Experimental And Theoretical Studies On Acoustic Agglomeration Of Coal-fired Fly Ash

The suspended particulate matter in the atmosphere not only has bad impact on climate but also causes serious harm to human health. It has caused wide public concern over the recent years. Therefore, numerous countries have promulgated strict air quality standards to control the concentration of particulate matter. However, the conventional particle filtering devices currently used in industry, including electrostatic precipitators and cyclone, is inefficient for fine particles. As a result, a lot of PM2.5 has emitted into the atmosphere.Acoustic agglomeration is an aerosol preconditioning technology used before it enters the particle filtering devices. In the high-intensity acoustic field, aerosol agglomeration occurs in several seconds. As a result, the number concentration reduces and the averaged particle size increases. Then, the treated aerosol can be efficiently removed in the particle filtering.devices.However, the investigation on acoustic agglomeration is still insufficient. The influences of the main operating parameters on agglomeration process, especially the optimum acoustic frequency, are not clear. The research work on the main agglomeration mechanisms, including orthok-inetic interaction mechanism, refilling mechanism and acoustic wake effect, contradict each other. Also, little work have been done on the numerical simulation of acoustic agglomeration. The theoretical models are oversimplified. The numerical methods are inefficient and low-precision.Based on the above analysis, the technology of acoustic agglomeration has been studied both experimentally and theoretically in this paper.Firstly, the influences of the main operating parameters are studied using the coal-fired fly ash experimentally. The sound devices can establish acoustic field at both low and high frequencies. The results show that the effect of sound waves is very sensitive to the frequency change. Sound field at low frequencies were more effective than that at high frequency. There exists an optimum frequency for an aerosol with a given particle size distribution. The effectiveness of agglomeration falls off fairly rapidly below or above the optimum frequencies. A 70% decrease in total number concentration is gained under an SPL of 147 dB and a frequency in the range of 1400～1700 Hz. It is found for the first time that the optimum frequency decreases slightly as SPL increases. In addition, with the increase in SPL, residence time and initial total number concentration, the effectiveness of agglomeration is enhanced.Secondly, some of the agglomeration mechanisms, which are still controversial and not well understood, are studied theoretically in the paper. The results show that the collision efficiency has a great effect in the acoustic agglomeration process. In the literature, collision efficiency was set to unit to simply the calculation. That is unreasonable for PM2.5. In the paper, the collision efficiency. is considered to improve the orthokinetic interaction model.The acoustic wake effect is also studied. The particle trajectories in acoustic fields due to this effect are numerically simulated. It is shown that particles experience strong attraction and converge within several acoustic cycles and eventually collide. The numerical results are in good agreement with the photos of the particle trajectories using Charge-coupled Device video cameras in the literature. The study shows that the acoustic wake effect plays a significant role in acoustic agglomeration. It can be either the major agglomeration mechanism of monodisperse aerosols or the major refill mechanism for polydisperse aerosols to supplement the orthokinetic interaction.Then, the numerical model of acoustic agglomeration is developed based on the theoretical studies in this paper. A modified sectional algorithm is used to solve the equations numerically. In order to verify the numerical simulation, the computational results are compared with the experiments data. It is shown that the computational results agree with the experiments data well. The influences of the main operating parameters on the acoustic agglomeration are studied numerically, which agree with the experimental ones.At last, the new developed quadrature method of moments (QMOM) is applied to the numerical simulation of acoustic agglomeration, since the sectional algorithm is not accurate and fast enough. The results show that QMOM is more accurate and faster than sectional algorithm. Therefore, QMOM is potential for the numerical simulation of acoustic agglomeration.