Acoustic Agglomeration Research Articles

Research on enhanced mass transfer using audible acoustic agglomeration technology

Currently, tail flues of coal-fired power plants are treated using wet desulfurization process to remove pollutants. But, the treated flue gas has a significant moisture loss and this process also contributes to environmental pollution. Acoustic agglomeration is a non-contact treatment process that can effectively solve the problem of moisture loss in flue gas; however, there are few theoretical and experimental studies on acoustic droplet agglomeration applied to coal-fired power generation. In this paper, an experimental platform for acoustic droplet agglomeration is demonstrated to recover moisture. Further, the effects of important factors such as acoustic frequency, sound pressure level and droplet flow rate on the agglomeration efficiency are investigated. The experimental results show that there are two optimal frequencies exist, namely 500 Hz and 1200 Hz, for the droplets used in this experiment with acoustic droplet agglomeration. A highest efficiency of 58% is achieved at 500 Hz. The acoustic action differs between the 500 Hz and 1200 Hz cases. The analysis suggests that the primary force driving droplet agglomeration at 500 Hz is likely the acoustic flow effect. On the other hand, the main mechanism behind acoustic agglomeration at 1200 Hz could be related to the acoustic wake effect. Sound pressure level is the primary factor affecting droplet agglomeration, and the agglomeration efficiency increases with sound pressure, reaching 96.7% at 150 dB. Furthermore, the agglomeration effect shows a positive correlation with droplet flow rate, while the agglomerated particle size also increases simultaneously. This research provides a new idea for moisture recovery, which could offer valuable insights for implementing acoustic droplet agglomeration and moisture recovery in power plants.

An improved DSMC method for acoustic agglomeration of solid particles assisted by spray droplets

The direct simulation Monte Carlo (DSMC) method was improved for describing the process of acoustic agglomeration assisted by spray droplets. The agglomeration and rebound as consequences of inter-particle collisions were modeled by considering all possible particle types, in particular, mixed-phase particles associated with the immersion and distribution mechanisms. Numerical predictions by the present method were validated by both analytical solutions and experimental data. The dynamic process of the acoustic agglomeration in terms of the size and type evolution as well as the evolution of number concentration of different particle types were examined. Results suggest that the strong interaction between the solid particles and the spray droplets causes the significant enhancement of acoustic agglomeration. Furthermore, the effect of frequency varying from audible to ultrasonic range on the acoustic agglomeration was evaluated. Overall, a better performance of acoustic agglomeration can be achieved at a lower frequency, regardless of addition of spray droplets. Agglomeration efficiency of 64.8 % can be achieve at acoustic frequency of 1000 Hz in case with spray droplets. This study provides not only a numerical model for describing the agglomeration of complex particles in particle-laden gas flows, but also important insights into the acoustic agglomeration assisted by spray droplets.

Ultrasonic aerosol agglomeration: Manipulation of particle deposition and its impact on air filter pressure drop

Acoustic agglomeration is a technique that leverages on sound waves to promote the collision of aerosol particulate matter, thus leading to the formation of larger particle agglomerates. In this study, this acoustics-driven phenomenon is demonstrated for its usefulness as an aerosol pre-conditioning method to significantly enhance the efficiency of filtration systems in particle treatment processes. Specifically, favorable changes in pressure drop across the filters are observed as a result of receiving less particle mass, for which filters are shown to be able to have their operational life extended remarkably by more than 50%. The involved ultrasonic aerosol agglomeration mechanisms are unveiled through numerical simulations, and the effects of residence time, sound pressure level, and initial particle number concentration on agglomeration performances are experimentally investigated. In addition, validations and measurements of filter pressure drop are obtained through a series of experiments. This study provides a comprehensive overview to the design and performance characterization of acoustics-agglomeration-enhanced filtration systems, which could potentially derive energy savings for fan power in ventilation systems and be scaled up for applications in industrial plants for reducing carbon emissions.

