固態燃料在自然對流環境下火焰行為之研究

Abstract

本論文係以數值分析方法來研究在自然對流及微重力場中固態燃料平板之火焰行為。論文內容可分為四個部分。第一部份主要在探討垂直擺放之熱薄型纖維質材料板之引燃及隨後之向下火焰傳播的現象。研究結果顯示，固態燃料溫度在加熱階段會逐漸上升且裂解反應會逐漸加劇，當可燃性氣體已經形成且氣相溫度足夠高的情況下，引燃隨之發生。引燃過程可分為兩個時期，分別為誘發時期(induction period)和熱爆發時期(thermal run-away)。在誘發時期，氣相化學反應和氣相溫度間彼此相輔相成，在熱爆發時期，火焰為預混火焰，之後火焰會逐漸從預混火焰轉變為擴散火焰，當燃料尾端部分開始燒盡時，火焰會達到一穩定之傳播速度向下移動。引燃時間主要是受到可燃性氣體形成的時間所影響和誘導流的強度及外界氧濃度無關。引燃時間會隨著燃料厚度增加或最大外界熱通量的減少而增加。火焰傳播速度會隨著重力場及固態燃料厚度的增加和外界氧濃度的減少而降低，但和最大外界熱通量之大小無關。當重力場增為地球重力場的6.7倍或外界氧濃度降至 時，則火焰會熄滅。 第二部份主要在探討垂直擺放之熱厚型PMMA板受到一反向氣流之火焰向下延燒的現象。模擬結果顯示，引燃時間在混合對流場中會隨著進氣溫度的降低或進氣速度的增加而增加，但當進氣速度小於30 cm/s時，引燃時間則保持一定值。在定性上，火焰傳播速度和熱邊界層厚度在模擬結果中與Pan(1999)之實驗量測相吻合。在定量比較上，除了在較低的進氣速度時，數值預測之火焰傳播速度和實驗結果相近，而熱邊界層厚度的差異則會隨著進氣速度增加而降低。此結果顯示在較高之進氣速度下，逆向火焰延燒主要是受到火焰前端沿著固態燃料方向之熱傳導所影響，而在較低之進氣速度下，則證明了輻射，有限燃料長度及三維效應的重要性，而這在本研究中是沒有被加以考慮的。本研究亦預測到在火焰前端會有一迴流產生，而Pan(1999)之實驗亦證實了此一現象。 第三部份主要係以數值分析方法來研究在靜止無重力場環境中，考慮固相輻射效應下熱薄型固態燃料之火焰傳播現象。同時得到在輻射效應下火焰熄滅之臨界厚度。研究結果顯示固定外界氧濃度下，火焰傳播速度根據固體燃料厚度的不同可分為兩個區域：當 時，火焰傳播速度會隨著燃料厚度減少而降低，當厚度小於臨界厚度時，則火焰會熄滅；當 時，火焰傳播速度會隨著燃料厚度增加而逐漸降低。在這兩個區域中，主要的控制機制分別為燃料供應以及熱傳，同時利用理論分析方式推導出當 時，火焰會因輻射熱散失而熄滅。另提供在燃料表面之固相溫度，密度，燃料裂解通量，氣相傳導至固相之熱通量，固相輻射散失熱通量以及淨熱通量之分佈。火焰傳播速度會隨著外界氧濃度的增加而加快，當外界氧濃度低於0.195時，則火焰會熄滅。火焰和燃料板之間的距離會隨著外界氧濃度降低而增加，此結果和Olson (1987)的實驗結果相當吻合。 第四部份主要在探討在低重力場中，輻射效應對垂直放置之熱薄型固態燃料之向下火焰延燒的影響。研究結果顯示，重力場大小和輻射效應對引燃時間沒有影響，火焰傳播速度在重力場為地球重力場的0.01倍時達到最大，之後隨著重力場的增加或降低而逐漸減小，當 時，其主要的控制機制為火焰拉伸效應，而在 時，輻射及氧氣傳輸則為主要的控制機制。在 時，火焰會熄滅，此結果和Sacksteder and T’ien (1994)的實驗相近。輻射效應同時有兩種貢獻，一為將熱散至外界而降低火焰強度，另一為結合沿著固態燃料方向之熱傳導加強此方向之總熱傳量來預熱未燃之固態燃料，這兩種效應互相競爭。因為輻射效應的影響，固態燃料溫度會隨著重力場降低而逐漸下降，同時燃料殘留的現象更為明顯。由能量分析顯示，熱量經由熱傳導傳從火焰傳至固態燃料主導了火焰行為，但隨著重力場降低，輻射效應已可逐漸與之匹敵。

