強制對流下固態燃料引燃及火焰傳播之研究

Abstract

本論文係以數值分析方法來研究暫態強制對流環境下，纖維質材料垂直熱厚型板之引燃及火焰傳播現象。材料引燃及火焰傳播歷程可以分成兩個階段，第一個階段是加熱階段，第二個階段是火焰發展階段。在加熱階段，固態燃料的最高溫度發生在燃料表面，且會隨加熱時間增加而上升。在火焰發展階段，主要包含了引燃過程及一個過渡的過程，其中引燃過程又可分為誘發時期(induction period)和熱暴升時期(thermal run away)。在誘發時期，固態燃料表面可燃混和氣體形成，但還不足以產生足夠的熱量。到達熱暴升時期，之前預混的可燃氣體發生化學反應，釋放出大量的熱，因此，氣相溫度在短時間內急速上升。在過渡過程初期，火焰型態由預混火焰快速的轉變為擴散火焰。其後，火焰隨時間逐漸成長，並開始往上游及下游傳播，最後，向下火焰傳播速度達到一穩定值。本論文改變了進氣溫度、速度和外加熱源的峰值等參數來討論對火焰的影響。針對進氣溫度，研究發現，引燃時間和達到引燃時燃料表面的最高溫會隨著進氣溫度的增加而減少，而向下火焰傳播速度則呈現相反的趨勢。考慮進氣速度的影響，在不同的進氣速度下，引燃時間沒有顯著的變化，向下火焰傳播速度則隨著進氣速度的增加而減少。對外加熱源的峰值而言，引燃時間會隨著外加熱源的峰值的增加而減少，而達到引燃時燃料表面的最高溫則呈現相反的趨勢。和Lin (1999)自然對流環境下的研究相比，本論文發現在火焰前端有迴流流場產生，而Lin (1999)並沒有，除此之外，引燃時間和達到引燃時燃料表面的最高溫均比Lin (1999)的結果還小。最後，本論文使用穩態和暫態燃燒模式來模擬Pan (1999) 及Chen (1999)的實驗，預測結果的趨勢，例如火焰傳播速度隨著進氣速度的減少而增加、隨著進氣溫度的增加而增加和燃料厚度的減少而增加等，都與兩實驗的結果相符合。另外，在進氣速度較高的條件下，火焰傳播速度的預測值與實驗值幾乎完全相同。

An unsteady combustion model is developed and solved numerically to investigate the ignition and subsequent flame development behaviors over a vertically-oriented cellulosic thick fuel, subjected to a specified incident heat flux in a forced convection environment. The whole process is divided into two distinct stages, which are heating up and flame development, respectively. In the heating up stage, the maximum temperature, occurred at interface, increases with time. The flame development stage consists of ignition and transition processes. Ignition includes an induction period and a thermal run away process. During the induction period, a flammable fuel/oxidizer mixture is establishing at the pyrolyzing fuel surface, but chemical reaction is not strong enough to generate significant heat. Upon to thermal run away process, a burning of premixed flame, the temperature raises sharply. In transition process, the flame initially is in a transition from a premixed flame to a diffusion flame. Subsequently, flame starts to spread downward and upward, simultaneously, and grows with time. A steady downward flame spread is reached eventually. The parametric study is based on the variation of the incoming flow temperature and velocity and the peak of imposed heat flux, respectively. Results show that the ignition delay time and the maximum interface temperature at the instant of ignition decrease with an increase of the incoming flow temperature, and steady downward flame spread rate shows the opposite trend. Considering on the effect of the incoming flow velocity, the ignition delay time is invariant under the different flow velocities and steady downward flame spread rate decreases as the opposed forced flow velocity increases. For the effect of the peak of imposed heat flux, the ignition delay time decreases and the interface maximum temperature at the instant of ignition increases as the peak heat flux increases. A comparison with Lin' results (1999) is given. It exists a recirculation flow just ahead of the flame front in the present study, whereas it does not appear at all in Lin (1999). Ignition delay time and the interface temperature at the instant of ignition are smaller than those in Lin (1999) under the same peak heat flux. Finally, a set of computations for steady and unsteady combustion models, simulating the experiments of Pan (1999) and Chen (1999), is carried out. In general, the qualitative trends are completely the same, that is, the flame spread rate increases as the opposed flow velocity decreases, the inlet flow temperature increases and the fuel thickness decreases, separately. The predicted flame spread rates have an excellent quantitative agreement with the measurements in high velocity regime.