Combined effects of fluid selection and flow condensation on ejector operation in an ejector-based chiller

• Ejector-based chiller with R134a and R245fa demonstrates COPs between 0.1 and 0.2.
• COP increase >12%, >14 K driving temperature decrease with shift in ejector operation.
• One-dimensional analytical models of flow in motive nozzle developed to predict two-phase flow.
• COP depends on wet or dry working fluids and magnitude of detrimental two-phase flow effects.
• Best COP when motive nozzle inlet condition slightly inside the saturation dome.

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.