NUMERICAL THREE-DIMENSIONAL MODEL OF ULTRASONIC COAGULATION OF AEROSOL PARTICLES IN VORTEX ACOUSTIC STREAMING

Abstract

Separation of highly dispersed systems with huge liquid-gas or liquid-solid interfaces is relevant for practical tasks of gas purification from the most highly dispersed and difficult-to-detect dispersed fraction PM2.5, and separation of nanoparticles (including their small agglomerates) in fine chemical technology processes. One of the most effective ways to separate highly dispersed systems with a large interface surface is to combine each of the closed subsurfaces (surfaces of individual dispersed particles) under the influence of hydrodynamic effects in the gas phase, arising both near the interface surfaces and at a considerable distance from them, due to the superposition of ultrasonic vibrations. Since the efficiency of ultrasonic coagulation decreases with a large distance between closed subsurfaces from each other in PM2.5 aerosol and the small size of these surfaces, it is necessary to create conditions for the emergence of new nonlinear effects that contribute to the local compaction of the dispersed fraction. In a resonant and significantly inhomogeneous ultrasonic field (with a scale of inhomogeneity on the order of the wavelength), vortex acoustic flows arise, which, due to inertial forces, locally compact the dispersed phase in the form of an increase in the concentration of aerosol particles. A numerical model of ultrasonic coagulation of PM2.5 aerosol particles in three-dimensional (3D) vortex acoustic streaming is proposed in this paper. The model is designed to identify the possibility of increasing the efficiency of ultrasonic coagulation in 3D streaming by virtue of the following mechanisms: (1) local increase in concentration caused by the inertial transfer of particles to the periphery of 3D vortices in the gas phase; (2) increase in the frequency of particle collisions due to 3D turbulent disturbances in ultrasonic fields; and (3) increase in productivity and ensuring uninterrupted implementation of the process in a flow mode owing to transfer of particles between the streamlines of the main vortices initiated by ultrasonic vibrations. The listed mechanisms for increasing the efficiency of coagulation in 3D streaming are taken into consideration by introducing two stream functions, considering turbulent chaotic disturbances of the flow resulting in dispersion of particle velocities. It was possible to establish based on numerical analysis of the model using the example of PM2.5 that laminar vortex flows begin to influence at sound pressure level from 160-165 dB, and turbulent disturbances make an additional contribution in the range of sound pressure levels from 140-170 dB. At the same time, as a result of 3D turbulent disturbances, the efficiency of coagulation reaches almost 100% at a sound pressure level 5 dB lower than with laminar flows (sound pressure amplitude, 3 times lower).