Numerical and experimental assessment of the CO₂ ejector for a small-scale refrigeration system for mobile/automotive applications.

The design of small-scale CO2 ejectors with throat diameters below 1 mm for mobile CO2 refrigeration units is demanding particularly when prototypes are required in limited quantities for pilot deployment. In this study, a numerical analysis was performed to evaluate design constraints, focusing on six key dimensions of the motive nozzle and mixing section. Simulations were conducted at motive conditions of 86 bar and 35◦ C and two suction nozzle conditions representative of low-temperature (LT) and medium-temperature (MT) evaporator outlets. The optimized geometry achieved efficiencies of approximately 20 % at pressure lifts of 4-5 bar, fulfilling the cooling power requirements under rated conditions (ambient 30◦ C). Experimental tests on a dedicated labscale test rig validated the design and provided insight into manufacturing tolerances and general performance characteristics. Ejectors demonstrated mass entrainment ratios between 0.45 and 0.50 at the design point (86 bar, 35◦ C motive; -21◦ C LT and -10◦ C MT evaporation). Flow stability varied between configurations: the MT ejector exhibited higher stability, whereas the LT ejector–despite more fluctuation–achieved higher efficiency (∼26 %). Manufacturing quality was assessed using numerical sensitivity analysis, revealing that deviations in tolerance and surface roughness could lead to reductions of up to 9 % in motive and suction nozzle mass flow rates. These novel findings underscore the critical importance of precision machining for scaled-down CO2 ejector components and demonstrate the feasibility of integrating such systems into compact mobile refrigeration applications.