本研究係以實驗的形式，探討以聲波振動的控制方式對火焰穩定性之影響。在研究中使用一個雙環同軸噴流燃燒器，於燃燒器的中心圓管提供氣態甲烷充作燃料，而外環圓管則供給空氣，以產生雙環同軸噴流擴散火焰。並且在施予各種不同的聲波振動頻率與振動振幅之情況下，分別振動氣態燃料、振動空氣以及同時振動氣態燃料與空氣，觀察雙環同軸噴流擴散火焰之結構的變化與噴流火焰的穩定性。

實驗中的火焰屬於完全發展均勻層流之擴散火焰，觀察中發現原本在管口的附著火焰(Attached Flame)在受到外界特定頻率強制振動時，將會產生上飄火焰(Lift-Off Flame)及飄離火焰(Blow-Out Flame)。其火焰型態也由原本的噴流擴散火焰轉變為預混火焰，在此稱這個特定頻率為共振頻率(resonance frequency)。

在聲波振動燃料的實驗中發現，當振動火焰之振動頻率逐漸接近共振頻率範圍時，燃燒器管口位置的火焰口徑尺寸會隨著振動頻率的增加而縮小，並且在稍微離開管口的位置，會發現振動火焰呈現內外雙層不同的淡藍色火焰；當振動頻率為共振頻率時形成上飄火焰，甚至產生飄離的現象。而增加外環空氣的出口流速與振動振幅之大小時，不僅擴大了上飄火焰的振動頻率範圍，增加碳顆粒生成的高度，而且上飄火焰距離燃燒器管口的平均高度亦隨之增加，甚至使得火焰更不容易回到管口的位置。然而，將聲波振動的頻率做升冪與降冪的改變時，對於被振動的實驗氣體而言，共振頻率發生的範圍以及振動火焰的火焰型態，並不會產生特殊的影響。

本研究另外進行對燃燒器外環圓管的空氣振動之探討，發現振動火焰隨著振動頻率逐漸增加時，火焰根部的藍焰區域外環將慢慢浮現一圈淡藍色的火焰並逐漸內縮，最後產生上飄火焰，但並沒有發生火焰被吹走並飄離的現象。當改變空氣的出口流速以及振幅的大小，除了影響上飄火焰的振動頻率範圍之外，在其他所探討的振動火焰之特徵長度方面，並沒有產生明顯的變化。

最後，針對聲波同時振動燃料與空氣的噴流擴散火焰進行火焰特性分析。所造成的共振頻率範圍與燃燒的火焰型態，是聲波振動燃料實驗以及聲波振動空氣實驗之結果的合成效果。並且當振動頻率的相位差為 時，比相位差在 時，其振動效果更為顯著。

In this study, the influence of acoustic modulation on flame stability has been investigated experimentally. A co-axial jet, in which methane as fuel flows at the inner tube, while air flowing at the outer tube is utilized in the experiment in order to produce a co-axial diffusion jet flame. The tube is long enough for obtaining a fully-developed flow.

Results show that as the attached flame is modulated by the woofer at the characteristic frequency, the jet flame turns out to be a lift-off flame and blow-out flame, the diffusion flame becomes premixed flame as well. The characteristic frequency of modulation is called the resonance frequency. It is found that the flame width at the exit of the burner decreases and the flame becomes light blue as the modulation frequency increases closed to the resonant frequency. As the modulation frequency is the resonance frequency, the flame becomes lift-off and even blow-out. It is also noted that as the air velocity and power increase, not only the frequency band increases, but also the height of soot production and the lift-off distance.

The effect of acoustic modulation of air on the diffusion jet flame is also under investigation. It is observed that there exists a light blue ring near the exit of the jet with acoustic modulation of air. When the modulation frequency increases, the diameter of the ring decreases. Eventually, a lift-off flame occurs. However, there is no blow-off condition with increasing modulation frequency. In addition, the effect of acoustic effect on both fuel and air is also studied. Result show that the effect of modulation of both fuel and air is the combined effect of individual modulation of fuel and air. Synchronized and 180° phase angle operating conditions are carried out in the experiment. It shows that the effect of modulation of both fuel and air with 180° phase angle is stronger than that with synchronized operating condition.

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