黏彈性流體在扁平矩形管道之層流熱傳研究

Abstract

黏彈性流體(非牛頓流體)在扁平矩形管道中流動時，會因為第二正向力差的原因而使流體產生二次流的現象，此現象增加了流場的混合，因此使熱傳係數提升，但也因為黏彈性流體的黏性現象與壁面產生較大的剪切力，使得壓力降也比純水(牛頓流體)高。本論文是以實驗方法探討純水，以及濃度為500ppm、1000ppm、1500ppm和2000ppm的聚丙醯胺水溶液，在矩形管道中廣義雷諾數80~300的層流範圍內，三種不同的邊界條件：上板加熱，下板加熱，與上下板同時加熱的情況下的熱傳與壓降關係。實驗結果顯示隨著質量流率的增加，聚丙醯胺高分子溶液的紐塞數會增高，與純水比較，上板加熱有較好的熱傳增加效果。而當濃度大於1000ppm時，所產生的壓力差降增加幅度遠高於熱傳增強效果。在此廣義雷諾數範圍內，同時考慮熱傳增加與壓力差增加的情況下，以邊界條件為上板加熱，水溶液濃度為500ppm和1000ppm時，在相同質量流率的情況時比較純水，熱傳增強效益比，可分別達到1.22與1.19倍。

Secondary flows exist due to second normal stress difference when a viscoelastic fluid flows in a rectangular channel. Therefore, the heat transfer coefficient is enhanced as a result of fluid mixing. However, viscoelastic fluids will have higher pressure drops than pure water because of viscosity effects. In this research, experimental studies on the laminar heat transfer and pressure drop of pure water, 500ppm, 1000ppm, 1500ppm and 2000ppm aqueous polyacrylamide solutions have been conducted. The range of values of general Reynolds number lies between 80~300. There are three different boundary conditions including upper heated wall, lower heated wall and upper-lower heated walls. Present results showed that values of local Nusselt number were enhanced with increasing mass flow rates with higher heat transfer enhancement for upper heated wall. On the contrary, when the concentrations of aqueous polyacrylamide solutions were higher than 1000ppm, the magnitudes of pressure drop increment were much larger than those of heat transfer increment at the same mass flow rates. In addition, when we considered both heat transfer and pressure drop increments with upper heated wall at the same mass flow rates, the maximum heat transfer enhancement indexes of 500ppm and 1000ppm aqueous polyacrylamide solutions were 1.22 and 1.19 respectively.