Numerical Study On The Ejecting Performance Of The Ejector

Ejector has been widely used in many industrial areas which are radioactive, flammable and explosive. In the operation of nuclear power unit, the non-condensable gas leaked or carried into condenser would significantly reduce the condensing capacity, thereby affecting the condenser vacuum and the equipment efficiency. Ejector is used to extract the non-condensable gas. The existing studies indicate that structural and operating parameters are the key parameters that influence the performance of the ejector. Moreover, there are strong couplings between the influencing parameters. Currently, the system analysis of the internal flow field of the ejector is not extensively studied. In this paper, the internal flow field and performances of the ejector were studied by means of computational fluid dynamics (CFD),in order to analyze the influence of various factors and provide reference for the design and modeling of ejectors.The main contents are as follows:Firstly, the ejecting and compression characteristics of the ejector were modeled based on the critical cross section theory. The analysis indicates that the ejecting and compression characteristics are determined by the cross section ratio fP1/fP*,fm/fP1 and f3/f2 and the ratio of operating parameters such as PP/PH, PC/PH, and TP/TH, rather than the absolute value of geometric and operating parameters.Furthermore, the internal flow field and ejecting characteristics of the ejector was studied via FLUENT simulation. The flow filed characteristics and the flow development of the mixing zone under three typical conditions of pressure mismatch degree at nozzle exit section,and the dual congestion state and single congestion state in ejector were studied.Finally, factors affecting the performances of the ejector were analyzed through a single factor analysis.The results indicate that the ejecting characteristics were determined by the effective ejecting area in the front of the mixing zone. When entrainment vortex is the dominant factor affecting the mixing layer characteristic, the two-phase distribution boundary line agrees well with that of eddy distribution. Therefore it is appropriate to use eddy boundary line to delineate two-phase boundary line. Moreover, turbulent kinetic energy k and eddy dissipation rate ε are directly related with momentum and component transportation in the mixing layer. Therefore it is appropriate to study the eddy characteristics via the analysis of k and ε. Besides, radial velocity contributes to the radial components transportation, and thus it is capable of predicting the mixing intensity based on radial velocity.When the pressure mismatch degree is no larger than 1, the compressibility of high velocity steam inhibits the eddy formulation. Therefore longer mixing length is needed to balance the flow field. However, there is an upper limit of the influence of compressibility on the development of mixing area. When the pressure mismatch degree is greater than 1, the shock wave at the nozzle exit would increase the growing speed of mixing layer,making the flow field to reach a balanced state in a short time, and greatly reducing the length of ejector. Increasing the nozzle expansion angle effectively enhances the intensity of initial shock wave too, advancing the development of mixing layer, shortening the length of ejector with little influence on the ejecting characteristic. At constant ejected liquid pressure, decreasing of operating steam pressure increases the ejecting flow area and the effective ejecting pressure. The tradeoff effects of the two phenomena are that the operating steam pressure does not have much influence on the ejecting characteristics.It was found through the analysis of geometric parameters that the cross-section ratio of the mixing chamber to that of the nozzle outlet has the most distinctive effects on the ejecting characteristics. Under constant value of the pressure mismatch degree, the influence of the ratio of nozzle outlet cross section to the critical cross-section on the ejecting characteristic is almost the same with that of the ratio of the mixing chamber cross-section to the nozzle outlet cross-section. In the domain of this paper,the ejecting characteristic increases with the distance increase between the nozzle and mixing chamber.