Wang, Zheng; (2018) Incorporation of a vortex tube in thermal systems - refrigerants screening and system integrations. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The temperature separation effect (TSE) is a unique thermal phenomenon occurring in a vortex tube (VT). This creates the possibilities of incorporating a VT in various thermal systems to improve their overall system efficiency. Any improvement will be strongly dependent on the working fluid choices, VT geometric parameters, and the system configurations and operating conditions. However, there appears that no systematic approach for selecting the possible working fluid and for evaluating the performance of a VT when operating in a system is available. Therefore, this research aims at developing a systematic approach to screen possible choices of working fluids, and a system integration procedure to achieve optimal matching of the working fluid choice, the VT geometries and the operation conditions, based on using a combined thermodynamic and CFD simulation analysis. A 2-D CFD VT model, created using Ansys Fluent, is used to assess the influence of the VT boundary conditions on the TSE, and to provide detailed information on the flow velocities, temperature and shear stress distributions inside the VT, as well as the cooling/heating effect of the VT. The shape of refrigerant’s T-s diagram is initially used for grouping various refrigerants to either cooling or heating applications of VT. The fluid state at the VT nozzle exit is set as the criterion to identify the suitable VT entry regions on the T-s diagram for individual refrigerants. The thermal-physical properties including isentropic expansion exponent, J-T (Joule-Thomson) coefficient, thermal diffusivity, kinematic viscosity and density are employed to appraise the relative heating or cooling performance of individual refrigerants. One cooling and one heating system are chosen to illustrate the development and implementation of the proposed system integration procedure. In developing the procedure, a boundary line concept is introduced, which allows suitable VT entry conditions in a system be identified for cooling applications. An iteration procedure is designed to identify the best combination of the VT inlet pressure and degree of superheat for the heating applications for individual refrigerants. A guideline for re-selecting alternative refrigerants and re-dimensioning of VT for improving heating or cooling effect is presented, based on examining their thermal-physical properties under system conditions. The results show that the pressure drop in the VT plays an important role in determining the final heating effect. Key thermal-physical properties, such as thermal diffusivity and kinematic viscosity, are shown to be able to reliably assist the evaluation of the relative cooling/heating performance of different working fluids in closed VT systems. The proposed integration procedure is developed in such a way that it could be easily adapted for evaluation of different system configurations.

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