|Title:||An investigation on the coagulation and deposition of combustion particles in an enclosed chamber|
|Subject:||Hong Kong Polytechnic University -- Dissertations.|
Combustion -- Mathematical models.
Aerosols -- Measurement.
|Department:||Department of Mechanical Engineering|
|Pages:||xxxvi, 338 p. : ill. ; 30 cm.|
|Abstract:||The temporal development of size distributions of diesel exhaust particles and fly ash particles in an enclosed chamber of 1.6 m3 capacity was investigated experimentally and analytically under still conditions (no air movement), stirred conditions (stirred with a fan operating at 2,800 rpm) and ventilated conditions (ventilation rates from 0.448*10-3 s-1 - 4.20*10-3 s-1). Particles were introduced into the chamber, where concentration, mean particle size and distribution shape over time was monitored. Number concentration was measured with a Scanning Mobility Particle Analyzer and a Dustmonitor. Mass concentration was monitored with a Tapered Element Oscillating Microbalance. The generated aerosol concentration was sufficiently high to allow simultaneous coagulation and deposition. The development of size distributions under still conditions was compared with that under stirred conditions and that during ventilation. The particles below 0.05 um in diameter dropped rapidly below background level under still and stirred conditions, indicating the high susceptibility of the small size range to diffusion controlled processes. Coarse particles larger than 0.5 um did not decrease in concentration below background level under still conditions. Under stirred conditions their concentration dropped considerably faster than under still conditions and reached levels below background concentration. Experimental results showed that stirring and ventilation enhanced the surface deposition. Heavy particles larger than 0.4 um in diameter showed an enhanced decay. Coagulation coefficients and deposition velocities regarding mass and number concentration decay were determined from the experimental data by means of nonlinear regression. Determined average coagulation coefficients were in the range of 1.27*10-15 - 2.23*10-15 m3 s-1. No significant difference was found among still, stirred and ventilated conditions. Deposition velocities for paper ash particles and diesel exhaust particles derived from number concentration were 2.07*10-5 and 3.35*10-5 m s-1 under still conditions, 12.7*10-5 and 13.7*10-5 m s-1 under stirred conditions. Ventilated conditions yielded 10.3*10-5 m s-1 for paper ash particles. The mass based deposition velocity of paper ash particles was 3.67*10-5 m s-1 under still conditions, 36.4*10-5 m s-1 under stirred conditions and 37.7*10-5 m s-1 under ventilated conditions. The number concentration based deposition velocities were significantly different among still, stirred and ventilated conditions. Also, the two types of particles were not significantly different in terms of coagulation coefficient or deposition velocity. Experimentally determined coagulation coefficients for a size distribution were higher than Brownian coagulation coefficients, which was attributed to electrostatic charges on the particles, causing mutual attraction by opposite charges. The analytical study combined two existing models, one for deposition and the other for coagulation, and established an algorithm to simulate the two processes simultaneously. Two different diameter definitions were used to calculate deposition and coagulation; the hydrodynamic diameter and the geometric diameter respectively. The analytical investigation showed very good agreement of simulated values with experimental data on the temporal change of concentration and geometric mean diameter of size distributions under still conditions and stirred conditions. The agreement was within 15 % for the temporal change of total concentration, within 5 % for the geometric mean diameter and within 10 % for the change of the lognormal distributions under both still and stirred conditions. The influence of the mass fractal dimension, electrostatic particle charge and turbulence level on concentration and particle size was elaborated. The simulations confirmed that the assumption of diffusion alone is not sufficient to account for the coagulation and deposition, and that the effect of particle charge must be taken into consideration.|
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