Combined microjet control of air ejectors with surrogate model-based optimisation

With the continuous increase in global energy demand and growing environmental concerns, the development of high-efficiency and more sustainable refrigeration technologies has become crucial. Eject refrigeration systems have drawn great attention for their ability to be incorporated with various renewable energy sources, although often suffering from low-performance issues, particularly during off-design conditions. In this study, a microjet control approach together with surrogate model-based optimisation framework has been developed, and its effectiveness on ejector performance enhancement has been examined. In the optimisation process, numerical simulations are used to predict the entrainment ratio of the ejector at the given operating condition for the construction and update of the surrogate model, which is then used to obtain the most suitable microjet control parameters. The results indicate that while the adoption of microjet control offers limited help during a critical mode, it leads to a remarkable enhancement of the ejector performance up to 85.8% under subcritical conditions. It has been found that a buffer layer between the main primary fluid and the nozzle wall was created, causing a delayed and weakened choking at the outlet of the nozzle, which further established a local low-pressure zone, enhancing interaction between the primary and secondary fluids. The findings and the established global optimisation framework for microjet control have the potential to provide insights and solutions for similar problems in a broad range of engineering applications.