Two-phase flow visualisation in the R744 vapour ejector for refrigeration systems

The dynamic development of R744 refrigeration cycles has been observed throughout recent years. State-of-the-art R744 is equipped with ejectors or ejector modules to increase the overall system coefficient of performance (COP). That performance improvement can be achieved only with the proper ejector design and corresponding control strategies. Therefore, numerous computational studies on the ejector performance and fluid flow inside the device have been conducted by various groups of researchers. Unfortunately, experimental studies on two-phase R744 are fairly limited, especially in terms of fluid flow visualisation. For that reason, this study is focused on two-phase flow and mixing visualisation inside the R744 ejector designed for an industrial-scale refrigeration system. Experiments were conducted in the experimental R744 vapour compression rack with the dedicated ejector tests section. The mixing section of the ejector used in this study was made of transparent material to enable flow visualisation experiments. A high-speed camera and additional light sources were used to capture the flow inside the mixing section. The experiments were conducted for the ejector working with unsteady operation in two modes: the suction nozzle closed and standard operation with both ports open. Then, the experimental results were used to evaluate the expansion angle for the subcritical, critical, and supercritical MN inlet conditions. The collected results showed that the expansion angle increased with increasing mass flow rate in the motive nozzle. In particular, the expansion angle for the subcritical case was equal to approximately 2.75°, while for the supercritical case, it was equal to 5.95°. The mixing angle for most of the investigated cases was equal to approximately 25°. The increase in the pressure lift increased the mixing angle up to 40°. The unsteady operation of the ejector mixing section showed a pressure lift equal to approximately 10.0 bar, which resulted in a decrease in the SN mass flow rate but did not affect the vapour compression test rig performance.