空氣噴流衝擊至一大直徑加熱圓盤之渦流結構與熱傳特性研究

Abstract

本篇論文利用實驗流場觀測方法及溫度量測方法來探討空氣噴流至一16吋直徑的加熱圓盤之渦流結構之流場進行研究。在本實驗操作範圍分別是：噴流到圓盤的距離 25.0 mm∼12.5 mm，噴流的直徑固定為20.0 mm，流量變化是0.4∼10.0 slpm，所相對的噴流壘諾數變化為 27∼676，而加熱圓盤與入口冷空氣間的溫度差範圍0∼14˚C，所相對的雷利數0∼7,340。 由流場觀測對於噴流到圓盤的高度為25.0 mm可以清楚顯示渦流的特性有一次渦流、二次渦流及數個浮力渦流影響流場。而浮力造成的渦流個數與強度隨著浮慣比的增加而變化，其中個數變化為1∼5個。此外在低的浮慣比之下，浮力渦流有很顯著的變形，而渦流流場隨著浮慣比的增加會變成不規則渦流。 在本實驗的結果中，流場經過一段長時間都是非穩態，浮力渦流的變形、互相推擠、合併分裂，都是非週期性。然而流場的非穩態與不規則渦流，對於溫度場有很大的影響，溫度震盪的非常劇烈。對於渦流定量的部份，針對浮力渦流的個數、位置以及尺寸將做成表格並且求得經驗公式。針對渦流種類分類經實驗結果繪製流譜亦求得經驗公式。 當噴流到圓盤的高度降到12.5 mm 以及冷空氣與加熱圓盤間的溫度差為8∼14˚C，所對應的雷利數為1,470∼2,670 ，會有環形水波在徑向方向移動以及徑向渦流出現在流場當中，這些水波會有點變形並且不對稱。而徑向渦流從停滯區開始往徑向方向成長，但並不是非常均勻性的成長。此外，這些水波跟徑向渦流會有同時存在的現象。將這些現象繪製流譜含有所有的慣性渦流、浮力渦流和水波以及徑向渦流，並且將這些渦流邊界求得經驗公式。

An experiment is carried out in the present study to investigate the vortex flow patterns resulting from a round air jet impinging over a large confined heated horizontal circular disk of 16 inch in diameter. In the present experiment the jet flow rate is varied from 0.4 to 10.0 (standard liter per minute) for the jet Reynolds number ranging from 27 to 676 with the injection pipe diameter Dj = 20.0 mm and jet-disk separation distance H = 12.5mm and 25.0 mm. The temperature difference between the disk and the air injected at the inlet of the jet is varied from 0 to 14.0℃ for the Rayleigh number Ra ranging from 0 to 7,340.

The results from the flow visualization for H=25.0 mm indicate that the vortex flow is characterized by the primary and/or secondary inertia-driven circular rolls along with several buoyancy-driven rolls. The number and strength of the buoyancy induced rolls increase with the buoyancy-to-inertia ratio, which can vary from 1 to 5. Besides, at a low Gr/Rej2 slightly deformed circular buoyancy rolls prevail in the test section. For slightly higher Gr/Rej2 the flow is dominated by the highly deformed curved rolls. The roll pattern becomes somewhat irregular for a further increase in Gr/Rej2 . For all cases examined here the vortex flow is unsteady at long time after the initial transients have died out and is irregular to a certain degree since the buoyancy effect is rather strong for the large impinging plate tested here. In addition to the roll deformation, the mutual pushing, merging and splitting of the buoyancy rolls occur nonperiodically in time. This unsteady and irregular vortex flow is also reflected in the data for the air temperature variations with time. Some quantitative buoyancy-driven vortex flow characteristics such as the size and location of the buoyancy rolls are summarized and correlated empirically. Moreover, a flow regime map is provided to delineate various induced vortex flow patterns and the boundaries separating various vortex flow patterns are also correlated empirically. When the jet-disk separation distance is reduced to 12.5 mm for ΔT varied from 8.0℃ to 14.0℃ ( corresponding to the Rayleigh number ranging from 1,470 to 2,670 ), circular waves traveling in the radial direction and radial vortex rolls appear in the confined impinging jet flow. These waves are somewhat deformed and are not axisymmetric. The radial rolls originate in the stagnation region of the impinging jet and grow in size in the radial direction. The size of the radial rolls is also nonuniform in the circumferential direction. Besides, the moving waves and radial rolls can coexist in the flow. Moreover, the ranges of the jet Reynolds number and Rayleigh numbers leading to the new vortex flow patterns are determined. Flow regime maps delineating various vortex flow patterns including all inertia- and buoyancy-driven roll and wave patterns found here are given. And the boundaries among various vortex flow patterns are empirically correlated.