稳态-解耦测算多孔材料热质传递系数及变化机理研究

It is very difficult to accurately measure the porous material temperature and moisture content due to their continuous changes accompanied with the shape and size. And there is also an inherent deviation in computational model. It is not only low precision but also poor reliability to estimate the porous material heat and mass transfer coefficients with unsteady-state method. The project aims at estimating the values of the porous material heat and mass transfer coefficients and exploring their change mechanisms. Firstly, both of the porous material heat and mass transfer coupling form and coupling mechanism are studied, and the steady and decoupling experiment platform is established for the estimation of heat and mass transfer coefficients. Then the convective heat transfer coefficient and the thermal diffusivity are estimated and their change mechanisms are explored through steady convective heat transfer experiment; and the convective mass transfer coefficient and the water diffusion coefficient are estimated and their change mechanisms are explored through steady isotherm convective mass transfer experiment on steady and decoupling experiment platform. The quantitative relationship equations between the convective heat transfer coefficient (thermal diffusivity, convective mass transfer coefficient, and water diffusion coefficient) and the physical properties of hot air and the porous material are revealed. Finally, the steady and decoupling method is verified through numerical simulation of convective drying porous material. The obtained data will be of very important value for promoting the development of heat and mass transfer equipment, process control, simulation and product quality improvement in terms of theory and application.

针对非稳态法测算热、质传递系数，多孔材料温、湿度连续变化并伴随形状、尺寸变化，温湿度实时精确测量困难、计算模型固有偏差等致使测算结果精度低、可靠性差。本项目创新探索稳态解耦实验测算多孔材料热、质传递系数及变化机制：首先研究多孔材料热、质耦合形式，耦合机制，试制热、质传递系数稳态解耦测算实验平台。然后通过稳态解耦实验平台，实现稳态等温对流传质系数和水分扩散系数测算与稳态等湿对流传热系数和热扩散系数测算，探索热空气和多孔材料物性对热扩散系数、水分扩散系数、对流传热系数和对流传质系数变化影响机制，揭示热扩散系数、水分扩散系数及对流传热系数、对流传质系数与热空气和多孔材料物性间量变关系及科学规律。最后根据测算结果对多孔材料热风对流干燥进行数值模拟仿真验证稳态解耦多孔材料热、质传递系数测算方法。本项目开展将对推动传热传质过程新设备开发、过程控制、模拟计算以及产品质量提高等具有重要理论意义和应用价值。

针对目前瞬态法和稳态法测算热、质传递系数，测算结果精度低，传热、传质数值计算可靠性差，首先通过实验和Luikov模型对多孔试样，两侧分别施加不同温度和水分含量，测试和计算了多孔试样内部水分含量及温度变化，分析了多孔材料温度梯度对水分扩散影响及多孔材料水含量梯度对热量传递影响。结果显示：封闭试样施加不同温度，不但初始水分含量较低试样高温侧局部水分含量计算值低于多孔试样平衡水分含量而且小于0，初始水分含量较高试样低温侧局部水分含量计算值高于其饱和水分含量；而且其平均水分含量计算值不等其初始平均水分含量；开放试样两侧施加与试样初始温度相同的不同水分含量，多孔试样内部温度计算值不恒定，与对应条件实测水分分布均匀及温度分布均匀相矛盾。表明Luikov模型中温度梯度驱动水分（湿）传递假设及多孔材料内部相变及吸附/解吸影响热传递或者水分含量梯度驱动热传递的假设与实际不符，表明多孔材料温度梯度不是其水分扩散驱动力，多孔材料水分含量梯度不是其热扩散驱动力。然后，利用改进稳态杯及本项目资助设计、申报并授权的国家发明专利稳态法测算多孔材料传质系数装置对黏土水分扩散系数及对流传质系数进行了测算和分析。结果显示黏土试样水分扩散系数数量级10-9m2/s，自然对流传质系数量级10-4kg/(m2s)，强制对流传质系数数量级10-2kg/(m2s)。测算分析结果表明多孔材料水分扩散系数不受其厚度、介质速度、介质相对湿度等影响，对流传质系数不受试样厚度和介质相对湿度影响，但对流传质系数受介质速度影响较大。研究成果深化了人们对多孔材料热、质耦合机制认识，提供了更高精度的多孔材料传质系数测算方法，将对推动传热传质过程新设备开发 、过程控制、模拟计算以及产品质量提高等具有重要理论意义和应用价值。