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重力場中自然對流環境下，纖維質材料平板之引燃及火焰傳播現象。論文內容主要可分為兩個部分，第一個部份是針對垂直及水平擺置的固相燃料在受到輻射熱後的引燃及火焰傳播特性作研究。第二個部份則是在相對座標中針對固態燃料厚度變化對向下火焰傳播特性所造成的影響作研究。 第一部份的研究結果顯示，在垂直及水平引燃模式下，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，此階段主要包含了引燃過程及一個過渡的過程。在加熱階段，固態燃料的最高溫度會隨加熱時間增加而上升，但其上升速率有減小的趨勢，這主要是由於在加熱階段後期，固態燃料開始進行裂解反應所造成。在火焰發展階段的引燃過程，因為在加熱階段累積的預混可燃氣體發生化學反應，釋放出大量的熱，因此，在引燃過程氣相溫度會在短時間內急劇上升。同時火焰的型態也在引燃過程中由預混火焰快速的轉變為擴散火焰（火焰前端除外）。在過渡的過程中，火焰同時扮演引燃源及熱源的角色，用來加速固態燃料的裂解，並維持火焰的存在。對垂直引燃模式而言，延長加熱時間可使材料從一暫態引燃(transition ignition)轉變成持續性的引燃。針對不同厚度的材料，研究結果發現，當厚度小於2.5mm時，引燃延遲時間會隨厚度增加而增加，但當厚度大於2.5mm時，引燃延遲時間與固態燃料的關係則視外加熱源的加熱速率而定，當加熱速率小於固態燃料內部的熱擴散速率時，引燃延遲時間會隨厚度增加而增加。反之，則厚度變化對引燃延遲時間造成的影響非常小。對水平引燃模式而言，引燃延遲時間會隨重力加速度增加而增加。另外，在相同的加熱速率下，水平燃料板較垂直燃料板容易被引燃。並且此兩者的引燃延遲時間差會隨加熱速率減小而增加。 第二個部份的研究結果顯示，火焰傳播速度與厚度的關係可以分成四個區域：（I）熱薄型區域(1.0mm< < 1.5mm)：在此區域火焰傳播速度與厚度成反比。（II）過渡區域(1.5mm< <2.2mm)：在此區域固態燃料二維的特性逐漸顯現。（III）熱厚型區域(2.2mm< £7mm)：在此區域火焰傳播速度隨厚度增加而減少，但其減小的程度有減緩的趨勢。在此區域中，固態燃料在火焰前端的預熱長度較氣相為長，此結果顯示固體中之熱傳導對火焰傳播特性有重要的影響。（IV）不穩定區域( 37.5mm)：在此區域我們發現火焰傳播速度與熱厚型區域相比較有突然下滑的區勢。

This study numerically investigates the ignition and flame spread behaviors of cellulosic materials in gravitational field under a natural convective environment. This dissertation includes two parts, the first part explores ignition and transition to flame spread phenomena over vertical and horizontal solid fuels subjected to a radiant heat flux. Then, the effects of solid fuel thickness on downward flame spread characteristics by using a constant flame spread rate approximation are investigated in the second part. For both ignition modes, the entire process is delineated into two distinct stages. In the heating up stage, the maximum temperature increases with time but at a decreasing rate because of the pyrolysis reaction. The flame development stage consists of ignition and transition processes. In the ignition process, the maximum temperature in gas phase increases dramatically within a short period of time because a large amount of heat is generated from chemical reaction of the accumulative, flammable mixture. The flame is in a transition from a premixed flame to a diffusion one, except for the small region around the flame front. In transition process, the flame serves as an extra heat source to further pyrolyze the solid fuel in order to sustain itself. For vertical solid fuel, prolonging the imposed radiative heat time leads the ignition from a transition one to a persisting ignition. The effect of varying solid fuel thickness indicates that the ignition delay time increases with an increase of solid fuel thickness while £2.5mm. For 32.5mm, the relationship between ignition delay time and solid fuel thickness depends on the external heating rate. If the external heating rate is the same order as the heat diffuse rate, the ignition delay time increases with an increase in solid fuel thickness. On the other hand, if the external heating rate is greater than the heat diffuse rate in the solid fuel, the ignition delay time remains nearly a constant For horizontal solid fuel, the ignition delay time is decreased with a decrease in gravity level because of larger flow velocity induced in high gravity environment. In addition, the ignition delay time is shorter for horizontal solid fuel than for vertical one under the same external heat flux, and the ignition delay time difference between vertical and horizontal modes under the same incident heat flux is increased with a decrease in external heating rate. In the second part, the computational results show that thickness can be categorized into four regions according to corresponding flame spread behaviors. Region (I) is the thermally-thin one (1.0mm < < 1.5mm), whose flame spread rate is approximately inversely proportional to fuel thickness. Region (II) is the transition one (1.5mm < < 2.2mm), where the two-dimensional feature becomes apparent. Region (III) is the thermally-thick one (2.2mm < £ 7mm), where the flame spread rate decreases with an increasing fuel thickness but with a decreasing descendent rate. In this region, the preheated length in solid phase exceeds that in gas phase, indicating that the solid conduction profoundly influences the flame spreading over the thick fuel. Region (IV) is an unstable one. ( 3 7.5mm), where a sudden drop occurs in flame spread rate in comparison with region (III).