Research On The Performence Of Two-stage Ejector In A Multi-evaporator Refrigeration System

With the rapid development of China's economy and the improving standards of our lives,the food transportation industry,especially the cold chain transportation,has been increasingly developed.While the refrigerated truck has been a thrill expanded,however,increased fuel consumption leads to a growing energy tension.In this paper,a two-stage ejector with large boost capacity is introduced into a multi-evaporator refrigeration system of a refrigerated truck,which forms a two-stage ejector-based multi-evaporator refrigeration system.It will not only solve the difficult problem of the large pressure difference among multiple evaporators,but achieve a big pressure lifting without any additional mechanical energy consumption.There are significant values in both theory and application for energy saving and emission reduction of refrigerated trucks.The main contents of this study are as follows:Firstly,the structures of the two-stage ejector-based three-evaporator refrigeration system and the two-stage ejector are designed by the refrigeration characteristics of the three-evaporator refrigerated truck.In this system,R134a is chosen as the working fluid.The ambient temperature is set at 35,and the temperature range of the three evaporators is-14^-5?,-34^-28?and 4¹3?,respectively.Then the effects of key geometric parameters,variable operating conditions and cooling loads on the performance parameters of the two-stage ejector are investigated by using two-dimensional CFD simulation.On the one hand,the simulation results of key geometric dimensions including the nozzle diameter,the length of the constant-pressure mixing section,the length of the constant-area mixing section and area ratio show that:1)the optimum diameter of the 1^st-and 2^nd-stage primary nozzles determined by the cooling requirement of the three evaporators in a refrigerated truck is 1.8mm and 2.4mm respectively;2)the effect of area ratio on the performances of the two-stage ejector is both significant,while it is worth noting that the two-stage ejector will be failure of function with the 1^st-stage area ratio is too low;3)there are both relatively small influences on the performances of the two-stage ejector when the lengths of the constant-pressure and constant-area mixing sections are changed;4)the optimal 1^st-and 2^nd-stage entrainment ratio of the two-stage ejector is 2.183 and 1.201,respectively.On the other,the simulation results of variable operating conditions such as back pressure and temperature and cooling loads on each stage of the two-stage ejector indicate that:1)three operational modes:critical mode,subcritical mode and back flow mode are available for each stage when two stages combine with each other,but the range of pressure lifting ratio generating subcritical and back flow modes is not the same with each other;2)the entrainment ratio of the 1^st-stage is mainly influenced by the variation of the evaporating temperature of refrigerating and freezing chambers,while the entrainment ratio of the2^nd-stage of is more influenced by the change of the evaporating temperature of the air-conditioning chamber;3)the effect of the mass flow rate of the 1^st-stage primary flow on the entrainment ratio of the 1^st-stage is more significant,while the effect of the mass flow rate of the 2^nd-stage primary flow on the entrainment ratio of the 2^nd-stage is more evident.4)the interstage pressure of the two-stage ejector is obviously affected by the mass flow rate of the 1^st-and 2^nd-stage primary flows,while the mass flow rate of the 1^st-stage primary flow cannot be selected too low,or the two-stage ejector will not work.