Numerical analysis of the performance of a thermal ejector in a steam evaporator

Abstract

Ejectors have been widely used in many applications such as water desalination, steam turbine, refrigeration systems, and chemical plants. The advantage of an ejector system lies in its extremely reliable operation due to the complete absence of moving parts. However, the performance depends on a number of factors, among which the flow channel configuration/arrangement is critical. To improve the performance of an existing thermal compressor in a steam evaporator, a comprehensive study was conducted in this paper with a main focus on the sensitivity of performance to the geometric arrangement. Numerical simulation was employed to investigate the thermal-flow behavior. The performance is measured by the entrainment ratio, i.e., the secondary (suction) flow rate from a vapor plenum over the primary steam jet flow. It is observed that any downstream resistance will seriously impede the suction flow rate. In addition, the entrainment ratio is sensitive to the location of the jet exit, and there is an optimum location where the primary flow should be issued. A well-contoured diffuser can increase the entrainment ratio significantly. However, the size of suction opening to the plenum is less important, and a contoured annular passage to guide the entrained flow shows little effect on the overall performance. Based on the numerical results the steam entrainment rate of the best case in the confinement of the current study is approximately 430% of the jet flow rate, while some cases with mediocre design can only produce an entrainment of 24% of the primary jet flow.