Return Air Channel Numerical Simulation And Research Of Forced-air Precooling Experiment For Fruits And Vegetables Precooling Equipment

Cold chain logistics,as the main circulation mode of fruits and vegetables agricultural products,has been increasingly applied after years of research and development in China.The most critical issue is the "first mile" and "last mile".Precooling is a vital part of solving the "first mile" problem.There are various ways of precooling,among which the forced-air precooling has the advantages of speed,uniform and wide applicability to fruits and vegetables.At the same time,the forced-air precooling operation is complicated,and the precooling process technology for different fruits and vegetables also needs detailed research.Aiming at the characteristics and shortcomings of the existing forced-air precooling technology,The following studies were carried out in this paper.(1)This paper uses CFD simulation software to construct the return air channel models of the forced-air device under different palletizing methods,and simulates its precooling process.Simulation of the flow field distribution under the conditions of nine different length and height return air channel parameters.According to the simulation results,it can be seen that the air pressure near the return air outlet of the forced-air return air channel is large.As the number of stacking rows increases,the overall precooling uniformity decreases,but the increase of the number of stacking layers has a small impact on the overall precooling effect.At the same time,the precooling process of four different sizes differential pressure forced-air ventilation equipment was simulated on this basis.The return air passage was too narrow,and the precooling effect of a single row of vent holes was poor.Based on this,the width of the return air passage was selected to 150 mm.Two rows of 70 mm diameter holes.(2)At the same time,some experiments were carried out to optimize the precooling process by using the existing forced-air precooling equipment and taking grapes as experimental objects.In order to verify the simulation results,the precooling time,precooling uniformity,and weight loss rate of the grapes in a four-row one-layer,two-row two-layer,and one-row fourlayer palletizing method were analyzed.It can be known from the experimental results that with the increase of the number of stacking layers and rows,the precooling rate will decrease accordingly,but the difference in the increase in the number of layers is not obvious,and the precooling rate with the increase in the number of columns is more obvious.The stacking method also has a certain effect on the weight loss rate.The closer to the return air outlet,the higher the weight loss rate.As the number of rows increases,the precooling uniformity also decreases.Considering a variety of factors,the overall precooling effect of the four-layer,one-row palletizing method is the best,which is basically consistent with the simulation results.(3)In order to further optimize the forced-air precooling technology,half-cooling time and 7/8 cooling time are taken as important indicators.The experiments were performed under the conditions of air supply speed of 0.5m/s,1m/s,1.5m/s,2m/s,2.5 m/s and air supply temperature of 0?,0.5?,1?,1.5?,and 2?.It is found that the lower the air supply temperature,the larger the air supply speed,and the shorter the precooling time;the larger the air supply speed,the higher the weight loss rate,and the air supply temperature has no significant effect on the weight loss rate;The cooling speed has a greater effect before the half-cooling,and the supply air temperature has a greater impact on the precooling speed after the half-cooling time.Based on the results of this experiment,double factor variable air supply parameters was designed,and factors such as precooling time,weight loss rate,precooling uniformity,and power consumption were considered to verify the optimal precooling process combination.The air supply speed is 2m/s,the air supply temperature is 1.5?before the half-cooling time,,the air supply speed is 0.5m/s,the air supply temperature is 0? after the half-cooling time,and the overall pre-cooling effect is the best.This paper optimizes from the aspects of palletizing method,forced-air ventilation device,and different air supply strategies to ensure the precooling efficiency while playing a certain energy-saving effect,which has Guide the production process to a certain extentcertain.