基于孔隙尺度的纸张干燥过程热质耦合传递机理与模型研究

Paper drying is a complicated physical process involved coupled heat and mass transfer mechanism. It is influenced by internal and external factors. However, there is still not a consensus on the moisture migration mechanism which determine the drying rate. As a result, many paper drying models could not reflect the nature of heat and mass transfer and simulate the drying process correctly. In this proposal, we will mainly focus on the internal factors. Differential scanning calorimetry (DSC), scanning electron microscope (SEM), and low-field nuclear magnetic resonance (LF-NMR) will be employed to measure the pore size and its distributions. Fractal theory will be used to characterize the effect of pore structure on fluid movement and heat transfer. Force balance analysis, phase balance analysis, and thermal analysis kinetics will be combined to study the different status and distribution as well as the evaporate order of water within paper web. In addition, the influence of pore structure changes in drying and cellulose fiber surface characteristics will also be considered. Then, moisture transfer mechanism are studied from the perspective of capillarity theory and adsorption theory, based on which, the related constitutive relations will be developed for various kinds of water. The proposed moisture migration mechanism will be coupled with different heat transfer mechanism and will be validated via contact/convection paper drying experiment at laboratory. Finally, the analytical mathematical model of the paper drying process will be constructed based on the proposed mechanism and the specific external factors. Numerical technology will be utilized to simulate the drying kinetic during paper drying. This study will have a significant influence on optimizing the operation conditions and energy saving for industrial paper machine drying process.

纸张干燥是传质与传热相互耦合的复杂物理过程，受内部和外部因素的共同作用，但由于对内部水分迁移机理认识的局限，很多干燥模型无法准确反映其热质传递的本质，导致模拟结果与实际过程偏差较大。本项目拟在研究纸张内部结构因素的基础上，采用DSC、SEM、LF-NMR等手段定量分析孔隙尺度及其分布规律，运用分形理论研究微观结构特征对纸张内部流体流动和传热过程的影响。在考虑孔隙结构变化和纤维表面特性影响的前提下，综合力平衡、相平衡和热分析动力学等理论研究纸张内水分的不同存在形式及其分布和蒸发次序。在此基础上，借助毛细管现象与吸附现象研究纸页内不同形式水分的迁移机理，构建相应的传质本构关系。通过接触/对流干燥实验分析纸张干燥热质耦合传递规律，结合相应的外部因素，建立纸机干燥部的解析数学模型，并运用数值分析技术开展干燥动力学模拟。本项目对于造纸干燥过程的工艺优化与节能降耗将具有重要的理论指导意义和应用价值。

项目主要以纸张（纸板或浆板）为研究对象，研究在干燥过程中发生在其内部的水分迁移蒸发特性和热质耦合传递机理。主要研究了纸张内水分分布形态及其孔隙结构变化特性，建立了描述水分迁移的传质本构关系；实验确定纸张干燥过程的动力学特性，并对其动力学模型进行了综合分析；通过结合传统的纸张不同干燥方式，建立热质耦合传递模型，考察了不同干燥因素的影响规律，研究阐明了干燥过程中纸张孔隙结构特性决定的水分传递机制；建立了造纸干燥实际过程的能量模型，实现了对造纸干燥过程的数值模拟；根据纸机干燥部实地测试数据，对比分析了数值模拟结果与实验结果的差距，验证了热质传递模型，通过模型仿真，研究了干燥部结构和操作参数对造纸干燥性能和能效水平的影响，阐明了某些关键变量对纸页干燥过程性能的影响，为指导实际纸张干燥过程的节能降耗工作提供了理论支持。