用于液体除湿的膜流道传热传质强化及其共轭机理研究

The membrane-based liquid desiccant air dehumidification technology is an effective method to overcome the drawback of liquid desiccant droplets carryover encountered in the traditional air/liquid directly contacting liquid desiccant air dehumidification. The air and the liquid desiccant streams are seperated from each other by membranes, which can prevent other gases and liquid desiccant from escaping into the processing air, while selectively permitting the transports of heat and water vapor between the two streams. To improve the air dehumidification performances of the membrane dehumidifiers, the methods of changing the constructions and adding fins are employed to enhance the heat and mass transfer in the flowing channels, which are used to realize liquid desiccant air dehumidification. The free surface model, periodic unit cell model and fractal model are employed to establish the mathematical models of the fluid flow and heat mass transfer in the channels. The equations governing the momentum, heat and mass conservations are solved by some self-built common program packages. The friction factors, Nusselt and Sherwood numbers in the channels are then obtained. A continuous membrane-based liquid desiccant air dehumidification system is desgined and experimented to validate the established mathematical models. The mechanisms of the conjugate heat mass transfer and the heat mass analogy in the membrane-formed channels are disclosed. The effects of the dynamic factors (such as air and liquid desiccant velocities and transport parameters in membranes) and status factors of the air and solution (temperature, humidity ratio and concentration) on the conjugate heat mass transfer and the heat mass analogy are investigated. Further, the key factors are summed up. The results of this research project can provide fundamentals for performance improvement of the membrane contactors used for liquid desiccant air dehumidification, contactor desgin and structural optimization.

膜式液体除湿是解决传统气液直接接触式液体除湿过程中除湿剂液滴夹带问题行之有效的一种方法。空气和除湿溶液被选择透过性膜隔离，该膜只允许水蒸气的透过，而严格阻止其他气体和液体的渗透。为了提高膜接触器液体除湿性能，本项目拟通过改变流道结构与形式、增加翅片等方式构建强化传热传质膜式液体除湿流道，采用自由表面模型、周期性单元模型以及分形模型建立流道中流体流动与传热传质数学模型并求解，计算流道中的阻力系数、努塞尔数以及舍伍德数等准则数并实验验证，揭示出膜除湿流道中传热传质共轭及其类比机理，分析动力学参数（空气和溶液流量、膜的热湿传递性能等）和非动力学参数（流道结构参数、除湿溶液种类及物性参数、空气和溶液温度、空气含湿量等）对热质共轭传递及其类比规律的影响，找出其关键影响因素，为膜接触器液体除湿性能的提升和膜接触器的设计优化提供理论基础。

本项目研究了一种能彻底解决传统气液直接接触式液体除湿过程中除湿剂液滴夹带问题的膜式液体除湿技术。依据项目计划书的研究目标和研究内容，课题组从两个方面进行了深入研究：.（1）膜制备、传热传质强化膜流道构建以及膜式液体除湿实验研究.以PVDF和PTFE为原材料，采用相转化法成功研制强疏水性、高透湿性的选择性透过膜，通过表面涂覆PDMS（聚二甲基硅氧烷）或液体硅胶对膜进行强疏水改性，形成涂层/PVDF或涂层/PTFE复合膜，膜表面接触角可以达到113°，水蒸气扩散系数1.3m2/s。搭建了一套能实现连续除湿的膜式液体除湿实验装置，装置经过除湿、溶液加热、再生和冷却四个过程，能够实现连续除湿。从实验手段上证实了膜式液体除湿技术的可行性。.（2）膜流道共轭传热传质机理研究.对于平行板膜接触器，构建了侧进侧出、六边形结构的准逆流平板膜流道。空气和溶液在流道中以错流和逆流相结合的流动形式（准逆流）流动。另外，还构建了间壁式、溶液流道带冷却管的平板膜流道，及时带走溶液吸收空气中的水蒸气产生的潜热，形成内冷型平板膜接触器。建立了流道中的流动与传热传质三维数学模型，计算出了共轭传热传质边界条件下的努塞尔数和舍伍德数，揭示出了空气和溶液通过膜相互影响产生的共轭传热传质机理。通过建立侧进侧出平板膜接触器内传热传质活塞流数学模型并求解分析，结果表明，当入口长度比和流道长宽比等于0.1时，相比错流流道，除湿效率提高了10%左右。.对于中空纤维膜接触器，构建了逆流和错流椭圆形中空纤维膜流道。采用自由表面模型、周期性单元模型和分形模型建立了流道中的流动与传热传质数学模型，计算出了共轭传热传质边界条件下的努塞尔数和舍伍德数，并与等壁温或等热流密度边界条件下的准则数相比较，揭示出其共轭传热传质特性，结果表明，当纤维管分布越集中，阻力系数和努塞尔数越小。当椭圆半轴比越小，综合传热能力得到提升；随机分布是影响椭圆管管间传递现象的主要因素，阻力系数和努塞尔数分别比四边形排列的小24.7-65.4%、19.5-91.8%。