為發展底吹轉爐煉鋼，底吹元件的優劣是此技術之核心。良好的底吹元件設計能使轉爐中的鐵水在噴吹過程中，在噴嘴周圍冷卻形成固凝物，達到保護底吹元件的效果。
本研究以水模實驗中的柱狀噴流流場為依據建立二維軸對稱模型，並以丙烷作為底吹雙重管噴吹之外管保護氣體，搭配多步驟燃燒反應以計算流體力學、熱傳學與燃燒學模擬底吹雙重管噴吹碳氫氣體與氧氣時的反應模式，並藉此探討碳氫氣體的裂解反應對於保護底吹元件之功效。除此之外，亦針對鐵水固凝物的厚度、雙重管噴吹之碳氫氣體流量這兩者對氣體裂解反應的影響做討論。
研究結果發現，由於丙烷噴吹流速快，因此僅於氣柱周圍與高溫交界面附近達到裂解溫度、發生裂解吸熱的反應。而丙烷跟氧氣更是在噴出管口之後一小段距離便得到良好的混合效果，因此在高溫加熱下燃燒形成火焰。即便丙烷裂解比例不高，裂解反應仍然為氣體增加了約30%的吸熱量。在固凝物生成的狀態之下，由於固體阻隔了高溫鐵水對氣體的直接熱傳，使得丙烷達到裂解所需的能量不足，造成丙烷裂解效果下降。隨著固凝物厚度的增長，逐漸使得丙烷無法在與氧氣混合之前就產生裂解反應，因此裂解反應所額外增加的吸熱效果亦逐漸減少。而調整丙烷流量，能使丙烷與氧氣混合的距離拉長，增加吸熱的面積及總量，但當流量持續增加時，因噴吹氣體流速加快以至於丙烷無法被有效的加熱，裂解反應也會趨緩，終至消失。
依據目前的研究結果，能推斷出影響底吹雙重管噴吹丙烷裂解吸熱的主要因素有：丙烷的流量、加熱程度以及與氧氣的混合距離。若能有效的將丙烷與氧氣隔絕，延緩其混合的速率，就能使丙烷有更好的裂解效率，進而達到冷卻鐵水保護噴吹元件之效果。

To develop the process of bottom blowing steelmaking, a good tuyere design is the core technology. Such a good design can cool down the molten iron around the tip of a tuyere to be solidified, and this solidified region named “mushroom” can protect the tuyere and the furnace refractory from erosion.
In this study, a 2D axisymmetric computational fluid dynamics model was built, according to the flow fields observed from the water model experiments using a double tuyere composed of an inner tube and an outer tube. This model was to investigate the influence of the mushroom thickness and hydrocarbon flow rate on the hydrocarbon pyrolysis and understand the role of co-injected hydrocarbon gas in the bottom-blown oxygen steelmaking. In a practical process, oxygen was injected from the inner tube, and hydrocarbon, which is propane in this study, was between the outer tube and inner tube. Propane is used as the protecting gas around oxygen from an early combustion. It was assumed that a main stream of the blowing gases is formed through the vent to the pressure outlet boundary. The propane pyrolysis was computed using a multistep combustion model.
The simulation results showed that the adopted propane flow rate in this study is so large that the propane decomposed at the brink of main stream. Moreover, propane is mixed well with oxygen after blown out the tuyere. Thus the location of combustion occurred after that of pyrolysis. Despite the fraction of pyrolized propane is low, the pyrolysis still can increase the heat absorption from the molten iron by around 30%.
When the mushroom is formed, it blocks the heat convection from molten iron to gases and causes the pyrolysis rate lower. Thus the thicker the mushroom is, the lower the pyrolysis rate is. When the mass flow rate of propane increased, the mixing distance of gases at the wall was longer and resulted in more heat absorption. However it would result in a lower temperature on the wall and the pyrolysis rate was lower.
As a result, the key factors of hydrocarbon pyrolysis in bottom blowing process are the flow rate of hydrocarbon, wall heat flux as well as the mixing condition of hydrocarbon and oxygen. The cooling effect from hydrocarbon pyrolysis can get better by separating the hydrocarbon and oxygen. This would induce longer mixing length to allow a larger propane pyrolysis rate and adsorb more heat from the molten iron to improve the mushroom growth.

論文審定書 i
誌謝 ii
摘要 iii
Abstract iv
目錄 vi
圖目錄 viii
表目錄 xi
符號說明 xii
第1章、 序論 1
1.1 前言 1
1.2 文獻回顧 3
1.2.1 轉爐發展與廢鋼回收技術 3
1.2.2 底吹元件的磨損 4
1.2.3 底吹元件種類 6
1.2.4 底吹氣體的選擇 7
1.2.5 固凝物形成機制 7
1.3 研究目的 9
1.4 本文架構 10
第2章、 研究方法 11
2.1 基本假設 11
2.2 模型介紹 13
2.2.1 底吹雙重管與固凝物模型 14
2.3 統御方程式 16
2.4 碳氫化合物裂解與燃燒反應 19
2.5 數值方法 21
第3章、 結果與討論 22
3.1 雙重管底吹碳氫氣體與氧氣反應模式 22
3.1.1 無反應下噴吹狀況 22
3.1.2 加入反應後噴吹狀況 25
3.1.3 比較氣體反應對於冷卻效果的影響 29
3.2 固凝物厚度對於噴吹氣體的影響 31
3.2.1 固凝物厚度對於流場性質的影響 31
3.2.2 固凝物厚度對於交界面冷卻效果的影響 35
3.3 流量對於裂解區域長度的影響 40
3.3.1 未加入反應前不同丙烷流量的流場性質 40
3.3.2 加入反應後不同丙烷流量的流場性質 47
3.3.3 不同丙烷流量下壁面溫度及吸熱量 55
第4章、 結論與未來展望 59
參考文獻 61
附錄 63
水模固凝物模型 63
模擬結果 65

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