超疏水翅片结霜初期表面特性与热气流综合作用除霜机理研究

The current defrosting methods for air source heat pumps all implement defrosting at the frost layer fully growth period. As a result, the heating performance decays because of the frost layer growth. Moreover, the heating cycle of the air source heat pump is interrupted when defrosting starts, which leads to the discontinuous heating and decrease in efficiency. The frost layer grows on the surfaces of the condensate droplets which form at the initial stage of the frosting process. If the condensate droplets are removed before they are frozen, the process of the frost layer growth will be cut off. Thus, a comprehensive defrosting method, combining the effects of surface characteristics of superhydrophobic fin and hot air, was proposed in this project. Firstly, the space and size distribution of condensate droplets on superhydrophobic fin surface will be investigated by methods of theoretical analysis, experiment and simulation. Secondly, the transient behavior characteristics of condensate droplets (including rolling, merging and shedding) under the effect of fin surface characteristics and high speed flow will be explored, and the comprehensive effect of fin surface characteristics and hot air on shedding droplets with big sizes will be revealed. Then, the unsteady heat and mass transfer mechanism of residual micro-scale condensate droplets under the mulriple effect of hot air and cold fin surface will be investigated, and the unsteady heat and mass transfer model will be built. Furthermore, an air source heat pump system and its model, which use the supercooling refrigerant as the heat source for heating air, will be established based on the comprehensive defrosting, and the system efficiency of comprehensive defrosting will be evaluate and optimized. Finally, the mechanism of the comprehensive defrosting method will be revealed to provide theoretical technological foundation for efficient and stable operation of air source heat pumps. All research achievements will enrich and improve the defrosting theory for air source heat pumps.

空气源热泵冬季制热运行一般是基于蒸发器表面霜层充分生长后中断制热进行除霜，由此导致其存在制热性能随霜层生长而衰减和因进行除霜引起供热不连续等问题。为此，本项目基于翅片表面结霜初期所形成的凝结液滴是霜层生长的基础，创新性地提出超疏水翅片结霜初期表面特性与热气流综合作用的除霜新方法，以期达到在凝结液滴冻结前将其去除，从而阻断霜层生长的目的。采用理论与实验研究相结合，研究表面特性与热气流综合作用下超疏水翅片表面凝结液滴脱除的瞬态行为特征及综合作用机制，探索尚滞留翅片表面的微尺度凝结液滴蒸发的非稳态传热传质特性，揭示超疏水翅片表面特性与热气流综合作用除霜机理，构建实现综合作用除霜并以制冷剂过冷为热气流加热热源的空气源热泵系统，揭示综合作用除霜特性与系统性能的耦合机制，为探索出可实现制热性能不衰减和除霜供热不中断的除霜方法及其系统实现提供理论基础和关键技术支撑，有望丰富和发展空气源热泵除霜理论。

针对空气源热泵常规除霜思路与除霜方法所存在的不足，提出了一种超疏水翅片结霜初期表面特性与热气流综合作用的除霜方法，并构建出实现该除霜方法的空气源热泵系统。课题以铜、铝翅片为对象，探索了表面特性可控的超疏水翅片制备方法，并探究增强表面耐久性的制备方法。制得表面水静态接触角均高于155°，滚动角均低于5°，具有良好的耐久性、耐酸碱性、抗紫外线性以及自清洁能力。通过理论、实验和数值仿真结合的方式，研究了结霜初期翅片表面凝结过程的细微观物理特征，揭示了翅片表明凝结液滴直径分布规律。根据液滴凝结现象，探究出了翅片表面特性对其表面凝结成核机理和凝结过程特性的作用机制及影响规律，获取了液滴数量、液滴半径均匀程度及不同疏水表面状态对液滴合并弹跳过程的影响规律。在液滴凝结研究基础上，构建了翅片表面液滴凝结成核能障模型，预测了超疏水表面优先成核位置与分布特征，获得了凝结成核难易程度的影响规律。通过观察液滴冻结过程的微观特性，构建了液滴与冻结液滴之间的传质模型，揭示了冻结阶段“冰桥”现象的影响规律。.在结霜初期通过风力吹除与蒸发联合作用去除凝结液滴是抑制结霜的创新性方法。通过课题研究，建立了超疏水表面微尺度凝结液滴吹除、蒸发数理模型，并获得了液滴在外加风力作用下的脱除特性以及气流作用下液滴蒸发过程中液滴特性的变化规律。基于以上思想，以超疏水换热器为研究对象，搭建了在翅片管换热器表面间歇性使用热气流作用吹除结霜初期凝结液滴的除霜系统，研究了该系统的抗结霜性能和作用机制。并在此基础上，构建了基于超疏水翅片管换热器的可实现热气流综合作用的空气源热泵系统，获得了综合作用除霜特性与系统性能的耦合机制，可实现无霜运行。对比逆循环除霜，制热量提高约16.30%，COP提高达18.97%。该课题研究成果为探索可实现制热性能不衰减和除霜供热不中断的除霜方法及其系统实现提供理论基础和关键技术支撑，可进一步丰富和发展空气源热泵除霜理论。