Takahama [3] that the flow inside a vortex tube behaves as a forced vortex based on measurements of the swirl velocity. To explain the existence of the secondary flow in a vortex tube, Ahlborn and Groves [4] measured both azimuthal velocity and axial velocity. Their results suggested that the flow consisted of a Rankine vortex, with a forced vortex in the centre and free vortex in the periphery. Detailed measurements of the flow in a counter-flow vortex tube, including the 3-D velocity distribution, temperature and pressure gradients, were conducted by Gao et al. [5]. However, due to difficulties in obtaining experimental measurements inside the vortex tube, there has not been a consis- tent understanding of the flow behaviour, so further clarification of the flow properties is required. Energy transfer between different layers of flow inside the vortex tube is believed to be the main reason for the thermal sep- aration as discussed previously [1]. Therefore, an energy analysis needs to be included in a thorough investigation of the vortex tube. Saidi and Allaf Yazdi [6] derived a equation based on the thermo- dynamic principles to calculate the rate of entropy generation in a vortex tube and provided a new method to optimize the tube’s dimensions and operating conditions. Moreover, in their numerical study, Frohlingsdorf and Unger [7] reported that it is possible to analyse the energy separation by calculating the work done on fluid due to viscous shear. Dincer et al. [8,9] performed an analysis of the exergy performance of a vortex tube, in which the effects of 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2012.12.009 ⇑ Corresponding author. E-mail address: yun.xue@adelaide.edu.au (Y. Xue). Experimental Thermal and Fluid Science 46 (2013) 175–182 Contents lists available at SciVerse ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs