From the polytropic compression work formula, we can find that the consumed polytropic work will reduce with the decrease of inlet temperature while compressing the refrigerant to the same compression ratio. However, the refrigerant may condense if the inlet temperature is low enough. Though the principle that the acceleration of fluid may result in condensation has been proved by numerical simulations and experiments, and the liquid formation inside the supercritical carbon dioxide (SCO2) centrifugal compressor has been widely studied, there is still not a user-friendly method to predict whether the inlet condition may cause liquid formation inside the compressor.

The fluid flow in the space near the blade suction face of the leading edge (SNSL) is assumed to the similar flow in a converging nozzle when the mass flow is larger enough; the fluid impinges on the suction surface of blades, and the absolute velocity of fluid will not be greater than sound velocity. The fluid turns to impinge on the pressure surface with the decrease of mass flow rate, which is similar to the flow in a converging-diverging nozzle, and the maximum absolute velocity in the SNSL may be greater than the sound speed.

A method is proposed to predict the lowest inlet temperature of refrigeration centrifugal compressor to avoid phase change, which is called the limit temperature. The predicted lowest temperature shares the same trend with the numerical results. The condensation will occur inside the compressor when the inlet temperature is lower than the limit inlet temperature. The lowest temperature will first increase and then decrease as the mass flow increases, which should be taken into account while designing a refrigeration centrifugal compressor or adjusting the operating condition.

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