【作者】 卢涛；

【导师】 沈胜强；

【作者基本信息】 大连理工大学 ， 动力机械及工程， 2003， 博士

【摘要】 基于体积平均理论，把含湿毛细多孔介质视为宏观意义上的虚拟连续介质。分析了含湿毛细多孔介质干燥过程中流体的基本传输形式。液体和气体在多孔介质中的流动遵循达西定律，同时考虑了液体传输时的毛细效应和蒸汽与空气分子的相互扩散效应。 在多孔介质流体基本传输机制基础上，假设多孔介质相变传热传质过程中各质点处于局部热力学平衡，考虑了多孔介质干燥过程中介质内部三相：固相、液相和气相在质量、动量和能量守恒关系的相互作用，分别建立了由多孔介质干燥过程中湿份和流体质量守恒方程以及多孔介质能量守恒方程共同组成的干燥过程相变传热传质三方程数学模型。在建立多孔介质各相质量守恒方程时，液体蒸发量相对于蒸汽相而言为质量源，对液相本身而言为质量汇。 在三方程数学模型的基础上，以残余饱和度S_ir为判据把对流干燥过程划分为湿区和干区两个区域。根据不同干燥区域中传热传质的不同，分别建立湿区和干区干燥模型，并以“蒸发界面”动态边界作为两干燥区域模型的耦合边界。得到了湿区关于含湿饱和度S、温度T、混合气体压力P_g和干区关于水蒸汽压力P_v、温度T、混合气体压力P_g为基本参数的湿区干区耦合模型。 运用数值计算方法，采用全隐式有限差分，对控制方程、边界条件、“蒸发界面”动态边界进行了离散，得到了一系列非线性方程组。在同一时间层内，逐步将非线性方程组线性化，求解迭代方程组得到下一时层的基本参数预测值，使用预测值求解时间推进方程组得到下一时层的校正值，如此循环运用预测—校正系统，最终得到整个干燥过程的数值解。 本文以上边界对流传热传质边界和下边界绝热绝湿边界的含湿石英砂填充床为实验对象，进行了对流干燥实验。实验测量了多孔介质对流干燥过程中平均含湿量和介质内部温度变化。平均含湿量和介质温度的实验值与对流干燥模型的模拟值吻合良好，证明了模型是合理的。同时数值模拟了多孔介质干燥过程各状态参数的变化规律，对干燥过程进行了传热传质分析。另外，通过改变下边界的边界条件为恒温绝湿边界，上边界仍为对流传热传质边界条件，数值预测了该干燥条件下干燥过程各干燥状态参数的变化规律，分析了其干燥特点。 运用本模型，根据纸机干燥和操作特点，建立了相应的边界条件，预测了工业造纸过程中纸张在纸机干燥部的整体干燥过程和状态参数沿纸张厚度的分布。对比分析了实测值和模拟值，两者表现出相当好的一致性，说明基于多孔介质干燥过程传热传质理论的纸张干燥模型是适用的。进一步探讨了干燥参数对纸机干燥能力影响。掌握纸张详细干燥行为对理性理解干燥参数对纸机干燥性能的影响，科学评估干燥参数对干燥效果和目标的实现，指导干燥参数的调整和运行管理，甚至对纸机的优化设计具有重要意义。更多还原

【Abstract】 Based on volume-averaging theory, wet capillary porous media is treated as virtual continuous body in macro-scale. Primary mechanisms of transportation for liquid and gas in drying process of porous media are analyzed. Flow of fluid, including liquid and mixture gas of vapor and air, is presented with Darcy’s law. Capillary transportation of liquid and diffusive transportation of vapor and air are considered in heat and mass transfer in porous media during drying.Assume porous media in local thermodynamics equilibrium and based on the mechanisms of fluid flow, interactions of mass, momentum, and energy conservation for three-phase: solid, liquid, and gas in porous media are taken into account. A universal three-equation mathematics model of drying of porous media is established, which is composed of two mass conservation equations of mixture gas and moisture-included liquid and vapor, and one energy conservation equation. Evaporation quantum of liquid is expressed as mass source for vapor in vapor mass conservation equation, while it is dealt with mass sink for itself.Following the universal model, wet and dry regions appearing in the drying process are separated by the criterion, irreducible saturation Str. Drying modelsof wet and dry regions are constituted with different mechanisms of heat and mass transfer in different regions. Two models are coupled via moving boundary of evaporation front. Wet region model is described by three state-variable: saturation of liquid S, temperature T, and pressure of mixture gas pg, whiledry region model is also denoted by three state-variable: pressure of vapor pv, temperature T, and pressure of mixture gas pg.A series of non-linear equation group are obtained through discretization of governing equations, boundary conditions, and evaporation front boundary condition with fully implicit finite difference technique. Predicted values of state-variable are achieved by solving iterative equation group in same time layer, and corrected values of state variable are calculated by solving time advancing equation. Following the circle, numerical results of drying process are realized.Experiment of convective drying of wet quartz sands as porous media, which had adiabatic-impermeable boundary of undersurface and convective boundary of upside, was carried out in laboratory. Averaging moisture content and temperature within the porous media were measured during drying. Comparison between experimental findings and numerical results shows a good coherence, which proves the model reasonable. Moreover, Distributions of drying state-variable were simulated with numerical technique, and intrinsic causes were discussed. In addition, numerical simulation was executed for another drying model, which had isothermal-impermeable boundary of underside and convective boundary of upside. The work as former was done.According to characteristic of operation of paper machine, drying model of paper in dryer section on paper machine is found. Detail behavior of paper drying was forecasted by numerical approach. Numerical results are identical in experimental ones fairly, which approves the drying model of paper sheet adaptable even if the boundaries are changed alternately and periodically. Some drying parameters influencing drying capability of paper machine are farther probed. Seizing knowledge of paper drying on modern paper machine is helpfuland significative to understand the drying parameters affecting drying capacity of paper machine, to evaluate drying purpose and objective, to guide adjustment of drying parameters and operation of paper machine, and to optimize design of paper machine.更多还原