Novel Bluff-Body on Improving Temperature Uniformity and Combustion Efficiency of Non-Premixed Flame (Ii)

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Project title

Code/計畫編號

MOST106-2221-E019-055

Translated Name/計畫中文名

新式鈍體提升非預混火焰溫度均勻性及燃燒效率之分析 (II)

Project Coordinator/計畫主持人

Shun-Chang Yen

Funding Organization/主管機關

National Science and Technology Council

Department/Unit

Department of Mechanical and Mechatronic Engineering

Website

https://www.grb.gov.tw/search/planDetail?id=12239710

Year

2017

Start date/計畫起

01-08-2017

Expected Completion/計畫迄

01-07-2018

Bugetid/研究經費

633千元

ResearchField/研究領域

機械工程

"本計畫申請人基於目前所執行 ‒新式鈍體提升非預混火焰溫度均勻性及燃燒效率之分析‒ 等溫流場 (冷流場) 的105 年度計畫延續；擬議以不同星芒型(三芒、六芒、圓型)鈍體進行燃燒溫度場的實驗研究。目前進行冷流場的研究，以煙線流場可視化技術來觀測低雷諾數下巨觀的流體行為；及熱線風速儀以量測流場的速度場特性與紊流性質，偵測旋渦尾流與剪流層中的頻率特性及高低頻之間的相關聯。目前結果發現，六芒型鈍體的尾流區渦旋頻率最大，且紊流強度能比圓盤鈍體提升70%。推論如能將芒型鈍體被動式調控機制，應用於非預混式燃燒爐，可提升燃燒效能。這是因為，申請人基於先前對於非預混式燃燒爐的冷流、燃燒效率的多種鈍體被動控制、聲波激擾主動控制等方式的研究與實務經驗。發現紊流擾動、及適量的旋流效果能產生可加速燃料與空氣混合，進而增加燃燒能力。但亦發現三個弱點，第一為非預混式燃燒爐燃燒時，當冷的燃料氣體是以大量快速注入燃燒場時，燃料氣體滯留時間短，燃料氣體和空氣沒有足夠時間完成化學反應，致使火焰上飄離噴口，產生火焰的不穩定性。第二為燃料噴流與鈍體機構間會產生停滯鞍點(Stagnation-saddle point)的流場，對燃料與空氣的混合與燃燒有相當的影響。第三為當旋流機制過大時，熱的火焰燃氣與周圍的冷空氣快速混合，旋渦火焰卻也因此容易造成燃燒反應中斷，而衍生大量的CO 產生，這是旋渦火焰的一大問題；這缺點起因是燃燒時冷的空氣與熱的火焰，因溫度場的不均勻性所衍生的影響。因此本計畫以「技術發展」層面的觀點，將星芒狀調控機構，裝置在傳統的燃燒爐的出口處。致使預混式燃燒爐因燃料與空氣滯留時間短所產生的火焰不穩定性、燃料噴流對鈍體停滯鞍點的影響、旋流機制過大時冷熱氣體易造成燃燒反應中斷等，溫度場不均勻性影響因素最小化。有鑑於此，本專題計畫是將進行燃燒場的研究；是繼而以本年(105 年)度星芒狀控制器在冷流場的研究結果，直接應用於非預混式燃燒爐中。藉由拍照攝影、Schlieren 光學技術、PIV 量測系統、熱電偶溫度量測、及燃燒氣體濃度分析等實驗方法，研究星芒狀鈍體調制非預混燃燒爐時的火焰行為、火焰長度、燃燒場速度、溫度分佈、燃燒特性與氣體濃度分析等變化。拍照攝影觀察火焰形狀，可由火焰外觀判讀空燃比之間增減對火焰行為的影響。Schlieren 光學技術可擷取噴嘴出口附近流場的密度在空間中一次微分的影像，可清楚觀看噴嘴出口下游凝聚性旋渦尾流結構的輪廓。PIV 系統藉由光學濾鏡的輔助將火焰光譜濾掉，可量化的量測時間相關的火焰非穩態流場之演化過程，及與渦旋逸放之交互作用。以熱電偶探針進行溫度的量測，得到燃燒場的溫度分佈，並計算燃燒釋放率，用以判斷火焰模態對燃燒的效應。氣體濃度分析可測的燃燒後氣體的組成包含一氧化碳、二氧化碳、碳氫化合物與氮氧化物，用以瞭解改善燃燒是否完全。預期達到星芒狀鈍體機構，可快速、經濟的架設應用於燃燒爐出口處，增進燃燒效率及降低碳氫化合物污染，結果在能源節約與環境污染上有極高的價值與應用性。實際例子如:工業用燃燒器、燃汽渦輪機、或民生用瓦斯爐、熱水器等都是有如此需求。" "This project will utilize the results of the previous study ‒ Novel Bluff-Body on Improving Temperature Uniformity and Combustion Efficiency of Non-Premixed Flame ‒ to present a series experiments and analyses on the combustion thermal fields. In this project, the tested bluff bodies will designed as three-astral, six-astral, and circle types. The previous results of cold flow utilized the smoke-streak visualization to detect the macroscopic flow behavior at low Reynolds numbers. The hot-wire velocimetry to measure the flow-field velocity and probe the connection between the wake vortex and shear layer frequencies. The previous results show that the highest vortex frequency occurs behind six-astral bluff body and increases the turbulence intensity higher than that of circle configuration about 70%. The applicant has experimentally studied the cold-flow behavior of the non-premixed combustor, the improvement of combustion efficiency using passive bluff-body control and active acoustic-excitation modulation. The experimental results show that the turbulent disturbance and appropriate swirl flow can enhance the mixing of fuel and airflow, and then improve the combustion capacity. However, the previous research experimental experience also showed