相变微/纳米胶囊对溶液除湿过程热质传递的影响机理研究

Liquid desiccant technology can utilize low grade heat effectively to reduce energy consumption in refrigeration and air conditioning system. However, in the adiabatic liquid desiccant dehumidifier, the condensation heat released in the moisture removal process leads to the temperature rise of the solution and the weakening of dehumidifying capacity consequently. Moreover, in the existing common internally cooled dehumidifier by using external coolant, the area of heat and mass transfer is relatively small, and the cooling capacity is limited since only the sensible heat is exchanged. A novel idea to enhance the dehumidification performance in adiabatic liquid desiccant dehumidifier, by adding micro/nano encapsulated phase change material(PCM) into the desiccant solution, is proposed in this project. The temperature rise in desiccant solution can be restrained because that the condensation heat is absorbed isothermally by the phase change material, which results in the "auto internal-cooling" in dehumidification without using external coolant. Thus, the performance of dehumidifier can be improved. Besides, some physical properties and transportation properties of desiccant solution can be changed by adding the micro/nano-scale capsules, which may affect the flow of the solution falling film as well as the heat and mass transfer in the "auto internal-cooling" dehumidification. The effect mechanism on the heat absorption during phase change and the changes in physical and transportation properties of desiccant solution by various factors, such as the capsule diameter, the material of capsule core as well as the capsule concentration and so on, will be investigated theoretically. The mathematical model of the falling film dehumidification considering the effects of "auto internal-cooling" as well as the physical and transportation properties’ changes of desiccant solution will be developed. The desiccant solution containing micro/nano encapsulated PCM with good dispersability and stability will be prepared, then the experiments of falling film dehumidification and the static measurement of solution physical properties will be conducted. Under different conditions of capsule scale, mixing proportions of solution and varied moisture removal loads, the heat and mass transfer characteristics in the falling film dehumidification process with the proposed "auto internal-cooling" effect will be studied theoretically and experimentally. The effect mechanism of the micro/nano encapsulated PCM on the heat and mass transfer in the liquid desiccant dehumidification process is expected to be revealed. The results and findings will provide theoretical foundation for this novel idea on performance improvement of liquid desiccant dehumidification.

溶液除湿技术可有效利用低品位热能降低制冷与空调系统能耗。针对现有绝热型除湿器中溶液温升导致除湿能力下降的问题，以及采用外部冷媒的内冷型除湿器存在热质交换面积和冷却能力受限的不足，本项目提出在除湿溶液中添加相变微/纳米胶囊，利用其等温吸热特性抑制溶液温升，实现“自内冷”除湿，提高除湿性能；微/纳米尺度的胶囊还会改变溶液物性和输运参数，对溶液降膜流动及“自内冷”除湿过程热质传递产生影响。本项目将从理论上分析囊芯材料、胶囊粒径和浓度等因素对“自内冷”效应和溶液物性的影响；建立考虑相变吸热及物性和输运参数变化的降膜除湿数学模型；制备分散性和稳定性良好的相变微/纳米胶囊除湿液，进行物性测量和除湿实验；在不同胶囊尺度和溶液配比、不同除湿负荷条件下，模拟和实验研究降膜除湿过程热量传递及组分迁移规律，揭示相变微/纳米胶囊对溶液除湿过程热质交换的影响机理，为这种提高溶液除湿性能的新方法提供理论支撑。

溶液除湿技术可有效利用低品位热能实现空气温温度独立处理，但除湿过程存在溶液温升问题。项目提出在除湿溶液中添加相变微/纳米胶囊，利用其等温吸热特性降低溶液温升，实现“自内冷”溶液除湿。项目对这种新型除湿强化技术所涉及的微胶囊制备、微胶囊悬浮液分散稳定性、悬浮液物性变化及其在降膜除湿中的热质交换特性等科学问题进行了研究。.首先探索了适用于溶液除湿过程的相变微胶囊制备方法，得出了囊芯材料、乳化剂、乳化方式等因素对相变微胶囊性能的影响；采用原位聚合法制备了壁材为三聚氰胺-甲醛、芯材为正十八烷的微胶囊，其平均粒径为0.636 μm，相变潜热为147.59 J/g。之后，分析了表面活性剂种类、用量等对相变微胶囊除湿悬浮液分散稳定性的作用，加入表面活性剂可以显著改善悬浮液稳定性，但存在最佳用量。进而，理论分析和实验测量了相变微胶囊悬浮液物性和输运参数的变化规律，悬浮液导热系数随微胶囊用量增大而减小，粘度随稳定性和温度的升高而降低，随氯化锂质量浓度增加而升高；芯壁比越大、微胶囊质量分数越高时，在相变温度段内悬浮液的比热容越大；采用直接测量静态法得出加入微胶囊可降低溶液表面蒸汽压，降低程度随温度升高而增大。建立了微胶囊吸热相变数值模型，得出微胶囊内部温度随时间和空间的分布情况；对微胶囊悬浮液除湿过程模拟结果显示，除湿性能的改善作用随微胶囊用量的增加而减弱，不能无限制增大悬浮液中微胶囊的含量。搭建了相变微胶囊悬浮液除湿实验系统并进行实验，发现添加微胶囊可以显著提高除湿量与除湿效率，当微胶囊用量为2%时，悬浮液的除湿量和除湿效率比氯化锂基液分别提高了18%和17%；当温度低于微胶囊熔融终了温度时，悬浮液除湿能力有较大提升；加入微胶囊能够有效抑制除湿过程溶液温升，当悬浮液温度接近相变起始温度时，微胶囊对溶液温升的抑制最为显著。.项目研究验证了添加相变微胶囊以提升溶液除湿性能的可行性，为该新型强化溶液除湿技术的应用提供了理论支撑。