Conference Year

2014

Keywords

Ejector, R134a, Primary nozzle, Throat area ratio

Abstract

A vapour jet refrigeration system (VJRS) is an alternative to the conventional mechanically driven vapor-compression refrigeration system. The VJRS utilizes a supersonic ejector as a thermal compressor and has the potential to reduce energy consumption in refrigeration systems [1]. In the present study, the performance characteristics of VJRS ejectors with R134a as refrigerant have been investigated numerically using ANSYS Fluent. VJRS works on the principle that the high-pressure vapour from generator gets expanded through the convergent-divergent nozzle to produce high velocity stream which entrain the refrigerant vapour from the evaporator. Both the streams mix together in the mixing chamber. It results in pressure rise in the mixing chamber due to formation of shock waves followed by flow through the diffuser. The constant pressure mixing chamber where primary and entrained fluids mix together is the area of concern in the ejector. In the present work, two-dimensional analysis on the ejector geometry of VJRS (capacity = 3.5 kW) is carried out to examine the turbulent behavior and boundary layer distribution in the constant pressure mixing chamber. Also the performance of ejector at low generator temperature and low evaporator temperature has been evaluated. In addition, the present study includes the effect of area ratio and mixing chamber length of the ejector. Turbulence effects in the ejector have been modeled using the standard k-epsilon turbulence model [2]. The primary nozzle profile is developed by method of characteristics. The geometry presented here uses five different values of throat area ratio for fixed diameter of constant area mixing chamber. The exit diameter of the convergent-divergent nozzle, diameter and length of diffuser and length of constant area mixing chamber have been optimised. The ejector geometry is assumed axisymmetric and quadrilateral mesh is used for the present study. The computed results are initially validated with available literature and experimental data. For the numerical simulations on the selected ejector, generator temperature is varied from 340 K to 370 K, the condenser temperature is varied from 290 K to 310 K and the evaporator temperature is varied from 263 K to 283 K. The numerical results obtained contribute to understanding the local structure of the flow and demonstrate the role of the secondary choking in deciding the entrainment ratio. Moreover, the results help in identifying the optimum operating condition for each value of nozzle throat area ratio.

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