Abstract In this study, hiTRANTM wire matrix tube insert was used to improve the heat transfer deterioration of supercritical nitrogen (N2) in a heated tube caused by the effects of thermo-physical property variations and buoyancy force. Heat transfer experiments were carried out with N2 flowing upwardly in a vertical circular tube with and without the wire matrix insert under a sequence of experimental conditions including pressures of 35 and 40 bar, mass flow rate of 27.6 and 41.3 g/min and constant heat flux conditions at 6.8, 8.0 and 9.3 kW/m2, respectively. Experimental results show that N2 exhibits similar heat transfer behavior when transiting across the pseudo-critical point as other fluids such as water and CO2. Due to the addition of the wire matrix insert, that intensified the overall fluid mixing in the test tube, the heat transfer performance of N2 was enhanced by more than 42% and up to 2.35 times while the pressure drop increase was negligible compared to the system inlet pressure. Moreover, it has been demonstrated that there might exist an optimum combination of experimental conditions leading to the maximum performance of the wire matrix insert. Furthermore, as the results show, with the Dittus-Boelter correlation and correlations for water and CO2 falling short of fitting the heat transfer data of N2, a heat transfer correlation, exclusively for N2 going through the pseudo-critical point is needed. The findings in this study also reveal that both supercritical N2 internal flow heat transfer and the use of wire matrix insert to enhance the deteriorated heat transfer of supercritical fluids are promising topics and require more significant attentions in future studies.

中文翻译：

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增强带有线矩阵插入件的垂直管中超临界氮的劣化传热

摘要 在本研究中，hiTRANTM 线基管插件用于改善受热管中超临界氮（N2）由于热物理特性变化和浮力的影响而引起的传热劣化。在包括 35 和 40 bar 压力、27.6 和 41.3 g/min 质量流量和恒定热通量在内的一系列实验条件下，在带有和不带有线基插入件的垂直圆管中向上流动 N2 进行传热实验条件分别为 6.8、8.0 和 9.3 kW/m2。实验结果表明，N2 在穿越伪临界点时表现出与其他流体（如水和 CO2）相似的传热行为。由于添加了线矩阵插件，这加强了试管中的整体流体混合，N2 的传热性能提高了 42% 以上，高达 2.35 倍，而与系统入口压力相比，压降增加可以忽略不计。此外，已经证明可能存在导致线矩阵插入物的最大性能的实验条件的最佳组合。此外，如结果所示，由于 Dittus-Boelter 相关性以及水和 CO2 的相关性无法拟合 N2 的传热数据，因此需要专门针对通过伪临界点的 N2 的传热相关性。