单级弹热制冷循环性能优化研究

Elastocaloric cooling is a novel solid-state cooling technology based on the martensitic phase transformation process found in shape memory alloys. Compared with materials used in magnetocaloric cooling and thermoelectric cooling systems, elastocaloric materials, i.e. shape memory alloys, have remarkably larger latent heat, and more advanced research and development status, which guarantee a good application potential in the future. Up-to-date, the fundamental operating principles of elastocaloric cooling technology have been established, and a few prototypes with more than 5 K system temperature lift were published. The theoretical system temperature lift, however, can be more than 20 K, which suggests substantial performance enhancement potential. Therefore, the scope of this research proposal focuses on improving the system performance of elastocaloric cooling systems. Theoretical analysis and experimental studies will be carried out from three aspects, including cycle design, more comprehensive screening of materials, and reducing unnecessary system heat transfer and cyclic losses. First, theoretical analysis on the physical process of shape memory alloys undergoing coupled phase changing and heat transfer processes will be conducted, to determine the equations of the theoretical limit of system operating frequency. Besides, the materials ranking method will be re-evaluated from the perspective of system dynamic performance, other than the conventional method which were only based on static state indices such as latent heat, to determine the best shape memory alloy. Furthermore, dynamic model will be established to study the operating principle of elastocaloric cooling systems, and to investigate the effect of a few operating and geometric parameters on the system performance. Consequently, the most significant parameter limiting the enhancement of system performance will be found, to guide the direction of improving the system performance. Based on the suggested direction, applicable methods to enhance the system performance will be proposed, which will serve as the foundation for the development of next generation elastocaloric cooling prototypes in the future.

弹热制冷是基于记忆合金马氏体相变的新型固态制冷技术。相比目前广受重视的磁热及热电材料，以镍钛合金为代表的记忆合金的单位质量制冷量可达其数倍，且材料研发更为成熟，使得弹热制冷技术具有良好的应用前景。目前弹热制冷的基本原理已经明确，已有可实现5K系统温升的原型机，相比超过20K的理论温升，系统性能仍有巨大的提升空间。本项目拟从循环流程设计、材料性能的综合筛选、减小系统传热及交变损耗三方面展开理论分析与实验研究。分析记忆合金相变及传热相耦合的物理过程，得到弹热制冷系统的理论极限运行频率的解析表达式；从动态特性角度重新审视记忆合金的评价方法，完善目前以相变潜热等静态参数为主的评价体系，找到综合性能最优的材料；建立研究弹热制冷系统运行规律的动态模型，探寻制约弹热制冷系统性能提升最关键的参数或条件，明确增大系统性能的方向；在此基础上提出切实可行的系统集成设计方案，为下一代弹热制冷原型机的研发奠定基础。

弹热制冷是基于记忆合金马氏体相变过程的潜热制冷的新型固态制冷技术，具有高能量密度、低噪声、可用于回收动能或低品位热能等突出优势，在17项非蒸气压缩制冷技术中被美国能源部评定为最具潜力。现有弹热制冷装置的制冷温差尚不能达到制冷产品的需求，究其原因，减小弹热工质在相变潜热释放的同时涉及交变流动传热，减小该部分损耗是提升系统制冷温差的关键。基于上述背景，本项目开展了以下三部分研究工作：（1）通过理论分析建立了描述耦合相变与传热的弹热工质在交变流动条件下的规律：提出了描述传热速率与相变释放速率比值的相似准则数Φ，揭示了弹热工质在耦合相变与传热过程的温度变化与相似准则数Φ的关系，理论推导出弹热工质理论运行周期的表达式，在此基础上进一步发现了弹热工质在交变流动传热过程中传热系数的时域相关性，提出了对流传热系数与雷诺数、普朗特数、交变雷诺数的新关联式（2）通过数值仿真系统性地描述了弹热工质涉及的六种损耗并提出了优化方向：基于相变动力学理论，建立了弹热工质及弹热制冷系统的动态仿真模型，通过实验测试了镍钛合金管在不同温度下的应力-应变特性和出口水温变化特性，验证了模型精度（±10%），仿真结果表明弹热工质长度、水力直径、流体置换量是制约弹热制冷系统温差的三个最主要参数，优化后弹热制冷系统温差可达70K。上述参数中流体置换量最为重要，反映了流体储热损失和系统回热损失的平衡关系，该参数与系统制冷温差和循环频率均相关。在极端工况下，工质的相变温度和驱动器驱动力会成为限制系统温差的因素。（3）以减小损耗为目标提出了两种新型弹热制冷系统集成方式：热驱动型弹热制冷系统集成方式使用了形状记忆合金热机替代机械驱动器，可将驱动器与制冷剂的质量配比从500:1减小至10:1，是未来弹热制冷系统紧凑化的一个发展方向。固-固传热单电机驱动的集成方式可以充分利用加载相变阶段固-固接触高散热热流密度的特性，实验结果表明该方案制冷温差可达8K。