苹果垂直送风式压差预冷性能模拟与分析

Simulation and Analysis of Forced-air Precooling of Apples with Vertical Air Supply

压差预冷是广泛应用于果蔬保鲜的预冷措施,采用垂直送风方式相较于传统的水平送风方式具有更好的冷却效果。通过建立单箱苹果垂直送风式压差预冷过程的数值模型,分析了送风速度、送风温度及箱体开孔率对预冷性能的影响。模拟结果表明,当送风速度由0.5 m/s增大至2.5 m/s时,苹果的冷却时间由127 min下降至90 min,苹果中心温度平均偏差由0.51℃下降至0.20℃;当送风温度由2℃升高至6℃时,7/8冷却时间不变,冷却终温与送风温度接近;当箱体开孔率由15%增大至30%时,冷却速率相近,压降由418 Pa降低至86.8 Pa,苹果中心温度平均偏差由0.73℃上升至1.11℃。结合实际情况,在15%开孔率下垂直送风式压差预冷的最佳送风速度约为2.0 m/s,低于相同工况下水平送风的最佳送风速度;在压降满足设计要求的前提下,出于结构强度考虑,宜采用较小的箱体开孔面积;为了减小质量损失率,应使气流均匀流过所有苹果表面,从而使其表面温度快速下降。

Forced-air precooling is helpful in preserving the quality of fruits and vegetables.Compared with the traditional horizontal pattern,the vertical forced-air precooling has higher cooling efficiency.Computational fluid dynamics model of vertical forced-air precooling of apples was built to analyze the influence of air-inflow velocity,air-inflow temperature and the ratio of vessel aperture area on the performance of vertical forced-air precooling.It was found that when the air-inflow velocity was increased from 0.5 m/s to 2.5 m/s,the cooling time of apple was decreased from 127 min to 90 min,the main variation of the center of apples was decreased from 0.51℃to 0.20℃,and the weight loss rate was decreased from 0.00786%to 0.00570%.When the air-inflow temperature was increased from 2℃to 6℃,the cooling time remained unchanged and the finishing temperature was close to the air supply temperature.When the opening area of box was increased from 15%to 30%,the cooling rate was almost unchanged,the pressure drop was decreased from 418 Pa to 86.8 Pa,the mean variation of the center of apples was increased from 0.73℃to 1.11℃,and the weight loss rate was about 0.006%.Concerned with the actual situation,the optimal pre-cooling air-inflow velocity of vertical forced-air precooling was about 2.0 m/s,which was lower than that of horizontal forced-air precooling.It was advisable to adopt the smaller box opening area to ensure the structural strength if the pressure drop was acceptable.In order to reduce the weight loss rate,air should flow evenly over all apple surfaces to ensure the surface temperatures drop as quickly as possible.By comparing with the experimental data,it was found that the results obtained by the model adopted were basically consistent with the trend of the experimental results,and the maximum temperature difference did not exceed 1.5℃,which can be used to evaluate the performance of precooling with air pressure difference in vertical delivery of apples.