Conference Year

2016

Keywords

numerical, flashing, vortex, ejector, nozzle

Abstract

Ejectors are known to be beneficial to vapor compression cycle performance as they can recover the kinetic energy released during the expansion instead of dissipating it in a throttling process. It is desirable to introduce an adjustable feature to the ejector so that ejector cycle performance can be optimized under different working conditions, which could make ejector technology more suitable for real world applications. Vortex control is a nozzle control mechanism which can possibly be applied to the control of ejector cooling cycles. It utilizes an adjustable vortex at the nozzle inlet to control the nozzle restrictiveness without having to change physical dimensions of the nozzle geometry. In this paper, two different approaches are employed to model the initially subcooled flashing vortex flow in a convergent nozzle at steady state. The first approach assumes that bubble nucleation during the depressurization in the nozzle all occurs at the nozzle wall. Bubbles are regarded as spherical particles that grow and move in the liquid flow field. The second approach assumes that there is an evaporation wave at the nozzle throat. The bubble generation in the upstream of the evaporation wave is neglected, thus the fluid in the upstream of the evaporation wave is assumed to be single-phase incompressible liquid. The modeling results are presented and compared with the experimental results. It has been concluded that bubble nucleation may not all occur at the nozzle wall at high degree of metastability. Nucleation in the bulk of the liquid might be dominant and should possibly be taken into consideration in the modeling. Pressure reduction is required for the kinetic energy increase of the nozzle flow in the azimuthal direction when there is inlet vortex introduced. For the same mass flow rate through the nozzle, the pressure difference from the nozzle inlet to the center of the nozzle throat increases as the inlet vortex becomes stronger. Therefore, less mass flow rate can be driven through the nozzle with stronger inlet vortex for the same degree of metastability at the throat and the same inlet conditions. The change in total mass flow rate is smaller with larger surface roughness for the same inlet vortex strength.

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