Effect of flow obstruction on single-phase heat transfer

本研究的目的是为流动障碍物下游强化传热提供实验数据基础，并将实验数据与现有的传热预测方法进行比较，提出一种改进的预测方法。 很少有实验研究已经进行，实际测量的传热增强由于单相流中的障碍物，这些传热实验只涵盖了一个狭窄的范围内的障碍物的形状和测试条件。一般的假设是传热增强遵循Yao和Hochreiter（1982）的定律，该定律规定Nu = Nu（1+A exp{-B z/Dhy}，其中Nu是装有障碍物的通道的Nusselt数，Nu是没有任何障碍物的参考通道的Nusselt数，A和B是几何相关常数，z是间隔件下游的轴向距离，Dhy是水力等效直径。 由于在所有核燃料棒束中都使用了诸如栅格和隔板之类的流动障碍物，并将其用作热交换器和锅炉管的支撑件，因此，了解障碍物效应具有相当大的实际重要性。 本文研究了以下参数对障碍物强化传热的影响：（i）不同形状的前缘和后缘，（ii）流动阻塞比，（iii）雷诺数，（iv）普朗特数，（v）与流动障碍物的距离，（vi）串联流动障碍物的数量。试验将在直接加热的试验段中进行，试验段沿着长度配备有各种热电偶。 为了在测量站保持相同的条件，除了与流动障碍物的距离，将使用一种独特的实验方法：使用位于（非磁性）Inconel测试段外部的强陶瓷磁体非侵入性地移动障碍物。还将测量对压降的影响。 在分析中，将研究压降和传热强化之间的关系。还将研究引入品质因数的可能性，即用压降的最小增加来表示最佳增强。

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.