Thermal Simulation Of Temperature Field In Cold Storage Box And Effect On Preservation Quality Of Fruits

China is a big country in the production and consumption of fresh fruits and vegetables.In recent years,with the continuous improvement of people's requirements for the quality of fresh fruits and vegetables,the cold chain has been fully developed as a way to meet the quality assurance in the whole process of storage and transportation of fruits and vegetables.Phase change cold storage technology is an important application part of the cold chain.At present,domestic and foreign scholars have done a lot of research on the mechanism of phase change cold storage,the temperature uniformity in the cold storage,and the cold storage packaging box.However,due to the large space,complex and random storage form and the dynamic change of external environment temperature in the actual cold storage,it is often difficult to achieve the purpose of uniform temperature field in the storage by a single optimization method.In view of the problem of poor temperature field in traditional storage tanks,a cold storage packaging box was proposed.By using the cold storage layer directly contacting the environment as a buffer,the temperature field inside the packaging box was uniform when the temperature of the storage tank fluctuated greatly.At the same time,the fluctuation of the temperature of the storage tank could be used to dynamically cool the cold storage solution,so that the large ambient temperature field was optimized locally in the cold storage packaging box.In this paper,the cold storage packaging box is designed and optimized by numerical calculation and experimental research.A variety of experimental studies were carried out to eliminate the supercooling of the solution;fluent software was used to simulate and optimize the roof selection,the thickness of the cold storage layer,the ambient temperature and the loading mode.Freeze-thaw tests were carried out on the cold storage packaging box under no-load and load conditions to verify the reliability of the model and the practical application value of the box.Finally,the storage experiment of sand pit radish was carried out in the cold storage packaging box to further verify the practicability of the cold storage packaging box.(1)The simulation results show that compared with the thickness of 5mm,15mm and30mm cold storage layer,the temperature field is more uniform when the thickness is 20mm;with EPS foam as the top cover,the temperature uniformity of the cold storage container is much better than that of the organic glass and PE film group,and the internal temperature is the lowest at the same ambient temperature.Under the loading condition,the temperature field of basket-mounted fruit stacking mode is better than that of close packing mode,and there is no local high temperature area in the box and the overall temperature is low.When the ambient temperature changes,the uniformity of the temperature field in the cold storage packaging box remains unchanged,and the overall temperature changes only slightly.(2)The experiment of reducing the supercooling degree of the cold storage solution showed that the higher the purity of the solution was,the greater the supercooling degree was.At the ambient temperature of-6°C,the supercooling degree of deionized water reached6.2°C,while that of tap water with high impurities was-5.2°C.In the screening of nucleating agent,the comprehensive performance of Ti O₂ nano solution was the best,slightly better than that of sodium polyacrylate gel and borax solution,and the performance of diatomite solution was the worst.The effect of ambient temperature on the supercooling degree is obvious.In the low temperature environment of-18°C,the supercooling degrees of the above various solutions are basically eliminated,and the supercooling degree is maintained in the range of 0.1–0.4°C.Retention of ice-assisted nucleation greatly improves the supercooling degree,and does not depend on the low temperature environment.The container material has no significant effect on the supercooling degree.(3)The experimental study on the cold storage packaging box shows that under the no-load condition,the cold storage packaging box with PE film at the top of±5°C can maintain a stable temperature field of about 50 h.Under the same conditions,the simulation calculation can maintain a cold storage time of 52.1 h,and the error between the experiment and simulation is 4.2%.Under the loading condition,in the environment of-5°C,the simulated temperature in the box is close to the experimental results in the range of-0.2°C–-0.4°C.(4)Fruits and vegetables experiments show that compared with the fruits and vegetables in the storage tank,the quality of fruits and vegetables in the cold packaging box is basically better than that in the cold storage,and the water content and lignin index are the most obvious.However,there was no significant difference in VC and chlorophyll content between the two groups,and the cold storage packaging group was slightly better than the storage group.