Non-invasive attempts to extinguish flames with the use of high-power acoustic extinguisher

1 Introduction

For many years, effective methods of fire extinguishing have been sought. Traditional methods of fire extinguish are based on cutting off the oxygen supply from a burning surface. These include powder extinguishers, traditional water-extinguishers, and CO₂ fire extinguishers [1, 2]. Despite their proven effectiveness in the fight against fire, they pose a real thread of serious damage equipment installed in the rooms. For this reason, contemporary fire-fighters are looking for new methods to reduce the damage caused by fire-fighting incidents [3,4,5,6,7,8]. It is possible to extinguish the flames using an acoustic wave [9]. The operational principle of this type of the fire extinguisher depends on disturbing of the flames crown. Air turbulence resulting from variable acoustic pressure breaks the continuity of the flame and leads to its extinguish [10, 11]. Against this background, contemporary achievements in signal analysis with the use of computer methods and fire extinguishing using drones seem to be interesting [12,13,14,15]. The extinguishing technology described in the article can also be used in automation systems to extinguish flames in case of fire, also in the free space. Currently, many scientific centres are working on the use of robots in the event of natural disasters or detection of flames [16, 17].

In 2007, the popular science program “Myth busters” verified the effectiveness of flame extinguishing using acoustic waves. It was found that this can be done because the sound waves disrupt the air enough to snuff out the flame [18]. In 2008, American Defense Advanced Research Projects Agency (DARPA), launched the Instant Fire Suppression (IFS) research programme aimed at a better understanding of the nature of sound waves in terms of their potential application in military applications. In 2012, the research team arranged two speakers opposite each other, to demonstrate the effectiveness of the extinguish process depending on the sound parameters of the acoustic wave [19]. As research has shown, effective extinguish tests were carried out using low frequency acoustic waves [20]. It is related to that effectiveness of the extinguish process depends on the amplitude of the air vibrations. Another parameter that determines the effectiveness of the extinguishing process is the acoustic power.

2 Assumptions and theoretical basics

The main assumption was to build a fire extinguisher that would produce a directional acoustic stream capable of extinguishing flames. This is essential for improving the effectiveness of the extinguish process by improving the range of extinguishing and reducing the required acoustic power. For this reason, a proper acoustic system is needed. The simplest examples of this are a closed end tube and an open tube with a circular cross-section called a waveguide [21]. The principle of the waveguide is to strengthen the acoustic wave as a result of acoustic resonance. This resonance occurs due to reflecting of the acoustic wave inside the waveguide. At certain frequency (called resonance frequency) a standing wave occurs. The standing wave arises as a result of the interference of two same waves, moving in the same direction, but having opposite turns. Distribution of standing waves in the closed end tube and open tube is shown in Figure 1.

Figure 1

Distribution of standing wave in the waveguide for the first f₁ and second f₂ resonant frequency a) closed end tube, b) open tube

The minimum wavelengths required for the resonance phenomenon are different. It will be respectively:

where λ₁ – the length of the closed end tube, λ₂ – the length of the open tube, V – air velocity, f – sound frequency.

It can be noticed that the required length of the waveguide is two times smaller in the closed end tube [22, 23]. This is especially important for practical reasons – for low frequencies a significant waveguide length is required. The generation of a 20 Hz sound wave requires, for example: λ₁ = 4.28 m for the closed end tube, and λ₂ = 8.57 m for the open tube.

For this reason, previous attempts to fire extinguish with acoustic waves have been carried out with a closed end tube. However, these tests were carried out only with the use of low power and low range of acoustic extinguishers [24, 25]. These devices had a small extinguishing range and included a small working area – for this reason it was necessary to build an innovative high power and high range fire extinguisher, to get the potential possibilities of this extinguishing method [26,27,28,29,30].

3 Test station

Investigations of the influence of acoustic wave parameters on the efficiency of flames extinguishing were carried out on a test station shown in Figure 2.

Figure 2

Experimental set-up diagram 1) signal generator, 2) power amplifier, 3) waveguide, 4) loudspeaker, 5) sound level meter, 6) source of fire

The measuring station consists of Rigol DG4102 function/arbitrary waveform generator, Proel HPX2800 power amplifier, SVAN 979 sound level meter, and an acoustic extinguisher. To clearly determine the influence of the acoustic wave parameters on the extinguishing process, a burning candle was used. Unlike a diffuse fire source which can be obtained by using a gas fire-pit pan, the use of a burning candle allowed to clearly identify the extinction process. In the experiments carried out by using a gas fire-pit pan, it could be observed that when the diffuse flames were not fully extinguished, they were re-ignited. For this reason, it was decided that a point source of fire in the form of a burning candle will be more suitable for experimental researches. The flame height was about 2 cm.

