基于微元回热的复叠式室温磁制冷循环机理诠释

As overcoming the nature shortage existed in nowadays magnetic active regenerator, that is irreversible regenerative process with big temperature difference, cascade magnetic refrigerator near room temperature hybrid with micro-unit regenerator might face the challenge of urgent need in practical application, named temperature span and refrigeration capacity collaborative increasing. Cascade magnetic refrigerators combined solid magnetic refrigeration technology with micro-unit regenerator, adopted thermoelectric thin films advantages such as compact structure and high speed heat transfer, build novel high efficiency regenerative cycle. The contents of the project include: 1) to establish the theory of multi-stage cascade magnetic refrigeration cycle by means of thermal dynamic principle in multi hybrid energy potential field like magnetic energy and heat, to evaluate accurately the maximum refrigeration performance in novel regenerative cycle; 2) Research on the running mechanism of high efficiency heat transfer micro-unit, to understand the design method of thermoelectric thin films placed into micro-unit, to understand how to balance the structure parameter and running parameter to meet design requirement; 3)to explain the working mechanism of multi-stage cascade refrigerator, to demonstrate the effect of material，thermodynamic performance, system structure on the system characteristics, furthermore to increase the magnetic refrigeration temperature span and refrigeration capacity. The project achievement would reveal principle of micro-unit magnetic regenerative cycle, and might explain working mechanism of the high temperature span magnetic refrigeration system.

基于微元回热的复叠式室温磁制冷系统，克服现有磁制冷主动式回热器的固有缺陷，即蓄热流体与工质之间的大温差不可逆回热损失，解决制冷量和温跨难以同时达到应用需求的难题，将固态磁制冷微型热电薄膜强化换热技术与微元回热复叠循环相耦合，结合热电薄膜结构微型化、高速导热的特性，创新性地构造了新型高效回热循环，内容包括：1）采用热力学理论进行磁热势场耦合分析，构建磁制冷多级复叠循环理论，准确评价磁制冷回热循环制冷潜力；2）研究高效传热结构的运行机制，研究半导体薄膜热电技术强化换热结构设计方法，研究如何平衡高低温微元之间合理的结构配置和运行参数配置以达到最佳的设计目标。３）诠释多级复叠磁制冷机工作机理，从材料、热力学特性、结构等方面研究参数对复叠式系统的影响，构造多层微元回热串并联复叠技术，从而提高室温磁制冷循环温跨及制冷量，项目诠释微元回热循环原理，揭示大温跨多级复叠回热循环磁制冷工作机理。

本项目针对现有主动磁回热循环存在的回热损失大、运行频率低、流路复杂等弊端，提出了基于微元回热的全固态室温磁制冷循环。采用数值模拟和实验相结合的手段，开展微元格内和格间的强化换热研究，深入机理层面分析循环内部的回热特性; 构建熵产率评价体系，从能量“质量”角度评估磁制冷循环的热力学性能。 本项目基于微元回热室温磁制冷循环展开研究，围绕循环理论分析、循环内部强化换热、样机设计搭建及性能测试、评价指标建立等方面进行探索。. 在仿真研究层面，先是基于仿生优化理论，建立导热问题的拓扑优化程序，得到树状分叉形式的格内高效传热结构。而后构建耦合热电半导体的微元回热循环模型，研究在不同格内传热结构和不同格间传热方式下的循环制冷性能。仿真研究结果表明：在格间采用铜块回热时，连续旋转模式下，拓扑优化结构的最大系统温跨比纯钆和平行片（紫铜）结构分别提高了66.67%和7.34%；在格内为纯钆结构时，耦合热电半导体的系统温跨值与未耦合相比提升了210%。. 在实验研究层面，设计并搭建微元回热室温磁制冷样机，探究不同结构参数（格内传热结构、格间传热结构）和运行参数（回热时间、输入电压）对样机制冷性能的影响规律。实验测试结果表明：优化格内及格间传热结构是提升样机制冷性能的有效手段，在平行片结构（石墨烯）下系统温跨值与未优化时相比提升了37.5%，在耦合热电半导体强化传热时对应的系统温跨值与未耦合相比提升了100%。在磁场强度0.65 T，输入电压1 V，回热时间25 s工况下，样机系统温跨值为3.2 K，该值与样机内工质块去磁温变（0.4 K）相比，增大了7倍。. 此外，以熵产率作为评价指标，对比分析微元回热磁制冷循环和主动磁制冷循环的热力学性能。结果表明：在输出相同能量数量的条件下，微元回热循环的总熵产率值小于主动回热循环，总熵产率值降低约52%，即基于熵产率最小分析，微元回热循环对能量的有效利用程度更高。. 本项目的研究工作具有理论意义和应用价值，为推动室温磁制冷技术的发展提供支撑。