利用電漿輔助氣體轉換穩駐層流跳脫火焰

Application of Plasma-Assisted Gas Conversion to the Stabilization of Laminar Lifted Non-Premixed Jet Flames

郭信德(Hsin-Te Kuo)

指導教授 ： 廖英皓

國立交通大學/工學院/機械工程系所/碩士(2018年)

摘要

電漿因有著產生活躍自由基之特性，應用在燃燒中能改變所需之化學反應途徑與速率，藉此提升燃燒的效率，成為了近年來備受矚目之研究方向。由於燃燒之實際應用常為高壓之作業環境，會使得電漿放電不易，因此本研究是在氣體燃燒前，先利用介電質放電電漿去分別轉換燃料及氧化劑氣體，探討其對跳脫火焰之增益。實驗結果顯示轉換氧化劑所產生之臭氧，能降低火焰跳脫高度及提升火焰吹熄速度，若能提升臭氧濃度或伴隨較高氮氧比之氧化劑則有較為顯著之影響效果。電漿轉換甲烷之產物如乙烯、乙炔等，其層流火焰燃燒速度較甲烷快，也因產物密度較大之緣故，在相同質量流率下其燃料噴流速度會變小，使得火焰能夠穩駐在較上游位置，其穩駐火焰之效果更甚於電漿轉換氧化劑。電漿轉換較小流量之丙烷因有著較長的駐流時間，會有較高之丙烷轉換率，生成產物以氫氣與甲烷為主，由於較高之電漿能量密度利於斷開C-H鍵結，其氫氣選擇性會較高。若考慮到丙烷轉換後氣體膨脹之影響，在修正管口燃料噴流速度後，能發現電漿轉換丙烷與電漿轉換氧化劑有著相同穩駐火焰之效果，但前者對於火焰跳脫速度有著更為顯著之改善。由於氫氣層流火焰燃燒速度極大之緣故，能夠增加火焰之傳遞速度，並延遲火焰產生跳脫，其比例多寡對於跳脫火焰有明顯的影響。本文也利用未經反應之噴流理論解去計算火焰的傳遞速度，其值會等同於當量輪廓上跳脫高度位置之噴流速度，根據其分析可證實電漿轉換氧化劑與電漿轉換丙烷都能有效提升火焰之傳遞速度。

關鍵字

電漿輔助燃燒 ； 氣體轉換 ； 層流火焰 ； 跳脫火焰

並列摘要

Application of plasma discharges to combustion has recently gained great attention due to its unique capability in producing abundant active species and radicals, subsequently altering reaction rates and pathways, resulting in combustion enhancement. However, challenges are present to produce stable plasma discharges in elevated-pressure environment where practical combustion systems commonly operate. Therefore, the present study is to apply plasma to separately convert fuel and oxidizer before the combustion take place, and to investigate effects of plasma-assisted gas conversion on characteristics of laminar lifted non-premixed jet flames. Results show that the addition of ozone, partly converted through oxygen by plasma, leads to a decrease in lift-off height and an increase in blowout velocity. The enhancement is more effective with the ozone concentration and the ratio of N2 to O2. For fuel conversion, results of methane flames are different than those of propane flames. Methane flames with fuel conversion exhibit similar behaviors to those with ozone addition, but with a smaller lift-off height and a larger blowout velocity. However, propane flames are seen to have inconsistent lift-off characteristics, presumably due to a combination of gas expansion, conversion efficiency and product selectivity. The gas expansion is due to the fuel activation that produces lighter hydrocarbons than propane. The conversion rate and the H2 and CH4 selectivities are found to be associated with the plasma energy density. The flame propagation can be greatly enhanced to produce a nozzle-attached flame when the plasma energy density is high enough to effectively decompose C3H8 into H2. Nevertheless, a decrease in plasma energy is more sufficient for the cleavage of C-C bonds, resulting in a significant decrease in H2 selectivity, but a nearly unaltered CH4 selectivity. The reduced conversion due to the lower plasma energy, along with the gas expansion, produces a lifted flame with a larger lift-off height than the non-activated flame injected with the same initial amount of propane.

並列關鍵字

Plasma-assisted combustion ； Plasma conversion ； Fuel activation ； Ozone ； Lifted flame

參考文獻

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