Structural Optimization Of A Two-stage Ejector With High Coupling Degree Under Constant/Variable Back Pressure

In this paper,a compression refrigeration system for a three-evaporator refrigerated truck based on two coupled ejectors is proposed.The ejector can solve the problem of excessive pressure difference between multiple evaporators,reduce throttling loss and increase the suction pressure of compressor to further reduce compressor power consumption.The refrigerant of the system is R134A.Evaporator 1 simulates the air conditioning room and connects the primary inlet of the first stage injector.Evaporator 2simulates the refrigerating room and connects the secondary inlet of the first stage injector.The outlet of the first-stage ejector is the primary inlet of the second-stage ejector,so the two-stage ejector has a strong coupling effect.Based on the 2D axisymmetrical geometry model of the two-stage ejector established by GAMBIT,the influence of the key geometry of the two-stage ejector,the type of the main nozzle of the second stage ejector and the variable back pressure on the performance characteristics of the two-stage ejector is discussed by CFD simulation.Main studies and finding are made below:First of all,according to three secondary main nozzle throat diameter（PNTD₂）,key geometric shapes（such as area ratio or AR,main nozzle export position or NXP and constant-pressure mixing angle）on the ejector performance is studied.The results show that:1)PNTD₂has the most significant influence on the ejection rate of the first stage,followed by AR₁,followed by NXP₁,and the smallest is the Angle of the mixing section at constant pressure of the first stage.2)The optimal values of AR₁,NXP₁,the angle of the first stage constant pressure mixing section and the peak value of ER₁increased with the increase of PNTD₂;3)AR₂has the most obvious influence on the second-stage ejection rate,followed by NXP₂,and the smallest is the second-stage constant pressure mixing angle;4)With the increase of PNTD₂,the optimal AR₂decreases,the optimal NXP₂does not change,and the optimal Angle of the second stage constant pressure mixing section increases.Then,under three PNTD₂conditions,the influence of the second stage main nozzle on the performance of the injector is studied when the nozzle of second stage ejector is respectively used with a decreasing nozzle and a scaling nozzle.The results show that:1)regardless of the PNTD₂value,the optimal range of the length of the nozzle is 15 mm～25mm,and the optimal range of the angle of the nozzle is 10°～14°;2)With the increase of PNTD₂,the influence of the length and Angle of the decreasing nozzle on the ejection rate of the first and second stage will be weakened;3)The optimal throat length,expanding segment length and expanding segment angle of the scaling nozzle are 0 mm,1 mm and 14°,respectively,and will not change with PNTD₂;4)The throat length of the scaling nozzle has the most significant influence on the performance of the two-stage ejector,followed by the length of the expanding segment,and the smallest is the Angle of the expanding segment.Finally,the influence of varied back pressure conditions on the performance of the two-stage ejector and the key geometry of the second stage（area ratio AR₂and main nozzle outlet position NXP₂）is investigated.Variable back pressure is obtained by changing the percentage pressure lift（PLR）.The results show that:1)PLR has a significant effect on the second-stage ejector rate,but almost no effect on the first-stage ejector rate;2)As PNTD₂rises,the PLR range of the ejector in the critical mode becomes wider;3)PLR has the most significant effect on the second-stage ejection rate,followed by AR₂,and NXP₂has the least effect;4)With the increase of PLR,the optimal value of AR₂decreases,and the effect of AR₂on the second-stage ejection rate is weakened.