| Under the general trend of energy saving and emission reduction,enhanced evaporator heat transfer was great significance to the optimization of refrigeration system.Visual experimental table of glass tube evaporator was set up in the experiment.Two refrigeration systems with different liquid supply modes were compared and studied,which named direct expansion liquid supply refrigeration system and gravity liquid supply refrigeration system.The system performance,heat transfer performance and fluid flow pattern in the evaporator were analyzed.The experimental device improved the flange connection of the original glass tube evaporator,thus solved the problem of leakage in the past.At the same time,under the same working condition,the influence of different superheat on the system performance and heat transfer performance of the direct expansion liquid supply system were studied.The heat flow meter was added to measure the change of heat flow along the pipe length,and the experimental results were more sufficient.The working conditions of two liquid supply refrigeration system were:-25℃,-20℃,-15℃,-10℃,-5℃,and 0℃,among which,three kinds of superheat were taken for each working condition of direct expansion refrigeration system: 0℃,5℃,and 10℃,respectively.The thermocouple was used to measure the temperature of each measuring point,the pressure transmitter was used to measure the suction and exhaust pressure of the compressor,the heat flow meter was used to measure the change of heat flow along the pipe length,the visual camera was used to shoot the flow state inside the glass tube evaporator,and the cooling capacity of the measuring system was measured by the heat balance method.Simulated and calculated the heat transfer coefficient of gravity liquid supply refrigeration system.The air side selected the calculation formula of heat transfer coefficient of crossl sweep single tube.Five correlations were selected for the refrigerant side.The correlations of the convective heat transfer coefficient on the refrigerant side were as follows: Shah correlation,Kandikar correlation,Gungor and Winterton correlation,Lui and Winterton correlation,and J.Chawla correlation.And the total heat transfer coefficient was calculated by the combination of them and formula of thermal conductivity of pipe wall.By comparing the direct expansion liquid supply refrigeration system with the gravity liquid supply refrigeration system,the following conclusions were drawn through experiments and theoretical calculations:(1)The suction and exhaust temperature of gravity liquid supply refrigeration system was higher than that of direct expansion liquid supply refrigeration system.The average increase rate of suction temperature and exhaust temperature was 71.61% and 25.30%,respectively.The performance of the compressor in gravity liquid supply refrigeration system was lower than that in direct expansion refrigeration system,but the parameters of gravity feed liquid refrigeration system change stably.The evaporation temperature of gravity liquid supply refrigeration system was higher than that of direct expansion liquid supply refrigeration system,and the temperature difference of heat transfer was smaller than that of direct expansion liquid supply refrigeration system.Under the condition of-25℃ to 0 ℃,the evaporation temperature of the gravity supply liquid increased by 0.71 ℃,1.73 ℃,1.36 ℃,1.14℃,0.88 ℃and0.27 ℃,respectively.When the storage temperature was low,the difference between them was large.Under the conditions of-25 ℃ to 0 ℃,the refrigerating capacity of gravity liquid supply refrigeration system was increased by12.69%,15.35%,8.11%,4.87%,0.77% and-0.91%,respectively.The gravity liquid supply refrigeration system had more advantages in low temperature condition.With the increase of storage temperature,the gravity supply system was close to the direct expansion system.(2)The temperature of the direct expansion refrigeration system increased continuously along the length of the glass tube evaporator.The refrigerant temperature changed gently in the first half of the pipe and rises rapidly in the second half.The temperature difference of heat transfer decreased along the direction of pipe length,and the heat flow gradually decreases along the direction of pipe length.The change of heat flux under low temperature condition was relatively stable.The heat flux was high before the refrigerant evaporates completely.The heat transfer coefficient of direct expansion refrigeration system decreased along the pipe length direction.The average heat transfer coefficient of the whole system increased with the work temperature rising.The average heat transfer coefficient decreased with the increase of superheat.