The acoustic extinguisher was made in the form of a folded tapered, closed end waveguide with a rectangular cross-section of 4.28 m in length – this type of waveguide was used in all experiments. The B&C 21DS115 loudspeaker with a nominal power of 1700 W was installed at the beginning of the waveguide.

4 Experimental results

Experimental researches have been divided into two main parts: determination of basic fire extinguisher parameters and extinguishing model fire source.

4.1 Identification of fire extinguisher parameters

Determining the parameters of an acoustic extinguisher is important to ensure the most effective operation of the device in the event of fire. The following parameters were determined: an impedance curve, a sound pressure level curve, and the directional characteristics of the device.

a) Impedance curve

The impedance curve was measured in the range of 10–90 Hz. On the basis of the impedance curve, the operating frequency was specified.

Taking into account Figure 3, the frequency was set at 17.25 Hz. The minimum impedance (Z_min = 11.4Ω) of the fire extinguisher was measured at the indicated frequency. At this frequency, the speaker cone was the most acoustically stressed, which significantly reduced its vibration amplitude. That allows for more efficiency use of the speaker's power capabilities.

Figure 3

Impedance curve of the extinguisher

b) Sound pressure level curve

The impedance curve was measured in the range of 10–90 Hz in the waveguide axis. The accuracy of measurement was 1 Hz. The distance of the sound level meter from the waveguide output was 1 m. The extinguisher was powered by a sinusoidal voltage of 13 V RMS value.

Considering Figure 4, we can observe that the maximum sound pressure level 103.1 dB occurred at 17 Hz. It can be noticed that this frequency, previously defined as the operating frequency, is the optimum value for both the maximum mechanical load on the speaker cone and the acoustic efficiency.

Figure 4

Sound pressure level curve of the extinguisher

c) Directional characteristics

During designing devices for acoustic fire extinguishing, it is necessary to determine their directional characteristics. These characteristics allow for the optimal positioning of the extinguisher relative to the source of the fire (concentration of the acoustic beam allows to reduce the required acoustic power and dimensions of the extinguishing device) [31, 32]. The measuring station is shown in Figure 5.

Figure 5

Test station 1) waveguide output, 2) sound level meter

Measurement conditions

The measurements were carried out in an open air for three different distances of the sound level meter: R = 2 m, R = 2.5 m and R = 3 m. The accuracy of measurements was Δ = 22.5°. The intensity of the acoustic background during the measurement was 64.7 dB. The measurements were taken at the working frequency of 17.25 Hz and 150 W of power supplied to the speaker.

Figure 6 shows the directional characteristics for three different distances of the sound level meter: R = 2 m, R = 2.5 m, R = 3 m. It can be seen that the fire extinguisher emits the sound omnidirectionally, however the main acoustic stream is emitted in the waveguide axis. Obtaining a oneway emission would require building a waveguide of a length equal to the wavelength.

Figure 6

Directional characteristics for three different distances of the sound level meter: R = 2 m, R = 2.5 m, R = 3 m

4.2 Extinguishing a model fire source

The main goal of this study was to investigate the influence of acoustic wave parameters on extinguishing efficiency. The measuring station is shown in Figure 7.

Figure 7

Test station 1) waveguide output, 2) fire source

Measurement conditions

Investigations were carried out under wind-free conditions in the open air. Attempts to extinguish the fire were made in the axis of the waveguide output. The accuracy of measurement was 0.1 m. The intensity of the acoustic background during the measurement was 64.7 dB.

a) Influence of distance between fire source and waveguide output on extinguishing effect

The measurements were made with the use of three frequencies: 14 Hz, working frequency 17.25 Hz, and 20 Hz of sine wave.

Considering Figure 8, it can be observed that the most efficiency frequency of the extinguishing wave is the operating frequency. The maximum extinguishing range obtained during the measurement was 1.2 m. The power delivered to the fire extinguisher was 1000 W. More powerful tests were not performed due to the power limitation of the loudspeaker and amplifier, which nominal parameters are given according to the guidelines for musical signals (AES standards). Another limitation that appeared during extinguishing attempts was the significant vibration amplitude of the speaker's diaphragm, which appeared (especially) at 14 Hz. This can be observed both in Figure 8 and Figure 9 (blue curves) as a limited range of extinguishing – at 50 cm a significant amplitude vibration of the speaker's diaphragm was observed. Higher vibration amplitude was also observed at 22 Hz. The vibration amplitude was lower than it was for 14 Hz.