Experimental study on the removal of submicron droplets by fibrous filter media in a sound field

The acoustic agglomeration technology is proposed as pretreatment to effectively reduce the concentration of particles produced in the industrial process, thus to improve the filtration efficiency of subsequent filter devices. However, the removal of submicron droplets by sound waves has not yet received attention. In this paper, a new method is proposed for high-efficient filtration of submicron droplets based on sound waves. According to the entrainment mechanism of sound waves on the submicron droplets, a sound field is introduced in the filtration system to enhance the collision probability between the droplets and the fibers in filter materials. The effects of sound pressure level (SPL) and sound frequency on the droplet removal were experimentally investigated at different filtration gas velocities. The results show that there is no shift for the upstream particle size distribution, however, the downstream droplet concentration decreases when the sound field is introduced. The effect of the sound field on filtration performance is determined by the vibration amplitude and the vibration period of the droplets. In the SPL range of 130–145 dB, the improvement of the filtration performance is better with the increase of the SPL for a fixed sound frequency. In the frequency range of 500–900 Hz, the optimal filtration performance of the filter material is obtained at 700 Hz. When the filtration gas velocity is between 0.2 and 0.4 m/s, the improvement of the filtration performance gradually weakens as the filtration velocity increases. Furthermore, the reduction of the downstream droplet concentration is more significant for low-efficient filters or those with fewer layers under the same condition in the sound field.

Agglomeration of oil droplets assisted by low-frequency sonic pretreatment

The metalworking process in industrial buildings produces small droplets of oil. The efficient purification of small oil droplets can protect the built environment and human health. Purifier performance is generally related to particle size, and increasing the particle size by pretreatment significantly improves purification efficiency. Acoustic agglomeration is a widely recognized technique for increasing dust particle size. In this study, the agglomeration efficiency (decrease in the number concentration) of oil droplets with a particle size of <5 μm under a sound wave was investigated. The operating parameters studied included sound wave frequency (1000–4000 Hz), waveform (sine, square, and triangular waves), sound pressure level (130–145 dB), and oil droplet residence time (2.5–4 s). The results show that the agglomeration efficiency peaks with frequency and increases linearly with the sound pressure level, first increasing sharply and then leveling off with residence time. The following were the optimum parameters:1800 Hz sine wave, 145 dB, and residence time of 3 s. Under these operating conditions, the agglomeration efficiency is 9.9%. The enhancement of acoustic agglomeration on purification by a wire mesh filter was verified, and it was shown that the purification efficiency of a wire mesh filter for 0.3–5 μm oil droplets increased by over 20%.

An efficient three-dimensional numerical simulation of particle acoustic agglomeration with fine-grained parallelization on graphical processing unit

To overcome the challenge of large particle number concentration in the simulation of realistic particle acoustic agglomeration process, the implementation and validation of an efficient three-dimensional simulation method with a fine-grained Graphical Processing Unit (GPU) based parallel strategy is proposed. In detail, the motion of the simulated particles is solved with the Discrete Element Method (DEM) that includes three major particle acoustic agglomeration mechanisms, two particle collision processes and varying agglomerate porosity. Under the framework of the spatial decomposition method, the fine-grained parallel algorithm allocates the computation workload of each simulated particle one-by-one to one GPU thread. Speed test shows that the developed algorithm could achieve relatively elevated efficiencies and high speedup ratios. For method validation, the predicted particle agglomeration rate is compared with experimental measurements in the literature. The agreement of the results demonstrates that the developed method could reproduce realistic particle agglomeration rate as in the experiment.

Theoretical analysis of acoustic and turbulent agglomeration of droplet aerosols

This study meticulously explores the agglomeration mechanisms in microscale droplet aerosols, specifically focusing on acoustic and turbulent agglomeration mechanisms. Our theoretical analysis reveals a significant impact of orthokinetic and hydrodynamic processes on acoustic agglomeration. The acoustic wake effect elucidates the swift replenishment of small particles subsequent to an orthokinetic phase. An optimal frequency, varying for different droplets, was identified in orthokinetic agglomeration within the 50–250 Hz range. Hydrodynamic agglomeration remained relatively stable at an acoustic frequency exceeding 1000 Hz. The aggregation kernel function, denoted as Kij, exhibited a significant increase with increasing sound pressure levels, reaching up to 10−8 s−1. Environmental temperature had a predominantly positive effect on orthokinetic and Brownian agglomeration, although it exhibited an inhibitory effect on hydrodynamic agglomeration. For raindrops, a correlation was identified between particle spacing and Kij; a larger particle spacing corresponded to a smaller Kij. Despite an increase in particle spacing to 50 times the particle diameter, the hydrodynamic effect persisted. The aggregation kernel function linked to Brownian thermal motion was found to be 3–4 orders of magnitude lower than that of orthokinetic and hydrodynamic interactions. Additionally, the turbulent agglomeration kernel function for fog, cloud, and rain droplets with corresponding parent nuclei of 100 μm was of the same order of magnitude as the acoustic agglomeration kernel function.

