Effect of flow obstruction on single-phase heat transfer

本研究的目的是(i)为流动障碍下游的传热增强提供一个实验数据库，（ii）将该数据库与现有的传热预测方法进行比较，（iii）提出一种改进的预测方法。设计合理的湍流促进流动障碍可以显著增强下游换热。很少有实验研究实际测量了在单相流动中由于障碍物而引起的传热增强；这些传热实验只涵盖了很小范围的障碍物形状和测试条件。一般假设传热增强遵循Yao和Hochreiter（1982）的定律，即Nu = Nuo (1+A exp{-B z/Dhy}，其中Nu为有障碍物通道的努塞尔数，Nuo为无障碍物参考通道的努塞尔数，A和B为几何相关常数，z为间隔器下游的轴向距离，Dhy为水力等效直径。障碍效应的知识具有相当大的实际重要性，因为流动障碍，如栅格和间隔板，在所有核燃料束中都使用，并用作热交换器和锅炉管的支撑。拟研究以下参数对障碍物引起的换热强化的影响：(i)不同形状的前后缘，（ii）阻塞比，（iii）雷诺数，（iv）普朗特数，(v)与流动障碍物的距离，（vi）串联流动障碍物的个数。测试将在一个直接加热的测试部分进行，沿着其长度配备各种热电偶。除了与流动障碍物的距离外，为了保持测量站的相同条件，将使用一种独特的实验方法：使用位于（非磁性）Inconel测试部分外的强陶瓷磁铁非侵入性地移动障碍物。对压降的影响也将被测量。在分析中，将探讨压降与强化传热之间的关系。此外，还将研究引入一个性能指标的可能性，即用最小压降增量来表示最佳增强。

The objective of the proposed investigation is  (i) to provide an experimental data base for  heat transfer enhancement downstream of flow obstacles, (ii)  to compare the data base with current heat transfer prediction methods, and (iii) to present an improved  prediction method.Properly designed turbulence-promoting flow obstacles can enhance the  downstream heat transfer significantly.  Very few experimental studies  have been performed that actually measure the heat transfer enhancement due to obstacles in single phase flow; these heat transfer experiments covered only a narrow range of obstacle shapes and test conditions. The general assumption is that the heat transfer enhancement follows the law of Yao and Hochreiter (1982) which states that Nu = Nuo (1+A exp{-B z/Dhy}where where Nu is the Nusselt number for the obstacle-equipped channel, Nuo is the Nusselt number for the reference channel without any obstacles, A and B are geometry-dependent correlating constants, z is the axial distance downstream from the spacer, and Dhy is the hydraulic-equivalent diameter.  Knowledge of the obstacle effect is of considerable practical importance since flow obstacles such as grids and spacers plates are used  in all nuclear fuel bundles and are used as supports heat exchanger and boiler tubes.  It is proposed to investigate the impact of the following parameters on heat transfer enhancement caused by obstacles: (i)  various shapes of the leading and trailing edges, (ii) flow blockage ratios,  (iii) Reynolds numbers and (iv) Prandtl numbers and (v) distance from the flow obstacle, (vi) number of flow obstacle in series. The tests will be performed in  a directly heated test section,  equipped with various thermocouples along its length.  In order to maintain the same conditions at the measuring station except the distance from the flow obstruction, a unique experimental approach will be used:  the obstacle will be moved  non-intrusively using strong ceramic magnets  located outside the (non-magnetic) Inconel test section.The impact on pressure drop will also be measured.  In the analysis the relationship between pressure drop and heat transfer enhancement will be investigated. The possibility of  introducing a figure of merit, i.e.  expressing the optimum enhancement in terms of minimum increase in pressure drop will be  examined as well.