Performance and modeling of ejectors in transcritical CO₂ heat pump systems for electric vehicles considering bubble nucleation and growth

Developing electric vehicles (EVs) serves as a critical initiative to alleviate energy shortages and safeguard the ecological environment, within which high-efficiency integrated thermal management stands out as a paramount research focus, making transcritical CO₂ ejector heat pump systems a mainstream trend in thermal management architectures. However, the internal non-equilibrium phase change within the ejector employed to mitigate throttling losses and recover expansion work, as well as the underlying mechanisms influenced by geometric parameters, remain obscure. A numerical model was established and experimentally validated for a two-phase carbon dioxide ejector considering cavitation-evaporation and condensation effects, focusing on investigating its internal microscopic bubble nucleation and growth and further extending to macroscopic thermodynamic predictions. Through model capturing, with the increase of the ejector mixing chamber diameter, the axial range of the high-speed and low-temperature core region is shortened during shock wave oscillations. The mixing chamber isentropic efficiency exhibits an identical variation trend to the entrainment ratio, both peaking at 3.2 mm with maximum values of 0.9271 and 0.8111, respectively, before showing a downward trend. Furthermore, with the enlargement of the primary nozzle diverging angle, bubble growth accelerates, leading to an increase in the bubble number density. The throat isentropic efficiency shares a consistent variation rule with the entrainment ratio, reaching maximums of 0.9401 and 0.6704 at 2° divergent angle, exhibiting a non-linear pattern of initially increasing and then decreasing. Finally, the data acquired by the model are utilized to fit a one-dimensional thermodynamic isentropic efficiency prediction formula for optimizing the ejector modeling parameters.