鈍體燃燒器冷態流場之研究

Studies on the Cold Flow of a Bluff-Body Combustor

本研究使用雷射都卜勒測速儀並配合折射率配合法，對一軸對稱鈍體燃燒器之冷態流場進行量測，探討在不同中心噴流與外環噴流之燃氣對空氣流動量比下諸流場之特性，同時並完整取得一般實驗方法無法獲得之噴流入口及近壁區流場之細部物理變量。實驗結果發現當噴流動量比由低改變至高時(MR=0.15,0.58,1.4)，鈍體燃燒器分別對應產生三種不同流場型態：(1)外環主導噴流，(2)非單一主導之混合噴流，(3)中心主導噴流等。此三種型態之流場在鈍體表面之迴流區內皆有兩個漩渦，而在前、後停滯點附近及噴流邊界之剪力層有較大之擾動速度、紊流動能及雷諾剪應力分佈。隨著動量比之增大，迴流區渦漩之形狀、大小、位置皆會隨之改變；而擾動速度、紊流動能則具相似之變化趨勢且亦隨動量比之增大而增大；但最特殊的是雷諾剪應力並未隨動量比之改變而改變，而是當動量比居中間值時(MR=0.58)流場具最大之雷諾剪應力。

Laser Doppler velocimetry, coupled with refractive index matching,were used to measure some important fluctuation statistics for the coldflow of an axis-symmetrical bluff-body combustor. Detailed characteristicsof the flow were obtained for different momentum ratios, especially at theentrance and near-wall regions where conventional measurement apparatusfailed because of disturbing the flow field. Results show that, as themomentum ratio increases ( MR =0.15,0.58,1.4), three different flowpatterns are found:(1) Jet flow with outer-ring flow dominating, (2)Mixed-type jet flow, (3) Jet flow with center flow dominating. Around thestagnation zone and the shear layers adjacent to the jet flow, the velocityfluctuation, turbulent kinetic energy, and Reynolds stress all have largervalues than in other areas. As the momentum ratio changes, the shapes,sizes, and locations of all separating bubbles change significantly. Similar distributions were found for turbulent fluctuation velocity and turbulentkinetic energy. For the whole field, the fluctuation velocity and theturbulent kinetic energy increase as the momentum ratio increases.Specifically, the Reynolds stress doesn’t show the same tendency and itshighest value occurs when there is moderate momentum ratio ( MR =0.58).