Abstract The strong variation of thermophysical properties of working fluids operating in the vicinity of the critical point makes this thermodynamic domain attractive to several energy applications. Therefore, herein a two-dimensional numerical method is used to investigate the effect of local thermophysical property variations on the local and overall thermal performance of internal convective heat transfer in a pipe in 324 operational conditions. Focusing on carbon dioxide and water as heat transfer fluids, an association of the variation of key thermophysical properties with thermohydraulic performance metrics is proposed, namely: (a) the local and (b) mean convective heat transfer coefficient and (c) the maximal temperature obtained at the tube wall. It is shown that there is an optimal combination of parameters such as mass flow rate, operating pressure, wall heat flux, and inlet temperature that, when properly selected, allow for a minimal maximal wall temperature. As expected, optimality is strongly associated with the Widom—or pseudo critical—line that extends from the critical point. Interestingly, however, contrary to what is observed in constant-property fluids, high heat transfer coefficient or minimal maximum temperature lead to different sets of optimal operating conditions. This difference is explained by how thermophysical properties vary locally along heat exchangers, which significantly affects overall heat transfer.

中文翻译：

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热力学和热物理效应使超临界流体中的高强制对流传热系数成为可能

摘要 在临界点附近工作的工作流体的热物理性质的强烈变化使该热力学域对多种能源应用具有吸引力。因此，本文采用二维数值方法来研究局部热物理特性变化对 324 种操作条件下管道内部对流传热的局部和整体热性能的影响。关注作为传热流体的二氧化碳和水，提出了关键热物理特性的变化与热工水力性能指标的关联，即：(a) 局部和 (b) 平均对流传热系数和 (c) 最高温度在管壁处获得。结果表明，存在质量流量等参数的最佳组合，操作压力、壁面热通量和入口温度，当正确选择时，允许最小的最大壁面温度。正如预期的那样，最优性与从临界点延伸的 Widom（或伪临界）线密切相关。然而，有趣的是，与在恒定特性流体中观察到的情况相反，高传热系数或最低最高温度会导致不同的最佳操作条件集。这种差异可以通过热物理特性如何沿热交换器局部变化来解释，这会显着影响整体传热。然而，有趣的是，与在恒定特性流体中观察到的情况相反，高传热系数或最低最高温度会导致不同的最佳操作条件集。这种差异可以通过热物理特性如何沿热交换器局部变化来解释，这会显着影响整体传热。然而，有趣的是，与在恒定特性流体中观察到的情况相反，高传热系数或最低最高温度会导致不同的最佳操作条件集。这种差异可以通过热物理特性如何沿热交换器局部变化来解释，这会显着影响整体传热。