黏彈性流體在寬高比2:1之水平矩形管道內的熱傳增強實驗

Abstract

黏彈性流體屬於非牛頓流體，在扁平的矩形管道內流動時，由於正向力差效應而產生二次流，此效應增加流場混合，提高熱傳係數，但是黏彈性流體的黏度比水大，流動時增加流體與壁面的剪切力，亦即增加管內的壓力差降。本論文則是以實驗的方式探討純水以及濃度1000wppm的聚丙醯胺水溶液，在寬高比為2：1的水平矩形管道中，廣義雷諾數60∼540的層流範圍內，三種不同熱邊界條件：上板加熱、下板加熱以及上下板同時加熱情況下的壓力差降與熱傳係數。實驗結果顯示，聚丙醯胺水溶液與純水的摩擦因子與雷諾數的關係，皆符合 關係式。聚丙醯胺水溶液具有比純水高的熱傳係數，原因為上板加熱時，因為正向力差效應以及輕剪力效應與軸向速度扭曲，使熱傳係數提高；下板加熱時，除了正向力差效應以及輕剪力效應與軸向速度扭曲外，尚有自然對流效應的加成；上下板同時加熱時，因為速度梯度具有對稱性，因此軸向速度不向任何一加熱面扭曲，所以主要熱傳增強機制為正向力差效應與自然對流效應。

The viscoelastic fluid has non-newtonian fluid behavior in which secondary flows exist in a rectangular duct due to the normal stress difference. Because of the secondary flows, the flow field results in more mixing and the heat transfer coefficient is therefore enhanced. However, the viscoelastic fluid has the higher pressure drop than that of the pure water as a result of its higher viscosity. Experimental studies of the laminar pressure drop and heat transfer behavior of the pure water and 1000wppm separan solutions were carried out in a aspect ratio of 2:1 rectangular duct. The range of the value of generalized Reynolds number lies between 60~540.Three different thermal boundary conditions were studied, including the heated upper wall, heated lower wall and heated upper-and-lower walls. The experimental friction factors of the pure water and the separan solutions agreed with the value predicted from the correlation: .The measured Nusselt numbers of the separan solutions were considerably higher than the values of the pure water. It was due to the normal stress difference, shear-thinnig effect and axial velocity distortion for heated upper wall. For heated lower wall, the heat transfer coefficient enhancement was the same as the heated upper wall,as well as the influence of natural convection effect was also included. In the heated upper-and-lower walls case, the axial velocity distortion vanished because the symmetry of the velocity profile. The mechanisms of heat transfer enhancement were normal stress difference and natural convection effects.