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Abstract: This paper presents a comprehensive study on a controllable two-phase flow ejector component efficiencies and ejector performances in transcritical CO2 refrigeration cycles. A two-phase flow ejector expansion model was developed and experimentally validated. A method of determining the efficiencies of ejector motive nozzle, suction nozzle, and mixing section based on the measured performance data external to the ejector was developed. It was found that motive nozzle efficiency decreases as ejector throat area decreases, and that suction nozzle efficiency is affected by outdoor air temperature. Empirical correlations of ejector component efficiencies were reduced. Parametric modeling study results show that the operation of an ejector reaches its optimum performance at ejector throat diameter approximately of 2.3 mm, mixing section constant-area diameter approximately of 3.5 mm, and diffuser diameter ratio a little bit larger than 2. Higher efficiencies of motive nozzle and suction nozzle as well as higher motive nozzle inlet pressure result in better ejector performances in certain ranges of motive nozzle throat diameter and mixing section constant-area diameter. [Copyright &y& Elsevier]