多孔圓柱上逆流擴散火焰的實驗觀測

Abstract

本碩士論文主要是利用燃料（甲烷）通過多孔燒結圓柱（直徑3公分、長度4公分）表面，在一個層流環境下產生擴散火焰，再加以強制對流的流場（增加進氣速度），以觀測火焰在圓柱上各種結構的特徵及發生吹離的現象。實驗參數分別為進氣速度和燃料噴出速度，以及兩種不同燃料噴出面積（半噴和全噴）的圓柱。在不同燃料噴出速度下改變進氣速度，可以得到各種不同的火焰結構（例如包絡焰、尾焰和吹離火焰），並定義火焰跳距距離（stand-off distance）、火焰厚度（flame thickness）、方位角（attached angle）、火焰長度（flame length）以及火焰吹離高度（flame lift-off height）等，以進行各種火焰的物理現象探討。而詳細的火焰種類以及發生吹離現象後又落回圓柱的轉變過程都會攝影記錄。並針對火焰吹離發生的原因及現象做詳細的說明。最後，將實驗所得到的結果和數值模擬的結果作比較分析來確認彼此的正確性，以對火焰有更完整清楚的認識和瞭解。火焰在半噴的圓柱上可以分成四個區域來討論，在I 、II及III的區域內，火焰初始呈現包絡焰，進氣速度增加後，火焰轉變成尾焰。而這三區的劃分在於火焰顏色隨著燃料和空氣間的比例不同會有所區別。在第IV區內，火焰隨進氣速度增加，依序為包絡焰、尾焰、吹離火焰，最後火焰又回到圓柱成為尾焰。火焰在全噴的圓柱上，分成V、VI兩個區域來討論，在第V區，火焰隨進氣速度增加，與第IV區火焰出現的順序一樣。而在第VI區，火焰在包絡焰後，直接變成吹離火焰，最後又回到圓柱，變成尾焰。

This work built the apparatus, consisting of a wind tunnel and porous sintered cylindrical burner, to observe the flame behaviors over the cylindrical burner. The parameters in the experiment are the changes of inflow air velocity (Vair) and fuel ejection velocity (Vfuel), respectively, under two different fuel ejection areas (S). The flame configurations and transition processes are photographed by digital video. The flame characteristics can be categorized into four regions (I-IV) for half fuel ejection area case and two regions (V and VI) for full area case. For region I, II and III, at the initial incoming velocity 0.41 m/s, an envelope flame is established surrounding the cylindrical burner. By increasing the inflow velocity to a critical value, the envelope flame is blown off from the forward stagnation area and transformed into a wake flame. The major difference among these regions is the appearance of flame colors. For region IV and V, as the inflow velocity increases, the envelope, wake, lift-off and late wake flames show up in order. Because the fuel can be directly injected into the lift-off flame for the case of full-ejection area cylindrical burner, its survival domain is much greater than that of half-ejection area one. In region VI, the lift-off flame directly appears after the envelope flame, no wake flame is observed between these two. Finally, as the inflow velocity increases, the wake flame appears again after lift-off flame.