Numerical Simulation Of Structural Optimization And Multi-nozzle Structure Of Steam Ejector

Steam ejectors often replace mechanical compressors in certain systems to achieve steam compression due to its simple structure,no moving parts,stable operation,etc.In the MED-TVC seawater desalination system,the low-pressure steam from final effect can be recovered and boosted into high-pressure steam,realizing the conversion of low-grade energy into high-grade energy.The entrainment ratio of the steam ejector directly affects the performance of the system.When the pumped low-pressure steam pressure remains unchanged,the critical back pressure determines the ejector’s ability to boost pressure.Therefore,the structure of the steam ejector was optimized,and the use of multiple nozzles was considered to enhance the internal mixing process of the ejector.Using Fluent,two-dimensional axisymmetric numerical model and calculation method were established for the steam ejector used in the MED-TVC seawater desalination system,and the results were compared with the existing experimental results to verify the reliability of the numerical model used.The flow conditions under different back pressures were calculated,and the relationship between the second shock wave and the back pressure was determined.Numerical simulations of steam ejectors under different structural parameters are carried out,and the entrainment ratio and critical back pressure in the double critical state are used to determine the structure that can maintain the optimal performance to optimize the structure of the steam ejector.The optimal value of the primary nozzle diameter ratio is 3.98.At this time,the internal flow loss of the ejector is the smallest.After the diameter ratio deviates from the best value,the vortex area of the mixing chamber becomes larger,and the flow loss becomes larger.The loss of mechanical energy caused by the primary shock wave chain is also greater.When the primary nozzle moves in the direction of the mixing chamber,the vortex area of the convergence section of the mixing chamber first becomes smaller until it disappears,and the flow loss is reduced.When it reaches the optimal position,△x/dt=30;and when the primary nozzle continues to move in the direction of the mixing chamber,the vortex area of the secondary steam decreases,the primary and secondary shock waves interfere with each other,and the flow loss increases.As the length of the convergence section of the mixing chamber increases to the optimal length,the energy loss caused by the interference of the two shock waves is reduced,and the primary and secondary steam can be fully mixed.At this time,the length of the mixing chamber is 44 times the diameter of the primary nozzle throat.With the decrease of the convergence angle,the vortex area disappears,the velocity of the secondary steam in the mixing chamber increases,and the loss of the momentum exchange of the primary and secondary steam decreases.When the optimal angle is reached,tanθ=0.058,and the angle continues to decrease as the secondary steam circulation area is reduced,the entrainment ratio is slightly reduced.As the diameter of the constant cross-section throat increases,the velocity of the secondary steam in the mixing chamber increases,and the energy loss of the primary and secondary steam mixing decreases.The friction loss of the mixed steam at the constant cross-section throat also gradually decreases,until the optimal diameter is reached,dc/dt=13.64.The length of the constant cross section throat and the diffusion chamber has no effect on the entrainment ratio of the steam ejector.As the length of the constant cross section decreases,the critical back pressure decreases slightly.Therefore,reducing the length of these two parts can be considered to reduce the overall size of the steam ejector.A three-dimensional model was established to study the performance and internal flow characteristics of the multi-nozzle steam ejector.Under certain operating conditions,the entrainment ratio and critical back pressure decrease as the number of nozzles increases.The length of the primary shock wave becomes shorter and the pressure in the cross section increases as the number of nozzles increases,and the distance for momentum exchange between the primary steam and the secondary steam becomes shorter.Finally,change the inlet pressure of the primary and secondary steam.The entrainment ratio decreases as the pressure of the primary steam increases,while the critical back pressure changes on the contrary,and the primary shock wave becomes longer.The maximum Mach number to the primary shock wave chain can be close,and the secondary shock wave is more obvious.As the secondary steam pressure increases,the primary shock wave chain becomes shorter,the secondary shock wave is gradually enhanced.The entrainment ratio increases due to the increase in the driving force of the differential pressure,and the critical back pressure also increases.The multi-nozzle ejector can further reduce the length of the mixing chamber compared to the single nozzle ejector,thereby further reducing the overall size of the ejector.The multi-nozzle steam ejector can operate at a higher primary steam pressure than a single-nozzle ejector.