| Steam ejector is a kind of fluid machinery which pumps low-pressure fluids using high-pressure steam.It has been widely used in many industries,such as hydraulics,petroleum,chemical industry,refrigeration,desalination and vacuum system for its simple structure,low running cost,little maintenance and obvious energy saving effect.However,despite of its simple structure,the supersonic mixing process inside is extremely complex,including the interaction of shockwaves,boundary layer and shear layer.The chart method is adopted to determine the entrainment ratio in most industrial design,and the critical size is determined by the rough empirical estimation method.The ejector designed by this method has a large error,cannot acquire the ideal working performance,and even fail to work stably,thus the stable operation of production is affected.For the purpose of improving one-dimensional design theory and complex supersonic mixing,the design calculation and structure optimization of steam ejector are theoretically analysed numerically studied.The primary research work and conclusions are summarized as follows:The steam ejector is designed based on fundamental principle of steam ejector and Sokoloff aerodynamics theory which is based on one-dimension steady state flow and constant-pressure mixing theory.Meanwhile,the empirical coefficient method and thermodynamics method used in the design process is also elaborated briefly.The mathematical models for steam ejector are established and the grid independence is tested.The correctness of the numerical models is verified by the mixed steam pressure p_c,and the internal flow field distribution characteristics of the ejector are studied numerically.The primary nozzle structure of supersonic steam ejector designed is numerically simulated with computational fluid dynamics software FLUENT,the optimal geometry of the primary nozzle is determined,the influence of shockwave and choking phenomena on the internal flow characteristics are also studied in detail.The results show that for given operation conditions and remaining other geometry parameters of the steam ejector unchanged,the diameter ratio that is defined as the ratio of the outlet diameter to the throat of the nozzle has a very strong influence on the entrainment ratio of the steam ejector.There exists an optimum diameter ratio range that ensures the steam ejector acquires its best performance.The divergent section length of the nozzle can be designed in a much broader range than the conventional method in which the cone angle of the divergent section is limited in the range of 10?-12?.It is possible to improve the performance of the ejector and increase the entrainment ratio by setting the proper throat length.Obviously there is an optimum nozzle distance corresponding to the peak value of entrainment ratio.There is a very complicated mixing process in the mixing chamber of the ejector,such as supersonic shockwave and boundary layer.The convergent section of mixing chamber and throat section structure for the steam ejector are studied and analyzed.The mixing process of two steams in the mixing chamber is also discussed and the optimal geometric dimensions for each part of the mixing chamber are determined.The results show that the mixing chamber has an optimal convergent length,which is about 21 times the diameter of the primary nozzle throat.For a given mixing chamber convergent length,there exists an optimum convergence angle for the mixing chamber,which makes the primary steam and entrained steam mixed rapidly and evenly.On the other hand,the throat section length of the mixing chamber has a very small influence on the steam ejector performance when the convergent length and convergence angle achieves the best.Simultaneously,the throat section diameter of mixing chamber has a big impact on the steam ejector.To achieve the optimal performance of the ejector,the diameter ratio of mixing room section to the nozzle throat diameter should be ranged from 4.8 to 4.85 under above design conditions. |
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