Drag reduction and turbulent characteristic of rough surface with fins for heat transfer enhancement

为了改善换热管的性能，对带翅片的粗糙表面的减阻和湍流特性进行了实验研究。用热线法测量了翅片管周围的平均速度分布、速度脉动强度、速度脉动谱和相干性分布，并用烟丝法进行了流动显示。由此得出以下结论：1.实心冷杉粗略地安装在换热管表面，既起到增加换热面积的作用，又起到导叶的作用。当翅片间距较小时，主流与翅片的夹角较小，管壁表面的流动倾角较小。管子表面的流动被翅片倾斜。当锯齿翅节距较大时，与实心翅片管相比，阻力增大，因为流动与锯齿翅片边缘分离。翅片表面不易与主流接触，导致换热性能下降。当锯齿翅的节距变小时，锯齿翅的边缘对流动有很大的倾斜作用。强化换热的原因不仅是翅片增加了湍流，还增加了主流与管子或翅片表面的接触距离。与实心翅片管相比，锯齿翅片管的流动倾角较大，阻力较大。当锯齿状翅片节距进一步变小时，流体不会在翅片之间的间隙内流动。因此，流体主要流出翅片部分，换热性能并没有提高。

Drag reduction and turbulent characteristics of the rough surface with the fins were experimentally investigated to improve the performance of heat exchanger tube. We measured the distributions of the mean velocity, the intensity of velocity fluctuation, the spectrum of velocity fluctuation and the coherence distribution around the fin tubes by using hot wire, and conducted flow visualization by smoke wire method. As a result following conclusions were obtained.1. The solid fir which mounted roughly on the surface of heat exchanger tube worked as not only the increase of heat transfer area but also the guide vane.2. When the fin pitch was small, the angle between the main flow and the fin was small, and the inclination of the flow on the surface of a tube was small. The flow on the surface of the tube was inclined by the fin.3. When the serrated fin pitch was large, the drag increased compared with the solid fin tube because the flow separated from the edge of sertated fin. And the heat transfer performance decreased because the surface of the fin did not contact easily with the main flow.4. The flow was inclined greatly by the edge of serrated fin when the pitches of serrated fin become small. The cause of the heat transfer enhancement was not only the increase of turbulence by the fin but also the increase of the contact distance between the main flow and the surface of the tube or the fin. The drag increased compared with the solid fin tube because the inclination of the flow was large at the serrated fin tube.5. When the serrated fin pitch become small further, the fluid did not flow inside of the clearance between the fins. Therefore fluid flowed chiefly outside of the fin parts, the heat transfer performance did not increase.