輻射效應對熱厚型固體燃料逆向火焰傳播特性之研究

Abstract

本論文係以數值分析方法來研究輻射對熱厚型固體燃料逆向火焰傳播特性。逆向火焰包含在自然對流中向下火焰傳播以及強制對流中逆向火焰傳播。而輻射效應模式包含由二維P-1近似法所模擬的氣相輻射和固相輻射。論文內容主要分為三個部份，第一部份為輻射對向下火焰傳播特性的影響；第二部份為輻射對在強制對流下逆向火焰傳播特性的影響；最後一部份則和Pan and Liou(1999)的實驗數據作比較。第一部份的研究結果顯示，熱厚型固體燃料其吹滅限發生在g=7.5ge，該值遠大於熱薄型固體燃料者(g=4.5ge) ，主要原因是厚板能保留較多的熱量使火燄能存活於較高的重力場。如同Lin and Chen(1999b)，在高重力時輻射的效應並不明顯；反之，在低重力時輻射明顯的降低火焰傳播速度。第二部份的研究結果顯示，在強制對流下發現火燄前端存有一迴流的流場，而這個迴流流場的產生可幫助火焰更加穩定而使得強制對流的吹滅限大於自然對流的吹滅限；至於在輻射效應，其影響類似於自然對流者。第三部份的研究結果顯示，在火焰傳播的特性方面，預測的結果完全符合Pan and Liou(1999)的實驗觀察到之現象，如進口溫度升高或速度降低以及固體燃料厚度減少皆會導致逆向火焰傳播速度增加。在越高速的強制對流中所得之模擬結果和實驗數據幾乎完全相同。

This dissertation numerically investigates the radiation effects on flame spread over thermally-thick solid fuel in the opposed air flows. The opposed flame spread includes downward flame spread and flame spread against the forced convective flow. It consists of three parts: the first part explores radiation effects on downward flame spread; the second one investigates radiation effects on the flame spread in an opposed air flow. Finally, a simulation by using PMMA as the solid fuel is carried out, and the predicted results will compare with the corresponding measurements obtained by Pan and Liou (1999). Comparing with the blow-off limit (g = 4.5 ge) for thermally-thin solid fuel, the thermally-thick solid fuel has higher one (g = 7.5ge) for downward flame spread. The reason is that the thicker fuel can maintain more energy in the solid to support the flame spread. Similar to Lin and Chen (1999b), the radiation does not affect the flame spread rate in high gravity regime. However, it can lower the flame strength and lead it to extinction in the very low gravity level. There exists a recirculation flow just ahead of the flame front for the flame spread in an opposed oxidizer flow which is not found in natural convection. It can stabilize the flame and enhance the mixing and heat transfer to aid the upstream flame propagation. Therefore, the spreading flame in forced convection environment can sustain to a much higher blow-off limit comparing to that in natural convection one. The radiation has similar influence on flame spread as that in downward flame spread. Finally, a simulation similar to the experiment of Pan and Liou (1999) is carried out. The qualitative trend is completely the same as what observed in the last reference. The flame spread rate increases as the opposed flow velocity decreases, the inlet flow temperature increases or the fuel thickness decreases. The predicted results have a good agreement with the corresponding measurements, especially when the incoming flow velocity becomes faster.