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Title:Vortex control of two-phase refrigerant flow in nozzles and its application in ejector vapor compression cycles
Author(s):Zhu, Jingwei
Director of Research:Elbel, Stefan
Doctoral Committee Chair(s):Elbel, Stefan
Doctoral Committee Member(s):Hrnjak, Predrag S.; Jacobi, Anthony M.; Wang, Xinlei
Department / Program:Mechanical Sci & Engineering
Discipline:Mechanical Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Two-phase refrigerant
Flow control
Vortex flow
Pressure profile
Motive nozzle
Transcritical R744
Vortex control
Ejector efficiency
Coefficient of performance
Computational fluid dynamics
Abstract:Vortex control is a novel two-phase convergent-divergent nozzle restrictiveness control mechanism which achieves flow control by adjustable nozzle inlet vortex strength. The nozzle can become more restrictive with increasing inlet vortex strength. Vortex control can potentially provide flow control with less sacrifice of nozzle efficiency, which is important for two-phase ejector cooling cycle performance. It is also less vulnerable to clogging since the flow control is achieved without changing the flow area. Different nozzle flows with initially subcooled R134a were experimentally investigated. The effects of nozzle geometry variations and operating conditions were studied. Flow visualizations were performed to gain further understanding of the complex two-phase flow characteristics in the vicinity of the nozzle throat. A maximum mass flow rate control range of 42% has been obtained by vortex control, which appears to be large enough to be suitable for numerous technical applications. To understand the underlying mechanisms behind the vortex control effect, static pressure profiles of vortex flashing R134a flow expanded through nozzles under various conditions have been measured. A 1D model for the estimation of vapor qualities in the initially subcooled flashing nozzle flow based on the measurement results was also developed. It was found that after the introduction of the inlet vortex to the initially subcooled flashing nozzle flow, the pressure drop across the divergent part of the nozzle has been increased, which is caused by the increased vapor generation rate in the divergent part. The elevated nozzle throat pressure results in the nozzle behaving like being more restrictive. However, this is achieved without variation of the physical nozzle geometry. The influence of inlet vortex on the nozzle restrictiveness and the nozzle pressure profile is significant when the inlet vortex is applied to flashing nozzle flow with single-phase liquid at the nozzle inlet. The nozzle isentropic efficiency can be significantly increased by applying inlet vortex, which could be beneficial to ejector performance when the vortex nozzle is used in ejectors. The vortex nozzle divergent part length as well as the divergent angle seem to be the most important geometric parameters that need to be appropriately sized in order to achieve satisfactory vortex control range and high nozzle isentropic efficiency. 3D CFD simulation of vortex flashing R134a flows in convergent-divergent nozzles has been conducted. Good agreement was found between the simulation and experimental results. When there is no vortex applied, void fraction at the nozzle center remains low. Due to the much lower density of the vapor compared to the liquid, when vortex is applied, vapor bubbles are driven towards the nozzle center. The applied vortex significantly increases the interphase mass transfer near the nozzle outlet with more uniform interfacial area per unit volume and better utilization of liquid superheat at the nozzle center for evaporation. Thus, after the introduction of inlet vortex, more vapor is generated in the divergent part of the nozzle such that the nozzle outflow vapor quality is much closer to thermodynamic equilibrium. As a result, the pressure drop across the divergent part of the nozzle is increased due to the increased vapor generation. There is negligible vapor content upstream of the throat even though the pressure is already below saturation pressure. Flashing starts near the nozzle throat. Finally, the influence of vortex control on transcritical R744 ejector and cycle performance has been experimentally investigated. Vortex control was applied to ejectors with different geometric parameters. The performance of ejector and ejector cycle with vortex control was compared with those of the other motive flow control methods (series expansion valve control and needle control). It was found that the total work recovery efficiency of ejector with vortex control can be better than series expansion valve control and is close to needle control. The decrease of suction nozzle length significantly improves the performance of ejectors with vortex control. Shorter motive nozzle divergent part is more favorable for ejector with vortex control in terms of the work recovery efficiency. The decrease of motive nozzle divergent part angle results in increase of ejector work recovery efficiency. Vortex control can be used to improve system performance by adjusting the high-side pressure of the transcritical R744 ejector cycle. Under off-design conditions, system capacity and COP can be improved by 11.0% and 8.1%, respectively, by applying vortex control.
Issue Date:2020-01-10
Rights Information:Copyright 2020 Jingwei Zhu
Date Available in IDEALS:2020-08-26
Date Deposited:2020-05

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