Optimization Design Of Steam Ejector For MED-TVC Desalination System

Ejector has been widely utilized in the fields such as desalination system,ejector refrigeration system,carbon capture,vacuum equipment,fuel cell system,thrust augmentation and so on,of which the optimization is of great theoretical significance and application value to improve the whole system performance.However,there are so many geometric parameters which are independent on one another that trial and error is the prevailing optimization design method,not only laborious and time consuming,but also hard to guarantee the global optimum.To address the problem,the thesis proposes an analytical method to perform the optimization of the primary nozzle by considering the flow mechanism and thermodynamics of its internal working fluid.Then,according to the developed design method,a streamline primary nozzle steam ejector under the typical operational conditions of MED-TVC desalination system is devised to validate its feasibility and superiority.Firstly,in consideration of the flow mechanism and thermodynamics of the working fluid,a one-dimensional isentropic steady-state mathematical model for the primary nozzle is formulated and the corresponding optimization design method is also presented.Secondly,to verify the effectiveness of the proposed model,considering the axial static temperature distribution along the nozzle,a streamline primary nozzle of the steam ejector under the typical operative conditions of MED-TVC desalination system based on the proposed method is devised.Finally,the performance and interior flow structure of the designed ejector are numerically investigated under various conditions and performance comparison of the proposed design method to conventional methods from the perspective of ejector performance and design cycle is thoroughly discussed.Conclusions are drawn as follows:(1)At fixed primary pressure,the entrainment ratio(ER)at critical mode and critical back pressure(CBP)increase almost linearly with the ascending secondary pressure and the individual linear regression empirical formula is attained with coefficient of determination of 0.9989 and 0.9991,respectively.(2)Under settled primary pressure,the normal shock position can serve as the indicator of the ER at critical mode and CBP.The normal shock wave within the diffuser can keep the ejector operating at critical mode and the ER biggest,otherwise the ER decreases with its rearward movement away from the diffuser entrance to the mixing chamber.The ER at critical mode and CBP also increase as the normal shock wave moves forward from the entrance to the exit of the difruser.(3)Increasing the secondary pressure at critical mode under specific motive condition can cause the forward movement of the normal shock wave from the entrance to the exit of the diffuser,consequently increasing the ER at critical mode and CBP.In addition,increasing the discharge pressure to its counterpart CBP at fixed primary and secondary pressure can cause the normal shock wave to move towards the diffuser entrance,keeping the ejector working at the critical mode,and further increase of the back pressure will cause the normal shock wave to move away from the diffuser entrance towards the mixing chamber,resulting in the decrease of ER at the sub-critical mode and even the malfunction of the ejector.(4)Compared with conventional methods,numerical simulation results show that the proposed design method can improve the overall ejector efficiency by 14.41%through at 600 kPa primary pressure and 15 kPa secondary pressure condition.