Flame structure and oscillation characteristics of ethanol pool flame under transverse acoustic force
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摘要: 為研究聲波滅火的機理，分析聲波對非封閉火焰的具體控制行為，對3、4和5 cm直徑油池火焰在30 ~ 90 Hz聲波作用下的火焰形態及燃燒特征進行了分析。對火焰圖像的分析發現，橫向聲波加劇了渦旋的不穩定運動，聲強迫下的火焰形態可歸結為間歇截斷、偏轉和穩定燃燒三種狀態。火焰幾何尺寸的數值分析表明：間歇截斷和偏轉狀態下的火焰表面高度扭曲皺褶，具有更高的分形維數。對火焰面積、高度和寬度的頻域信號分析表明：在間歇截斷狀態下火焰信號極不穩定，頻域峰值集中在0 ~ 10 Hz之間，聲頻率在火焰寬度信號的頻域分布中始終突出。基于火焰傾角和Richardson數的關系提出了Ri數在聲波作用下的形式，Richardson數分析表明：在50 ~70 Hz之間，火焰對聲波頻率的響應尤為顯著，聲頻高于或低于該段頻率時對火焰的影響存在邊際效應，間歇截斷和偏轉狀態的臨界Ria?1數（聲波作用下的火焰Richardson數的倒數）分別為10.32和2.92。
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關鍵詞:
- 聲波強迫 /
- 油池火焰 /
- 分形維數 /
- 振蕩頻率 /
- Richardson數
Abstract: Clean and efficient fire extinguishing technology has been a hot research topic in fire science. The research of acoustic extinguishing technology originates from the discovery that the different acoustic modes of combustion noise can lead to an unstable oscillation of the flame and local flame extinction. Recently, acoustic extinguishing technology has gradually entered the research field of scholars because it is clean and exhibits no secondary pollution. To study the fire extinguishing mechanism and analyze the specific control behavior of the acoustic wave on an unclosed flame, the flame shapes and the combustion characteristics of the pool flame with 3, 4, and 5 cm diameters under a 30–90 Hz acoustic force were analyzed. The experimental system includes a high-speed camera, signal generator, power amplifier, loudspeaker, and acoustic signal analysis device. The flame image analysis shows that the transverse sound wave intensifies the unsteady flow of the vortex, and the flame shape under an acoustic force could be divided into three types of state: intermittent, deflective, and stable. The numerical analysis of the flame geometry shows that the flame surface is highly twisted and wrinkled under the intermittent and deflective states with a higher fractal dimension. The frequency-domain signal analysis of the flame area, height, and width shows that the flame signal is very unstable in the intermittent state, and the peak frequency domain is concentrated in the range of 0–10 Hz. The acoustic frequency is always prominent in the frequency distribution of the flame width signal. Based on the relationship between the flame inclination angle and Richardson number, the form of the latter under the action of the acoustic wave was proposed. In the 50–70 Hz range, the response of the flame to the acoustic frequency was particularly considerable, and there may be a marginal effect when the acoustic frequency is higher or lower than the said range. The critical Ria?1 of the intermittent and deflective states are 10.32 and 2.92, respectively.-
Key words:
- acoustic forced /
- pool flame /
- fractal dimension /
- oscillation frequency /
- Richardson number
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圖 1 實驗裝置示意圖及聲波參數圖. (a)裝置示意圖；(b)聲參數圖
Figure 1. Diagram of experimental system and sound parameters: (a) experimental system; (b) sound parameters
圖 2 無聲波強迫下與聲波強迫下不同直徑油池火焰部分火焰圖像（左、中、右分別為3，4和5 cm直徑油池火焰；a,b,c,d,e序列分別為無聲波，30，50，70和90 Hz聲波）
Figure 2. Partial flame images of pool flame under an acoustic force (left, middle, and right pool flames are 3, 4, and 5 cm in diameter, respectively; a, b, c, d, and e sequences are nonacoustic with 30, 50, 70, and 90 Hz acoustic force, respectively)
圖 3 油池火焰在聲波作用下火焰形態變化示意圖（I、II、III、和 IV分別表示無聲波作用、間歇截斷、偏轉和穩定燃燒的狀態）
Figure 3. Schematic diagram of flame shape change with acoustic force (I, II, III, and IV represent the nonacoustic, intermittent, deflective, and stable states, respectively)
圖 4 火焰圖像二值化處理及火焰面積像素點數. (a)圖像二值化處理；(b)火焰面積像素點數
Figure 4. Binarization of flame image and number of flame area pixels: (a) binarization of flame image; (b) number of flame area pixels
圖 5 不同直徑油池火焰在不同聲頻率下的火焰面積、火焰寬度、火焰高度以及分形維數的平均數值. (a)火焰面積; (b)火焰寬度; (c)火焰高度; (d)分形維數
Figure 5. Average flame area, flame width, flame height and fractal of pool flames with different diameters under acoustic wave of different frequencies: (a) flame area; (b) flame width; (c) flame height; (d) fractal dimension
圖 6 不同直徑油池火焰在聲波強迫下的頻域信號(a、b和c前綴分別表示在30、50和70 Hz聲波作用下；1、2和3后綴分別表示火焰面積、火焰寬度和火焰高度信號)
Figure 6. Frequency signals of pool flames with different diameters under an acoustic force (a, b, and c prefixes denote acoustic wave at 30, 50, and 70 Hz, respectively; 1, 2, and 3 suffixes indicate the flame area, flame width, and flame height signal, respectively)
圖 7 聲波頻率、火焰傾角和Ria?1值之間的關系. (a)不同頻率下的火焰傾角正切值; (b) Ria?1值與火焰傾角正切值的擬合關系; (c)火焰Ria?1值
Figure 7. Relationship between acoustic frequency, flame angle, and Ria?1: (a) tangent value of flame inclination angle at different frequencies; (b) fitting relationship between Ria?1 and the tangent value of flame inclination angle; (c) flame Ria?1 value
表 1 不同直徑油池火焰在不同頻率聲波作用下的燃燒狀態
Table 1. State of pool flames with different diameters and different frequencies of acoustic force
Frequencies of acoustic force/Hz State of pool flames with different diameters 3 cm 4 cm 5 cm 0 Ⅰ Ⅰ Ⅰ 30 Ⅱ Ⅱ Ⅲ 40 Ⅱ Ⅱ Ⅲ 50 Ⅱ Ⅲ Ⅲ 60 Ⅲ Ⅲ Ⅲ 70 Ⅳ Ⅳ Ⅳ 80 Ⅳ Ⅳ Ⅳ 90 Ⅳ Ⅳ Ⅳ 
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