Open AccessArticle

Smoke Precipitation by Exposure to Dual-Frequency Ultrasonic Oscillations

Vladimir Khmelev

Andrey Shalunov

Sergey Tsyganok

and

Pavel Danilov

Biysk Technological Institute (Branch), Altay State Technical University, 659305 Biysk, Russia

Author to whom correspondence should be addressed.

Fire 2024, 7(12), 476; https://doi.org/10.3390/fire7120476 (registering DOI)

Submission received: 9 November 2024 / Revised: 4 December 2024 / Accepted: 13 December 2024 / Published: 15 December 2024

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Figure 1
Design and simulation results fora disk emitter. (a) Distribution of amplitude; (b) distribution of stress. 1—emitter; 2—emitting pad of the piezoelectric transducer; 3—piezoceramic rings; 4—reflecting pad; 5—tightening bolt; 6—copper electrode; 7—tightening screw. "> Figure 2
The manufactured emitter with an electronic generator for supplying its power. "> Figure 3
Dual emitter for equal frequency action on smoke. "> Figure 4
Emitters for multi-frequency action on smoke. "> Figure 5
Stand for measuring the directional pattern of ultrasonic emitters. 1—Ultrasonic disk emitter, 2—electronic generator; 3—emitter stand, 4—microphone; 5—noise meter measuring unit; 6—microphone stand; 7—microphone direction point. "> Figure 6
Experimental setup; (a) with one emitter; (b) with two emitters. 1—Ultrasonic disk emitter; 2—electronic generator; 3—smoke chamber; 4—smoke generator; 5—infrared radiation source; 6—photodetector. "> Figure 7
Directivity pattern for a single emitter. "> Figure 8
Attenuation in relation to distance from the source in a smoke chamber (one emitter). "> Figure 9
Directivity pattern of dual disk emitters.Red color—two simultaneously operating disks at the same frequency; blue color—two simultaneously operating disks of different frequencies. "> Figure 10
Attenuation over distance in a smoke chamber (two emitters). Blue color—two simultaneously operating disks of different frequencies; red color—two simultaneously operating disks of equal frequencies. "> Figure 11
Difference frequency directivity pattern. "> Figure 12
Beat frequency attenuation over distance in smoke chamber. "> Figure 13
Results of visual observation of ultrasonic smoke agglomeration. "> Figure 14
Measurement of relative visibility from the time of ultrasonic exposure for different distances (in m) from the emitter. "> Figure 15
Measurement of relative visibility from the time of ultrasonic exposure for different distances (in m) from the emitters. "> Figure 16
Histogram of agglomerate size distribution. "> Figure 17
Images of smoke particle agglomerates (100×). (a) Single-frequency action; (b) dual-frequency action. ">

Versions Notes

Abstract

The analysis conducted herein has shown that the efficiency of smoke precipitation can be improved by additionally making smoke particles interact with ultrasonic (US) oscillations. Because the efficiency of US coagulation lowers when small particles assemble into agglomerates, the authors of this work have suggested studying how smoke particles interact with complex sound fields. The fields are formed by at least two US transducers which work at a similar frequency or on frequencies with small deviations. To form these fields, high-efficiency bending wave ultrasonic transducers have been developed and suggested. It has been shown that a complex ultrasonic field significantly enhances smoke precipitation. The field in question was constructed by simultaneously emitting 22 kHz US oscillations with a sound pressure level no lower than 140 dB at a distance of 1 m. The difference in US oscillations’ frequencies was no more than 300 Hz. Due to the effect of multi-frequency ultrasonic oscillations induced in the experimental smoke chamber, it was possible to provide a transmissivity value of 0.8 at a distance of 1 m from the transducers and 0.9 at a distance of 2 m. Thus, the uniform visibility improvement and complete suppression of incoming smoke was achieved. At the same time, the dual-frequency effect does not require an increase in ultrasonic energy for smoke due to the agglomeration of small particles under the influence of high-frequency ultrasonic vibrations and the further aggregation of the formed agglomerates by creating conditions for the additional rotational movement of the agglomerates due to low-frequency vibrations.

Keywords:

smoke precipitation; ultrasonic action; acoustic agglomeration; disk transducer; sound pressure level; piezoelectric ultrasonic oscillator; ultrasonic oscillations; two-frequency effect

1. Introduction

Smoke formation during closed and large-area open fires sufficiently effects the ability of people to evacuate quickly and slows down firefighting. Smoke is a product of incomplete combustion and it usually forms simultaneously with carbon monoxide. The smoke’s poisonous properties are caused not only by presence of small hard particles, but also by carbon monoxide which is absorbed onto their surfaces. This eliminates the possibility of the presence of people and firefighters without special breathing equipment and requires the preliminary, or implemented simultaneously with extinguishing the fire, removal of smoke from the premises or fire zone.

The smoke control methods currently used are ineffective. Thus, the use of smoke removal methods proposed in works [1,2] and based on the formation of air flows leads to an additional influx of fresh air, which can accelerate the spread of fire.

