The use of ejectors as a gas-liquid contacting device has been reported to give higher mass transfer rates than conventional contactors. Computational fluid dynamics (CFD) modeling studies were undertaken to understand the hydrodynamic characteristics with reference to the ejector geometry. The CFD model also provides a basis for quantifying the effects of operating conditions on the ejector performance. CFD studies show that at low value of area ratio (ratio of throat area to nozzle area), due to the larger diameter of the water jet, the annular area available for air flow reduces, causing recirculation of the entrained air within the converging section of the ejector. On the other hand, for higher values of area ratio, due to smaller diameter of the water jet, the momentum transfer to the air decreases and all the entrained air cannot be forced through the throat. As a result, the net air flow rate going into the throat for both area ratios is small. Thus there is an optimum area ratio for the maximum air entrainment rate. The air entrainment rate correlates with pressure difference between the air entry and throat exit for a wide variety of ejector geometries and operating conditions. The overall head loss factor and the ejector efficiency can be predicted a priori. (c) 2005 Elsevier Ltd. All rights reserved.