A CFD-based numerical analysis of the flow irreversibility in R744 ejectors is presented. A validated CFD tool was used to investigate three cases that were differentiated by the mass flow rate per unit area (mass flux) that passed through the mixer, which represented three dissimilar flow patterns. The mixer mass flux was found to significantly affect the ejector performance both locally and globally. An original approach was introduced to assess the contribution of the local irreversibilities to the overall entropy increase. A new factor was proposed to evaluate the ejector performance based on the reference entropy increase in a classic expansion valve. In addition, the influence of the mixer diameter and length on the ejector performance was numerically analysed, which showed that the effects of both geometric parameters may be significant. Namely, in the conditions considered, both enlargement of the mixer cross section area by 17.4% as well as shortening the mixer length by 33.3% resulted in the increase of the overall entropy growth rate by 8.9% and 5.4%, respectively. © 2013 Elsevier Ltd and IIR. All rights reserved.