Influence of operating conditions on the optimal nozzle exit position for vapor ejector

Abstract

Vapor ejector cooling systems have attracted a lot of interest in the last 25 years, given that their electrical consumption is much lower than that of conventional refrigeration systems. However, ejector systems have suffered from a low coefficient of performance. Among ejector design parameters, the nozzle exit position (NXP) has a strong impact on system performance, but so far, limited and contradicting observations in terms of its optimal value have been reported in different literary works. In this work, a large-sized and well instrumented experimental setup was used to determine the impact of the nozzle geometry and operating conditions on the optimal NXP values. The results showed that the optimal NXP values were significantly impacted by the ejector nozzle geometry and the critical entrainment ratio (ωcrit), however the primary, secondary and outlet pressures had negligible effect. Up to four optimal NXP values were observed over the tested NXP range. This number was found to increase with the critical entrainment ratio, with two optimal NXP values for ωcrit < 0.25 and four optimal NXP values for ωcrit > 0.4. Proper adjustment of the NXP value increased the entrainment ratio by >34%. For the first time, the wall pressure along the constant area section of the ejector was used to study the optimal NXP values. This led to a new finding: while the critical entrainment ratio was going from peaks to valleys along the NXP range, the wall pressure at the inlet of the constant area section varied inversely, this pressure being at a minimal value when NXP is close to an optimal value. It was also observed that the critical compression ratio decreased at higher NXP values. This work is the first to show that the optimal NXP is linked to the critical entrainment ratio, which provides an explanation to the contradictory observations reported in the literature regarding the impact of operating pressures on the optimal NXP values. Finally, this is the first study that shows the importance of adjusting the NXP whenever operating conditions are modified or a different ejector is used.