Experimental Study on the Agglomeration of Oily Fine Particles by Sound Wave

Oily fine particles are an important air pollutant in industrial environments. Workers exposed to oil mist for a long time face great health risks. Particle growth pretreatment is a technical principle to increase particle size and improve purification efficiency. Acoustic waves are commonly used to acheive particle growth, and a large number of acoustic wave agglomeration experiments have been carried out on non-oil fog. However, studies on oily particles are few. On the basis of previous studies on acoustic agglomeration of non-oily particles, this experiment designed a set of experimental equipment to compare the agglomeration effect of oily and non-oily particles. It was found that the agglomeration effect ratio of oily and non-oily particles to φ1oiliness/φ1non-oily particles was greater than 1. Therefore, the agglomeration effect of oily particles under stationary acoustic waves was more obvious. Results clearly show that oily particles have a higher agglomeration ability. In this study, a traditional ventilation and purification technology was expanded to include sound agglomeration technology into the pretreatment stage of purification and dust removal, thereby demonstrating feasibility of improved purification efficiency of an oily fine particle purification system, and laying a foundation for engineering applications.

Investigation of the acoustic agglomeration on ultrafine particles chamber built into the exhaust system of an internal combustion engine from renewable fuel mixture and diesel

Reducing the pollution of internal combustion engines is a very important problem that can be solved in various ways. However, the acoustic agglomeration method is not used in diesel engines. The study used a 1.9 TDI diesel internal combustion engine supplied with a mixture of diesel (D100) and a 90% of rapeseed methyl ester - 10% propanol fuel mixture (ROMEP). The study also changed the position of the exhaust gas recirculation (EGR) valve by adjusting the 20% EGR throughput limits and maintaining a constant engine load of 90 Nm. It should be noted that the use of biofuels produces less particulate matter, which reinforces the relevance of this study. Measurements were performed using Measurement System: The Testo 380 fine particle analyzer system was used to determine the mass concentration, and a six-channel Fluke 985 particle counter with an isokinetic sampling probe was used to determine the fractional numerical concentration of the particulates. Six particle size distribution regimes in the size range of 0.3 to 10 μm were observed, controlling the transmittance of the EGR system by 20%. The direction of the sound pressure throughout the flow and the excitation frequency 21400 Hz and 33800 Hz were also investigated and compared with the results without agglomeration. The article examines the possibility of using the developed acoustic chamber in the exhaust systems of various objects that uses diesel or various alternative fuel mixtures as fuel. The acoustic field reduces the number of particles by up to 92.5% for 10 μm and up to 44.5% for 0.3 μm at an excitation frequency of 21400 Hz.

Study on aerosol agglomeration using the airborne ultrasonic transducer

Acoustic agglomeration technology use high-intensity acoustic field to make aerosol particles collide and condense rapidly. Existing studies have shown that 70%–90% of fine particles can be eliminated within minutes using compression drives and air-jet generators. Currently, there are limitations to the sound sources used. In this paper, an airborne ultrasonic transducer with a resonant frequency of 15 kHz is designed, followed by the corresponding numerical simulation and experiments for the evaluation of the vibration modal and sound pressure field. The sound pressure levels (SPL) of the open space and the agglomeration chamber can reach 150 dB and 156 dB, respectively. The agglomeration effect of water droplets, liquid phase smoke, solid phase smoke and mixed smoke is experimentally investigated, and the light transmittance rapidly increases from 8% to 60% within 4 s, 8 s, 5 s and 6 s, respectively. Agglomeration is also effective in the high-frequency range, and we infer that the acoustic wake effect is the predominant mechanism. The elimination effect is promoted with the increasing of SPL until the corresponding secondary acoustic effect is enhanced. Moreover, the agglomeration rate of higher concentration aerosol is significantly better than that of diluted aerosols in ultrasonic agglomeration process.

