Experimental Analysis of a Variable Geometry Ejector for Solar Air Conditioning

The concept of a variable geometry ejector (VGE) that operates at near optimal performance under changing working conditions, such as the case of using solar energy as primary heat source, has already been discussed by several authors. Its benefits, for the application in solar cooling system, were demonstrated in previous theoretical studies. This paper presents the experimental results with the first laboratory scale VGE using isobutane (R600a). In the VGE, internal geometry can be changed by active control of two geometrical factors: the area ratio and the nozzle exit position. The area ratio is modified by a movable spindle installed in the primary nozzle. Variation of the nozzle exit position is achieved by a movable ejector tail. Ejector performance was assessed by calculation of cooling cycle COP, entrainment ratio and critical back pressure. The results indicate very good ejector performances for the considered generator and evaporator temperatures. The entrainment ratio observed was as high as 1 for a generator temperature of 83 °C and evaporator temperature of 9 °C. The experimental data also indicate that both geometrical factors, spindle position (SP) and nozzle exit position (NXP), has an important effect on COP. The existence of an optimum SP, dependent on the back pressure is identified. By changing the SP, measured cooling cycle COP varied in a range from 0.4 to 0.8, at fixed generator, evaporator and NXP conditions. Also the critical back pressure varied with SP, in a range of about 1 bar. Likewise, the existence of an optimum NXP is identified, though the dependence of the ejector performance parameters are less significant when compared to the SP effect. By changing the NXP form 5 to 15 mm, COP varied about 20 %.