# Experimental and Numerical Investigation of Spray Scrubber Dust Collection Efficiency
## 2 Materials and Methods
Such a phenomenon is unfavorable for the operation of the scrubber, so a water nozzle with a relatively large size of the generated drops had to be selected. During the tests, several nozzles were tested using the Droplet Size Analyzer (AWK D) from the manufacturer \"Kamika Instruments\" (Warsaw, Poland). It is designed to measure the distribution of liquid droplets in air under the laboratory conditions in the range from 50 \(\upmu\)m to 4 mm. After analyzing the distributions of several water and water-air nozzles (atomizers), it was decided that the Lechler 490.928 nozzle would be the most suitable for the planned experiments. The measurement results are shown in Figure 7.  
The nozzle generated relatively large droplets, and the average droplet dimensions were 741 \(\upmu\)m for the quantitative distribution and 1276 \(\upmu\)m for the volumetric distribution. As can be seen in Figure 7, the droplet fraction below 200 \(\upmu\)m is relatively small and it can be expected that only a small part of the droplets will be entrained by the air flow. The nozzle was also selected to deliver the appropriate volumetric flow rate, which was approximately 20 L/min. A control valve was placed before the nozzle, which allowed the water flow rate to be adjusted. Figure 6: Particle size distribution in the scrubbing chamber. In order to enable the measurement of separation efficiency, it was necessary to measure the mass of dust introduced into the system by the fluidizer and the mass of dust that was captured on the filter. A dryer was used before filter mass measurement, which was necessary due to the presence of some smaller droplets that were entrained by the flow. A \"Hepa 10\" paper filter was mounted on the scrubber. To determine the separation efficiency, the following formula was used:  
\[\eta=\frac{(m_{pp}-m_{pk})-(m_{fs}-m_{fp})}{m_{pp}-m_{pk}}.100\% \tag{1}\]  
where:  
\(m_{pp}\)--Initial mass of dust introduced into the fluidizer [g];  
\(m_{pk}\)--Mass of dust left in the fluidizer after the test [g];  
\(m_{fs}\)--Weight of the dried filter after the test [g];  
\(m_{fp}\)--Initial filter weight [g]. To determine the dust fraction that was separated on the filter, an analogous relationship was used:  
Figure 3.0  
In order to acquire more information about the phenomena occurring inside scrubbing chamber, a simple numerical model of spray scrubber was prepared. However, for the purpose of this paper, only air flow was solved and analyzed. The mechanism of droplet and dust particles interaction was not analyzed. The geometry of scrubber investigated in laboratory was mapped as a 3D model and implemented into computational software, which is presented in Figure 8a, while the zoomed computational mesh in the vicinity of demister vanes is shown in Figure 8b. Figure 3.1 Distribution of droplet sizes generated by the nozzle. The computational domain was divided into approximately 28 million cells. Mesh in the channels of demisters was refined in order to perform proper calculations. A boundary layer was also implemented in the model. Mesh independence tests were carried out on three grids with a different number of cells. Standard deviation of velocity at lower demister cross-section was taken as a criterion as well as pressure drop on the whole scrubber. The results of the grid independence test are presented in the Figure 3.2 The pressure drop was nearly the same for all cases while standard deviation of the velocity slightly decreased for the biggest mesh. Steady state analysis was performed. In the calculations, dry air of constant density and a temperature of 20 \({}^{\circ}\)C was flowing through rectangle inlet in the lower part of scrubber, while the outlet was set on the top of scrubber. The solver software used was Ansys Fluent 2020 r2 and the realizable k-epsilon model with enabled enhanced wall treatment option was applied. The boundary conditions were set as \"mass flow inlet\" and \"pressure outlet\".