Study On The Dual-ejector Performance Under Switchable Mode In The Refrigeration Systems For Refrigerated Trucks

In this paper,a novel ejector-based multi-evaporator refrigeration system using R134a is proposed to meet different working modes of refrigerated trucks running in tropical regions.It can not only realize the air-conditioned refrigeration in the driver's chamber,but also meet the requirements of food refrigeration and fresh freezing.The research focus of this paper is to study the performance of the ejector under two working conditions:air-conditioning-refrigerating mode?ejector 1?and air-conditioning-freezing mode?ejector 2?.According to the influence of cooling load on the performance of ejectors,two initial ejector models were designed.A computational fluid dynamics?CFD?simulation model was established.The experimental system was built and the reliability of the model was verified.The main research contents and main conclusions are as follows:First,under the condition of a given operation,the optimal primary nozzle throat diameter?PNTD?was determined with considering the cooling load requirements of the air conditioning chamber.Consequently,crucial ejector geometrical parameters such as area ratio?AR?,nozzle exit position?NXP?,lengths of primary nozzle and diffuser are optimized for each mode.The results show that the ejector 1 and ejector 2 have selected the primary nozzle diameter of 2.8 mm;The optimal AR₂ is 8.16,almost twice that of the optimal AR₁.The optimal value of NXP₂ of 27 mm is much longer than that of NXP₁ of 17mm.when the length of the primary nozzle of the ejector changes,the geometric parameters corresponding to the maximum value of the primary mass flow rate and the entrainment ratio are different.At this time,the performance of the ejector should be evaluated by the secondary mass flow rate.Secondly,a novel approach by taking the deviation of the entrainment ratio or secondary mass flow rate over the deviation of the geometrical parameter into account to evaluate the ejector performance sensitivity to each geometrical parameter is first time presented to the best of the authors'knowledge.The results show that the performance of both ejectors is primarily influenced by area ratio and NXP,NXP impact is more pronounced;the influence of main nozzle length and diffuser length can be neglected,and the influence of primary nozzle length is the least;the proposed sensitivity indicator can better evaluate the sensitivity of the ejector performance to geometrical parameters.Then,for air-conditioning-freezing mode,numerical investigations on optimization of ejector primary nozzle geometries with fixed and varied nozzle exit position?NXP?.The main contribution is:for the fixed NXP,the detailed effects of converging portion angle and length of two types of nozzle as well as diverging portion angle and length of converging-diverting nozzle on the ejector performance and relevant optimizations on these geometries were conducted.The results show that the optimal geometric parameters of the converging section of the converging nozzle and converging-diverging nozzle are angle 8°and length 16 mm.The geometry of the diverging section of the converging-diverging nozzle is A_d22 of 6°and L_d22 of 2 mm.It is the first time to study the influence of NXP on the optimal geometric parameters of the nozzle.The results show that the optimal values of L_c11 and A_d2are affected by different NXP,and L_c11 is more obvious.NXP has no effect on the optimal values of A_c11 and L_d2.Finally,based on the post-optimized converging nozzle type ejector,the influence of ejector auxiliary entrainment performance is studied.The entrainment ratio of the double opening and triple opening ejectors based on the optimal position of each chamber of the ejector is lower than that of the single opening ejector in the diffuser,and the entrainment ratio of the single opening ejector in the diffuser is 59.41%higher than that in the case of no opening.The optimal central opening position of the single hole ejector in the diffuser is59.5 mm,the opening angle is 45°,and the effect of the opening position on the performance of the ejector is stronger than the opening angle.The main thing is that the optimum opening position and angle are affected by the change of area ratio.In addition,due to the limitation of time and experimental conditions,more research work is needed to further prove the potential of dual ejector switchable cycles in refrigeration systems,such as the research on off-design condition and system automatic control.