Numerical study on the flow noise and performance of trans-critical carbon dioxide ejector

The flow noise and the performance of trans-critical carbon dioxide ejector were simulated to summarize the correlation between them. This correlation is the basis of judging working condition of ejector by monitoring the flow noise. In this article the homogeneous model (HEM) was employed to simulate the inner flow in the ejector. Based on the flow field, the flow noise distribution was derived using broadband noise simulation (BNS). The simulation results showed that major noise sources locate in the premix and diffuser section. The noise source in the premix section is caused by the great velocity difference between primary and secondary flow. The noise sources in diffuser are the back flow and vortexes caused by flow separation. The characteristics of flow noise under variable back pressure are as following: the premix noise remains unchanged while the diffuser noise decreases with the increase of back pressure. Comparing the trend of flow noise and the performance parameters under variable back pressure, it was found that with the decrease of diffuser noise, the pressure ratio and the efficiency of ejector were increased. The correlation between the entrainment ratio and diffuser noise lies in: before the critical back pressure, the entrainment ratio remains constant with the decrease of noise. After that, the noise and entrainment ratio decrease simultaneously. Especially, the back pressure range when the entrainment ratio starts to decline is consistent with the range when the diffuser noise rapidly drops. Therefore, the significant decline of flow noise in diffuser can be viewed as an important judgment basis of ejector entering the subcritical working condition.