A flux equation of diffusion for bi-disperse porous catalyst pellets was proposed by modifying the previously developed model equation over fractal trajectories. The proposed fractal model equation considered the same...A flux equation of diffusion for bi-disperse porous catalyst pellets was proposed by modifying the previously developed model equation over fractal trajectories. The proposed fractal model equation considered the same tortuous degree for both micro-and macro-pores. The experimental data of diffusion over a bi-disperse Ni/gamma-alumina pellet were obtained with a standard Wicke-Kallenbach diffusion cell for both carbon monoxide- ethylene and carbon dioxide-ethylene binary mixtures. The fitting between experimental results and the fractal model equation leads to a fractal dimension of 1.11. The prediction of diffusion flux over the bi-disperse Ni/gamma- alumina pellet by the proposed fractal model equation is much better than the traditional tortuosity-based model equation by comparison with the measured flux through the pellet.展开更多
Homogenization theory provides a rigorous framework for calculating the effective diffusivity of a decaying passive scalar field in a turbulent or complex flow.The authors extend this framework to the case where the p...Homogenization theory provides a rigorous framework for calculating the effective diffusivity of a decaying passive scalar field in a turbulent or complex flow.The authors extend this framework to the case where the passive scalar fluctuations are continuously replenished by a source (and/or sink).The basic structure of the homogenized equations carries over,but in some cases the homogenized source can involve a non-trivial coupling of the velocity field and the source.The authors derive expressions for the homogenized source term for various multiscale source structures and interpret them physically.展开更多
为了解莲子干燥过程中水分传递过程,监控、预测水分变化,该文通过开展莲子薄层热风干燥试验,考察了莲子在不同干燥温度（50、60、70、80、90℃）下干燥特性,建立了莲子热风干燥试验模型;利用低场核磁共振技术（nuclear magnetic resonan...为了解莲子干燥过程中水分传递过程,监控、预测水分变化,该文通过开展莲子薄层热风干燥试验,考察了莲子在不同干燥温度（50、60、70、80、90℃）下干燥特性,建立了莲子热风干燥试验模型;利用低场核磁共振技术（nuclear magnetic resonance,NMR）,弛豫时间（transverse relaxation time,T2）和成像（nuclear magnetic resonance imaging,MRI）,考察了干燥过程中莲子内部水分分布状态与变化规律。结果表明,莲子干燥一直处于降速干燥段;干燥温度显著影响干燥过程（P〈0.05）,干燥温度升高,干燥时间缩短;通过比较4种数学模型,发现莲子干燥过程采用Midilli模型（决定系数R2〉0.998）进行准确模拟（相对误差E〈10%）;有效扩散系数在6.056 7×10^（-10）～1.660 3×10^（-9） m2/s之间,并随着干燥温度的升高而增大;活化能为24.268 5 k J/mol。核磁共振试验表明,半结合水是莲子的特征水分,占新鲜莲子总水分的85.59%,其脱除过程呈现指数特征（R2〉0.91）;干燥过程中,不同状态的水分流动性变差。莲子内部存在水分梯度,表层最先失去水分,莲芯水分最后脱除;干燥终止时,剩余水分主要存在于莲芯部位。MRI为确定莲子干燥终点提供了直观的参考依据。研究结果可为控制莲子热风干燥过程、优化干燥工艺参数提供理论依据。展开更多
基金Supported by the National Natural Science Foundation of China (No. 50228203 and No. 20476076)
文摘A flux equation of diffusion for bi-disperse porous catalyst pellets was proposed by modifying the previously developed model equation over fractal trajectories. The proposed fractal model equation considered the same tortuous degree for both micro-and macro-pores. The experimental data of diffusion over a bi-disperse Ni/gamma-alumina pellet were obtained with a standard Wicke-Kallenbach diffusion cell for both carbon monoxide- ethylene and carbon dioxide-ethylene binary mixtures. The fitting between experimental results and the fractal model equation leads to a fractal dimension of 1.11. The prediction of diffusion flux over the bi-disperse Ni/gamma- alumina pellet by the proposed fractal model equation is much better than the traditional tortuosity-based model equation by comparison with the measured flux through the pellet.
基金Project supported by the National Science Foundation "Collaborations in Mathematical Geosciences"(No.OCE-0620956)
文摘Homogenization theory provides a rigorous framework for calculating the effective diffusivity of a decaying passive scalar field in a turbulent or complex flow.The authors extend this framework to the case where the passive scalar fluctuations are continuously replenished by a source (and/or sink).The basic structure of the homogenized equations carries over,but in some cases the homogenized source can involve a non-trivial coupling of the velocity field and the source.The authors derive expressions for the homogenized source term for various multiscale source structures and interpret them physically.
文摘为了解莲子干燥过程中水分传递过程,监控、预测水分变化,该文通过开展莲子薄层热风干燥试验,考察了莲子在不同干燥温度（50、60、70、80、90℃）下干燥特性,建立了莲子热风干燥试验模型;利用低场核磁共振技术（nuclear magnetic resonance,NMR）,弛豫时间（transverse relaxation time,T2）和成像（nuclear magnetic resonance imaging,MRI）,考察了干燥过程中莲子内部水分分布状态与变化规律。结果表明,莲子干燥一直处于降速干燥段;干燥温度显著影响干燥过程（P〈0.05）,干燥温度升高,干燥时间缩短;通过比较4种数学模型,发现莲子干燥过程采用Midilli模型（决定系数R2〉0.998）进行准确模拟（相对误差E〈10%）;有效扩散系数在6.056 7×10^（-10）～1.660 3×10^（-9） m2/s之间,并随着干燥温度的升高而增大;活化能为24.268 5 k J/mol。核磁共振试验表明,半结合水是莲子的特征水分,占新鲜莲子总水分的85.59%,其脱除过程呈现指数特征（R2〉0.91）;干燥过程中,不同状态的水分流动性变差。莲子内部存在水分梯度,表层最先失去水分,莲芯水分最后脱除;干燥终止时,剩余水分主要存在于莲芯部位。MRI为确定莲子干燥终点提供了直观的参考依据。研究结果可为控制莲子热风干燥过程、优化干燥工艺参数提供理论依据。
