Optimization Of Pre-cooling Air Parameters For Cherries

With the development of Chinese economy,it caused rapidly expanded of demanding for fruits and vegetables.However,due to the high environment temperature,how to rapid reduce the field heat of fruits and vegetables had become an important step to improve the shelf life.The forced air pre-cooling was one of the most effective method which had been used in the pre-cooling process of agricultural products.In the forced air pre-cooling,the special air-stacking method of fruits and vegetables was combined with the felt to form an intermediate airflow path,and the cold air is forcibly fed through the differential pressure fan.By using this method,it could achieve the purpose of rapid cooling of fruits and vegetables.In this paper,in order to obtain the optimal air parameters,the pre-cooling time,energy consumption and temperature uniformity were analyzed by using cherries.The main tasks were as follows:1.Establishment of a three-dimensional mathematical model of cherry pre-coolingThe numerical simulation of the forced air pre-cooling process of cherries was carried out by using CFD software.At the same time,in order to get closed to the real situation,the physical properties of the cherries as a function of temperature has been obtained by ASHRAE manual and the mixture calculation formula.Finally,the simulation data of the air velocity of 1.45m/s and 0.72m/s were verified.The temperature curves of the experimental and simulated were basically the same,and the maximum temperature difference between the two situation was within 2?.2.Effect of air velocity on pre-coolingThrough experiment and simulation,when the range of air velocity was 0.5m/s~3.0m/s,the higher air velocity was,the shorter the pre-cooling time,the greater the pre-cooling energy consumption and the smaller the temperature unevenness were.When the air velocity exceeded 2m/s,the increase of air velocity had little effect onthe pre-cooling time.When the range of air velocity was 0.5m/s~1m/s,the pre-cooling energy consumption decreased gradually.When the range of air velocity was1m/s~3m/s,the pre-cooling energy consumption increased rapidly.When the air velocity exceeded 1.5m/s,the influence of air velocity on the pre-cooling unevenness was little(the change rate was less than 0.59%).Therefore,the best air velocity of cherries' pre-cooling was 2m/s.3.Effect of air temperature on pre-coolingWhen the range of air temperature was-2?~2?,the higher air temperature was,the longer the pre-cooling time,the smaller the pre-cooling energy consumption and the smaller the temperature unevenness were.With the unite air temperature increased,the decrease rate of unevenness was 14.44%,15.22%,28.57% and 42.51%,the decrease rate of total energy consumption was 2.10%,2.29%,1.23% and 0.30%,and the increase rate of pre-cooling time was 10.88%,13.21%,17.33% and 27.27%respectively.So the optimal temperature was 0? in the cherries' pre-cooling.4.Effect of air direction on pre-coolingThrough the experiment and simulation analysis,the influence of the direction of the air on the pre-cooling was obtained.The air direction had no obvious influence in the pre-cooling time(its range was 25min~26min).The more frequently the air direction changed,the more energy consumption was.When the periodic of air direction was 10min(the change of air direction 5 times),the pre-cooling energy increased significantly,and the maximum energy consumption of pre-cooling reached44.42kJ/kg,when the periodic of air direction was 10 minutes,which was 25.18%higher than single air direction.In the hole process,the temperature unevenness decreased significantly,and the unevenness was only equal to 50.22% to 61.92% of the single air direction.Therefore,in the process of cherries' pre-cooling,the air direction was once.6.Optimal air parameters for pre-coolingThe air parameters(air velocity,air temperature,air direction)of pre-cooling were analyzed comprehensively by using pre-cooling time,energy consumption and unevenness.The optimal air parameters were obtained as follows: air velocity,airtemperature and change times of air direction were 2m/s,0? and once,respectively.