以水霧做為海龍替代物滅火劑性能評估之數值模擬

Abstract

一理論模式用來解析一水霧流過由Tsuji Burner所造成擴散對沖火焰之火焰 與液滴交互作用。其統御方程式由兩個部份構成。其中，氣相燃燒模式採用由 陳俊勳和翁芳柏[10]所發展的理論模式。在液相方面，液滴的運動由牛頓第二 運動定律來描述，並同時考慮液滴在流場中的熱傳及質傳；一些實驗關係式也 被用來描述液滴的行為。最後，再用PSI-Cell模式來處理二相之間的藕合。 當液滴沿其軌跡運動時，會從火焰中吸收熱量，並在流場中產生水蒸氣。 由液滴蒸發所產生的水蒸氣會稀釋氧和燃料的濃度而由相變化所吸收的熱量也 會降低火焰的溫度，兩者都會使火焰強度減弱。若採用50微米的液滴，當水霧 的流量增加到空氣流量的7%時，在停滯區的包封火焰前端會被熄滅而後退到尾 流區形成尾流火焰。當水霧的流量大於15%，火焰就完全熄滅。至於80微米的 液滴，在流量4.5%時會產生尾流火焰，在10%以上的流量則沒有火焰存在。從 研究中也發現，在相同的水霧流量下，較小液滴情況下的火焰強度較強。此外 ，也對液滴大小的效應做一研究，藉由同時改變液滴大小及相對應的水霧流量 以固定液滴的數量流率。液滴大小為55微米時會產生尾流火焰，在70微米以上 則沒有火焰存在。本論文亦討論了該模式在未來可以繼續研發的方向。

The interaction between the water mist and counterflow diffusion flame over Tsuji burner is studied numerically. The governing system comprises two parts. The gas phase combustion model adopted one developed by Chen and Weng [10]. In the liquid phase, the motion of droplet is described by the Newton's second law, and its heat and mass transfer is considered by including some empirical correlations. Finally, the PSI-Cell model is used to handle the two-phase coupling. The evaporation of droplets absorbs heat from flame and subsequently generates vapors in the flow field. The generated vapors dilute both the oxidizer and fuel concentrations and the heat absorption by phase change lowers the flame temperature. For 50 mm droplets, the flame front of envelope flame is extinguished from the forward stagnation area and retreats to downstream to become a wake flame as the mist flow rate increases to 7%. The wake flame no longer exists when mist flow rate is greater than 15%. The critical values of the envelope flame transforming into wake one and flame extinction for 80 mm droplets are 4.5% and 10% separately. It is found that the flame is stronger for the smaller droplet sizes under the same mist flow rate. Besides, the effect of varying droplet size are studied by changing the size of droplet and corresponding mist flow to keep the number flow rate constant. The critical sizes of the envelope flame transforming into wake one and flame extinction are 55 mm and 75 mm separately. Finally, the possible future extensive works are discussed as well.