In Situ Experimental Study of Cloud-Precipitation Interference by Low-Frequency Acoustic Waves

Since acoustic agglomeration is an effective pre-treatment technique for removing fine particles, it can be considered as a potential technology for applications in aerosol pollution control, industrial dust and mist removal, and cloud and precipitation interference. In this study, the cloud-precipitation interference effect was evaluated in situ based on a multi-dimensional multi-scale monitoring system. The variations in the spatial and temporal distribution of rainfall near the surface and the characteristics of precipitation droplets in the air were investigated. The results indicate that strong low-frequency acoustic waves had a significant impact on the macro-characteristics of rainfall clouds, the microphysical structure of rain droplets and near-surface precipitation, and various microwave parameters. In terms of physical structure, the precipitation cloud’s base height decreased significantly upon opening the acoustic device, while agglomeration and de-agglomeration of raindrops were in a dynamic equilibrium. When the sound generator was on, the particle concentration at a sampling attitude of 500−1700 m and the proportion of particles with diameters of 1–1.5 mm decreased significantly (by 1–5 ln [1/m3·mm]). In contrast, the particle concentration increased by 1–3 ln [1/m3·mm] at a sampling attitude below 400 m. Moreover, during acoustic interference, the reflectivity factor surged by 2.71 dBZ within 1200 m of the operation centre. Overall, the spatial and temporal distributions of rainfall rates and cumulative precipitation within 5 km of acoustic operation were uneven and influenced by local terrain and background winds.

Acoustic agglomeration characteristics of fine solid particles under effect of additional droplets

Agglomeration of fine solid particles under the excitation of external acoustic field has potential applications in the field of ultra-low emission of combustion pollutants. It is expected that the performance of particle agglomeration can be improved by adding large sized liquid droplets. According to the dynamic process of acoustic agglomeration, including the particle motion, collision, agglomeration and rebound, a model of acoustic agglomeration for gas-liquid-solid three phase system with coexistence of liquid droplets and solid particles in gas phase is developed by using the direct simulation Monte Carlo (DSMC) method. Using this model, numerical simulations are performed to investigate the process and performance of acoustic agglomeration of fine solid particles under the effect of additional droplets. The numerical results are compared with experimental data, and the proposed model is validated. On this basis, the dynamic behaviors of acoustic agglomeration of fine particles in the case with additional droplets are explored. Furthermore, the influences of the diameter and number concentration of additional droplets on the performance of acoustic agglomeration of fine particles are examined. The results show that rapid agglomeration among the solid fine particles and additional droplets can be achieved by adding droplets into the acoustic field, yielding large sized liquid-solid mixed-phase particles. In this process, the agglomeration efficiency of fine particles increases significantly. It is also found that the diameter and number concentration of additional droplets are important factors that affect the acoustic agglomeration of fine particles. The agglomeration efficiency of fine particles rises, while the magnitude of increase tends to decrease with the droplet diameter and number concentration increasing. The research results can provide both theoretical basis for modeling the agglomeration process of complex particle systems and method guidance for achieving the ultra-low emission of fine particles from combustion sources.

Flow and sound field characteristics of a Hartmann whistle with flow-sound-separation feature

Acoustic agglomeration technology can arouse relative motion and rapid agglomeration for aerosol particles. Traditional electroacoustic transducers are commonly used in laboratories to produce high-intensity sound, but they are fragile and have low conversion efficiency. Hartmann whistle is a kind of air-jet generator with the characteristics of simple structure, absence of moving parts and high sound power. However, as a Hartmann whistle works, the spent airflow would be produced at the exit of the horn along with the sound field, which will push the aerosol particles away from the sound source and reduce the agglomeration efficiency. To solve the problem, this paper proposes a new Hartmann whistle with flow-sound-separation feature for the first time, and the influence of opening ratio (X/L) and inlet air pressure is studied by numerical simulation. Results show that when the opening ratio is 0.5, the air flow rate at the exit of the horn is reduced to 0 kg/h, which means that all of the spent air exhausts from the side opening. Meanwhile, the sound efficiency is decreased by 19 %. The air flow rate of the side opening increases with the increasing opening ratio or inlet air pressure. As the opening ratio exceeds 0.5, there will be backflow at the horn. The total sound power of the whistle maintains constant with the variation of opening ratio, and reaches the maximum value of 39 W at 0.275 MPa.

