深低温系统氦两相透平膨胀机内自发凝结流动特性研究

Helium cryogenic technology is the fundamental guarantee for high-tech technologies in many advanced fields. Through the substitution of part of the throttling process by two-phase helium turbo-expander, the system Coefficient Of Performance (COP) of can get a 20% to 40% increase, meanwhile, system liquefaction ratio can get a maximum increase up to 30%. Along with the near isentropic expansion process in a two-phase helium turbo-expander, the spontaneous condensation will occur to gas helium. Then the flow in high speed expansion wheel becomes the spontaneous condensation two-phase flow, which is the main problem for defining a two-phase helium turbo-expander. This project aimed at the spontaneous condensation phenomenon of two-phase helium flow in a helium turbo-expander which working in cryogenic temperature as low as 4.2 K. The attention will be put on the rule of spontaneous condensation nucleation and droplet growth for the no-polar monatomic molecule gas helium. The theoretic models for nucleation and droplet growth will be established from micro level. The experimental tests about Wilson points and the amount of condensing liquid will be performed to verify the theoretic models. This project will propose the design method of a two-phase helium turbo-expander through the revelation of the rule of nucleation and droplet growth in helium spontaneous condensation process. After that, a prototype for two-phase helium turbo-expander will be developed and the design method will be improved according to the test data. The wetness loss will be defined with the actual performance data, which could supply the crucial theoretic guidance for the design of a helium cryogenic system.

氦低温技术是众多领域尖端科技发展的技术保障。氦低温系统末端利用两相透平膨胀机替代部分节流过程，可以使系统COP提高20%~40%，使系统液化率提高近30%。氦两相透平膨胀机内深低温工质随着近等熵膨胀过程产生自发凝结现象，形成高速流道内氦自发凝结流动问题。该问题的求解，是攻克氦两相透平技术的基础理论前提。针对深低温（4.2 K温区）氦两相透平膨胀机高速流道内工质自发凝结流动现象展开研究，重点关注深低温区非极性单原子气体氦在高速流道内自发凝结成核及液滴增长规律，从微观层面求解氦自发凝结成核率及液滴增长率。同时开展Wilson点及带液量测量实验，检验修正理论模型，揭示高速流道内氦自发凝结成核和液滴增长及运动规律，完成氦两相透平膨胀机设计准则构建和实验样机开发。进而，开展氦两相透平膨胀机实验研究，修正完善设计理论，总结分析运行特性，量化带液损失关系，为氦低温系统流程优化设计提供基础理论指导。

由于氦低温系统末端利用两相透平膨胀机替代部分节流过程，可以使系统性能提高，同时提高系统的液化率，针对透平膨胀机内氦低温自发凝结及两相流动的核心基础问题，本项目目标要求的研究内容为：1. 深低温（4.2K 温区）氦自发凝结两相流动理论模型构建；2.研究深低温氦自发凝结 Wilson 点测量及氦自发凝结带液量测量的方法，对成核率模型和液滴增长率模型进行实验验证；3.研究氦两相透平膨胀机的优化设计方法及设计原则。.本项目针对透平膨胀机高速流道内氦自发凝结流动问题的基础问题研究，基于双流体模型，考虑了非平衡态气液两相间传热与传质过程，建立了高速流道内低温工质非平衡自发凝结两相流动的数学模型。针对Laval喷管中氦气凝结两相流动，完成了非平衡自发凝结流动的数值模拟，揭示了低温工质膨胀凝结成核及液滴生长的机理与规律，进一步开展了数学模型中不同成核理论与不同表面张力修正理论的适用性研究。之后开展了基于Laval喷管的氦自发凝结过程的实验研究，通过理论与实验的对比分析建立了非平衡自发凝结数学摸型，对透平膨胀机中氦的两相流动进行了数值研究，分析了工作轮流道内液滴数目密度、液滴直径及带液量的分布规律。将现有单相氦透平膨胀机运行在两相状态，利用实验数据对氦自发凝结的液滴增长模型进行验证，并最终修正和完善了氦自发凝结及两相流动理论模型。基于氦自发凝结两相流动模型，开展数值分析，总结分析了氦两相透平设计方法和设计原则，开发出氦两相透平样机。通过对低温下氦工质两相平衡态和非平衡态热物性的深入研究，设计了基于热平衡法的透平出口带液量测量装置，搭建了低温两相氦透平膨胀机性能测试实验系统，在实验测试范围内进行了带液量和带液损失量化分析。最终通过对实验结果的分析与归纳，验证理论研究的正确性，基于工作轮叶片型线对自发凝结过程的影响，对理论模型和仿真预测方法进行改进，完成了两相透平膨胀机主通流部分的优化设计，掌握了深低温氦两相透平膨胀机的基础理论和关键技术。