Numerical simulations of the flow in a vapour ejector have been carried out. Real gas effects are accounted for in the calculations. Ejection as well as flow-through studies have been performed. Effects of the generator and evaporator temperatures and position of the primary nozzle have been investigated. Predicted values of the suction pressure and COP have been compared with experimental values reported in the literature. In addition, secondary flow area has also been evaluated and correlated with the COP. By tracking the sonic line and the edge of the primary stream and flow separation, insights on the gas dynamic and fluid dynamic aspects of the flow field and how they influence the entrainment of the secondary stream and consequently the COP are brought out. The study reveals that, in addition to the choking of the secondary stream, the expansion of the primary stream and the area available for the secondary stream also plays a key role in affecting the performance of the vapour ejector. Copyright © 2020 Inderscience Enterprises Ltd.