Experimental study of acoustic agglomeration coupled with water droplets on eliminating cable fire smoke

Acoustic agglomeration to eliminate fire smoke is one of the most promising technologies at this stage, and we effectively solve technical stumbling blocks such as narrow acoustic frequency range, sound pressure level injury, and long agglomeration time using acoustic waves coupled with water droplets. According to the mechanism and experimental research, acoustic waves coupled with water droplets technology can broaden the optimal agglomeration frequency range, reduce the influence of acoustic power, and effectively improve agglomeration efficiency. Water droplets with a concentration of 40 g/m 3 were added at the acoustic frequency of 1.5 kHz and the acoustic power of 10 W to achieve the safe escape threshold for personnel within 30 s of working while reducing the damage to the human ear caused by the high sound pressure level. • A new method to eliminate fire smoke is proposed. • The addition of water droplets can expand the range of agglomeration frequencies. • 50% reduction in required agglomeration time. • The addition of water droplets can reduce the acoustic power input. • Reduces damage to the human ear from high sound pressure levels.

Investigation of critical response characteristics of micro-droplets under the action of low-frequency acoustic waves

For the droplets with different size distribution, reasonably selecting the frequency and period of acoustic waves are of great significance to acoustic agglomeration. To investigate critical responses of microdroplets under the action of low-frequency acoustic waves, laboratory experiments and numerical simulations of acoustic interference were conducted, and statistical test and theoretical analysis were carried out. A total of 1,680 sets of experiments were performed, from which about 300,000 particle size samples were collected, with sound frequency of 30–280 Hz and the sound pressure level (SPL) of 70–130 dB. Droplet size distribution (DSD), equilibrium response time (ERT), the nodal plane in the air chamber and entrainment coefficient were analyzed. The critical SPL of acoustic agglomeration was 110 ± 15 dB based on average droplet size increment, and the variation of droplet size indicated that the ERT of acoustic intervention on microdroplets under the critical SPL was 44 ± 12 s. In addition, lower sound frequencies corresponded to larger widths of droplet size with significant response (DSSR), which were jointly affected by sound pressure gradient (SPG), the entrainment coefficient and the droplet concentration. For microdroplets with unknown particle size distribution, acoustic intervention with variable frequencies is suggested for fog elimination and precipitation enhancement.

Interaction between Strong Sound Waves and Aerosol Droplets: Numerical Simulation

In this study, we attempted to eliminate atmospheric fog and aerosol particles by strong sound waves. The action of sound waves created an air disturbance, and the oscillation of the local air caused the micron-sized aerosol droplet particles to move. To provide guidance of the characteristics of the effective sound waves, this study numerically simulated aerosol droplet agglomeration under the action of sound waves, which was solved by coupling computational fluid dynamics (CFD) and discrete element methods (DEMs) as a typical two-phase flow problem in this study. The movements of aerosol droplet particles were simulated, as well as their agglomeration. The evolution process of the average particle size and the number of multimers were obtained, and the influence of different sound frequencies, sound pressure level (SPL), and particle spacing on agglomeration were studied. It was found that the promotion effect of low-frequency sound waves on aerosol droplet agglomeration was significantly higher than that of high-frequency sound waves, and the sound wave promotion effect of high SPLs was better than that of low SPL. In addition, the concept of the average agglomeration time required to quantify the acoustic agglomeration speed was proposed, and it was found to be positively correlated with sound frequency and particle spacing, while being negatively correlated with SPL.

