藉由底盤旋轉在垂直圓柱容器中空氣圓形噴流衝擊至一加熱圓盤抑制渦流結構之流場觀測研究

Abstract

本篇論文利用實驗流場觀測方法及溫度場量測方法探討藉由底盤旋轉在垂直圓柱容器中一空氣圓形噴流衝擊至一加熱圓盤抑制由浮力所驅動的穩態及非穏態渦流結構之流場特性進行研究。在本實驗研究操作範圍分別是：流量變化0〜12.0 slpm，加熱圓盤與入口冷空氣間的溫度差範圍0〜25.0℃，底盤轉速由0~50 rpm，所相對的噴流雷諾數變化為0〜1,623，相對於雷利數0〜63,420，而旋轉雷諾數0〜3,892另外噴流出口到加熱底板間的距離維持20.0 mm，而噴流管徑分別是10.0 與 22.1 mm。 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。在較高的底盤轉速下，由浮力所造成的渦流可以明顯受到抑制，甚至完全消除。此外，在高轉速下，由慣性力所形成的primary inertia-driven roll會變得較細長且強度較弱。而在足夠高的噴流雷諾數下所形成的secondary, tertiary, quaternary inertia-driven rolls亦會隨著底盤轉速提高而依序消失。由實驗結果可以進一步了解底盤旋轉對於由慣性力/浮力所造成的週期性與非週期性流場有穩定的效果，此種不穩定的流場可以藉由一定高的底盤轉速而達穩定，在溫度場的結果上，由底盤旋轉可以減低徑向的溫度變化，並且明顯使得Buoyancy-driven roll出現的臨界浮慣比數延遲。

An experiment combining flow visualization and temperature measurement is carried out in the present study to explore the possible suppression of the buoyancy-driven stable and unstable vortex flow resulting from a round jet of air impinging onto a heated horizontal disk in a vertical cylindrical chamber by the disk rotation. In this study the experiment is mainly conducted for the jet flow rate varied from 0 to 12.0 slpm (standard liter per minute) with two different injection pipes (diameter 10.0 and 22.1 mm) and the temperature difference between the disk and the air injected into the chamber is varied from 0 to 25.0℃ at a fixed jet-to-disk separation distance of 20.0 mm. The disk rotation speed is varied from 0 to 50 rpm. Thus the jet Reynolds number, Rayleigh number and rotational Reynolds number range respectively from 0 to 1,623, from 0 to 18,790, and from 0 to 3,892.

The results from the flow visualization clearly show that typically the steady mixed convective air jet impinging onto the 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 hence termed as the inertia-driven vortex roll. 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 resulting from the temperature difference between the heated disk and the inlet air prevails in the outer zone of the processing chamber. At a high disk rotation rate, the buoyancy roll can be significantly suppressed and even wiped out by the disk rotation. Besides, the primary inertia-driven roll is stretched out to become slender and weaker. Moreover, the inertia-driven secondary, tertiary, and quaternary rolls dominated at high Rej can be entirely wiped out. We further note that the disk rotation can effectively suppress the inertia-driven and/or buoyancy-driven time-dependent and nonperiodic vortex flows. The unstable vortex flows can be completely stabilized by the disk rotated at a high speed and the flows become steady. The disk rotation can also reduce the radial temperature variation in the flow and significantly delay the onset of the buoyancy-driven roll. Based on the present data, a flow regime map is provided to delineate the the axisymmetric and nonaxisymmetric vortex flows with various disk rotation rates for H=20.0 mm.