垂直圓柱容器中空氣經由多孔板衝擊至一加熱圓盤之混合對流渦流結構之研究

Abstract

本篇論文利用實驗流場觀測方法探討在垂直圓柱容器中空氣經由多孔板衝擊至一加熱圓盤穩態及非穏態渦流結構之流場特性進行研究。主要的目的是藉由多孔氣流的效應來降低在圓柱中因慣性力所造成的渦流結構，並闡釋多孔板放入入口處所造成的效應對於穩態及暫態過程中熱浮力驅動之渦流結構之影響以及探討慣性力渦流結構出現的機制。另外，熱浮力所造成的不穩定現象也一併討論說明。本實驗研究之操作範圍分別是：噴流至圓盤的距離20〜40 mm，多孔板的直徑固定為76.2 mm，流量變化0〜5.0 slpm，加熱圓盤與入口冷空氣間的溫度差範圍0〜25.0℃，整體的噴流雷諾數變化為0〜89，所相對的噴流雷諾數變化為0〜28，相對於雷利數0〜150,325。 從流場可視化可以很清楚地發現慣性力所造成的第一類和第二類渦流結構均有被抑制並且出現的機制也延後發生，原因是多孔板所造成的效果。當∆T ≠ 0時，慣性力所造成的第二類渦流並無出現且熱浮力所驅動的渦流結構會一直存在流場中。另外，針對不同高度(H = 20.0, 30.0及40.0mm)我們分別定義出四種型態的流譜型態：(1) 理想流譜，(2) 熱浮力所主導的流譜，(3) 慣性力所主導的流譜以及(4) 慣性力和熱浮力共同主導的流譜，並且界定不穩定的流場型態以及出現的範圍。其中，理想流譜僅出現在完全沒有加熱的情況且低流量。 除此之外，本研究還比較有和沒有加裝多孔板對於流場型態有多大的影響。從相關的流場圖分析結果得知，加裝多孔板確實能夠有效地抑制慣性力所產生的渦流結構在圓柱型腔體內出現。最後，我們將流場轉變以及熱浮力造成不穩定性的範圍以實驗結果做分析，求得經驗公式。

An experimental flow visualization is carried out in the present study to investigate how the installation of a showerhead at the jet injection nozzle affects the vortex flow resulting from multiple air jets impinging onto a heated horizontal circular disk confined in a vertical cylindrical chamber. The study is motivated by the fact that the multiple air jets of small cross section are less likely to induce inertia-driven vortex flow. Particular attention is paid to examining the effects of the showerhead installation on the characteristics of steady and time-dependent buoyancy-driven vortex flows. Besides, the onset of the inertia-driven vortex flow and the instability of the buoyancy-driven vortex flow will be inspected. In the present experiment three jet-disk separation distances are considered with H = 20.0, 30.0, and 40.0 mm for a fixed injection nozzle diameter (Dn = 76.2 mm). The jet flow rate is varied from 0 to 5.0 slpm (standard liter per minute) for the overall jet Reynolds number Ren ranging from 0 to 89 and the jet Reynolds number Rej ranging from 0 to 28. 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 150,325. The results from the flow visualization clearly show the significant effects of the showerhead on the critical Rej for the onset of the primary and secondary inertia-driven rolls. Specifically, the onsets of the inertia-driven rolls are delayed to higher Rej through the installation of the showerhead. Besides, the secondary inertia-driven roll does not appear in the chamber with ∆T ≠ 0 for HDn = 0.26 to 0.52. Moreover, we identify four different types of vortex flow (plug flow, buoyancy-driven vortex flow, inertia-driven vortex flow, and mixed flow) and delineate temporal state of the flow when HDn is varied from 0.26 to 0.52. Flow regime maps are presented. The plug flow only occurs at lower Rej for the disk unheated (Ra = 0). The buoyancy roll always appears for HDn = 0.39 and 0.52 even at a small ∆T of 1℃. Besides, we compared the vortex flows in the chamber with and without the showerhead installation. The result shows that the showerhead installation at the injection nozzle can effectively suppress the inertia-driven vortex rolls. Finally, empirical correlations are proposed for the boundaries separating the buoyancy-driven and mixed vortex flows and for the onset conditions of the vortex flow instabilities for HDn.