Experimental and numerical studies on the use of a needle for variable capacity single-phase ejectors

Vapor ejectors raise interest for cooling applications given their low electrical consumption, however, a single ejector suffers from a lack of capacity flexibility. The current study explores the use of an adjustable needle to control the primary nozzle throat area (At) in view to allow variable ejector capacity. Tests were done using a large scale test bench working with the refrigerant R245fa. CFD simulations using k-w SST and, for the first time, Scale-Adaptive Simulation (SAS), were also done. A comparison of the simulation results using the two different turbulence models (w SST and SAS) showed negligible difference. Also, the simulation results with the w SST model showed good agreement with the experimental values. For fixed operating conditions, the simulation showed that the reduction of the throat area from 1.00At to 0.61At increased the entrainment ratio from 24% to 56%, but reduced the critical outlet pressure from 440 kPa to 315 kPa. For all conditions, the transiting exergy efficiency eta tr,ex first increased with the increase of the outlet pressure (Pout), reached a maximum value when Pout is close to the critical outlet pressure (Pcrit), and then decreased sharply due to the decrease of the entrainment ratio. Finally, CFD modelings with a blunt tip needle to see the effect of the needle geometry and in order to have a lower throat area value of 0.39At. This novel study shows that the ejector equipped with a needle is able to maintain ejector performance with changes in operating conditions.