In the developments proposed by Li-Wei, P.A.N. [3], Ray, S.K. [4] and Morlon, R. [5], the use of highly dispersed water fog to reduce the concentration of smoke is proposed. This helps to temporarily improve visibility in the room for a short period of time (several minutes) [6], but, subsequently, visibility again decreases to lower values due to the absorption and scattering of light on small water droplets.

The urgent need for smoke precipitation and the lack of effective solutions to this problem necessitate the search for new methods of smoke control. They should significantly improve visibility in a smoke-filled room by settling smoke to ensure the work of firefighters and the safe evacuation of people can be carried out.

It is clear that the efficiency of smoke precipitation can be increased by applying additional physical effect on smoke particles. One of the most effective methods of affecting solid particles in gaseous environments is acoustic agglomeration.

The agglomeration process leading to the deposition of smoke particles by the action of acoustic vibrations was discovered more than 30 years ago during the study of sirens—acoustic alarms—that were triggered by a fire. It was found that deposits of soot or resinous residues from smoke were formed on the surface and around the openings of the acoustic alarm, which formed in the form of patterns of a certain shape, Chladni figures, characterizing the presence of oscillatory processes [7].

As shown in the works of J. Liu et al. [8], acoustic agglomeration is a technology that can be used to influence solid particles in gasses, in which acoustic vibrations accelerate the relative motion of particles along the direction of wave propagation, as a result of which these particles combine with each other into agglomerates. The sizes of the agglomerates significantly exceed the original particles. At the same time, many researchers (Z. Wang et al. [9] and G. Zhang et al. [10]) confirmed that the agglomerates formed under acoustic influence are deposited under the action of gravity forces, and the particle count concentration decreases. The processes occurring during acoustic agglomeration have been widely studied in recent years, both experimentally and theoretically. A large number of results have been obtained for the analysis and selection of optimal modes and conditions of influence.

The effectiveness of acoustic agglomeration has been demonstrated for the deposition of coal fly ash [11,12], diesel exhaust particles [13], TiO₂ particles [14], soot particles [15] and liquid droplets [16]. The acoustic frequencies in the experiments were varied over an extremely wide sound range from 46 Hz [17] to 18 kHz [18]. The proposed agglomeration mechanisms are explained by orthokinetic interactions, acoustic wave effects, etc. Acoustic agglomeration has shown great potential in improving the efficiency of dust emission collection, extracting valuable particles or droplets from flue gasses and preventing fogging at airports [19,20,21,22,23]. As studies have shown, the impact of acoustic vibrations on a soot aerosol with a mass concentration of 1.0 g/m³ [19,24] leads to the formation of soot aggregates in the form of levitating disks with a diameter of 1–2 cm during a long (more than 10 min) acoustic impact time.

The results presented allow us to state that acoustic agglomeration technology can improve visibility in smoke in a fire situation by reducing the smoke concentration at an early stage of a fire, and thus, aid evacuation before the escape routes are blocked by open combustion. In addition, compared with other smoke removal methods, this technology has a number of advantages related to the simplicity of operation and the high speed of reducing the concentration of smoke. However, there are a number of obstacles to its practical use:

-: Firstly, almost all of the positive results were achieved by combining particles with sizes greater than 3–5 μm;
-: Secondly, the main studies were carried out at an audio frequency and at a sound pressure level of 140 dB or more. However, high-intensity (> 140 dB) acoustic radiation in the audible sound range is unacceptable for practical use in open spaces (or without the use of personal protective equipment) due to the mortal danger posed to humans and animals.

At the same time, it is known today that the most effective way to agglomerate small particles (less than 2.5 µm in size) is to use ultrasonic vibrations with a frequency of more than 22 kHz, which is safe for humans and animals. It has been established that ultrasonic vibrations quickly combine small particles into agglomerates and increase their size to 10 µm [25,26,27]. However, for smoke particles, the largest number of which lies in the submicron range of 0.001–1 µm, even high-frequency ultrasonic vibrations are not effective enough. At the same time, the time it takes to reduce the concentration of particles by at least an order of magnitude can be tens of minutes, which is unacceptable in the case of fighting smoke during fires.

To improve the efficiency of acoustic agglomeration, various approaches have been proposed based on changing the characteristics of the acoustic wave or initiating secondary acoustic effects in the insonified medium, leading to additional convergence and an enlargement of the particles. Thus, in the study carried out in [26] the creation of nonlinearly distorted waves to increase the effective cross-sectional area of the collision is considered. It is shown that, with a nonlinearity coefficient of up to 0.8, the probability (area of effective particle collision) increases by 5–20 times [26]. There are also approaches based on initiating acoustic flows of various scales in the insonified medium (from the size of the insonified medium to the sizes corresponding to the pressure drop zones in the oscillatory process). It was found that such an approach allows the efficiency of submicron particle agglomeration to be increased by 25%.

However, the practical implementation of these approaches is possible only in the case of creating resonance in the air gap, and this is achieved only in small layers, the thickness of which does not exceed several of the lengths of the ultrasonic waves in the air. Real world rooms in which it is necessary to implement smoke precipitation differ significantly in size, so it is necessary to consider alternative approaches to increasing the efficiency of agglomeration, which are less demanding on the sound pressure level in the volume being used. Thus, to form nonlinearly distorted shock waves, it is necessary to create a sound pressure level of at least 180 dB, and to form intense acoustic vortices—at least 170 dB [26]. This is technically not feasible in rooms, tunnels and open spaces.

