# Experimental Studies on the Spraying Pattern of a Swirl Nozzle for Coal Dust Control
## 1 Introduction
The results showed that the separation efficiency reached over 80% when the droplet diameter was more than 1.5 um. For the large delta wing, the collection efficiency reaches 70% with 2 um particles, indicating a good separation performance of the proposed supersonic separator. [PERSON], et al. [24] carried out the numerical simulation with experimental separator by using numerical methods; the results show that the viscous heating and strong swirling flow cause adverse pressure in the annular channel, which may negatively affect the separation performance. [PERSON], et al. [25] set up an experimental system for testing the separation efficiency of three new designed separators with wet air. Under a strong swirling flow field, the separation efficiency reached over 95%. [PERSON], et al. [26] developed the computational fluid dynamics modeling to optimize the vane structures; results indicate that 45-60\({}^{\circ}\) swirl angle, 0.125-0.3 dimensionless height, and 8-16 vanes are reasonable for gas purification using the supersonic separator. Spray nozzles are used more than the rotary spray and ultrasonic atomization in coal dust control. Many scientific researchers have focused their work on spray nozzles. [PERSON] et al. [27] conducted an experimental study to analyze the influence of conical and cylindrical nozzle orices on injection rate behavior of a common-rail fuel injection system at maximum needle lift in a cavitation test rig. [PERSON] et al. [28] studied the spray penetration and liquid length under evaporating conditions for various nozzle geometries. [PERSON] et al. [29] measured a kerosene air blast atomized spray flame between two co-swirling air streams by a phase-doppler particle sizing system, and found a small liquid exists in the center of the combustor due to larger droplets with the swirl effect. Both particle image velocimetry (PIV) and phase-doppler anemometry (PDA) have been used for measurements on the droplet velocity and droplet size. Measurement were made by [PERSON] and [PERSON] [30] using PIV and PDA; they indicate that higher throw length can be achieved with a full cone nozzle compared to a hollow cone nozzle, when both nozzles have the same cone angle and flow rate. Swirl nozzle is one kind of spray nozzle that has been widely used in many industrial fields [31]. [PERSON] [32] performed an experimental investigation to explore the characteristics of sprays produced by a liquid-liquid coaxial swirl atomizer. The results indicate that the maximum Sauter Mean Diameter of droplets along the injection axis moves to the outer periphery of the spray and increases downstream. In this paper, a swirl nozzle is developed for generating water spray for dust control in coal mines, as shown in Figure 1. The nozzle can not only convert the liquid pressure into the kinetic energy of the liquid, but also produce the liquid eddy to form a conical cavity in the center of the swirl chamber so that the eddied liquid in the form of a liquid film sprays out at high-speed from the nozzle. The liquid, after leaving the nozzle, is no longer restricted by the inner wall of the nozzle and spreads into the air. The slight disturbance of the air acts on the high-speed jet to form a turbulent wave. The amplitude of the disturbance increases when the kinetic energy of the small volume liquid is greater than the surface tension of the liquid, and the liquid film breaks and forms small fog particles under strong vibrations. The mechanism for using spray to control dust is that the produced tiny droplets capture smaller dust particles in the air. According to the theory, for the tiny water droplets to capture dust particles, the relationship between the smallest dust particle size to be captured and conical cavity droplet size is expressed by the following formula [33].