This study consists of four parts. The first part studied the ignition and subsequent downward flame spread over a thermally thin solid fuel in a gravitational field. The solid fuel temperature rises gradually in the heat-up stage and the pyrolysis becomes more intense. Ignition, including the induction period and thermal run-away, occurs as soon as a flammable mixture is formed and the gas phase temperature becomes high enough. During the induction period, the reactivity and temperature in gas phase are mutually supportive. The thermal run-away consists of a premixed-flame burning. This is followed by a transition from a premixed flame into a diffusion flame. Finally, steady flame spread takes place as burnout appears. The ignition delay time is mainly controlled by the flammable mixture formation time, and is independent of the induced flow strength and ambient oxygen index. The ignition delay time increases with the solid fuel thickness or a decrease of the incident peak heat flux. The steady downward flame-spread rate decreases with an increase in the gravity level or fuel thickness and a decrement of the ambient oxygen index, but is independent of the incident peak heat flux. The blow-off limit is around 6.7ge, and the extinction limit is Yo=0.131. In the second part, the downward flame spread over a thick PMMA slab in mixed convection environment was studied theoretically. Simulation results indicate that the ignition delay time increases with decreasing opposed flow temperature or increasing velocity. the ignition delay time is nearly constant with a low opposed flow velocity, i.e. the opposed flow velocity is less than 30 cm/s. The qualitative variation trends of the flame-spread rate and thermal boundary layer thickness are identical between Pan’s measurements (1999) and the numerical predictions. From the perspective of quantitative comparison, the predicted and experimental flame-spread rates correlate well with each other, except at a low velocity regime. The discrepancies in thermal boundary layer thickness decrease with increasing flow velocity. The quantitative agreement in high velocity regimes indicates that the opposed flame spread is mainly controlled by the stream-wise heat conduction in flame front, whereas the discrepancies in the low velocity regime demonstrate the importance of radiation, finite fuel length and 3-D effects, which are not considered in the combustion model. Recirculation ahead of the flame front is predicted by the simulation and confirmed by Pan’s experiment (1999). In the third part, the flame spread phenomena over a thermally thin solid fuel, where the radiation from solid is under consideration, in a quiescent, zero gravity environment was studied numerically. A derivation of critical thickness for flame extinction including radiation effect was given. Under fixed the ambient oxygen index, a dividing point is identified to distinguish the flame-spread rate into two regimes against the solid fuel thickness. For fuel thickness lessthan 0.006cm , the flame-spread rate decreases with fuel thickness. The flame is extinction when fuel thickness is lower than a critical value. For fuel thickness greatthan 0.006cm , the flame-spread rate gradually decreases as fuel thickness decreases. The controlling mechanisms in these two regimes are fuel control and heat transfer control, respectively. The flame is extinguished due to radiation loss as fuel thickness is great than extinction thickness, derived by using a theoretical analysis. The responses of solid fuel, including temperature, density, vaporized mass flux, conduction heat flux from flame to the solid fuel, solid radiation heat flux and net heat flux are presented as well. The flame-spread rate increases with ambient oxygen index. The predicted extinction limit is at Yo=0.195. The standoff distance increases with a decrease of ambient oxygen index. This result consists with experimental results of Olson (1987). In the last part, the radiation effects for downward flame spread over a thermally thin solid fuel in partial gravity environment were explored. The simulation results indicate that the ignition delay time is influenced slightly by the radiation effect and gravity level. The flame-spread rate reaches a maximum at g = 0.01 and then decreases in spite of increasing or decreasing gravity level. For g > 0.01, the flame behaviors are mainly dominated by flame stretch effect. Radiation heat transfer and oxygen transport control the flame behaviors for g < 0.01. The predicted radiation quench limit is g=5x10E-6 that is close to the experimental results (Sacksteder and T’ien, (1994)). The radiation has two kinds of contributions simultaneously. One is to reduce the flame strength by losing heat to the ambient. The other one is to join the upstream conduction to enhance the total forward heat transfer rate and the subsequently preheat upstream virgin fuel. The solid temperature is low and the fuel leftover phenomenon is apparent in low gravity level due to radiation. Based on energy analyses, the conduction heat flux from the flame dominates the flame behaviors. However, the radiation effect can compete gradually with conduction heat flux when the gravity level decreases continuously.