As was shown previously, ultrasonic action allows for very rapid agglomeration of small particles and an increase in their size to 5–10 µm [25,26,27]. However, further ultrasonic vibrations practically cease to act on the formed agglomerates of such a size that could be combined due to sound action, as shown previously.

Therefore, in this paper, in order to create a highly efficient method for smoke agglomeration, we propose studying the effect of complex acoustic fields formed by several (at least two) emitters operating at the same or similar frequencies on smoke particles.

In this case, the intensification of smoke particle agglomeration will occur due to several factors. Since it is known that smoke particle agglomerates have an aspherical shape, in complex ultrasonic fields they will perform not only translational motion (due to the influence of a non-zero momentum during the collision of particles into an agglomerate), but also rotational motion (due to a non-zero angular momentum) [28]. It is clear that the rotational motion of an aspherical agglomerate can increase the cross-sectional collision area. In this case, the greatest rotation can be achieved by changing the vector of the oscillatory velocity of the acoustic field both in magnitude and in direction. A change in the vector of oscillatory velocity is possible in the case of the interaction of ultrasonic vibrations of two close frequencies with the formation of beats that arise and propagate over the area in which the ultrasonic vibrations interact.

Therefore, it is of great interest to study the influence of dual-frequency ultrasonic action (about 20 kHz), which is capable of forming beats at a different low frequency, effective for the sedimentation of agglomerates, when interacting with oscillations close in frequency. In this case, we propose forming beats at a frequency lower than human hearing, but higher than the infrasonic frequency and the resonant frequencies of human internal organs. This will make such action safe for humans [29].

The creation of conditions for the formation of complex ultrasonic fields, when two ultrasonic waves and a different low-frequency wave are directed at an angle to each other and have a phase shift, should ensure the most effective deposition of smoke. To create such an effect in practice, highly effective ultrasonic emitters have been proposed and developed, the design, adjustment features and application of which are discussed below.

2. Materials and Methods

Design of Emitters for Multi-Frequency Ultrasonic Action on Smoke

To implement the smoke precipitation process, an ultrasonic piezoelectric transducer with a disk emitter with a variable cross-section was developed and manufactured. During operation, such an emitter produces bending oscillations. During our study, it was found that a smooth increase in the thickness of the central and outer ring zones ensures not only an increase in their total area, creating in-phase oscillations, but also allows for the formation of oscillations with the same amplitude along the entire surface of the emitter. Figure 1 shows the developed bending–oscillating disk emitter with a piezoelectric oscillatory system for the excitation of oscillations. Also, Figure 1 shows the distribution of the emitter’s oscillation amplitudes. It is evident that the emitter operates on the second oscillation mode.

The design consists of a disk-shaped emitter performing bending oscillations (Figure 1, position 1) and a piezoelectric transducer consisting of a radiating pad, in this case an oscillation concentrator (Figure 1, position 2), piezoceramic rings (Figure 1, position 3) and a reflecting pad (Figure 1, position 4). The disk oscillates in the second bending-ring oscillation mode. At the same time, in order to reduce mechanical stresses and increase the thickness of the central and outer ring zone to optimal dimensions, the radial transition was increased to dimensions sufficient to ensure the design of the emitter without step changes in the thickness of the disk [30].

To ensure a multi-frequency effect for smoke precipitation, three experimental samples of ultrasonic disk emitters with different operating frequencies were manufactured. Two emitters had the same resonant frequency, and the third had a higher frequency (by 300 Hz). The resonant frequency was measured using an MS6100 frequency meter (Mastech-group, Taiwan, China), which provided a measurement accuracy of at least 1 Hz in the range from 10 kHz to 100 kHz. The emitters were used in three different versions of the experiment. For the control experiment (with which all other results were compared), a single emitter(Emitter 1)was used, with an electronic generator for its power supply (version 1). A generator with independent excitation and phase-locked frequency control was used. The resonant frequency of the emitter was 22,785Hz. A photograph of the emitter is shown in Figure 2.

A dual emitter was used to create acoustic fields with a complex structure and increase the power of acoustic impact. It consisted of two disk emitters (Emitter 1 and Emitter 2) powered by one generator with synchronization of frequency (22,785 Hz) and phase of disk oscillations (version 2). Compensation for the relative change in the resonant frequency of emitters (relative to each other) was carried out electronically by means of a tunable choke installed in the power supply circuit of each emitter. During operation, the difference in emitter frequencies did not exceed ± 2 Hz. A photograph of the generator with two emitters is shown in Figure 3.

For multi-frequency exposure, two emitters (Emitter 1 and Emitter 3) with frequencies of 22,785 Hz and 23,080 Hz were used. Two independent electronic generators were used to excite the emitters’ oscillations. Synchronization between the generators was not performed (version 3). Multi-frequency emitters with generators are shown in Figure 4.

The main characteristics of the emitters are given in Table 1.

To conduct comparative tests with the created emitters with various exposure options under identical conditions, experimental setups were developed and manufactured.

