Design And Experimental Study Of Low Temperature Logistis Box With Nitrogen Refrigeration System

With the rapid development of the global economy and the great improvement of people’s living standard,people’s diet structure is gradually changing.The problem is how to ensure the quality and safety of food.The cold chain is to ensure the perishable food standards of the important means of circulation,to ensure quality and safety of food has important significance.At present,the circulation of cold chain equipment is mainly to meet the needs of food freezing and cold storage above-18 ℃, the device of low-temperature frozen storage is very deficiency in the range of-18℃~-60℃.A kind of low temperature logistics box with liquid nitrogen refrigeration was designed and studied for the low temperature circulation of the food below-18℃,to make up some of the losses caused by the weak link in the end of cold chain.In this paper, the low temperature logistis box was designed according to the theoretical calculation,numerical simulation and experiment.Theoretical calculation is carried out according to the relevant literatures around word,then the FLUNT software was used to calculate the heat transfer of the thermal insulation layer,the expriment velocity on the outlet of fan and the temperature change of the tuna fish block.The dropping process of the temperature in the logistis box was meassured under full loaded condition, the temperature change of the fish block and the consumption of liquid nitrogen were studied under different inlet velocity.Finally,the exprimental results were compared to verify the correctness of the simulation method.The follow conclusions are obtained.(1) The simulation results for the insulation structure show that the temperature difference of the multilayer plate achieves the maxum value as the vaccume insulation plate(VIP) is located in the middle of the multilayer plate,which is larger by about 3.2℃ than that as the VIP is located outside.The temperature difference basically has no significant difference as the vacuum plate is located on the high temperature side and the low temperature side,that means the heat insulation performance of the vacuum plate in the middle is the best.The different insulation structure simulation results show that the insulation performance of the multilayer(VIP+PU) is significantly better than that of the polyurethane(PU) under the same thickness. The simulation results of the effects of different air velocity on the temperature in the logistics box show that the temperature changes of the food is ideal in the box under the air velocity of 1.5m/s and the freezing time of the food was 240 min.(2) The exprimental results of the temperature changes in the logistics box under no-load conditions show that the inside temperature decreases rapidly with time and the undercooling occures after reaching the set value of the temperature.The undercooling continues about 10 min,then gradually tends to the set value,finally the inside temperature(3) is maintained at the set value(-60℃). The experimental results under full loaded conditions show that the difference of vertical temperature distribution is small,there is a big difference in the horizontal temperature distribution.(3) The experimental results for the logistics box under full loaded show that the volume of the liquid nitrogen tank can meet the consumption of liquid nitrogen for 12 hours frozen storage transportation,under the air velocity of 1.5m/s is about by 1.6Kg more than that under the natural heat transfer,which relatives to 6.4% higher than that under the natural heat transfer conditions.(4) The experimental datas were compared with the simulational results under the full loaded conditions,and it can be seen that the temperature trend of the experimental results and the simulation results is consistent.The simulated freezing time is 162 min and the experimental results is 144 min,which the relative error between them is 11.11 %.The simulation results show that the cooling process for a single steak comply with the Coanda Effect,the decrease of temperature at both ends is significantly faster than that on the upper and lower sides. The numerical results are in good agreement with the experimental results.The simulated freen time is 123 min,and the experimental freezen time is 138 min,and the relative error between the two is about 10.87%.