Influence of large seed particle on acoustic particle interaction dynamics: A numerical study

Acoustic agglomeration as a promising preconditioning process for fine particle removal has been proven to be inefficient for submicron particles. In order to enhance the performance of acoustic agglomeration, large seed particles have been introduced into the flue gas. To understand the influence of seed particles on acoustic agglomeration, a nondimensional model of particle interaction was developed by taking into account all important particle interaction mechanisms. The simulated particle trajectories were validated against experimental data. The interaction dynamics of a seed particle and its neighboring fine particles was examined, and the dimensionless first collision time under different conditions was obtained. The results suggest that the enhancement of acoustic agglomeration with seed particle is mainly due to the acoustic wake effect. The overall collision kernel for the acoustic agglomeration was referred from the collision time. The results reveal that the dimensionless collision kernel almost increases linearly with the Stokes number of the seed particle, whereas its dependence on the Schmidt number of the fine particle varies significantly. The collision kernel for acoustic agglomeration was further compared with traditional collision kernels based on differential sedimentation, Brownian diffusion, laminar shear and turbulent shear. The results demonstrate that acoustic field indeed remarkably enhances the collision rate.

An adaptable direct simulation Monte Carlo method for simulating acoustic agglomeration of solid particles

An adaptable direct simulation Monte Carlo (DSMC) method that settles the challenge of coexistence of agglomeration and rebound as a consequence of inter-particle collisions was developed to simulate acoustic agglomeration of solid particles. The method was validated against experimental data under a range of operational conditions. Based on this, the evolution of particle size distribution, agglomeration efficiency and agglomeration behavior with time was numerically investigated. The effect of restitution coefficient on the performance of acoustic agglomeration was also examined. The results show that the occurrence of acoustic agglomeration yields a significant decrease in the number concentration of small particles and a small increase in the number concentration of large particles. Moreover, a worse performance of acoustic agglomeration at a higher restitution coefficient is observed. This study demonstrated the power of the proposed method in simulating the acoustic agglomeration and provided a fundamental approach for investigating the dispersed gas-solid two-phase flows.

Microphysical Characteristics and Environmental Isotope Effects of the Micro-Droplet Groups under the Action of Acoustic Waves

Acoustics can cause particles/droplets to agglomerate in the air medium, thereby accelerating gravity sedimentation. To assess the microphysical characteristics and environmental isotope effects of micro-droplet groups under the action of acoustic waves, an air chamber experimental platform was established, and 100 groups of controlled experiments were conducted. The characteristic particle size, size spectrum, isotope values, corresponding linear relationships with hydrogen and oxygen, and d values were analyzed. The isotope exchange equation between the micro-droplet groups and environmental water vapor inside the air chamber was investigated. The results showed that the peak size values of the micro-droplet groups increased under the action of acoustic waves. The characteristic particle size (D90) showed a “trigger effect” with the acoustic operation with a positive deviation in the size spectrum and isotope exchange between the micro-droplet groups and environmental water vapor. The relative variations in theoretical values for different sedimentation conditions were consistent with those of the experimental results. Environment isotopes could be used to trace the acoustic agglomeration process of micro-droplets in the future.

Fast elimination of cable fire smoke in underground tunnels using acoustic agglomeration technology

Reduction in light transmittance and visibility due to the obscuration of fine smoke particles has been regarded as the most critical threat to fire life safety. Therefore, it is essential for smoke control in underground spaces where electric cables are installed, especially for cable tunnels, underground utility tunnels, subway tunnels, etc. This paper presents a new method of fast elimination of cable fire smoke in underground tunnels using acoustic agglomeration technology. First, the smoke production characteristics of typical electric cables are studied. It is found that the light transmittance drops to 20% after burning for about 60 s. Comparing to vertical burning, more smoke is produced when the cables are burned horizontally. Then, the elimination effect of cable fire smoke is investigated experimentally. Typically, the light transmittance is rapidly increased from 10% to 60% in 11 s, and then reaching the threshold of safe escape in fire situation in only 60 s, at an optimal frequency of 1.5 kHz and an acoustic power of 12 W. In the acoustic agglomeration process, acoustic frequency significantly influences the time required for a light transmittance to reach a given value. The elimination effect is promoted with the acoustic power until the corresponding secondary acoustic effects become significant.