Design And Performance Investigation Of An Auto-Tuning Area Ratio Ejector For MED-TVC Desalination

As the most promising desalination technology at present,Multi-Effect Distillation with Thermal Vapor Compression(MED-TVC)could take the waste heat generated by thermal power plants as the heat source of distilling seawater to make full use of low-grade energy with high freshwater productivity,which could effectively solve the problems of freshwater shortage and energy shortage in China.However,due to the change of power generation in thermal power plants and the variation of steam flow rate in steam turbine thermodynamic system,motive steam would fluctuate sharply with the varying operating conditions of steam turbines.The traditional fixed structure ejector could not meet the varying operating conditions of MED-TVC desalination system.This paper presents a steam ejector which could adjust the area ratio adaptively according to the change of motive steam pressure in order to improve the freshwater production capacity of MED-TVC desalination system under varying working conditions.It has important theoretical significance and application value for MED-TVC desalination system.By studying the mechanism of ejector and bellows,a bellows-driven spindle is installed inside the primary nozzle on the basis of traditional steam ejector for MED-TVC desalination system,which could change the throat area of the nozzle according to the motive steam pressure.The structural design of the auto-tuning area ratio ejector was completed,and the overall dimensions of the spindle were designed.The CFD two-dimensional axisymmetric model of the proposed auto-tuning area ratio ejector was built,and the model grids was refined adaptively.The grid independence was verified and the model was validated with experimental data.Through numerical simulation,the effect of motive steam pressure and area ratio on the performance of steam ejector was studied,and the area ratio optimization strategy of auto-tuning ejector for MED-TVC desalination system was provided.The Mach contour distribution in the auto-tuning area ratio ejector was analyzed,and a correlation to calculate the optimal opening degree under varying motive pressure with different critical back pressures was established and identified by data regression method.The performance of the auto-tuning area ratio ejector was compared with that of the traditional fixed structure ejector in the MED-TVC desalination system.The results of numerical simulation show that the decrease of steam pressure and the increase of area ratio would increase the entrainment ratio of ejector,improve its ejecting performance,reduce the critical back pressure and decrease the working range of steam ejector.When the spindle of auto-tuning area ratio ejector is driving,it would affect the flow field inside the steam ejector and change the expansion status of the primary flow.By using the auto-tuning area ratio ejector,relatively higher entrainment ratios could be achieved under varying motive steam of MED-TVC system.When the critical back pressure is fixed at 20 kPa,the average entrainment ratio of the auto-tuning area ratio ejector could reach 1.39,which is higher than 0.69 of the fixed structure ejector while the motive steam pressure changes from 800 kPa to 2000 kPa.The auto-tuning AR ejector has the capacity to enhance performance up to 1.88 times with the varying primary pressure.The proposed auto-tuning AR ejector would have wide applications in MED-TVC desalination systems and other systems driven by low-grade energy.