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《Journal of Agricultural Engineering Research》1997,66(4):267-273
Previous research has shown that the variance in the moisture content of individual maize kernels is significant at harvest. In this series of three papers, the stochastic nature of the moisture content of maize before and after drying is explored. Part 1 presents experimental data of the distribution of moisture content of maize kernels in samples taken at harvest time, after thin-layer drying, and after crossflow drying. The moisture content in recently harvested and in thin-layer dried maize appears to be normally distributed and in crossflow-dried maize to be skewed. In Part 2, stochastic models are developed of the distribution of moisture content of maize kernels dried in concurrent-flow, counter-flow and crossflow dryers, and in Part 3 one of the models is used to analyse the process of grain drying in a crossflow dryer. 相似文献
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一种稻谷横流循环干燥数学模型的组建 总被引：1，自引：0，他引：1
谷物干燥过程具有多变量耦合、非线性、大时滞、因果关系错综复杂等特点,要实现干燥过程的精确控制,必须探明谷物干燥过程中主要因果变量间的内在关系.为此,以带有缓苏段的横流封闭循环式粮食干燥机为原型,以稻谷为干燥对象,介绍了稻谷的水分扩散系数方程、平衡水分方程、横流干燥方程、缓苏特性方程、缓苏时间确定等数学模型,从数学角度阐明环境温湿度、介质温度、谷物初始含水量、干燥时间等参数与干燥谷物含水量、谷温、缓苏谷粒表面含水量、缓苏时间的关系,以期为实现干燥系统智能精确控制提供理论依据. 相似文献
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对油茶果干燥脱蒲技术进行试验研究,比较分析了常规热风干燥、热泵干燥和微波干燥对油茶果脱蒲效果的影响。研究发现,由于干燥不均,微波干燥无法实现脱蒲。采用热泵干燥,油茶的脱蒲率较低,不到80%,而采用常规的热风干燥,在65 ℃下干燥6 h,油茶的脱蒲率可以达到96.97%。对油茶籽进行了热风薄层干燥试验并运用Arrhenius方程计算油茶籽干燥过程中的水分扩散系数和活化能。结果显示,油茶籽的水分有效扩散系数为1.681 4×10-9~3.046 7×10-9 m2/s,干燥活化能为21.323 7 kJ/mol。油茶籽干燥过程可以用Sutherland或Paul Singh模型进行描述,验证试验表明,理论与实际数据的平均误差为4.91%。因此,该模型可以用来预测不同温度下油茶籽干燥过程中的含水率变化情况。 相似文献
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半环对撞流干燥的流动与传热传质分析 总被引：4，自引：0，他引：4
建立了半环对撞流干燥过程中的流动与传热传质数学模型,针对模型各部分的不同特点,分别采用合适的数值处理方法进行了数值计算,不同工况下,模型计算与试验结果吻合良好,能较好地预测干燥过程中入口空气温度,载带率及物料初始含湿量等因素对干燥过程的影响。通过对模型计算和试验结果的分析,提出了提高半环对撞流干燥速率与降低干燥能耗和费用的有效途径。 相似文献
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为了解固定床花生换向通风干燥作业特性,为已研发的箱式换向通风干燥设备提供合适的花生干燥通风参数,以花生与空气间的传热传质为基础,建立干燥状态偏微分方程组,并以此为基础采用有限差分法对干燥过程进行数学模拟,分析左右干燥室同时从下向上通风干燥时长、换向干燥阶段换向时间、单位体积通风量对干燥行为的影响,并采用均匀设计和综合加权评分法确定最优通风干燥参数。结果表明:从下向上通风阶段,热量主要用于花生加热和表层水分快速蒸发,下、中、上层物料干燥升温速度差明显,易形成较大水分梯度;换向通风干燥阶段,物料温度呈类波浪状波动,有效控制整床物料干燥均匀性。缩短从下向上通风时间,有助于降低耗能和水分差,但对干燥耗时和生产率影响较小;换向时间的改变对耗时、生产率、耗能、水分差影响较小;单位体积通风量的增加有助于干燥耗时、水分差的降低及生产率的提高,但耗能亦将快速增加。均匀设计和综合加权评分表明,含水率>25%阶段通风量1 394 m~3/(m~3h),含水率15%～25%阶段通风量838 m~3/(m~3h),含水率0.98,模拟仿真可准确描述干燥过程物料温度和含水率变化。为花生换向通风干燥设备改进和工艺优化提供参考。 相似文献
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荔枝果肉热风干燥薄层模型 总被引：20，自引：1，他引：20
利用热泵干燥装置探讨了热风温度和热风风速对荔枝果肉干燥水分比MR和干燥速率U的影响。结果表明:荔枝果肉薄层热风干燥是内部水分扩散控制的降速干燥过程。对9种常见食品薄层干燥模型进行试验数据非线性拟合,通过比较评价决定系数R2、卡方χ2和标准误差eRMSE以及试验验证,结果显示Page模型是描述荔枝果肉薄层热风干燥过程的最优模型。不同干燥条件下有效水分扩散系数Deff和活化能Ea的求解结果表明,有效水分扩散系数Deff随热风温度和风速的增加而变大,平均活化能Ea为29.939 kJ/mol。 相似文献
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《Journal of Agricultural Engineering Research》1996,63(4):333-343
Heat and mass transfer characteristics of alfalfa pellets are needed in the optimization of coolers for freshly-made pellets and in managing storage schedules of the pellets in silos and bins. Moisture diffusivity and thermal properties are important parameters used to characterize the heat and mass transfer ability of a material. In this study, experimental thin-layer data on (a) moisture desorption, (b) moisture absorption and (c) rate of heating of alfalfa pellets were collected. By applying the inverse theory and using second order mass transfer and heat transfer equations in cylindrical coordinates, the moisture absorption and desorption data were used to estimate the moisture diffusivity as a function of moisture content of the pellets while the heating rate data were used to estimate the thermal properties (thermal conductivity and specific heat) as a function of pellet temperature.Better estimates were obtained when moisture diffusivity of the pellets was exponentially related to moisture content in comparison with a linear relation between moisture diffusivity and moisture content. Moisture diffusivity during desorption (2·40×10-9to 4·12×10-9 m2/s) was about three times that of the values of diffusivity during moisture absorption (7·50×10-10to 1·26×10-9 m2/s). A good fit to the experimental heating rate data was obtained when thermal conductivity and specific heat of the pellets were linearly related to temperature. Over a temperature range of 2 to 110°C, estimated particle thermal conductivities and specific heats of the pellets were in the range of 0·04 to 0·19 W/m K and from 962 to 2114 J/kg K respectively. 相似文献