3. Experimental Setups

The main characteristics of the emitters that determine the efficiency of their action on smoke are the characteristics of the acoustic field, which include the level of the generated acoustic pressure and the radiation pattern. To determine the characteristics of the acoustic field, a measuring setup (stand) was developed and manufactured, shown in Figure 5. Ultrasonic emitters (position, 1) were positioned vertically. The microphone (position, 4) of the noise meter (position, 5) was located in the center of the acoustic axis of the emitter or on the symmetry axis of one or two ultrasonic emitters. The sound pressure level was measured using the Ekophysika—110 A noise meter.

The ultrasonic emitter stand (position 3) was rigidly fixed. The microphone stand (position 6) could move, changing the angle of the microphone position relative to the symmetry axis. The angle varied from 0° to 90°. In this case, a distance of 1000 mm was maintained between the radiating surfaces of the ultrasonic emitters and the microphone.

To study the process of smoke precipitation due to ultrasonic action using the emitter we created, an experimental setup was developed and manufactured, a photo of which is shown in Figure 6. The setup includes the developed and manufactured emitter (position 1); an electronic generator (position 2); a smoke chamber (position 3); a smoke generator with the ability to adjust the smoke flow rate (position 4); light sources—infrared LEDs BL-L513IRAB (position 5); and WL-TDRB 1540031EC4590 photodiodes (position 6).

Each LED–photodiode pair was located directly opposite each other in the center of the side walls of the smoke chamber. A total of eight optical pairs were used, evenly spaced vertically at a distance of 0.25 m from each other. In the experiments, wood chips burned in a smoke generator were used as a smoke source. The smoke chamber measuring 0.3 × 0.3 × 2 m was filled with smoke for 5 min. The smoke flow rate was approximately 25 L/min for experiments with one emitter, and 50 L/min when exposing the smoke to two emitters. During the ultrasonic exposure, smoke was supplied to the chamber.

A multichannel system consisting of sources and receivers of optical radiation in the infrared range was used to quantitatively assess the degree of smoke deposition. According to the Beer–Bouger–Lambert (BBL) law, due to the scattering and absorption of light by smoke particles, the intensity of optical radiation of light passing through the smoke decreases. The relative visibility T was defined as follows:

T = \frac{I_{t}}{I_{0}},

(1)

where I_t—the intensity of infrared radiation passing through smoke; I₀—the initial intensity of infrared radiation in the absence of smoke.

4. Results and Discussion

4.1. Characteristics of the Acoustic Field Formed by Disk Emitters

Based on the results of the measurements, the directivity pattern and the dependence of the sound pressure level on the distance from the emitter (attenuation per unit length) were measured for all three combinations of ultrasonic action: action with one emitter at one frequency; action with a double emitter consisting of two disks operating at one frequency; and action with two disks at different frequencies.

When acting with one emitter, it was found that the opening angle of the main lobe of the directivity pattern was 20°, the sound pressure level at a distance of one meter was 141 dB (Figure 7).

Next, the linear attenuation of ultrasonic vibrations generated by a single emitter was determined. Measurements were taken in a smoke chamber in clean air (without smoke) and when exposed to smoke. It was found that, when propagating through smoke, ultrasonic vibrations attenuate much more strongly than when propagating through air. The results of comparative measurements are presented in Figure 8.

The presented dependencies show that the sound pressure level generated by the disk emitter is higher in the smoke chamber than in the open space. This is due to the fact that the chamber walls prevent the lateral divergence of the energy of the emitted vibrations and direct it along the chamber. Due to this, at a distance of 1 m from the emitter, the sound pressure level(SPL) increases from 141 dB to 160 dB. However, the presence of smoke leads to a sharp decrease in the sound pressure level with distance from the emitter: from 160 dB in an empty chamber to 141 dB in a chamber filled with smoke (the data comparison is given at the exit from the chamber, the length of which was 2 m). The difference of about 20 dB corresponds to a 100-fold decrease in the power of the ultrasonic vibrations generated in the chamber!

The obtained results confirm the necessity of influencing smoke with ultrasonic fields with a high level of sound pressure and a complex structure, increasing the probability of the convergence and interaction of smoke particles. For this purpose, the characteristics of the acoustic field formed by two emitters operating at the same frequency and two emitters of different frequencies were studied. The measurement results (in the form of a directivity pattern in open space) are shown in Figure 9.

It follows from the presented diagram that, due to the use of two emitters, the maximum sound pressure level increased by 3 dB. At the same time, a narrow central lobe of the diagram with a width of ±5° was formed. Two symmetrical side lobes can be distinguished in the diagram, extending from the acoustic axis of the emitter by 15°. The sound pressure level of these side lobes is equal to the central one and is ≈143 dB. When such an emitter acts in a limited space (smoke chamber), the presence of such lobes does not affect the radiation’s efficiency. As was shown earlier, the extended design of the chamber eliminates the divergence of ultrasonic vibrations. But when acting in open spaces, such lateral divergence must be excluded. This will be the direction of our future work.

Figure 10 shows the results of the attenuation in relation to distance to source for different and equal-frequency operating disk emitters. The attenuation was also measured in a chamber filled with smoke.

