Experimental and numerical analysis of R744 ejectors operated in part load

This paper presents an experimental and a numerical investigation of R744 ejectors. For different operating conditions, several experimental tests were carried out. The experimental results show a strong correlation between the mass entrainment ratio and pressure lift. The achieved ejector efficiencies reach from 22% to 33% for the pressure lifts between 3.5 bar and 9 bar. The experimental data was used to validate the numerical simulations. For a detailed investigation, three cases were chosen. To perform the simulations, the homogeneous equilibrium model (HEM) and the k-ε RNG turbulence model were used. The contour plots of Mach number, turbulent kinetic energy and pressure as well as plots along the flow path reveal the flow pattern details. The approach of a sectional and cumulative increase in entropy rate was used to investigate local and global flow irreversibilities. The results show that the diffuser section contributes little to the overall increase in entropy rate with values from 10.12% up to 28.73% and that the diverging part of the motive nozzle contributes a considerable amount more with values from 33.3% up to 37.77%. It can be concluded that changes in turbulence along the mixer and diffuser are the main reason for the different efficiencies.