Two-phase radial ejector for transcritical CO2 refrigeration

Transcritical refrigeration systems using natural refrigerants like CO2 operate on the ejector–expansion cycle to achieve high operating efficiency. The variability of ejector geometry dictates the adjustability of refrigerating capacity in such systems. Traditional spindle-controlled axial-flow ejectors significantly obstruct the high-pressure motive flow to regulate mass flow rate by controlling flow area. This study introduces the first radial-flow ejector geometry for transcritical CO2 refrigeration systems, designed to operate at high mass flow rates and entrainment ratios with minimal obstruction to motive flow. The study numerically investigates the performance of a radial-flow two-phase ejector for CO2, comparing it with an axial-flow ejector of similar dimensions under the same conditions. It explores the impact of applied compression ratio, motive nozzle throat spacing, and other geometries on radial ejector performance. An increase in the motive throat spacing improves the range of operable discharge pressures and achievable compression ratios, with a slight reduction in the secondary flow entrainment. A nozzle throat spacing of 0.6 mm allows a wider range of operable discharge pressures, and beyond this, the operable discharge pressure range drops. The addition of a diffuser section of sufficient length also improves the flow entrainment in the radial ejector. The radial ejector allows substantially high mass flow rates of even 15 times that of the axial ejector. This high mass flow capacity in the radial configuration can significantly compact ejector designs for large-capacity transcritical CO2 refrigeration and air-conditioning systems.