Ejectors performance with different design geometries and operating conditions

This study describes the experimental investigation of the performance of the ejector used in desalination applications, using air as the working fluid. An experimental test loop was designed and constructed to investigate experimentally the effect of some parameters such as; the primary fluid pressure, the suction fluid pressure, the shape of the primary nozzle and its size, and the constant area mixing length to diameter ratio (L/D) on the ejector performance (the entrainment ratio). The effect of the relative position of the primary nozzle exit within the mixing chamber on the performance of the ejector was also investigated. To study the effect of the primary nozzle throat size on the ejector performance, three convergent-divergent nozzles were designed and constructed. The first nozzle has a throat diameter of 2.5 mm, the second nozzle has a throat diameter of 3.0 mm, and the third nozzle has a throat diameter of 3.5 mm. The other main ejector geometries were kept constant, [while the nozzle was at the second position (just at the convergent inlet mixing section part). The convergent-divergent nozzle can be moved to study the effect of nozzle positions on the ejector performance]. Tests were carried out at three different positions for each nozzle. The first position, the exit section of the primary nozzle is located at 5 mm past the inlet plane of the mixing chamber section of the ejector. The second position, the exit section of the primary nozzle is just at the inlet plane of the mixing chamber section. While the third position, the exit section of the primary nozzle is shifted 5 mm before the inlet plane of the mixing chamber section. To study the effect of the ratio of the constant area mixing length to diameter ratio (L/D) on the ejector performance, the constant area mixing length to diameter ratio were varied to take the values 2.5, 5, and 7.5 while all the main ejector geometry was kept to constant. To study the effect of the primary fluid pressure on the ejector performance, the pressure of the primary fluid was varied from 2 to 7 bar. Moreover to study the effect of the suction fluid pressure, the suction pressure was varied from 0.36 to 0.89 bar.The results of the present work show that: the performance of the ejector, the entrainment ratio, for the convergent-divergent nozzle is higher than that given by the nozzle of convergent inner shape. Also, the ejector performance improves by decreasing the primary nozzle throat diameter and also by increasing the suction pressure. Moreover, the performance of the ejector is affected by the nozzle position, and the optimum position, which yields a maximum entrainment ratio for all nozzles, when positioned at the inlet of the mixing chamber section of the ejector. As the constant area-mixing length to diameter ratio increases, the entrainment ratio increases. And the increase in the entrainment becomes very small and can be negligible for L/D greater than 7.5. The entrainment ratio increases by increasing the primary pressure, and the entrained flow reaches a maximum at a certain primary pressure and beyond this value the entrained flow will decrease.