# Influence of Acoustic Streams on the Efficiency of Ultrasonic Particle Agglomeration
## 1 Introduction
For fine particles smaller than 2.5 \(\upmu\)m, the drag coefficient modulus is close to 1, i.e., all particles oscillate almost in-phase with approximately equal amplitude, regardless of the sound pressure value. Accordingly, the orthokinetic interaction is weakly realized particle [18]. In addition, for particles to aggregate, the distance between them should not exceed the displacement amplitude of gas molecules in the sound field. For these reasons, the low probability of particle collision, especially at low concentrations (large distances between particles) leads to the fact that, even at the maximum permissible sound pressure level (before the destruction of the formed agglomerates), increasing the efficiency of agglomeration of particles smaller than 2.5 \(\upmu\)m due to ultrasonic vibrations becomes virtually impossible. For example, in [19] it is shown that ultrasonic coagulation studies were carried out under conditions of high mass concentration of particles in the range from 0.2 to 5 g/m\({}^{3}\). Such conditions are typical during the production of any product and its capture in the form of highly dispersed particles. At the same time, due to ultrasonic exposure, the cleaning efficiency increased from 30-50% to only 40-85%. However, research results indicate that at relatively low concentrations (less than 0.2 g/m\({}^{3}\), which is typical for industrial emissions, for example at thermal power plants), the efficiency of agglomeration will be very low, the degree of agglomeration will not exceed 2-3. Various agglomeration systems utilizing standing wave aerosol impacts are proposed to address this problem. It is known that [PERSON] et al. developed a semi-industrial pilot-scale system combining an electrostatic precipitator and an acoustic agglomeration chamber to remove submicron particles (average particle size 0.8 \(\upmu\)m) from a flue gas flow [20]. In this system, four ultrasonic transducers were arranged at the bottom of the agglomeration chamber, and the standing wave sound field was generated perpendicular to the gas flow to ensure agglomeration. The disadvantage of this design is low efficiency at low concentration. This is due to the fact that during generation of the standing wave the particles almost do not interact with each other even within the node areas. [PERSON] et al. developed several types of airborne ultrasonic transducers that generate powerful sound waves in the air [21; 22]. Among them is an ultrasonic radiator in which a rigid wall was attached around the circumference of a circular transversely vibrating plate. This ultrasonic radiator shapes an intense standing wave sound field into a cylindrical sealed space perpendicular to the plane of the circular transversely vibrating plate. Therefore, the fine particles can be retained in enclosed space in which the powerful sound field is formed. Using this sound wave agglomeration equipment, the effect of fine particle concentration on agglomeration was studied, and the agglomeration effect was higher at higher particle concentrations. However, ultrasonic wave agglomeration for PM 2.5 achieved an agglomeration efficiency of no more than 63%. This is due to large distances between particles, at which the ultrasonic interaction processes do not take place. This determines the urgent need to find new ways to further increase the efficiency of ultrasonic agglomeration of particles smaller than 2.5 \(\upmu\)m.  
Understanding the actual mechanisms of ultrasonic agglomeration suggests that, if it is impossible to further increase the sound pressure level (the value of direct impact on the particles), the probability of collision of small particles can be increased by giving them additional movements that contribute to their collision and aggregation. In [19; 23], it was proposed to increase the efficiency of agglomeration by swirling the flow with dispersed particles at the entrance to the agglomeration chamber.