three weaknesses. Firstly, for the non-premixed combustion process, when the central cold fuel flew fast and massively into the flame field (i.e., at high central fuel-jet speed), an unstable lift-off flame occurred due to the short residence time of fuel flow. This short residence time caused an incomplete chemical combustion reaction between the fuel and air flows. Secondly, the three-way stagnation-saddle points occurred between the fuel jet and bluff body has a considerably effect on the combustion and mixing of fuel and air jets. Thirdly, at high swirl-flow speed, the rapid mixing occurred between the hot flame and ambient cold air. The swirling flame can interrupt the combustion reaction and caused a large generation of carbon monoxide. The cold air and hot flame forms an un-uniform temperature field. This project attempts to minimize the effect of un-uniform temperature field on the flame field by improving the flame instability resulting from the short residence time of fuel and air, by improving the swirling flame between the fuel and bluff body, and by decreasing the occurrence of combustion interrupt at high swirling speed. This project will concentrates on the study of combustion field and will utilize the previous results of cold flow modulated using the astral-type controller. For the non-premixed combustor, the fuel will be ignited and burned to verify the effects of the controlling parameters. The experimental methods/techniques utilized in the second program include the direct photo, Schlieren optical visualization, PIV, thermocouple measurement and burned exhausted gas analysis. The examination focuses are flame behavior, flame length, velocity contour of flame field, flame temperature distribution, combustion characteristics and exhausted gas distribution. The photographed flame configurations can verify the effect of air/fuel ration on flame behaviors. The Schlieren photograph utilized the first differential image to visualize the coherent structure of wake vortex. The PIV method utilizes the optical filter to remove the flame spectrum and quantize the evolution process of the unsteady flame field. The thermocouple detects the temperature distribution of flame field. The combustion capacity will be considered using these flame temperature distributions. The gas analyze detects the exhausted gas including the carbon monoxide, carbon dioxide, hydrocarbon and nitrogen oxide to improve the combustion completeness. Finally, this project attempts to utilize an astral-shaped bluff mechanism for lowering the generation/pollution of hydrocarbon and improving the combustion efficiency behind a non-premixed combustor. Furthermore, the research results will be applied in the issues of energy-saving and environmental pollution, such as the industrial burner, gas turbine, gas stove and gas heater."

Keyword(s)

星芒狀調控器
非預混式燃燒爐
燃燒場
旋渦火焰
Astral-shaped disk controller
Non-premixed combustor
Flame field
Swirling flame

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Novel Bluff-Body on Improving Temperature Uniformity and Combustion Efficiency of Non-Premixed Flame (Ii)

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