Abstracts This paper explores the performance effects of a novel small-scale multiple vortex generators (MVG) on heat transfer deterioration (HTD) mitigation and pressure drop using supercritical CO2 (sCO2) as the working fluid. The HTD mitigation and pressure drop are examined using an overall performance factor, R = ΔHTD/(Δp/Δp0). Numerical calculations are first conducted to investigate the effects of single conventional vortex generator's (VG) configurations (rectangular, triangular and trapezoidal). The results indicate that under the same aspect ratio, rectangular VG has the largest area facing the fluid flow and the largest angle of attack which induces the strongest longitudinal vortices leading to its highest value of R. Then, the morphological properties (density and height) of five pairs of evenly distributed rectangular VGs in an MVG are varied while their uniformity is maintained. With the increase in MVG's height, the channel's heat transfer coefficient (HTC) profiles increase significantly at the locations where MVG is installed while the opposite trend is found for R. The density and nonuniformity of the MVG have the highest performance effects on HTD mitigation, with R of more than 18% and 11% for the density of 2.5 and non-uniform MVG respectively. Analysis reveals that as the density of MVG increases, the fluid recirculation zone behind each VG diminishes correspondingly, causing a greater vortex intensity mixing by swirling flows which enhances the downstream production of turbulence kinetic energy (TKE), thus weakening the buoyancy forces and leading to HTD mitigations. In addition, by further increasing the MVG density beyond 2.5, it was found that the HTC decreases while the pressure drop increases, with the wall temperature peaks stabilizing at 178 °C, which demonstrates the breakdown of Reynold's analogy. Increasing the number of VGs in an MVG increases its delay and broadening effects on the wall temperature peaks by further delaying the boundary layer recovery caused by interaction of longitudinal vortices generated by each VG, but has relatively little effects on R. Also, analysis further reveals the presence of buoyancy-induced flow oscillations near the channel walls, which are attenuated by the VG and MVG effects. Overall, MVG offers superior performance than convectional single VGs in terms of HTD mitigation, and the results presented here can be employed as a reference guide in the design of highly efficient, safe and reliable supercritical heat exchanger systems.

中文翻译：

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使用新型小型多涡流发生器减轻具有超临界 CO2 的圆管中的传热恶化

摘要 本文探讨了使用超临界 CO2 (sCO2) 作为工作流体的新型小型多涡流发生器 (MVG) 对减轻传热恶化 (HTD) 和压降的性能影响。使用整体性能因子 R = ΔHTD/(Δp/Δp0) 检查 HTD 缓解和压降。首先进行数值计算以研究单个常规涡流发生器 (VG) 配置（矩形、三角形和梯形）的影响。结果表明，在相同的纵横比下，矩形VG面对流体流动的面积最大，攻角最大，引起最强的纵向涡流，导致其R值最高。那么，MVG 中五对均匀分布的矩形 VG 的形态特性（密度和高度）在保持均匀性的同时发生变化。随着 MVG 高度的增加，通道的传热系数 (HTC) 曲线在安装 MVG 的位置显着增加，而 R 的趋势相反。 MVG 的密度和不均匀性对 HTD 缓解的性能影响最大，对于 2.5 的密度和非均匀 MVG，R 分别超过 18% 和 11%。分析表明，随着 MVG 密度的增加，每个 VG 后面的流体再循环区相应减少，通过旋流引起更大的涡流强度混合，从而增强了下游湍动能 (TKE) 的产生，从而削弱浮力并导致 HTD 缓解。此外，通过进一步增加 MVG 密度超过 2.5，发现 HTC 在压降增加的同时降低，壁温峰值稳定在 178 °C，这证明了雷诺类比的失效。通过进一步延迟由每个 VG 产生的纵向涡流相互作用引起的边界层恢复，增加 MVG 中 VG 的数量会增加其对壁温峰值的延迟和展宽影响，但对 R 的影响相对较小。此外，分析进一步揭示通道壁附近存在浮力引起的流动振荡，这被 VG 和 MVG 效应减弱。总体而言，就 HTD 缓解而言，MVG 提供比对流单 VG 更优越的性能，