山药片红外联合热风干燥热质传递收缩模拟与品质

    Simulation of heat and mass transfer shrinkage and quality of yam slices dried using infrared combined hot air

    • 摘要: 为了准确揭示山药片红外联合热风干燥传热传质机理,在考虑山药片收缩变形特性的基础上,通过有限元软件COMSOL6.1建立了“温度场-湿度场”多场耦合的山药片红外联合热风干燥传热传质模型。模拟研究基于山药片在不同温度(50、60、70 ℃)下收缩变形的传热传质,并通过试验进行验证。分析不同温度对山药片品质(色差、复水比、多糖和尿囊素含量)的影响。结果表明:1)山药片体积比随干燥温度的升高而增加,在干燥温度分别为50、60、70 ℃时,其值分别为34.55%、37.23%、39.04%。2)在干燥温度为50、60、70 ℃时,红外联合热风干燥收缩模型可准确预测山药片干燥过程中干燥温度和含水率,其决定系数R2分别为0.973、0.976、0.981和0.983、0.984、0.974。3)山药片外部温度升高,表面水分开始蒸发,形成水分梯度。随着干燥的继续,红外热量在山药片内部不断积累,导致内部温度升高,水分向外扩散,进而减小了内外水分梯度。随着干燥温度的升高,增加了山药片温度和湿度梯度,促进了热量和质量的传递,提高了水分迁移的速率。4)在60 ℃时,干燥品质最优,其色差为7.49、复水比为2.65 kg/kg、多糖含量为24.17 mg/g、尿囊素含量为2.66 μg/g。该模型为其他物料在红外联合热风干燥技术的模拟研究提供有益借鉴。

       

      Abstract: This study aims to accurately reveal the heat and mass transfer of yam slices during infrared and hot air drying (IR-HAD). A multi-field IR-HAD model was established to couple the temperature and humidity fields in the yam slices using finite element (FE) software COMSOL 6.1, considering the shrinkage and deformation. The heat and mass transfer was simulated at different temperatures (50℃, 60℃ and 70℃) and then verified by a series of experiments. A systematic investigation was implemented to clarify the effects of different temperatures on the quality (color difference, rehydration ratio, polysaccharide content and allantoin content) of yam slices. The results showed that (1) the volume ratios of yam slices increased with the increase in drying temperature, which were 34.55%, 37.23%, and 39.04% at the drying temperatures of 50°C, 60°C, and 70°C, respectively. (2) The R2 values of the simulated temperature and moisture content considering shrinkage and deformation were 0.973, 0.976, 0.981, and 0.983, 0.976, and 0.974, respectively, indicating better agreement with the test ones. (3) The temperature field of yam slices presented consistent simulation and experimental drying. The surface temperature of the yam slice was higher than the internal temperature at the early stage of drying (0-15 min). The peripheral temperature was slightly higher than the center, leading to the forced hot-air heating. The infrared radiation further heated the inside of the yam slices, as the drying proceeded. As such, the internal and center temperatures were gradually higher than the external and peripheral ones. The moisture vapor diffusion of yam slices also showed a tendency to increase and then decrease during drying. There was an increase in the temperature of the yam slice and the rate of water evaporation in the pre-drying period (0-60 min); The temperature of the yam slice was stable in the mid-drying period (60-120 min). The infrared hot air was mainly used to evaporate the latent heat, and the amount of water evaporation and diffusion increased gradually to the maximum; The surface of the yam slice formed a hard layer in the post-drying period (120-195 min), leading to the less water evaporation and diffusion. At the same time, there was an uneven moisture concentration of yam slices during the whole drying. The internal moisture concentration was higher than the surface ones, where the center concentration spread to the outside. The reason was that the heating occurred mainly on the surface and periphery of the yam slices, leading to the vaporization of moisture in the formation of an internal humidity gradient. (4) The quality of dried yam slices was evaluated by the coefficient of variation. The highest performance was achieved at 60 ℃, with a color difference of 7.49, a rehydration ratio of 2.65 kg/kg, a polysaccharide content of 24.17 mg/g, and an allantoin content of 2.66 μg/g. This finding can provide a strong reference for the simulation of various materials in infrared and hot-air drying.

       

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