垂直圓柱容器中一低速空氣圓形噴流衝擊至一加熱圓盤之渦流結構流場觀測

Abstract

本篇論文利用實驗流場觀測方法對於在垂直圓柱容器中一低速空氣噴流衝擊至一加熱水平圓盤所產生的混合對流流場中的渦流結構進行研究。我們此次的研究焦點放在噴流入口流量、加熱圓盤與入口冷流體間的溫度差以及入口管徑對於渦流流場結構的影響。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由浮力、噴流慣性力、噴流衝擊效應所交互影響著的流場型態。實驗的操作範圍分別是：流量變化0∼5 slpm，加熱圓盤與入口冷空氣間的溫度差範圍0∼25℃以及兩種不同入口管徑（10 與 22.1 mm）。在本篇論文的實驗結果顯示了在實驗爐體中存在的典型流場結構是由三個圓形渦流卷所組成。在爐體內部以及中間部分的渦流卷主要是由噴流衝擊至圓盤表面而偏向所造成的結構，因此我們將之歸類為主要的以及次要的慣性力驅動渦流結構。而由浮力所引起的渦流卷則是出現在實驗爐體的外圍區域。在低浮力以及慣性力的實驗條件下，流場內的渦流結構為穩定並且呈現軸對稱的型態。但在高浮力或是高慣性力的實驗條件下，流場內的渦流結構則呈現不穩定以及產生稍微的變形，也因此發現了在流場型態中分別存在著慣性力、浮力所引起的不穩定。除此之外，在浮力所造成的不穩定流場結構中，存在於爐體中間部分的渦流卷不斷的進行生成以及衰退的變化型態。對於較大管徑的實驗條件下，爐體中間部分的渦流卷幾乎消失而不存在。以本次研究的實驗結果為依據下，我們提出了消除不穩定結構流場的關係方程式，並且建立了流場型態分佈圖以清楚顯示隨時間變化的流場結構的物理條件範圍。此外，我們也針對因慣性力所驅動之主要渦流卷尺寸大小與相關物理參數間的關係提出一關係公式。

Experimental flow visualization is carried out in the present study to investigate the vortex flow patterns in a mixed convective flow of a low speed air jet impinging onto a heated horizontal circular disk in a vertical cylindrical chamber. Attention is focused on how the jet flow rate, temperature difference between the heated disk and cold air jet, and diameter of injection pipe affect the vortex flow patterns. The flow photos taken from the side and top views of the processing chamber intend to unravel the vortex flow patterns driven by the combining effects of the buoyancy, jet inertia and jet impingement. Experiments are conducted for the jet flow rate varied from 0 to 5 slpm and disk-to-jet temperature difference from 0 to 25℃ for two different injection pipes with Dj = 10 & 22.1 mm. The results show that typically the flow in the processing chamber consists of three circular vortex rolls. The inner and middle vortex rolls are mainly generated by the deflection of the impinging jet at the disk surface and hence termed as the primary and secondary inertia-driven vortexes. The buoyancy-induced vortex roll is in the outer zone of the processing chamber. At low buoyancy and inertia the vortex rolls are steady and axisymmetric. But at certain high inertia and/or buoyancy the vortex flow becomes unstable and somewhat deformed, suggesting the presence of inertia and buoyancy driven instabilities in the flow. Besides, in the buoyancy driven unstable vortex flow the formation and decay of the middle vortex roll appear. For the larger injection pipe the middle vortex roll nearly disappears. Correlation equations are proposed to estimate the onsets of these two instabilities. Base on the present data, a flow regime map is established to reveal the temporal state of the flow. Moreover, a correlation equation is provided to show the effects of various parameters on the size of the primary inertia-driven vortex roll.