采后热风与热水处理杀虫的水果温度场有限元模拟

Finite element analysis on fruit temperature fields based on postharvest disinfestations with hot air and water treatments

热风和热水加热具有高效、环保且易于控制的优点,是一种替代化学熏蒸控制采后水果虫害的有效物理方法。但由于缺乏对加热速率、加热时间与温度分布等问题的深入与系统研究,常导致杀虫效果较差或对水果品质造成一定负面影响。为了研究水果采后热处理过程的传热机理,建立了水果热处理时的非稳态传热模型,利用有限元分析软件COMSOL建模求解,并进行了试验验证。结果表明,模拟值与实测值之间的均方根误差均低于8%,从而验证了模拟的可靠性。达到相同的热处理效果,水果热水(55℃)加热时的热处理时间仅为热风(55℃)的30%,所以热水是一种更有效的加热介质。增加热风的流速可以提高加热速率,而热水循环速度对传热速率影响较小。水果内部的传热速率主要受水果大小、介质流动速度和水果形状的影响,热扩散系数对水果加热时间的影响较小。所建立的水果传热模型及相关试验结果可为水果采后热杀虫工艺参数的确定及优化提供参考。

Hot air and hot water heating have been extensively studied as effective physical treatment methods to replace chemical fumigation for controlling insect pests in fruits and vegetables because of environmental benefit and easy control. However, it is lack of systematic research on heating rate, heating time and temperature distribution, the insect mortality requirement is not met or fruit quality is negatively affected. To study the heat transfer mechanism of postharvest fruit heat treatment process, a computer simulation model was developed to analyze the heating rate and the transient temperature distribution in thermally treated fruits using finite element-based commercial software, COMSOL, based on the unsteady heat transfer, a set of differential equations that govern heat transfer in fruits was reduced into a group of algebra equations in the simulation model. The measured surface and center temperatures of fruits during hot air and water heating were compared against to the results obtained from the finite element simulation. The root mean square error between simulated and measured temperatures was all below 8%. The comparison analysis showed that the simulation results were in good agreement with the measured values, which indicates the reliability of the simulation model. With the validated simulation model, the impacts of various parameters on heating rates were systematically studied using forced hot air and water treatments. To reach the same thermal effect, hot water （55~C） heating time was about 30% in hot air （55~C） treatments. Water was more efficient medium than air. Increasing air speed increased heating rates, but water circulation speeds had little impact on heat transfer rate. The most important parameters in the model included the fruit size, fruit shape and the heating medium followed by the heating medium speed and thermal diffusivity. This study demonstrated that the computer simulation model can be used to evaluate the impacts of various heating parameters on the temperature-time history in fruits. These parameters include fruit size, fruit shape, heating medium speed, and thermal diffusivity. Combining the hot air or hot water treatment with the fast heating method, e.g. radio frequency or microwave energy, further improves the fruit heating efficiency since electromagnetic energy may eliminate conduction as a major rate-limiting factor directly delivered to the fruit interior. Furthermore, when combined with insect mortality and quality kinetic information, the model can be used to support for the heat treatment process and optimize the process parameters in postharvest thermal disinfestations.