Multi-scale evaluation of an R290 variable geometry ejector

Highlights: • The performance of a propane variable geometry ejector has been investigated. • The influence of the area ratio and boundary conditions have been investigated. • A spindle-based control system has been presented. • Spindle position is found to affect the entrainment process and pump effect. • The VGE system allows a more efficient operation than fixed geometry ejectors. Although representing a promising alternative to mechanical compressor driven systems, ejector refrigeration has not widely spread due to its limits due to the system operation's rigidity. Ejectors are fluid-dynamics controlled devices and because of their fixed geometry, operates at their best efficiency in a narrow range of operating conditions, which is often in contrast with the dynamic pressure and temperature levels of real refrigeration systems. Variable geometry ejector represents a promising solution to provide the ejector cycle with increased flexibility and an overall performance improvement. The present study aims to extend knowledge on VGE systems, evaluating the impact of a spindle-provided ejector operated with R290 on the refrigeration system's performance. The analysis has been carried out using an integrated model which allowed us to interpret the results by CFD visualization. Eight spindle positions (in the range of 0–7 mm) have been tested for different primary and secondary boundary conditions to assess how the different nozzle area ratio (in the range of 2.25–4.41) affects the entrainment ratio and the critical temperature. Results showed how increasing primary nozzle area ratio moving the spindle towards the mixing chamber can effectively reduce the thermal input, increasing the average COP and reducing the critical discharge temperature. For instance, a + 33% increase of the nozzle area ratio, enhanced the COP by an average + 57.1% but lowered the average critical temperature by −6.7 K. Fluid dynamics significantly affect the entrainment process and pump-effect phenomenon with particular attention to the primary flow expansion. In detail, the entrainment ratio benefitted from over-expanded jets, whilst the compression effect increased for an adapted jet. In the end, the performance curves have been obtained, which correlate the critical temperature conditions to the system performance.