# Experimental Studies on the Spraying Pattern of a Swirl Nozzle for Coal Dust Control
## 2 Research Method
### Experimental Setup  
Experiments were conducted to study the performance of the swirl nozzle. A dedicated test system for spray characteristic analysis was established at the Mine Fluid Control Laboratory, Taiyuan University of Technology. Tap water was used as the working medium, the tap water from the tank was pressurized to the preset pressure by a high-pressure pump and then forced out from the swirl nozzle.  
At present, optical measurement techniques are the most suitable to measure the characteristics of water mist, and a number of different techniques are available. In this study, OMEC DP-2 particle size analyzer was used to measure the particle size of the fog field. Measurements were carried out at 1.0, 1.2, and 1.5 mm nozzle orifices and pressures from 2 to 6 MPa. A HX-6 high-speed camera was used to record and investigate the angle of water sprays. The experiment system is shown in Figure 2.  
Figure 2: Measurement setup with swirl nozzle and Laser Doppler Anemometry. (1: receiver, 2: swirl nozzle, 3: transmitter, 4: pressure gauge, 5: high pressure pump unit, 6: flow meter, 7: valve, 8: tank, 9: high-speed camera, 10: computer).  
The experimental measurement system was tested in the Mine Fluid Control Laboratory. The setup was surrounded within a plastic curtain to maintain an enclosure space and avoid air flow that could cause errors. The hydraulic pressure gauge was used to monitor and maintain the spray pressure at its preset value. A flow meter was used to record the nozzle flow rate and a high-speed camera was used to capture the velocity of movement and atomization angle of fog particles. Meanwhile, the distribution of fog particles size was measured by the DP-2 particle size analyzer at different locations by changing the relative position of the nozzle. The experimental site is shown in Figure 3.  
For analysis and recording, a coordinate system (Figure 4) was established in the spray area. The horizontal axis H and vertical axis L are in the radial and axial directions of the nozzle, respectively, and the unit is cm. For example, point A in Figure 4 indicates that laser position is 250 cm from the axial direction of the nozzle and 20 cm from the radial direction of the nozzle.  
The particle size measured by the DP-02 laser particle size analyzer is the distribution of particle size passing through the laser line. However, accurate measuring the size distribution and velocity of the droplets, as well as coalescence and break-up events are still difficult to achieve. The sizes of  
Figure 4: Coordinate system was established in the spray area. H = horizontal axis and L = vertical axis.  
Figure 3: Setup and testing of the swirl nozzle.  
droplets are usually characterized by D\({}_{10}\), D\({}_{50}\), D\({}_{90}\), D\({}_{[3,2]}\), and D\({}_{[4,3]}\), etc. The volumetric median diameter D\({}_{50}\) is used as the average particle size of droplets, and it is the diameter at which 50% of the total of the droplets in volume exhibit smaller diameter than this characteristic diameter [37; 38; 39]. Here the Sauter mean diameter \(d_{SMD}\) is adopted [4].  
\[d_{SMD}=\sum_{i=1}^{m}n_{i}d_{i}^{3}/\sum_{i=1}^{m}n_{i}d_{i}^{2} \tag{5}\]  
where \(m\) is the number of droplet diameter segments and _d\({}_{i}\)_ and _n\({}_{i}\)_. are the droplet diameter and number of the _i_th segments, respectively.