Abstract Previous numerical and theoretical results ( Chen et al., 2019 ; Liu et al., 2018 ; Zhao et al., 2019 ) based on the optimization theory of convective heat transfer reveal that the optimized flow structures in a straight circular pipe enhancing convective heat transfer are multiple longitudinal vortices. This conclusion encourages us to find out whether such flow structures really exist in some enhanced heat transfer pipes by means of advanced experimental techniques. Therefore, a typical enhanced heat transfer pipe was selected, namely a spirally corrugated pipe, and stereoscopic particle image velocimetry (SPIV) was employed to measure its internal instantaneous flow field. Moreover, the proper orthogonal decomposition (POD) method was used to extract the large-scale coherent structures from the measured instantaneous velocity fields. Besides the spirally corrugated pipe, the fully developed turbulent flow in a straight pipe was also analyzed as benchmark of the enhanced heat transfer pipes. The results reveal that longitudinal whirling flow with multi-vortices is formed in both the fully developed turbulent flow field of the straight pipe and the spirally corrugated one. It is thus easy to explain the heat transfer enhancement mechanism of the above flow structures from the perspective of momentum transfer. The flow structures of the fully developed turbulent flow in a straight pipe are quite similar to the optimal flow pattern from the optimization theory. More specifically, multiple longitudinal vortices are spontaneously generated due to turbulence without external heat transfer enhancement techniques. Furthermore, the flow structures similar to multiple longitudinal vortices also exist in the spirally corrugated pipe, although these flow structures deviate from symmetric multiple vortices. Moreover, the flow structures in the spirally corrugated pipe are much more energetic than those in the fully developed turbulent flow in a straight pipe. This is probably the reason why a spirally corrugated pipe can enhance heat transfer compared with a straight circular pipe.

中文翻译：

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螺旋波纹管内连贯结构的实验研究

摘要 以往基于对流换热优化理论的数值和理论结果（Chen et al., 2019 ; Liu et al., 2018 ; Zhao et al., 2019 ）表明，优化后的直圆管内流动结构增强了对流换热。传热是多个纵向涡流。这个结论鼓励我们通过先进的实验技术来发现这种流动结构是否真的存在于一些增强型传热管中。因此，选择了一种典型的增强传热管，即螺旋波纹管，并采用立体粒子图像测速仪（SPIV）测量其内部瞬时流场。此外，适当的正交分解（POD）方法用于从测量的瞬时速度场中提取大尺度相干结构。除了螺旋波纹管外，还分析了直管中充分发展的湍流作为强化传热管的基准。结果表明，在充分发展的直管和螺旋波纹管湍流流场中均形成多涡旋纵向涡流。因而很容易从动量传递的角度解释上述流动结构的传热增强机制。直管中充分发展的湍流的流动结构与优化理论中的最佳流型非常相似。更具体地说，在没有外部传热增强技术的情况下，由于湍流而自发产生多个纵向涡流。此外，螺旋波纹管中也存在类似于多重纵向涡流的流动结构，尽管这些流动结构偏离了对称的多重涡流。此外，螺旋波纹管中的流动结构比直管中充分发展的湍流中的流动结构更有活力。这可能就是螺旋波纹管比直圆管能增强传热的原因。