Non-Invasive Attempts to Extinguish Flames with the Use of High-Power

Open Eng. 2021; 11:349–355

Research Article

Paweł Stawczyk* and Jacek Wilk-Jakubowski Non-invasive attempts to extinguish flames with the use of high-power acoustic extinguisher https://doi.org/10.1515/eng-2021-0037 contemporary achievements in signal analysis with the use Received Jun 19, 2020; accepted Jan 03, 2021 of computer methods and fire extinguishing using drones seem to be interesting [12–15]. The extinguishing technol- Abstract: This paper presents an innovative method of ex- ogy described in the article can also be used in automation tinguishing of flames using a high-power acoustic extin- systems to extinguish flames in case of fire, also in thefree guisher. This method allows for effective and non-invasive space. Currently, many scientific centres are working on the extinguishing of the flames. Experimental results showing use of robots in the event of natural disasters or detection the effectiveness of the fire extinguisher for different dis- of flames [16, 17]. tances from the flame source and different frequencies of In 2007, the popular science program “Myth busters” the acoustic wave are discussed. The paper ends with the verified the effectiveness of flame extinguishing using description of the advantages, disadvantages, and limita- acoustic waves. It was found that this can be done be- tions of the proposed fire extinguishing method. cause the sound waves disrupt the air enough to snuff out Keywords: Acoustic extinguisher; fire detection; fire retar- the flame [18]. In 2008, American Defense Advanced Re- dation; fire suppression; resonant frequency; non-invasive search Projects Agency (DARPA), launched the Instant Fire extinguishing of the flames 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, 1 Introduction 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 For many years, effective methods of fire extinguishing wave [19]. As research has shown, effective extinguish tests have been sought. Traditional methods of fire extinguish were carried out using low frequency acoustic waves [20]. are based on cutting off the oxygen supply from a burning It is related to that effectiveness of the extinguish process surface. These include powder extinguishers, traditional depends on the amplitude of the air vibrations. Another water-extinguishers, and CO2 fire extinguishers [1, 2]. De- parameter that determines the effectiveness of the extin- spite their proven effectiveness in the fight against fire, they guishing process is the acoustic power. 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 [3–8]. It is possible to extinguish the 2 Assumptions and theoretical flames using an acoustic wave9 [ ]. The operational principle basics of this type of the fire extinguisher depends on disturbing of the flames crown. Air turbulence resulting from variable The main assumption was to build a fire extinguisher that acoustic pressure breaks the continuity of the flame and would produce a directional acoustic stream capable of leads to its extinguish [10, 11]. Against this background, extinguishing flames. This is essential for improving the effectiveness of the extinguish process by improving the range of extinguishing and reducing the required acoustic *Corresponding Author: Paweł Stawczyk: Department of In- power. For this reason, a proper acoustic system is needed. dustrial Electrical Engineering, Kielce University of Technology, 7 The simplest examples of this are a closed end tube and Tysiąclecia Państwa Polskiego. Ave, Kielce 25-314 Kielce, Poland; an open tube with a circular cross-section called a waveg- Email: [email protected] Jacek Wilk-Jakubowski: Department of Information Systems, uide [21]. The principle of the waveguide is to strengthen Kielce University of Technology, 7 Tysiąclecia Państwa Polskiego. the acoustic wave as a result of acoustic resonance. This res- Ave, Kielce 25-314 Kielce, Poland; Email: [email protected]

Open Access. © 2021 P. Stawczyk and J. Wilk-Jakubowski, published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 License 350 Ë P. Stawczyk and J. Wilk-Jakubowski onance occurs due to reflecting of the acoustic wave inside 3 Test station the waveguide. At certain frequency (called resonance frequency) a standing wave occurs. The standing wave arises Investigations of the influence of acoustic wave parameters as a result of the interference of two same waves, moving in on the efficiency of flames extinguishing were carried out the same direction, but having opposite turns. Distribution on a test station shown in Figure 2. of standing waves in the closed end tube and open tube is shown in Figure 1.

(a)

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 func- (b) tion/arbitrary waveform generator, Proel HPX2800 power amplifier, SVAN 979 sound level meter, and an acoustic ex- Figure 1: Distribution of standing wave in the waveguide for the first tinguisher. To clearly determine the influence of the acous- f f 1 and second 2 resonant frequency a) closed end tube, b) open tic wave parameters on the extinguishing process, a burn- tube ing candle was used. Unlike a diffuse fire source which can be obtained by using a gas fire-pit pan, the use of a burning The minimum wavelengths required for the resonance candle allowed to clearly identify the extinction process. In phenomenon are different. It will be respectively: the experiments carried out by using a gas fire-pit pan, it V could be observed that when the diffuse flames were not λ = (1) 1 4f fully extinguished, they were re-ignited. For this reason, it was decided that a point source of fire in the form of V a burning candle will be more suitable for experimental λ2 = (2) 2f researches. The flame height was about 2cm. where λ1 – the length of the closed end tube, λ2 – the length The acoustic extinguisher was made in the form of of the open tube, V – air velocity, f – sound frequency. a folded tapered, closed end waveguide with a rectangular It can be noticed that the required length of the waveg- cross-section of 4.28 m in length – this type of waveguide uide is two times smaller in the closed end tube [22, 23]. was used in all experiments. The B&C 21DS115 loudspeaker This is especially important for practical reasons – for low with a nominal power of 1700 W was installed at the begin- frequencies a significant waveguide length is required. The ning of the waveguide. 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. 4 Experimental results For this reason, previous attempts to fire extinguish with acoustic waves have been carried out with a closed Experimental researches have been divided into two main end tube. However, these tests were carried out only with parts: determination of basic fire extinguisher parameters the use of low power and low range of acoustic extinguish- and extinguishing model fire source. ers [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 [26–30]. Non-invasive attempts to extinguish flames with the use of high-power acoustic extinguisher Ë 351

