Investigation Into Agglomeration Processs Of Inhalable Particle In Coupling Acoustic Wave And Jet

Particles smaller than 10μm were defined as inhalable particles, which could penetrate deeply into the respiratory system. These particles cause serious healthy hazards due to being captured difficultyly by conventional filters such as electrostatic precipitators and cyclone separators. Inhalable particles were characterized by great adhesive forces due to very small size in diameter. External forces can accelerate the particles to collide and agglomerate together so that a significant shift of particle size distribution from small to large sizes. Large groups were convenient to be captured by conventional filters. Agglomeration process was an efficient preconditioning method for inhalble particles, which was easy in operation with low cost.FLUENT6.3, a CFD simulation software, was used to simulate the particle tracks of in flow field in order to evaluate the probability of collision. A macro-agglomeration setup was used for study the removal efficiency of inhalable particles in acoustic field, turbulent jet and the coupling effes of sound field and turbulent jet. The aerosol dynamics equation was solved by sectional methods to calculate the size distribution of inhalable particles during the agglomeration process.The discrete phase model in FLUENT6.3 was employed to calculate the tracks in flow field. Modified momentum equation containing acoustic field was first developed. Inhalable particles suspended in a gas flow were distributed honuniformly and even concentrated in local area due to the entrainment of acoustic field. The velocities of small particles were higher than those of large ones. For turbulent agglomeration process, inhalable particles could quickly acquire energy from turbulent jet to improve the particles velocity when particle moved to the core space of turbulent jet. The energy exchange between the fluid and particle varied with the particle size, which makes small particles have higher velocities.Each particle was entrained by the acoustic field in the propagation direction of the applied wave, and this entrainment served as the direct driving force leading to collision between the particles. Acoustic agglomeration showed that the maximum removal efficiency reached at the frequency of 1416 Hz. And the agglomeration of inhalable particles increased with increasing sound pressure level. The critic residence time of particles ranged from 9.0 s to11.0 s in acoustic field at 40-50% relative humidity. For inhalable particles, the small particles (<1μm) and large ones (5-10μm) were easy to remove, but mid-size particles (1-5μm) were difficult to be captured.A turbulent gas jet was introduced into the particle agglomeration chamber to generate the local turbulent field. Experimental findings indicated the mass and number removal efficiency increased with increasing Reynolds number in jet exit and velocity ratio of jet-to-crossflow. The gas jet at the injection angle of 90o could achieve high removal efficiency. Gas jet at the obtuse angle injection (120o or 140 o) favors the removal of mid-size particles compared with ones at the acute inclined injection (40o or 60o).The coupling effects of gas jet and acoustic field were employed to intensify the agglomeration process of inhalable particles. During the acoustic agglomeration and the coupling agglomeration process, the removal efficiency increased with an increase in sound pressure level, and reached to the maximum value at the frequency of 1416 Hz. During the jet agglomeration and the coupling agglomeration, the removal efficiency increased with increasing velocity ratio of jet-crossflow. The optimal agglomeraton was observed while gas jet was ejected vertical into acoustic agglomeration chamber. For jet agglomeration, increasing the Reynolds number in jet exit could enhance the removal of particles, whereas high Reynolds number in jet exit would decrease the removal value for the coupling agglomeration process.In agglomeration process the increased initial concentration of fly ash results in decreasing of removal efficiency of inhalable particles. When relative humidity in chamber approached 40-50%, the maximum removal efficiency was acquired, and higher or lower humidity could reduce the efficiency. After agglomeration, the mass mean median diameter of the aggregate group increased from 2-3μm to 5-7μm.Aerosol dynamics equation was solved by sectional method to simulate the evolution of particles number concentration. Classical orthokinetic coefficient and classical turbulent coefficient were choosen to calculate the agglomeration processes. The numerical removal efficiency in acoustic agglomeration or turbulent jet agglomeration process agreed well with experimental results and they fit well. In coupling agglomeration process, a linear combination of the acoustic kernel and turbulent kernel was used in aerosol dynamics equation. The simulation particle number concentration was slightly greater than the experimental ones for submicron particles, but they fit well for micron particles.