CHAPTER 4 RESULTS AND DISCUSSION
4.3 Numerical results based on Eulerian method for the nanoparticle collection
4.3.3 Comparing the particle collection efficiency in the present wet ESP
Figure 4.19 shows comparison of the particle collection efficiency between numerical values and experimental data for 10 and 50 nm NaCl particles in the present single-stage wire-in-plate wet ESP. As the figure shows, when Fuchs model is adopted to calculate the particle charge, the simulated collection efficiencies at the applied voltage of +3.6~+4.3 kV agree with the experimental data with deviation of 0.10-10.8 % and 4.50-14.1 % for 10 and 50 nm particles, respectively. For 10 nm particles, when the model of Marlow and Brock (1975) is used to calculate the particle charge, reasonable agreement between the predicted and the experimental collection efficiency is also obtained with a deviation of 0.10-8.71 %.
In Figure 4.19, the effect of the ion molecular weight on the particle collection efficiency is also shown. Within the range of positive ion molecular weight from 0.109 to 0.290 kg/mol, the corresponding α0 and α1 can be calculated based on Fuchs theory as shown in Table 4.3.
The calculated collection efficiency ranges from 74.2~79.4 % to 100 % for 10 nm particles, and from 40.9~44.9 % to 100 % for 50 nm particles, respectively, at the applied voltage of +3.7~4.3 kV. For comparison, the particle collection efficiency ranges from 60.0~64.4 % to 100 % for 10 nm particles at the applied voltage of +3.7~4.3 kV, based on Marlow and Brock’s charging model. Again, it shows that there are no significant differences in the particle collection efficiency when different ion molecular weights are used.
Figures 4.20 (a)-(c) show the number concentration distribution of 50 nm particles carrying 0, 1 and 4 charges in the wet ESP at an applied voltage of +3.7 kV, an air flow rate of 5 L/min, and an ion molecular weight of 0.290 kg/mol. As shown in the figure, the concentration of particles with 0 charge decreases with increasing x distance from the entrance of the wet ESP, where some particles are charged by positive ions to acquire 1-4 charges and some of which are collected by the collection electrodes due to electrical force.
This can be observed from the location of the highest charged particle concentration near the
collection electrode surface as shown in Figures 4.20 (b) and (c). Besides, large amount of particles carrying 1~4 charges penetrate through the present wet ESP because the electrostatic force is not high enough for collecting the charged particles. The average concentration of particles at the exit of the wet ESP was calculated to be 8.66×104, 1.27×107, 4.48×107, 1.03×107, and 2.60×105 m-3 for particles carrying 0-4 charges, respectively, leading to an average outlet particle charge of 1.97 when the inlet particle concentration was 8.70×107 m-3. When the applied voltage was increased to +4.3 kV, the collection efficiency reached up to 99
% and the particles carried an average charge of 3.19.
For 10 nm particles, particles with an average outlet concentration of 4.62×107 and 2.79×106 m-3 carrying with 0 and 1 charge, respectively, were found to penetrate through the wet ESP operating at + 3.7 kV. The average outlet particle charge was 0.06, or partial charging occurred which led to a decrease of particle collection efficiency (Figure 4.19).
When the applied voltage was increased to +4.3 kV, the partial charging effect became insignificant, and the collection efficiency of particles with an average charge of 2.00 reached up to 99 %.
Table 4.3. Combination coefficient of positive ion with 50 and 10 nm NaCl particles calculated by using different ion molecular weights shown in Table 1 based on Fuchs theory.
dp=50 nm dp=10 nm
Mion (kg/mol)
positive ion α0 (m3/s) α1 (m3/s) Mion (kg/mol)
positive ion α0 (m3/s) α1 (m3/s) 0.109 7.36E-13 1.97E-13 0.109 3.42E-14 1.00E-15 0.130 7.03E-13 1.86E-13 0.130 3.21E-14 9.81E-16 0.140 6.88E-13 1.81E-13 0.140 3.13E-14 9.71E-16 0.148 6.78E-13 1.78E-13 0.148 3.07E-14 9.62E-16 0.150 6.75E-13 1.77E-13 0.150 3.05E-14 9.60E-16 0.200 6.22E-13 1.60E-13 0.200 2.73E-14 9.03E-16 0.290 5.55E-13 1.39E-13 0.290 2.35E-14 8.13E-16
3.6 3.8 4 4.2 4.4 Applied voltage (kV)
0 20 40 60 80 100
Collection efficiency (%)
Experimental data (10 nm) Experimental data (50 nm) Fuchs model (10 nm) Fuchs model (50 nm) Marlow and Brock (10nm)
Mi=0.290 kg/mol
Mi=0.109 kg/mol
Figure 4.19 Comparison of particle collection efficiency in the single-stage wire-in-plate wet ESP between numerical results and experimental data at the air flow rate of 5 L/min.
0 0.1 0
0.004
6.0E+04 3.0E+05 6.0E+05 3.0E+06 6.0E+06 3.0E+07 6.0E+07
0 0.1
0 0.004
6.0E+04 3.0E+05 6.0E+05 3.0E+06 6.0E+06 3.0E+07 6.0E+07
0 0.1
0 0.004
6.0E+04 3.0E+05 6.0E+05 3.0E+06 6.0E+06 3.0E+07 6.0E+07
Collection plate Charged particle concentration distribution (#/m3)
Aerosol inlet
:Discharge wire
Collection plate Charged particle concentration distribution (#/m3)
Aerosol inlet
:Discharge wire
Collection plate Charged particle concentration distribution (#/m3)
Aerosol
Figure 4.20 Number concentration distribution of 50 nm particles carrying 0-4 charges in the single-stage wire-in-plate wet ESP when the applied voltage and air flow rate was +3.7 kV and 5 L/min, respectively. (a) 0 charge, (b) 1charge, (c) 4 charges. (Note: The scale in y direction is magnified 4.5 times relative to that in x direction.
Figure 4.21 shows comparison of the particle collection efficiency between numerical values and experimental data for silver particles in the size range of 5.23~107.5 nm. As the figure shows, the particle collection efficiency decreased from 64.2 to 36.9 % and 86.9 to 63.7
% with decreasing particle diameter from 45.3 to 8.06 nm when the applied voltage were +3.7 and +3.8 kV, respectively. The collection efficiency increased from 38.2 to 54.7 % and 63.7 to 68.8 % for particles with diameter of 5 nm when the applied voltages were +3.7 and +3.8 kV, respectively. It is because the higher diffusion deposition of 5 nm particles than the particles with dp≧8.06 nm. When Fuchs model is adopted to calculate the particle charge, the simulated collection efficiencies at the applied voltage from +3.7 to +3.9 kV agree with the experimental data with deviation of 0.70-18.9 %, 0.10-19.8 %, and 0.10-4.80 %, respectively.
At the applied voltage of +3.7-+3.9 kV, the partial charging effect on the collection efficiency for particles with dp≦20 nm was found. When the applied voltage was increased to +4.3 kV, the partial charging effect on the collection efficiency was diminished due to high electric field strength and ion concentration, and the particle collection efficiency reached up to 97.7-99.8 %.
0 20 40 60 80 100 Particle diameter (nm)
20 40 60 80 100
Collection efficiency (%)
Experimental data 3.7 kV 3.8 kV 3.9 kV 4.3 kV
Numerical values 3.7kV 3.8kV 3.9kV
Figure 4.21 Collection efficiencies of the polydisperse silver particles in the present wet ESP when the aerosol flow rate and the applied voltage are 5 L/min and +3.7-+3.9kV, respectively.
Each test was repeated 6 times.