|Abstract: ||本研究利用聲導納(acoustic admittance)之量測,探討長方形管道中,終端燃燒(end burning)火焰在外在聲波場下,火焰驅動/抑制聲波場之特性,並與火焰輻射量測結果比較。實驗中是將預混火燄燃燒爐置於管道之一端,幾何上模擬火箭終端燃燒之特性,探討燃燒不穩定之問題。實驗乃以雷射測速儀(Laser Doppler Velocimetry,LDV)及壓力轉換器(Pressure Transducer)分別作速度及壓力振盪之量測,直接得到燃燒爐面之聲導納;光電管系統作火焰輻射量測並配合芮氏準則(Rayleigh criterion),分析火焰驅動或抑制聲波場之特性。實驗結果顯示,甲烷與空氣混合氣流速約在 6cm/sec與28cm/sec之間,當量比約在0.6與1.16之間火焰產生自發性振盪。由聲導納之量測結果顯示,冷流爐面聲導納之實部值與燃燒穩定性有關;火焰改變冷流邊界聲導納之實部值;同時,當外在聲場頻率介於700Hz至900Hz時,火燄抑制聲波場壓力振盪;在400Hz至700Hz時火燄驅動聲波場壓力振盪。不同當量比下火燄驅動/抑制聲波場壓力振盪之趨勢相似,但當量比愈接近1時,其火燄驅動/抑制聲波場壓力振盪之能力愈大。此結果與火焰輻射量測結果非常接近,顯示藉由聲導納之量測,可用來探討火燄驅動/抑制聲波場壓力振盪之特性。研究結果同時顯示,本實驗中低流速平板火焰之存在並未明顯改變聲導納之虛部值,同時聲導納虛部值之正負變化與火焰驅動/抑制聲波壓力振盪趨勢相似。因此利用冷流邊界聲導納之量測,可判斷火燄是驅動或是抑制聲波場壓力振盪。若能應用此結果於固態火箭推進劑之燃燒,則只要進行推進劑聲導納冷流量測,將可探討推進劑燃燒之穩定性。|
This Research is concerned with the investigations of combustion instability in a geometrically simulated end- burning rocket combustor. Specifically, the driving characteristics of premixed flames located at one end of a rectangular duct were investigated by using acoustic admittance technique. The results were compared with those obtained using flame radiation measurements. These studies were performed by using Laser Doppler Velocimetry (LDV) and pressure transducer for direct acoustic admittance measurements, and using photomultiplier system for flame radiation measurements which were analyzed based on Rayleigh criterion, to determine the driving/damping characteristics of the investigated flames. The results of the acoustic admittance measurements reveal that the real parts of the cold flow acoustic admittance could be related to the combustion stability. In addition, the flames change the real part of the burner surface acoustic admittance. Both results show that the flame damps pressure oscillations with excited frequencies ranged from 700Hz to 900Hz, while the flame drives the acoustic fields with frequencies between 400Hz and 700Hz. The tendency of the flame driving characteristics are similar with flames of different equivalence ratios. However, the flame with equivalence ratio close to one exhibits stronger flame driving/damping behavior. The results obtained from both methods show good agreements. This study also shows that the flame only change little the values of the imaginary part of the burner surface admittance, and the imaginary part of the cold burner acoustic admittance can be used to qualitatively predict the driving/damping characteristics of the flames. Thus, by using cold flow acoustic admittance measurements, it would be possible for the stability studies of, for example, solid propellants.