From the presented dependencies, it is evident that for two equal frequency emitters the sound pressure in the chamber filled with smoke is higher than that for two multi-frequency emitters at 1.5–2 dB. An excess is observed both at a distance of 1 m from the emitters and at a distance of 2 m from the emitters, which should contribute to the accelerated agglomeration of smoke.

In general, the results of the sound pressure level measurements presented in Figure 9 and Figure 10 showed that both equal-frequency and multi-frequency ultrasonic action create practically identical complex acoustic fields on the deposited smoke. Thus, the various efficiency values of ultrasonic smoke agglomeration obtained in further experiments will be unambiguously determined only by the type of ultrasonic action (single-frequency or multi-frequency), and not by the sound pressure level.

However, when exposed to multi-frequency emitters, beats were formed at a difference frequency of 300 ± 5 Hz. Figure 11 shows the directivity pattern for the generated oscillations of the difference frequency.

The angle of the main lobe of the directivity pattern was ±10 degrees, while the sound pressure on the main lobe of the directivity pattern reached 97.6 dB. The results of measuring the linear attenuation of a system of two ultrasonic emitters with reflectors and horns are shown in Figure 12. The measurements were carried out in a smoke-filled chamber.

It follows from Figure 12 that low-frequency oscillations are attenuated less in a smoky environment. With a lower initial value, the relative decrease in SPL at a distance of 1 m and 2 m from the emitters is 10 dB less than for ultrasonic frequencies. Thus, at a distance of 1 m from the radiating surface of the ultrasonic emitters, the sound pressure intensity was 84.1 dB, and at a distance of 2 m it was 77.8 dB.

4.2. Qualitative Assessment of Smoke Precipitation Efficiency

Preliminary experiments have shown that, in less than 300 s of ultrasonic action, with the smoke generator operating in the chamber, complete deposition of smoke occurs, i.e., the chamber is completely cleared of smoke. Residual turbidity on the transparent walls of the chamber is caused by the deposition of smoke and resin particles on the walls under the action of ultrasonic vibrations. For comparison, when the smoke supply to the smoke chamber was turned off, the time taken for the natural deposition of smoke (without ultrasonic action) in the chamber was about 30 min.

The results of visual observation of ultrasonic agglomeration of smoke are shown in the photos (Figure 13).

Initially, the chamber is filled with smoke from burning sawdust (0 s). Ultrasonic action on the smoke is performed. Under the action of the acoustic field, the smoke particles come together and begin to unite into agglomerates with further precipitation under the action of gravitational forces. The process of acoustic coagulation begins in the immediate vicinity of the ultrasonic emitter, i.e., first, there is an effect on the smoke particles located in the upper quarter of the chamber. After 50 s of exposure, light in the upper quarter of the chamber is observed. Then, acoustic coagulation spreads along the chamber and, even after 100 s of ultrasonic action, almost complete precipitation of smoke particles occurs in the upper half of the chamber and acoustic coagulation begins in the lower half of the chamber. After 150–200 s of ultrasonic action, the precipitation of the smoke particles occurred in almost the entire chamber. Once again, it should be noted that during the experiments the smoke generator was on, thus smoke was entering the chamber constantly.

Thus, it has been qualitatively confirmed that there is an increase in the transparency of the air environment due to the agglomeration of smoke particles under the action of ultrasonic vibrations, and initial data have been obtained for conducting comparative tests. This confirms not only the fundamental possibility of using ultrasonic action as a means of combating smoke, but the need to study it to identify more effective modes and conditions of this action.

4.3. Results of Smoke Deposition by One Emitter

Using optical sensors installed in the smoke chamber, the relative change (relative to smoke-free air in the room) in visibility at different distances from the emitter was measured.

The results are shown in Figure 14. Data from every second sensor are shown. In Figure 14, the parameter shown by the curves is the distance in meters from the emitter to the optical pair measuring relative visibility.

The recording device obtains readings from the optical sensors at a frequency of 1000 Hz and averages over the last 20 measurements. The personal computer polls the device every 0.5 s for the last averaged value. As a result, the raw data of the experiment are an array of light irradiance measurements taken at a frequency of 2 Hz. A central moving average filter with a width of 10 s is applied to this array to smooth out periodic interference from the ultrasound device. The constant irradiance component is subtracted from the obtained data. Then, the values are normalized by the maximum irradiance value at the beginning of the experiment. The relative error in calculating illumination did not exceed 2.5–3%. The Python programming language with the numpy, scipy and pandas libraries is used for data processing.

Before the experiment, the chamber was filled with smoke to its full height. The moment the ultrasonic action began corresponds to the zero mark on the time axis. It is evident that at the initial moment of time (during the first 50 s) there is a significant increase in visibility in the smoke chamber, up to 0.8 of the visibility in the absence of smoke at a distance of 0.25 m from the emitter. At the same time, at a distance of two meters from the emitter, the relative visibility is no more than 0.6 (the supply of smoke to the smoke chamber did not stop). Visually, this corresponds to the second photograph in Figure 13, where a significant gradient of smoke density is observed as the distance from the emitter located at the upper end of the chamber increases.

