Combined effects of fluid selection and flow condensation on ejector operation in an ejector-based chiller
Abstract This work investigates the optimization of the coefficient of performance of an ejector-based chiller through changes in the two-phase flow characteristics inside the ejector using wet R134a and dry R245fa fluids. Reducing the superheat at the motive nozzle inlet results in a 12–13% increase in COP with a 14–16 K drop in driving waste heat temperature. The roles of momentum transfer, heat transfer, and two-phase flow on performance are delineated. The change in COP appears to be a combination of the choice of fluid and the effect of phase change on momentum transfer effectiveness. Larger degrees of condensation reduce momentum transfer effectiveness; however, energy savings from reduced motive superheating compensates for the effect of condensation, and causes a net increase in COP. It is recommended that ejector-based chillers be operated such that the motive nozzle inlet is near saturation, and dry fluids like R245fa are used to improve performance.