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標題: | 超音速燃燒室噴注液態癸烷並加入乙烯多孔性圓柱燃燒器之數值模擬與分析 Numerical Simulation of Liquid n-Decane Combustion in a Scramjet Combustor Aided by an Ethylene-fueled Porous Burner |
作者: | Wing-Ki Cheung 張永淇 |
指導教授: | 潘國隆(Kuo-Long Pan) |
關鍵字: | 超音速衝壓燃燒引擎,CFD,OpenFOAM,多孔性燃燒器,歐拉拉格朗日 方法,癸烷,多相流,煤油代表產物,乙烯,大渦模擬,LES,KHRT 破碎模型,噴霧燃燒, Scramjet engine,CFD,OpenFOAM,porous burner,Eulerian-Lagrangian method,n-decane,multiphase,kerosene surrogate,ethylene,Large Eddy Simulation,LES,KHRT breakup model,spray combustion, |
出版年 : | 2020 |
學位: | 碩士 |
摘要: | 本研究使用免費開源軟體OpenFOAM來分析模擬超音速沖壓燃燒室流場,由C++語言所寫成。在超音速燃燒室內,氣流速度非常高,導致燃料在超音速、冷流場的狀態下停留時間短,導致液態的烷類燃料更加難以在常溫下穩住火焰。本研究提供了如何在上述條件下使得癸烷(煤油代表產物)在燃燒室內能夠穩定燃燒的方法 – 改變噴注方式及加入充滿乙烯的多孔性燃燒器。對於本研究所用到的求解器和算法、論文包含了許多對於前人實驗的驗證案例,並透過對本研究進行了網格收斂測試,來證明研究的可信度。 液態癸烷被選為本研究的主要燃料,透過微小修改支架幾何組態來改變燃料噴射方式及位置,令燃料可以垂直噴注至橫流中。這種噴注的方式在燃料氣化進入下游之前,能更有效地預先被破碎成更小的液滴。本研究進行了主要三個案例的試驗,分別為沒有加入多孔性圓柱燃燒器,及在兩個不同長徑比(L/D)下加入充滿乙烯多孔性燃燒器,而對應的燃料的整體當量比皆設為0.077。 在沒有加入燃燒器的情況下,整體來說由於熱釋放率與熱損失率達不到平衡導致火焰無法在下游產生。局部雖然有火焰附在支架後方但也因為癸烷液滴持續的吸熱及氣化造成冷卻效應,使局部支架後方的迴流區的溫度隨時間步下降。局部火焰最後無法維持,完全熄滅。 加入燃燒器在長徑比11.7的案例下,由於燃燒器視為障礙物令前方出現弓形震波。火焰主要出現在燃燒器的後方。在前幾個時間步下,寬的高溫區在燃燒器後方形成,由於高溫、低速區過寬造成熱阻塞。背壓過高使弓形壓波被推往支架方向,發展成正震波,在液體持續噴汪的加持下令流場極度不穩定,導致最後燃燒器後方的整體火焰被吹熄成弱的小火焰。另外由於支架後方沒有形成高溫的迴流區,癸烷液滴無法在上游被預先蒸發成氣態,使癸烷到達下游燃燒器的高溫迴區才被徹底蒸發,由於整個氣化過程被延後導致化學反應無法及時進行,整體火焰無法形成。 加入燃燒器在長徑比1.7的案例下,鄰近支架後方的低速區可以消除弓形震波的形成。此外,能觀察到高溫、低速的迴流氣流在燃燒器後方出現,這為癸烷液滴提供了理想的環境進行氣化、燃料/氧氣混合。液滴到下游之前就能被徹底氣化並達到更早的化學反應。故此整體性火焰能大概在燃燒室中間位置形成。另外本研究發現液滴完全氣化需要時間及距離,加上形成可燃燒的混合物的延遲時間,癸烷火焰的形成會位於比較後方,這類似於火焰剝離的現象。為了更全面的探討,針對迴流區而言,我們對時間尺度進行分析。另一個方面淨推力及比衝值的計算也被包含在結果與討論的部份。 An open-source, free, and C++ written software OpenFOAM was adopted in the present study, which is capable of dealing with Computational Fluid Dynamics problems numerically. This research aimed to tackle the challenge of flame stabilization of kerosene surrogate - n-decane under the cold flow condition. Validations have been performed to ensure the solver and scheme is suitable for the flow solutions. The liquid n-decane fueled system in a scramjet combustion chamber has been investigated in this study. An altered configuration of the strut has been chosen for liquid injection in which the fuel will be injected from the strut vertically into the crossflow to have favorable conditions for the droplet breakup process. The global equivalence ratio (GER) is 0.077 in three major analyzed cases. In the case without a porous burner, the global flame does not form due to the counterbalance between the combustion heat release rate and heat loss rate due to droplets evaporative cooling effect. Then the local flame eventually was unable to sustain in the recirculation zone; thus, extinction was observed. With the addition of ethylene-filled porous burner installed at a downstream position ( ), a broad flame occurred in the rear-porous region for the early time. The blockage of the porous burner leads to the formation of a bow shock, causing a thermal choking of the combustor. It then developed a normal shock due to the high backpressure downstream. Thus, the flow field demonstrated extreme unsteadiness, and blown-off was observed (weak flame is detained). Moving a porous burner from downstream to a near strut base ( ) location could eliminate the presence of bow shock induced by the porous burner. Furthermore, a hot, low-velocity recirculating flow behind the porous burn provided an ideal environment for the vaporization of n-decane, as well as entrainment of fuel and air. It is observed that droplets could be evaporated thoroughly and sustainably, leading to a stabilized local and global premixed flame that takes place in downstream. It is of interest that lifted secondary flame is revealed due to the delay of the formation of unburnt n-decane/oxygen mixture. More comprehensively, time scales inside the recirculation zones are discussed. There is a net thrust generation by integrating the whole domain. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/66928 |
DOI: | 10.6342/NTU202003628 |
全文授權: | 有償授權 |
顯示於系所單位: | 機械工程學系 |
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