This may be due to both the continuous flow of smoke into the chamber through the lower end and the increased attenuation of ultrasonic vibrations in a dispersed medium containing smoke particles. With further ultrasonic exposure, the relative visibility increases and levels out along the height of the chamber, asymptotically approaching 1 near the emitter and reaching 0.9 at a distance of 2 m from the emitter (the flow of smoke into the chamber did not stop). The obtained dependencies allow us to state that ultrasonic exposure not only causes the agglomeration of smoke particles, but also suppresses the flow of new smoke particles from the source into the area filled with sound.

Thus, it was established that the action of ultrasonic vibrations ensures agglomeration and sedimentation of smoke. This leads to an increase in visibility, almost to the state of a smoke-free chamber. However, the required exposure time turns out to be unacceptably long for the practical application of ultrasonic agglomeration in real conditions (for example, for the rapid evacuation of people).

To increase the efficiency of the agglomeration of smoke particles, complex ultrasonic fields and the nonlinear effects initiated by them (rotation of agglomerates, and low-frequency acoustic beats that can enhance the effect of ultrasonic vibrations) were further used.

4.4. Smoke Precipitation in Complex Acoustic Fields

Such acoustic fields can be created using two emitters. The emitters must operate synchronously at one frequency or at two close frequencies to initiate the rotational motion of agglomerates (for an additional increase in the collision cross-section of smoke particles) and the additional formation of acoustic beats (for subsequent agglomeration and final sedimentation of large particles and primary agglomerates of sizes from 1 μm). The smoke flow rate was 50 L/min. Otherwise, the conditions of the experiment were similar to the previous one. The dependencies of the change in relative visibility for each type of ultrasonic action (at one frequency and different frequencies) are shown in Figure 15.

In this figure, the lines represented by the warm shades (curves 1, 2, 3) show the dependencies of relative illumination for two disks operating at the same frequency, and the lines in cold shades (curves 4, 5, 6) represent disks operating at different frequencies. In each experiment, signals from three sensors located at distances of 0.25 m, 1 m and 2 m from the emitters were compared.

As can be seen from the graph (Figure 15), for disks operating at one frequency, a significant gradient of relative visibility is observed depending on the distance from the emitters. Thus, at the initial moment of time (during the first 50 s), a significant increase in visibility is observed in the immediate vicinity of the emitters, up to 0.65 (curve 1). However, visibility deteriorates significantly as the distance from the ultrasonic emitters increases. At a distance of 1 m, the relative visibility does not exceed 0.5, and at 2 m it does not exceed 0.25 (in comparison with single-frequency exposure by two emitters 0.65 and 0.5, and with exposure by one emitter at a distance of 2 m no more than 0.6). Moreover, the resulting gradient of relative visibility remains almost constant throughout the experiment. This indicates that the use of two disk emitters, which provided an increase in SPL by 3 dB (the acoustic power increased by 2 times) does not compensate for a similar increase in the flow rate of smoke entering the chamber. The speed of the ultrasonic agglomeration of smoke particles is not high enough to compensate for the influx of new particles. This is why the minimum relative visibility (maximum smoke) is observed in the area where smoke enters the chamber. In the previous experiment with one emitter, such a visibility gradient was not observed.

Starting from 250 s, the visibility gradient along the height of the smoke chamber begins to decrease. Thus, the relative visibility at a distance of 2 m from the emitter begins to increase sharply. This is due to the burning of sawdust (in 250 s, a 20 g weight of sawdust burns completely) in the smoke generator and a decrease in the density of smoke entering the chamber. The smoke particles remaining in the chamber are gradually displaced in the direction of the emitters and agglomerate under the action of ultrasonic vibrations. This explains the fact that in the middle of the chamber (at a distance of 1 m), the longest time is required to achieve the maximum value of relative visibility.

Thus, the impact of two emitters at one frequency and increased by 3 dB SPL for a smoke supply capacity increased by 2 times does not allow suppressing the smoke supply from the source to the sounded (protected volume). In turn, for a two-frequency impact, significantly different dependencies were obtained. The increase in relative visibility occurs almost simultaneously throughout the entire volume of the smoke chamber. That is, the graphs of relative visibility for different distances from the emitter change almost identically.

This allows us to state that, due to the acoustic effects (rotation of agglomerates, acoustic beats) arising from the interaction of different-frequency ultrasonic vibrations, it is possible to achieve not only a uniform increase in visibility in the smoke chamber, but also suppression of smoke entering the chamber from the source. At the same time, the value of the generated sound pressure and, accordingly, the energy costs for the smoke agglomeration process did not increase.

The calculation of the real visibility S was carried out using the following formulas. Based on Labmert–Beer’s law, the constant K₀ was calculated:

K_{0} = - \frac{1}{L} \ln (T),

(2)

where L—distance between the source and receiver of infrared radiation (in the experimental setup it was 0.35 m); T—relative visibility of smoke particles.

To quantify the distance from the observer to identifiable objects in fire smoke, visibility was often used to describe that [31]. The visibility could be obtained from light transmission:

S = \frac{C}{K_{0}}

(3)

where c—constant characteristic of the type of object being viewed through the smoke; c = 3 foralight-reflectingsign was used in further calculations.

