Analysis of the internal flow features of a CO2 transonic nozzle and optimization of the nozzle shape profile

Abstract

Two-phase flow Laval nozzles are simple and have promising applications, but the internal flow is intricate and encompasses physical phenomena like phase transition, transonic and Mach waves. Therefore, a 1D mathematical model for validating and designing a CO2 two-phase flow transonic nozzle was developed and validated based on experiments. The findings demonstrated that a grid density of 0.292 mm was appropriate and that the maximum error of the model was less than 6.0 %, with an average error of less than 2.5 %. Additionally, it was discovered that when a phase transition occurred at the nozzle throat, whether through condensation or evaporation, there was a minimum critical pressure ratio in the nozzle; when the phase transition took place in the nozzle diffusion chamber, there was a minimum total pressure ratio. Finally, the Laval nozzle was designed to utilize the isostatic pressure gradient method (IPG), which gave a more significant pressure drop than a conventional nozzle while eliminating the shock wave strings at the nozzle throat. Under the typical operating conditions of a transcritical CO2 heat pump system, the ejector performance was boosted by an average of 5.44 %.