Numerical investigation on the mixing process in a steam ejector with different nozzle structures

Abstract

The effects of different nozzle structures on the performance of a steam ejector have been investigated numerically with the computational fluid dynamics (CFD) technique. The performance of the steam ejectors with five different nozzle structures, namely, conical, elliptical, square, rectangular and cross-shaped nozzles, have been compared under the same conditions. It is found that, compared with the CFD results of the ejector equipped with the conical nozzle, the entrainment ratio (ER) and critical back pressure (CBP) of the rectangular nozzle is 7.1% and 21.3% lower respectively; the ER and CBP of the elliptical nozzle is 7.9% and 21.3% lower respectively; the square nozzle has improved the ER by 2.0% and decreased the CBP by 2.1%; the ER and CBP of the ejector utilizing cross-shaped nozzle is 9.1% higher and 6.4% respectively lower. Based on the simulation results of the streamwise vortex and spanwise vortex distributions in the mixing chamber and the internal energy variations along the streamwise distance, the characteristics of the mixing process and the main factors accounting for the ejector performance change are clarified. The ER increase can be achieved by efficient mixing due to the interactions between the streamwise vortex and the spanwise vortex. The streamwise vortex helps to deform and rupture the spanwise vortex which has greater strength. Collides of the vortices to the mixing chamber wall at early stage would increase mechanical energy loss and reduce “effective area” for secondary flow to pass through, resulting in great decrease of the ER and CBP. This scenario should be avoided in the design of nozzles.