The experimental results are summarized in Table 2 to determine the reduction in time for establishing the maximum value of relative visibility and real visibility.

The results show that a high level of sound pressure is created near the emitter, sufficient for effective agglomeration, both with equal-frequency and with different-frequency ultrasonic exposure.

Thus, multi-frequency exposure allows the agglomeration process (if we consider a distance of 0.25 m) to be accelerated by approximately 1.9 times: from 175 s with an equal frequency to 90 s with multi-frequency exposure. If we consider a distance of 1 m, then the increase is by approximately 3.3 times: from 250 s with an equal frequency to 75 s with different frequencies. If we consider a distance of 2 m, then the increase is approximately 5 times: from 300 s with an equal frequency to 60 s with different frequencies.

Thus, it can be concluded that the multi-frequency impact (at a sound pressure level of no more than 140 dB) increased the probability of the collision of smoke particles and their agglomerates due to the interaction of ultrasonic vibrations of two frequencies with the formation of beats, which contributed to imparting rotational motion to the agglomerates in acoustic fields with different vibrational velocity vectors.

4.5. Visual Analysis of Smoke Particle Agglomerates

To assess the influence of rotational motion and acoustic beats on the shape and size of agglomerates, samples were collected by catching the formed agglomerates at a distance of 2 m from the emitters. The agglomerates were caught on a microscope slide covered with a layer of immersion liquid. A histogram of the distribution of agglomerate sizes (the agglomerate diameter was defined as the diameter of a circle with an area equal to the area of the agglomerate), obtained with equal-frequency and different-frequency ultrasonic exposure, is shown in Figure 16.

The histogram in Figure 16 shows that the sizes of the agglomerates obtained with equal-frequency exposure are shifted toward smaller values compared to multi-frequency exposure. The average diameter of agglomerates is 115 µm. In the case of dual-frequency exposure, the average size of agglomerates is somewhat larger and equals 128 µm. Also, with multi-frequency ultrasonic exposure, agglomerates of sizes greater than 230 µm (up to 290 µm) are formed, which are not observed with equal-frequency ultrasonic exposure. Since the exposure time and sound pressure level were the same for both cases, the larger average size of the agglomerates may be due to the increased frequency of collisions of smoke particles and agglomerates with each other. Figure 17 shows an example of photographs of smoke particle agglomerates obtained using the Lomo Mikmed−6 microscope.

Direct image analysis shows that multi-frequency (Figure 17b) exposure produces larger agglomerates of smoke particles. The agglomerates have a more porous structure, and their shape resembles a regular disk. This porous structure may be due to the fact that the final shape of the agglomerate was obtained by merging smaller agglomerates (which are formed by combining the original smoke particles with ultrasonic frequency oscillations) under the action of beats. In turn, the disk-shaped shape is due to the rotation of the agglomerate around one of the axes. Since the agglomeration time under different-frequency exposure is 1.6 times less than under equal frequency exposure, the agglomerate was exposed to ultrasonic vibrations for a shorter time and did not have time to compact due to the “sticking” of a large number of initial smoke particles.

In turn, the agglomerate (Figure 17a) obtained with equal-frequency action has an elongated shape due to the absence of rotation and the location of the agglomerate along the antinodes of the ultrasonic field. It is also known that, in the case of a polydisperse distribution of the size of suspended particles, the most likely result is the attachment of small particles to larger ones (compared to the probability of combining particles of similar sizes). This is why the agglomerate (Figure 17a) has a denser structure formed by the multiple “sticking” of small smoke particles to the agglomerate nucleus (initially a larger smoke particle). A detailed determination of the morphology of agglomerates depending on the type of ultrasonic action will be the direction of further research.

In general, the obtained results allowed us to establish that the effect of multi-frequency oscillations allows us to reduce the time required for complete removal (agglomeration of particles) of smoke by 2.5 times, compared to the action of one emitter, and by at least 1.6 times compared to the use of two emitters operating at the same frequency. This allows us to recommend this method of action when developing practical methods and designs for emitters for combating smoke during fires.

5. Conclusions

Based on the analysis, it was established that the efficiency of smoke precipitation is increased by using additional physical action on smoke particles with ultrasonic vibrations, which ensure the agglomeration of small particles into agglomerates.

To increase the efficiency of ultrasonic smoke precipitation, it is proposed to use acoustic fields with complex structures, formed by several (at least two) emitters operating at different frequencies, to act on smoke particles.

To form such fields, highly efficient bending–oscillating ultrasonic emitters have been proposed and developed, simultaneously generating ultrasonic (22 kHz) oscillations in the range of a sound pressure level of at least 140 dB (at a distance of 1 m) at two slightly different frequencies (no more than 300 Hz).

The practical implementation of smoke deposition in the experimental setup provided a relative visibility of 0.8 at a distance of 1 m, and 0.9 at 2 m (in comparison with the results for single-frequency exposure with two emitters of 0.65 and 0.5, and with exposure to one emitter at a distance of 2 m which was no more than 0.6).

In this case, complete smoke deposition occurred twice as fast as with ultrasonic action of two equal-frequency emitters.

