Abstract Hydrodynamic characteristics and the corresponding heat transfer performance are important features of hydrocarbons flow in regenerative active cooling systems. The main objective of this study is to clarify the intrinsic relationship between the hydrodynamic and heat transfer characteristics in a horizontal stainless mini-tube. Experiments were conducted using pure hydrocarbons of cyclohexane and pentane and their binary zeotropic mixtures (cyclohexane/pentane: 30/70, 50/50, and 70/30 by wt%). The test tube with the inner diameter of 2 mm is 670 mm long. The parametric conditions comprise heat flux range of 155–255 kW/m2, pressure range of 2.5–4.5 MPa, and inlet temperature range of 30–50 °C. The relationship between the static instability and the heat transfer process was discussed. Flow patterns and their evolutions were visualized at their critical pressure. Comparing the experimental results and visualizations, the similarities of the pure components and mixtures were verified under subcritical and supercritical pressures. The heat transfer characteristics coupled with the onset of flow instability were analyzed, and the mechanisms of mass transfer resistance for mixtures were explained. The correlations based on dimensionless numbers were determined to control the onset and offset boundaries of static instabilities for hydrocarbon mixtures, which provides guidance for preventing the flow instability for the complex working fluid in engineering applications.

中文翻译：

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二元烃在跨临界和超临界压力下的流体力学和传热特性

摘要 水动力特性和相应的传热性能是蓄热式主动冷却系统中碳氢化合物流动的重要特征。本研究的主要目的是阐明水平不锈钢微型管中流体动力学和传热特性之间的内在关系。使用环己烷和戊烷的纯烃及其二元共沸混合物（环己烷/戊烷：30/70、50/50和70/30重量%）进行实验。内径为2mm的试管长670mm。参数条件包括热通量范围为 155-255 kW/m2，压力范围为 2.5-4.5 MPa，入口温度范围为 30-50 °C。讨论了静不稳定性与传热过程之间的关系。流动模式及其演变在其临界压力下可视化。比较实验结果和可视化，在亚临界和超临界压力下验证了纯组分和混合物的相似性。分析了与流动不稳定性相结合的传热特性，并解释了混合物的传质阻力机制。确定了基于无量纲数的相关性以控制烃混合物静态不稳定性的起始和偏移边界，这为防止工程应用中复杂工作流体的流动不稳定性提供指导。在亚临界和超临界压力下验证了纯组分和混合物的相似性。分析了与流动不稳定性相结合的传热特性，并解释了混合物传质阻力的机制。确定了基于无量纲数的相关性以控制烃混合物静态不稳定性的起始和偏移边界，这为防止工程应用中复杂工作流体的流动不稳定性提供指导。在亚临界和超临界压力下验证了纯组分和混合物的相似性。分析了与流动不稳定性相结合的传热特性，并解释了混合物传质阻力的机制。确定了基于无量纲数的相关性以控制烃混合物静态不稳定性的起始和偏移边界，这为防止工程应用中复杂工作流体的流动不稳定性提供指导。