垂直圓柱容器中一空氣圓形噴流衝擊至一加熱圓盤之不穩定渦流特性

Abstract

本篇論文利用實驗流場觀測方法及溫度場量測方法對於垂直圓柱容器中一高速空氣圓形噴流衝擊至一加熱圓盤之可能產生新的慣性力驅動渦流和一些獨特的週期性混合對流渦流流場特性進行研究。在本實驗研究中，我們藉由拍攝流場結構的上視圖以及側視圖以釐清這些新的渦流特性。實驗的操作範圍分別是：噴流出口到加熱底板間的距離變化10.0〜30.0 mm，流量變化0〜12.0 slpm，相對於雷諾數變化0〜1,623，加熱圓盤與入口冷空氣間的溫度差範圍0〜25.0℃，相對於雷利數0〜63,420。 在本篇論文的實驗結果顯示了在實驗爐體中足夠高的雷諾數下，會產生慣性力驅動的三次和四次渦流。在更高的雷諾數，由於慣性力的驅動致使這渦流流場變成不穩定。在較大的浮慣比會形成浮力所驅動的不穩定，增加雷諾數則變成穩定，在較小的浮慣比下會形成慣性力所驅動的不穩定，此過程只發生在噴流出口到加熱底板間的距離為20.0 mm情況下。在高的噴流出口到加熱底板間的距離30.0 mm，由於慣性力及浮力所驅動渦流相當的大且相連在一起，所以整各流場不穩定的運動狀態則是由兩者互相推擠所造成。特別注意在噴流出口到加熱底板間的距離為10.0 & 20.0 mm慣性力所驅動的三次及四次渦流的臨界雷諾數隨著加熱圓盤與入口冷空氣間的溫度差的增加而增加，但是在噴流出口到加熱底板間的距離為30.0 mm則相反，並指出會隨著加熱圓盤與入口冷空氣間的溫度差的增加使渦流流場更容易產生不穩定。

An experiment combining flow visualization and temperature measurement is carried out in the present study to investigate the possible presence of new inertia-driven vortex rolls and some unique characteristics of the time-dependent mixed convective vortex flow resulting from a high speed round air jet impinging onto a heated horizontal circular disk in a vertical cylindrical chamber. The flow photos taken from the side and top views of the vortex flow in the chamber aim to unravel these new vortex flow characteristics. In the present experiment the jet-to-disk separation distance is varied from 10.0 to 30.0 mm and the jet flow rate is varied from 0 to 12.0 slpm (standard liter per minute) for the jet Reynolds number Rej ranging from 0 to 1,623. The temperature difference between the disk and the air injected into the chamber is varied from 0 to 25.0℃ for the Rayleigh number Ra ranging from 0 to 63,420. The results from the flow visualization clearly show that at sufficiently high Rej the inertia-driven tertiary and quaternary rolls can be induced. At even slightly higher Rej the vortex flow becomes unstable due to the inertia-driven flow instability. Only for H=20.0 mm the flow is subjected to the buoyancy-driven instability. Because of the simultaneous presence of the inertia- and buoyancy-driven instabilities, a reverse flow transition can take place in the chamber with H=20.0 mm. At the large H of 30.0 mm the flow unsteadiness results from the mutual pushing and squeezing of the inertia- and buoyancy-driven rolls since they are relatively large and contact with each other. It is also noted that the critical jet Reynolds number for the onset of tertiary and quaternary rolls increase with for H=10.0 & 20.0 mm. But for H=30.0 mm the opposite is true, indicating that raising can destabilize the vortex flow.