Study On Temperature Control And Temperature Field Numerical Simulation Of Liquid Nitrogen Refrigerated Box

In recent years,the consumption level of residents has increased,eating habits have changed,and the range of foods available for consumption has also expanded.In order to ensure food safety,cold chain logistics has developed rapidly.As an important part of cold chain logistics,refrigerated transport requires corresponding carrier to provide a relatively stable low-temperature environment for food,avoid food spoilage during transportation,and endanger consumer health.The commonly used mechanically refrigerated transportation equipment has serious pollution,while the refrigerated transportation equipment using liquid nitrogen vaporization refrigeration has simple structure,economic and environmental protection.Under the policy of green development,the market prospect of liquid nitrogen refrigerated transportation equipment is broad.The liquid nitrogen refrigerated box is a kind of refrigerated transportation equipment using nozzle jet liquid nitrogen to lower the temperature.This paper briefly summarizes the working principle of the liquid nitrogen refrigerated box.By analyzing the heat source absorbed by the liquid nitrogen refrigerated box during transportation,the liquid nitrogen storage capacity and the liquid nitrogen jar volume were calculated according to the refrigeration temperature,external temperature and transportation time during transportation.The paper introduces the basic theory of heat transfer.By analyzing the advantages of computational fluid dynamics,the simulation process of computational fluid dynamics is briefly described,and the relevant control equations are analyzed,which lays a foundation for the numerical simulation of the temperature field of liquid nitrogen refrigerator.In order to ensure the uniform temperature distribution in the liquid nitrogen refrigerated box during transportation and avoid the uneven temperature distribution that may cause frostbite or deterioration of the goods,the paper simulates the temperature field in the liquid nitrogen refrigerated box during the refrigeration process,and analyzes its impact on the uniformity of the temperature field in the box by changing the nozzle flow rate,injection angle and fan pressure.The study found that when the fan is not running,the greater the flow rate of liquid nitrogen,the faster the cooling rate in the box,but the uniformity of the temperature field is poor,and the top temperature is significantly higher than the bottom temperature.Increasing the injection angle,the uniformity of the temperature field in the box is improved,but the improvement effect is limited.When the fan starts to run,the temperature field uniformity is obviously improved,but the uniformity of temperature field is not obviously improved by continuously increasing the fan pressure.Various emergencies may occur during transportation,resulting in the failure of the refrigeration system of the liquid nitrogen refrigerated box.In order to analyze the change of the temperature field in the box with time after the failure of the refrigeration system,the temperature field in the box with different insulation layer thickness,thermal insulation material,cargo carrying quality and stacking mode after the failure of refrigeration system is studied by numerical simulation.Comparing the simulation results under different conditions,it can be seen that the greater the thickness of the insulation layer,the better the insulation performance of the thermal insulation material,and the slower the temperature rise in the box.However,the increment of insulation layer thickness is not proportional to the increment of insulation effect.Considering the insulation effect,the quality of the loaded goods and the cost of the insulation layer,the paper chooses 0.048 m rigid polyurethane material as the insulation layer of the box.The more the cargo in the box is loaded,the more concentrated the stacking is,and the slower the cargo temperature rises.By drawing the curves of temperature at different positions with time,it can be seen that the temperature field in the box is relatively uniform under no-load condition,and the heating rate in the box is faster.In case of loading cargo,the heating rate inside the cargo is lower than that at the edge of the cargo,and the heating rate of the top gas is higher than that of the bottom gas.The heating rate of the top cargo is higher than that of other positions during multi-layer stacking.Verify the correctness of the simulation conclusions and methods through experiments.Combined with the simulation results,relevant measures are proposed to reduce the heating rate of goods in the liquid nitrogen refrigerated box,avoid deterioration of goods and reduce economic losses.