空氣噴流衝擊至一加熱旋轉圓盤之穩定及不穩定渦流特性研究

Abstract

在本論文中，利用實驗流場觀及溫度場量測，探討底盤旋轉在垂直圓柱容器中空氣噴流衝擊至一加熱圓盤之穩定與不穩定渦流特性研究。主要關注底盤旋轉對由慣性力及浮力所驅動的穩定渦流特性及其發生條件之影響。此外，也對底盤旋轉如何抑制在較高的慣性力及浮力所驅動的渦流進行研究。特別的是，本實驗研究操作範圍分別是:雷諾數0~811，雷利數0~120,260，而旋轉雷諾數0~2335，另外噴流出口到加熱底板間的距離比上噴流的直徑HDj(=H/Dj)為2~5。 由流場觀測可以清楚顯示典型噴流衝擊旋轉圓盤的結果有三種渦流，靠近噴流中心，產生了由於慣性力所驅動的渦流稱為Primary inertia-driven roll，而流道中間的渦流是由於底盤旋轉的離心泵送效應，稱之為Rotation-driven roll。而靠近爐體壁面的渦流是由於加熱圓盤與入口冷空氣間的溫度所形成的浮力效應所引起，稱之為Buoyancy-driven roll。由實驗的結果清楚地了解在較高的底盤轉速下產生慣性力所驅動之渦流的臨界雷諾數也跟著提高而延遲。在HDj=3~4時，在足夠高的噴流雷諾數下所形成的secondary和tertiary inertia-driven rolls亦會隨著底盤轉數提高至大於10轉後而消失。而HDj=2時，在高轉速下( rpm)，由浮力所形成的Buoyancy-driven roll會變得較小且強度較弱。同時，由慣形力所形成的primary inertia-driven roll會變得較細長且較弱。此外，Secondary inertia-driven roll則完成被消除。但在HDj=3跟4，由於高度增加，需要更高的轉速才能有效抑制由浮力所驅動的渦流。因此，底盤旋轉台有效抑制由慣性力驅動的不穩定渦流，但對在HDj=5由於慣性力及浮力互相推擠所驅動的不穩定渦流則無法穩定流場。最後，針對在HDj=2，由於高浮慣比所驅動的type-1 buoyancy-driven unstable vortex flow可以藉由一定高的底盤轉速而達穩定。 此外，我們以流譜圖描述在爐體內存在三種型式的渦流結構，也發表了區分這些渦流結構之邊界的經驗公式。

An experiment combining flow visualization and temperature measurement is carried out here to explore the effects of disk rotation on the vortex flows resulting from a round jet of air impinging onto a heated horizontal disk confined in a vertical cylindrical chamber. Attention is paid to investigating how the disk rotation affects the onset of the inertia- and buoyancy-driven vortex rolls and their steady characteristics. Moreover, the unstable vortex flows prevailed at high jet inertia and buoyancy force affected by the disk rotation are explored to investigate the possible suppression of the unstable flows by the disk rotation. Specifically, the present experiment is conducted for the jet Reynolds number, Rayleigh number and rotational Reynolds number range respectively from 0 to 811, 0 to 120,260, and 0 to 2,335 for HDj(=H/Dj)=2 to 5. The results from the flow visualization clearly show that typically the steady vortex flow resulting from the mixed convective air jet impinging onto the heated rotating disk consists of three circular vortex rolls. The inner vortex roll is generated by the deflection of the impinging jet at the disk surface and is mainly driven by the jet inertia. The middle vortex roll is mainly formed by the centrifugal pumping action produced by the disk rotation and hence termed as the rotation-induced roll. The buoyancy-induced vortex roll results from the temperature difference between the heated disk and the inlet air and prevails in the outer zone of the processing chamber. The results from the present study clearly reveal that the critical jet Reynolds numbers for the onset of the inertia-driven rolls are slightly higher for the disk rotated at a higher speed. Besides, at the disk rotation rate ≥ 10 rpm the secondary and tertiary inertia-driven rolls are completely wiped out for HDj=3 and 4. For HDj=2 at higher disk rotation rate for ≥ 20 rpm the buoyancy-driven roll can be substantially squeezed by the disk rotation to become smaller and weaker. Meanwhile, the primary inertia-driven roll is stretched out to become slender and weaker. Moreover, the secondary inertia-driven rolls are completely wiped out. But for HDj=3 and 4, due to a stronger buoyancy-driven vortex flow at a higher jet-disk separation distance a higher is needed to stabilize the buoyancy-driven roll. The disk rotation can effectively stabilize the inertia-driven unstable vortex flow. But the mutual roll-pushing unstable vortex flow is not stabilized by the disk rotation for a larger jet-disk separation distance with HDj=5. Finally, we note that the type-1 buoyancy-driven unstable vortex flow prevailed at high buoyancy-to-inertia ratio for HDj=2 can be suppressed and even completely stabilized by the disk rotated at a high speed. Moreover, we identify three different types of vortex flows in the chamber. Flow regime maps delineating various types of vortex flow are provided. Besides, empirical correlations are proposed for the boundaries separating three different types of vortex flows.