CFD simulation of vortex flashing R134a flow expanded through convergent-divergent nozzles. (April 2020)

• Record Type: Journal Article
• Authors: Zhu, Jingwei; Elbel, Stefan
• Is Part Of: International journal of refrigeration. Volume 112(2020)
• Page Start: 56
• Page End: 68
• Publication Date: 2020-04
• Subjects: Nozzle -- Two-phase refrigerant -- Flow control -- Vortex flow -- Computational fluid dynamics -- Ejector; Tuyère -- Frigorigène diphasique -- Régulation du débit -- Écoulement en vortex -- Mécanique des fluides numérique -- Éjecteur; Refrigeration and refrigerating machinery -- Periodicals; 621.56
• DOI: 10.1016/j.ijrefrig.2019.12.005
• Languages: English
• ISSNs: 0140-7007

Abstract

Highlights: CFD simulation of vortex flashing R134a nozzle flow has been conducted. Good agreement was found between the simulation and experimental results. Inlet vortex increases the pressure drop across the nozzle divergent part. Inlet vortex increases the vapor generation rate in the nozzle divergent part. Inlet vortex can increase the nozzle isentropic efficiency and outflow velocity. Abstract: Vortex control is a novel two-phase convergent-divergent nozzle restrictiveness control mechanism. The flow control is achieved by adjustable nozzle inlet vortex strength. The underlying mechanism behind vortex control is still unclear. In this study, 3D CFD simulations of vortex flashing R134a flows in convergent-divergent nozzles have been conducted. Good agreement was found between the simulation and experimental results. When there is no vortex applied, the void fraction at the nozzle center remains low. When the 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 superheats at the nozzle center for evaporation. Thus, after the introduction of inlet vortex, more vapor is generated in the divergent part of the nozzle. As a result, the pressure drop across the divergent part of the nozzle is increased. There is negligible vapor content upstream of the throat even though the pressure is already below saturation pressure. When the inlet vortex is applied, with elevated pressure at the nozzle throat and constant inlet conditions, the pressure difference across the nozzle convergent part decreases and therefore the nozzle mass flow rate is reduced. The overall nozzle outflow mass flow averaged axial velocity can be increased by 30.1%, nozzle isentropic efficiency increases from 37.7% to 63.8% and nozzle mass flow rate drops from 16.0 g s −1 to 12.6 g s −1 when inlet vortex is introduced.