Figure 8

Minimum electrical power delivered to the extinguisher causing extinguishing effect in the function of the distance from the waveguide output

Figure 9

Sound pressure level causing extinguishing effect in the function of the distance from the waveguide output

Considering Figure 9, it can be concluded that the sound pressure capable of extinguishing effect was in the range of 120–130 dB. The decrease of sound pressure level observed with increasing distance between the flame and the waveguide output is caused by the overlapping of phase incompatible reverberant waves (reflected from the limiting surfaces of the measuring area) and waves directly emitted by the extinguisher. This resulted in sound pressure fluctuations which increased with increasing distance from the waveguide output. At 17.25 Hz and a distance of 110 cm from the waveguide output, the sound pressure level increased unexpectedly. It may result from a temporary change in atmospheric conditions.

b) Influence of sound frequency on extinguish effect

Investigations of the influence of sound frequency on the extinguishing efficiency were carried out for frequencies in the range of 14–21 Hz of sine wave. The accuracy of the measurements was 1 Hz. The distance of the fire source from the waveguide output was 0.5 m.

Experimental research has been divided into two parts:

• Measurement of the influence of acoustic wave frequency on the minimum extinguishing power

• Measurement of the influence of acoustic wave frequency on the minimum sound pressure causing extinguishing effect

The results show that acoustic wave frequencies between 14–21 Hz are capable of extinguishing flames.

It can be seen that with the increase in the frequency of the acoustic wave, the required electrical power supplied to the fire extinguisher also increases as is shown in Figure 10. The lowest extinguishing power of 125 W was measured at 14 Hz and the highest of 350 W at 20 Hz.

Figure 10

Minimum electrical power delivered to the extinguisher causing extinguishing effect in the function of sound frequency

The minimum extinguishing sound pressure increases from 116 dB for 14 Hz to 125 dB for 20 Hz as is shown in Figure 11. It can be seen that both the power and the sound pressure curves have a local maximum that may be measured near the operating frequency.

Figure 11

Minimum sound pressure causing extinguishing effect in the function of sound frequency

It can also be seen that the power and sound pressure curves are similar in shape. This is due to the fact that both of these values were measured in the same extinguishing attempts. Moreover, the sound pressure level is logarithmically dependent on the power supplied to the speaker.

5 Conclusions

Experimental researches confirmed the effectiveness of the innovative acoustic extinguisher in the fight against fire. The main assumption that variable sound pressure can cause turbulence to disturb the flame front, leading to its extension, has been confirmed. The use of higher frequency acoustic waves resulted in an increased electrical power supplied to the extinguisher causing the extinguishing effect. Acoustic waves of lower frequencies are more suitable because they cause a higher amplitude of flame vibrations and thus have a higher extinguishing efficiency. During the extinguishing tests the lowest of the tested frequencies, 14 Hz, showed the highest extinguishing efficiency. Then the necessary electrical power delivered to the fire extinguisher was the lowest. Therefore, it seems reasonable to build acoustic extinguishing devices with the lowest possible operating frequencies. However, generating a wave with a frequency of several Hertz requires the use of a much longer waveguide, which results in difficulties in practical implementation. The limit frequency of the acoustic wave above which it was difficult to observe the phenomenon of extinguishing flames, was 22 Hz. The use of higher frequency acoustic waves brings some benefits. One of them is the greater concentration of the acoustic beam, which appears as the frequency of the acoustic waves increases. The sound emission then becomes more directional, which directly improves the effective range of extinguishing. The second one is the limitation of the size of the extinguisher, which is particularly important in the context of mobile applications. Moreover, experimental research has shown that acoustic waves of higher frequencies, despite their advantages, require more electric power supplied to the extinguisher. A correlation between the increasing required power supplied to the loudspeaker increases with the frequency was observed – it can be seen in Figure 8.

Experimental research has also shown that a fire extinguisher emits sound omnidirectionally, however, the main acoustic stream is located in the axis of the waveguide. The dispersion of the acoustic beam limits the range of effective firefighting. The maximum distance with full extinguishing efficiency (1.2 m) was measured at operating frequency 17.25 Hz.

Significant size of extinguishing devices made with this technology and the unaffected impact of infrasound of such a high intensity on the health of rescuers and injured persons limit the scope of potential applications. For this reason, the fire-fighting technology with the use of acoustic waves could become element supporting the safety of server rooms or industrial halls as stationary fire extinguishing systems.

The device can be used for extinguishing fires of different classes, as the acoustic waves penetrate both solids, liquids, and gases. Nowadays it can be used in the case of fires of classes B and C, when gases or liquids are burning. However, it is unlikely to be as effective in the case of solid flames, as the flame may re-ignite due to the lack of heat absorption from the interior of the material.

To answer for all possible questions about the use of acoustic extinguishing technology, it is necessary to carry out tests on devices with much higher power and different operating frequencies – not only for one, 17.25 Hz, as it was shown in this paper. This will allow to define the limits of the range of operation both in the context of possible applications and to determine the impact of low-frequency acoustic waves on the human body. The second important issue is to determine the impact of such low-frequency waves on building structures. The risk that occurs here and which must be verified – how the resonance vibrations of walls or windows may contribute to damage or destruction of the building? – at this moment it has not been conducted.