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

为了提高换热管的性能，对带翅片的粗糙表面减阻和湍流特性进行了实验研究。采用热丝法测量了翅片管周围的平均速度分布、速度波动强度、速度波动谱和相干性分布，并采用烟丝法进行了流动显示。结果得出以下结论：固体冷杉粗粗地附着在换热管表面，既增加了换热面积，又起到了导叶的作用。当翅片节距较小时，主流与翅片之间的夹角较小，流动在管表面的倾斜度较小。管表面的流动被翅片倾斜。当锯齿形翅片间距较大时，由于流动从锯齿形翅片边缘分离，阻力较实心翅片管增大，而由于锯齿形翅片表面不易与主流接触，传热性能下降。当锯齿鳍节距变小时，锯齿鳍边缘对流动的倾斜度较大。导致换热增强的原因不仅是翅片增加了湍流度，还增加了主流与管或翅片表面的接触距离，由于流动在锯齿状翅片管处的倾斜度大，因此阻力比实心翅片管增加。当锯齿状翅片间距进一步变小时，流体不再在翼片间隙内流动。因此，流体主要在翅片外流动，换热性能没有提高。

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.