横掠椭圆管的换热和流动特性研究(英文)

对横掠不同长径比的椭圆管的换热和流动特性进行了研究。实验过程中考虑了8种不同类型的椭圆管,流动介质为空气。通过热质比拟和萘升华技术,获得了雷诺数在7000~65000范围内的局部和平均对流换热系数,通过比较不同类型椭圆管的实验结果,阐明了长径比k的影响,首次给出了Nu=f(k,Re,Pr)形式的实验关联式。同时,通过实验还获得了流动阻力系数。最后,使用油-灯黑可视化技术对横掠椭圆管的绕流特点进行了显示。结果表明,局部对流换热系数随长径比k变化,边界层的分离点随k的增大而向后移动。当k<6.0时,椭圆管的平均对流换热系数高于圆管;当k=6.0时,达到最大值,大约为圆管的1.6倍,同时,其阻力系数随着k的增大而减小;当k=2.0时,阻力系数约为圆管的60％。流型印证了实验结果的合理性。

Heat Transfer Characteristics and Flow Behaviors of Elliptic Cylinders in Crossflow

Heat transfer characteristics and flow behaviors of elliptic cylinders with different axis ratio k in crossflow are investigated experimentally. Eight kinds of cylinders have been examined, and the testing fluid is air. At first, the local and overall heat transfer coefficients are obtained with the Reynolds number ranging from 7 000 to 65 000, by heat-mass transfer analogy and the naphthalene sublimation technique. The effect of k on heat transfer is clarified by comparing the results of various types of elliptic cylinders. Furthermore, the hydraulic drag coefficients are also presented. Finally, the oil-lampblack technique has been employed to enable visualization of the flow features around the elliptic cylinder. The results show that the local heat transfer characteristics vary with k, and the separation of the boundary layer shifts downstream with the increase of k. It is found that the overall heat transfer coefficient of elliptic cylinder with k<6. 0 is higher than that of a circular cylinder. At k = 6. 0, it has the maximum which is about 1. 6 times that of a circular cylinder. At the same time, the drag coefficient drops down with the increase of k. For k=2. 0, the drag coefficient is about 60% that of a circular cylinder. At last, the fluid versions confirm the rationality of the experimental results.