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《Journal of Agricultural Engineering Research》1999,72(2):187-195
The results of measurements of loss of water in convection drying of slices of cut vegetables and mushrooms, not touching one another, indicate that the rate of drying of these products decreases from the beginning of the process. Consequently, almost all researchers have assumed that during the initial stage of drying, there is no stage of constant drying rate determined by conditions of external water exchange. Instead, it was assumed that models of the drying of these products from initial to final water content should be solutions of differential equations of internal mass diffusion or semi-empirical exponential equations. Such a view is not, however, compatible with the theory of convection drying of solids with high initial water content, such as vegetables and mushrooms. The results of studies presented in this paper, show that the initial stage of drying of red beet, onions, carrots, garlic and mushrooms, is a constant rate process determined by the known model of the theory of drying based on laws of external exchange of heat and water. Losses of water from 10 to 30% were compatible with the model, with a relative error of less than 1%, and from 40 to 50% with an error less than 5%. The introduction into the equation of a coefficient to allow for shrinkage of the products, extended the range of applicability of the model from 20 to 50% water loss with a relative error of less than 1%, and from 50 to 80% with an error of less than 5%. 相似文献
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根据非平衡态热力学和相平衡理论,建立了多孔介质对流干燥内部传质模型、传热模型,该模型充分考虑了传热与传质之间的相互藕合关系。用VB编程软件对该模型进行了分析求解,设计开发模拟软件。以切片土豆为例进行传质模拟计算,并与土豆片干燥实验数据进行对比,结果表明:模拟值和实测值十分接近,二者的最大相对偏差小于9.1%。模拟结果显示土豆对流干燥中内部水分由内往外迁移,以及干燥过程中产生的表低内高水分分布规律。 相似文献
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南瓜浆滚筒干燥动力学模型 总被引：1，自引：0，他引：1
以辊式布料的单滚筒干燥机为对象进行南瓜浆滚筒干燥试验,探讨了滚筒干燥过程中物料的干燥动力学特性及其干燥动力学模型。根据物料的状态将滚筒干燥过程分为浆状和膜状两个阶段,测定了不同蒸气压力下物料的含水率,分析了干燥速率随蒸气压力和时间的变化规律。结果表明:物料中的水分大部分在浆状区中蒸发,蒸气压力越高,干燥速率越快;物料在膜状区中为降速干燥阶段,蒸气压力越高膜状区起始含水率越低,起始阶段的干燥速率越慢,干燥速率下降也越慢,干燥时间越短。膜状区物料含水率的试验数据与主要薄层干燥模型进行拟合,Midilli-Kucuk模型可以很好地预测南瓜浆滚筒干燥的动力学特性。 相似文献
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《Journal of Agricultural Engineering Research》1994,57(3):199-205
Microwave drying of grains is fundamentally different from either convection or conduction drying. Drying of cereal grains appears to proceed mainly in the period when the drying rate is decreasing (in the falling rate period), a characteristic of internally controlled diffusion. However, the analytical solution to Fick's second law of diffusion for a homogeneous, isotropic sphere with constant initial and boundary conditions does not adequately describe the drying behaviour. In an attempt to overcome this problem, the moisture ratio was written in terms of the surface moisture content rather than the equilibrium value. The associated surface drying coefficient, as determined by an iterative technique, was found to be expressible as a linear function of the initial free moisture content of the grain. The resulting empirical model better described the observed drying kinetics. This approach also resulted in good fits to independent data from experiments on convective drying of rough rice, microwave drying of wheat and combined microwave-fluidized bed drying of wheat. 相似文献
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介电常数与含水率有很高的相关性,可通过测量农产品的介电特性参数预测其干燥过程含水率,高压脉冲电场可有效加快干燥过程,但介电常数和含水率在此过程中的影响机理尚待进一步研究。为解决高压脉冲电场作用下果蔬介电常数与含水率之间关系的微观解释不明确问题,建立有效的苹果含水率与介电常数关系模型,本文以苹果为研究对象,三维模拟其内部电场参数,使用蒙特卡罗法对高压脉冲电场下苹果的介电特性进行理论模拟计算,把苹果内部宏观和微观介质作为集总参数来等效替代其内部整体结构。实验结果显示,当修正系数为8.96时,控制苹果相对水含率从88%逐步下降到18%,介电常数计算值与实测值间相对误差稳定在10%以内。研究为脉冲电场处理果蔬加工提供理论依据。 相似文献