(3)The temperature of the gravity liquid supply refrigeration system increased along the length of the glass tube evaporator.The temperature change rates from-25℃ to 0℃ were1.78%,4.68%,8.75%,17.86%,28.76% and 51.19%,respectively.The temperature change rates in low-temperature conditions were small.The temperature difference of heat transfer increased with the increase of working temperature.The change rate of heat transfer temperature difference under low temperature condition was small.The heat flow decreases with the decrease of work temperature,and the overall trend of the heat flow decreased along the direction of pipe length.From-25℃ to 0℃,the change rates of heat flow along the pipe length were: 29.22%,46.80%,49.96%,63.88%,68.34%,and 72.83%,respectively.The decreasing rate of heat flow in low temperature condition was small.The general trend of the heat transfer coefficient of gravity liquid supply refrigeration system decreased along the pipe length direction.The change rates of heat transfer coefficient from25℃ to 0℃ were: 15.62%,27.03%,28.75%,38.25%,44.62%,47.44%,respectively.The change of heat flow and heat transfer coefficient at low temperature was stable and the fluctuation was small.The average heat transfer coefficient of the gravity liquid supply refrigeration system decreased first and then increases with the decrease of working temperature.The teperature change rate from-10 ℃ to 0 ℃ was 29.42%,and the temperature change rate from-10℃ to-25℃ was 89.75%.The rate of change of average heat transfer coefficient in low temperature condition was higher than that in high temperature condition.(4)Comparing the direct expansion liquid supply refrigeration system with the gravity liquid supply refrigeration system,the temperature of the gravity liquid supply refrigeration system changed more stably along the pipe length,but it gradually approached the direct expansion refrigeration system in the condition of-10 ℃ to 0 ℃.The heat flow rate of gravity liquid supply refrigeration system from-25℃ to-10℃ was higher than that of direct expansion liquid supply refrigeration system,and the heat flow rate of the two liquid supply modes was similar at-5℃ and 0℃.The heat transfer coefficient of the two liquid feeding methods decreased along the pipe length direction,but the reduction of gravity liquid feeding refrigeration system was smaller.Under the working conditions from-25℃to 0℃,the change rates of heat transfer coefficient of gravity liquid supply refrigeration system decreased by: 61.89%,42.67%,42.97%,29.79%,23.25%,11.54%,respectively.The average heat transfer coefficient of the direct expansion liquid supply system decreased with the decrease of working temperature.The average heat transfer coefficient of the gravity liquid supply refrigeration system decreased first and then increases with the decrease of working temperature.The heat transfer coefficient of gravity liquid supply refrigeration system under low temperature condition was larger than that under high temperature condition.It fully showed that the gravity liquid supply refrigeration system had more advantages at low temperature.(5)The main flow patterns of refrigerants in the direct expansion refrigerating system were:stratified flow,violent wavy flow,stationary wavy flow and irregular ring flow.In the direct expansion refrigeration system,it is found that the flow of single row tube was relatively stable,and even row tube was relatively violent.The main flow patterns of refrigerant in the tube of gravity liquid supply refrigeration system were bubble flow,air plug flow,airbomb flow,stratified flow and wave flow.Under the high temperature condition of gravity liquid supply refrigeration system,the nodes of flow pattern appeared earlier and the wavy flow will appear earlier and longer.The change of flow pattern of the two kinds of liquid supply modes was almost the same under different working conditions.The difference was that the length of the tube in each flow pattern was different.The two kinds of liquid supply methods had different flow types in the tube because the refrigerant flow into the evaporator was different and the liquid content was different.In contrast,the refrigerant flow and liquid content of the gravity liquid feeding refrigeration system into the evaporator were relatively large,so there will be a complete change process from bubble flow to gas plug flow,airspring flow,stratified flow and wavy flow.(6)The heat transfer coefficient of the glass tube evaporator was calculated by five kinds of correlation formulas of the heat transfer coefficient on the side of refrigerant,Shah correlation,Kandikar correlation,Gungor and Winterton correlation,Lui and Winterton correlation,and J.Chawla correlation,and the convection heat transfer coefficient of the air side was the air cross swept single tube and formula of thermal conductivity of pipe wall.The average error was 23.21%,33.33%,29.20%,24.43% and 19.28%,respectively.The average error of J.Chawla correlation was the smallest,Shah correlation was the second,and Kandikar correlation was the largest.The error of Shah correlation under high temperature was less than that of other correlations,and the error under-15 ℃ to 0 ℃was 5.65%,4.49% and 19.02%,respectively,which were less than 20%.At low temperature,J.Chawla correlation formula was the closest to the experimental value,and the errors from-25℃ to-15℃ were 14.67%,6.59% and 9.22%,respectively,all of which were less than15%.In push side,when the working condition was more than or equal to-10 ℃,the Shan correlation was used,and when the working condition was less than-10 ℃,the J.Chawla correlation was used.Combining the two correlations,the error was 9.94%,less than 10%. |
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