穩態液晶技術應用於旋轉內冷卻通道之熱傳量測傳量測

Abstract

現今渦輪機常見的散熱技術分為內部與外部冷卻，在內部冷卻通道設置肋條(rib)及鰭片(pin fin)為普遍增強熱傳的方法。本文以穩態液晶量測技術來分析旋轉內部冷卻通道的熱傳分佈。實驗利用CCD攝影機記綠通道表面液晶的變色過程，利用感溫液晶的顏色變化來得知此區域之溫度值，進而得知該區域之熱傳分佈。實驗模型為矩形單通道，將會在通道內部設置鰭片，冷卻通道的寬高比(aspect ratio)為4，冷卻通道的水力直徑(D_h)為20mm，鰭片直徑為5mm，鰭片的高度與鰭片直徑比值(H/D)為1，鰭片主流方向間距與鰭片直徑比值(X/D)為2，鰭片徑向方向間距與鰭片直徑比值(S/D)為2。實驗時在冷卻通道內部加壓並配合旋轉，提升旋轉數(Ro)，以利比較差異性。旋轉數(Ro)範圍為0~0.195，雷諾數(Re)範圍為10000~20000。實驗結果顯示，靜止時，高雷諾數的鰭片尾流區域較低雷諾數時的明顯。隨著旋轉數升高，鰭片前方的高熱傳區域與鰭片尾流區域形狀會產生變化並產生偏移，而鰭片尾端的紐賽數比值(Nu/Nuo)因為溫度較低而值較高，整體紐賽數比值大約介於2~3之間。

Steady state liquid crystal thermography was used to measure heat transfer in a one-pass rotating rectangular channel. The aspect ratio of the channel was 4:1 with two opposite walls roughened with pin fins. The test conditions of Reynolds number(Re) and rotation number(Ro) were in the range of 10000-20000 and 0-0.195, respectively. The diameter and height of the pin fins were 5mm and 5mm, respectively. A staggered array of pins with uniform streamwise and spanwise spacing(X/D=2,S/D=2) was installed. Stroboscopy was used as the photography technique. This photography technique is able to record images when the channel is in rotating condition. The result showed that in stationary condition, the wake region behind the pin fins became narrower with increased Reynolds number. When the rotation number increased, the shape of the pin fin wake was shifted in different directions.