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. Figure 4: Taking into account Figure 3, the frequency was set at Sound pressure level curve of the extinguisher 17.25 Hz. The minimum impedance (Zmin = 11.4Ω) of the fire extinguisher was measured at the indicated frequency. At can be noticed that this frequency, previously defined as this frequency, the speaker cone was the most acoustically the operating frequency, is the optimum value for both the stressed, which significantly reduced its vibration ampli- maximum mechanical load on the speaker cone and the tude. That allows for more efficiency use of the speaker’s acoustic efficiency. power capabilities.

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 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 maxi- Figure 5: Test station 1) waveguide output, 2) sound level meter mum sound pressure level 103.1 dB occurred at 17 Hz. It 352 Ë P. Stawczyk and J. Wilk-Jakubowski

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 ∆ ∘ = 22.5 . The intensity of the acoustic background during the measurement was 64.7 dB. The measurements were Figure 7: Test station 1) waveguide output, 2) fire source 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 Measurement conditions different distances of the sound level meter: R = 2 m, R = 2.5 Investigations were carried out under wind-free conditions m, R = 3 m. It can be seen that the fire extinguisher emits in the open air. Attempts to extinguish the fire were made in the sound omnidirectionally, however the main acoustic the axis of the waveguide output. The accuracy of measure- stream is emitted in the waveguide axis. Obtaining a one- ment was 0.1 m. The intensity of the acoustic background way emission would require building a waveguide of a during the measurement was 64.7 dB. length equal to the wavelength.

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

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 8: Minimum electrical power delivered to the extinguisher causing extinguishing effect in the function of the distance fromthe waveguide output Non-invasive attempts to extinguish flames with the use of high-power acoustic extinguisher Ë 353 of the speaker’s diaphragm, which appeared (especially) at • Measurement of the influence of acoustic wave fre- 14 Hz. This can be observed both in Figure 8 and Figure 9 quency on the minimum extinguishing power (blue curves) as a limited range of extinguishing – at 50 • Measurement of the influence of acoustic wave fre- cm a significant amplitude vibration of the speaker’s di- quency on the minimum sound pressure causing ex- aphragm was observed. Higher vibration amplitude was tinguishing effect also observed at 22 Hz. The vibration amplitude was lower The results show that acoustic wave frequencies be- than it was for 14 Hz. tween 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. 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

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 Figure 10: Minimum electrical power delivered to the extinguisher surfaces of the measuring area) and waves directly emitted causing extinguishing effect in the function of sound frequency by the extinguisher. This resulted in sound pressure fluctu- ations 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: Figure 11: Minimum sound pressure causing extinguishing effect in the function of sound frequency 354 Ë P. Stawczyk and J. Wilk-Jakubowski pressure curves have a local maximum that may be mea- extinguishing efficiency (1.2 m) was measured at operating sured near the operating frequency. frequency 17.25 Hz. It can also be seen that the power and sound pressure Significant size of extinguishing devices made with curves are similar in shape. This is due to the fact that both this technology and the unaffected impact of infrasound of of these values were measured in the same extinguishing such a high intensity on the health of rescuers and injured attempts. Moreover, the sound pressure level is logarithmi- persons limit the scope of potential applications. For this cally dependent on the power supplied to the speaker. 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 5 Conclusions systems. The device can be used for extinguishing fires of different classes, as the acoustic waves penetrate both solids, Experimental researches confirmed the effectiveness of the liquids, and gases. Nowadays it can be used in the case of innovative acoustic extinguisher in the fight against fire. fires of classes B and C, when gases or liquids are burning. The main assumption that variable sound pressure can However, it is unlikely to be as effective in the case of solid cause turbulence to disturb the flame front, leading to its flames, as the flame may re-ignite due to the lack ofheat extension, has been confirmed. The use of higher frequency absorption from the interior of the material. acoustic waves resulted in an increased electrical power To answer for all possible questions about the use of supplied to the extinguisher causing the extinguishing ef- acoustic extinguishing technology, it is necessary to carry fect. Acoustic waves of lower frequencies are more suitable out tests on devices with much higher power and different because they cause a higher amplitude of flame vibrations operating frequencies – not only for one, 17.25 Hz, as it was and thus have a higher extinguishing efficiency. During shown in this paper. This will allow to define the limits the extinguishing tests the lowest of the tested frequencies, of the range of operation both in the context of possible 14 Hz, showed the highest extinguishing efficiency. Then applications and to determine the impact of low-frequency the necessary electrical power delivered to the fire extin- acoustic waves on the human body. The second important guisher was the lowest. Therefore, it seems reasonable to issue is to determine the impact of such low-frequency build acoustic extinguishing devices with the lowest pos- waves on building structures. The risk that occurs here and sible operating frequencies. However, generating a wave which must be verified – how the resonance vibrations of with a frequency of several Hertz requires the use of a much walls or windows may contribute to damage or destruction longer waveguide, which results in difficulties in practical of the building? – at this moment it has not been conducted. implementation. The limit frequency of the acoustic wave above which it was difficult to observe the phenomenon Acknowledgement: The authors would like to thank the of extinguishing flames, was 22 Hz. The use of higher fre- company “Ekohigiena Aparatura Ryszard Putyra Sp.J.”, quency acoustic waves brings some benefits. 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