Two-frequency action for smoke deposition does not require an increase in energy costs and is achieved by combining small particles into agglomerates under the action of high-frequency ultrasonic vibrations and further combining the formed agglomerates when creating conditions for their additional rotational movement of agglomerates by low-frequency vibrations arising from the interaction of ultrasonic vibrations of different frequencies.

The results obtained indicate the effectiveness of two-frequency action for accelerating the process of smoke deposition and can be recommended for practical application.

Author Contributions

Conceptualization, V.K. and A.S.; data curation, P.D. and S.T.; formal analysis, A.S.; Investigation, A.S. and S.T.; methodology, V.K., A.S. and S.T.; validation, S.T.; visualization, S.T. and P.D.; writing—original draft, S.T. and A.S.; writing—review and editing, V.K. and A.S All authors have read and agreed to the published version of the manuscript.

Funding

The study was carried out at the expense of the grant of the Russian Science Foundation No. 24-19-00900, https://rscf.ru/project/24-19-00900/ (accessed on 14 December 2024).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data are contained within the article. The data presented in this study are available in Figure 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, Figure 14, Figure 15 and Figure 16 and Table 1 and Table 2.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Design and simulation results fora disk emitter. (a) Distribution of amplitude; (b) distribution of stress. 1—emitter; 2—emitting pad of the piezoelectric transducer; 3—piezoceramic rings; 4—reflecting pad; 5—tightening bolt; 6—copper electrode; 7—tightening screw.

Figure 2. The manufactured emitter with an electronic generator for supplying its power.

Figure 3. Dual emitter for equal frequency action on smoke.

Figure 4. Emitters for multi-frequency action on smoke.

Figure 5. Stand for measuring the directional pattern of ultrasonic emitters. 1—Ultrasonic disk emitter, 2—electronic generator; 3—emitter stand, 4—microphone; 5—noise meter measuring unit; 6—microphone stand; 7—microphone direction point.

Figure 6. Experimental setup; (a) with one emitter; (b) with two emitters. 1—Ultrasonic disk emitter; 2—electronic generator; 3—smoke chamber; 4—smoke generator; 5—infrared radiation source; 6—photodetector.

Figure 7. Directivity pattern for a single emitter.

Figure 8. Attenuation in relation to distance from the source in a smoke chamber (one emitter).

Figure 9. Directivity pattern of dual disk emitters.Red color—two simultaneously operating disks at the same frequency; blue color—two simultaneously operating disks of different frequencies.

Figure 10. Attenuation over distance in a smoke chamber (two emitters). Blue color—two simultaneously operating disks of different frequencies; red color—two simultaneously operating disks of equal frequencies.

Figure 11. Difference frequency directivity pattern.

Figure 12. Beat frequency attenuation over distance in smoke chamber.

Figure 13. Results of visual observation of ultrasonic smoke agglomeration.

Figure 14. Measurement of relative visibility from the time of ultrasonic exposure for different distances (in m) from the emitter.

Figure 15. Measurement of relative visibility from the time of ultrasonic exposure for different distances (in m) from the emitters.

Figure 16. Histogram of agglomerate size distribution.

Figure 17. Images of smoke particle agglomerates (100×). (a) Single-frequency action; (b) dual-frequency action.

Table 1. Comparative characteristics of disk emitters.

Name	f, Hz	Disk Diameter, mm	Disk Material	Q
Emitter 1	22,785	99.5	Titanium alloy	2850
Emitter 2	22,785	99.5	Titanium alloy	2870
Emitter 3	23,080	99.5	Titanium alloy	2810

Table 2. Relative visibility in relation to time for one- and two-frequency action.

Time, s	Type of Action	Distance from Emitters
		0.25 m		1 m		2 m
		Visibility		Visibility		Visibility
		Relative (T)	Real (S), m	Relative (T)	Real (S), m	Relative (T)	Real (S), m
50	one-frequency	0.65	2.4	0.45	1.3	0.3	0.9
50	two-frequency	0.7	2.7	0.62	2.2	0.45	1.3
300	one-frequency	0.8	4.7	0.65	2.4	0.5	1.5
300	two-frequency	0.9	10.0	0.88	8.2	0.9	10.0

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Khmelev, V.; Shalunov, A.; Tsyganok, S.; Danilov, P. Smoke Precipitation by Exposure to Dual-Frequency Ultrasonic Oscillations. Fire 2024, 7, 476. https://doi.org/10.3390/fire7120476

AMA Style

Khmelev V, Shalunov A, Tsyganok S, Danilov P. Smoke Precipitation by Exposure to Dual-Frequency Ultrasonic Oscillations. Fire. 2024; 7(12):476. https://doi.org/10.3390/fire7120476

Chicago/Turabian Style

Khmelev, Vladimir, Andrey Shalunov, Sergey Tsyganok, and Pavel Danilov. 2024. "Smoke Precipitation by Exposure to Dual-Frequency Ultrasonic Oscillations" Fire 7, no. 12: 476. https://doi.org/10.3390/fire7120476

APA Style

Khmelev, V., Shalunov, A., Tsyganok, S., & Danilov, P. (2024). Smoke Precipitation by Exposure to Dual-Frequency Ultrasonic Oscillations. Fire, 7(12), 476. https://doi.org/10.3390/fire7120476

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