# Experimental Studies on the Spraying Pattern of a Swirl Nozzle for Coal Dust Control
## 1 Introduction  
Coal dust of the mining industry causes the number one health problem in coal miners: pneumoconiosis (black lung). Excessive coal dust in underground coal mines can also cause disastrous explosions [1; 2]. Water spraying is one of the most efficient and cost-effective measures to reduce dust concentration in coal mines [3]. This method uses small water particles sprayed by a nozzle to capture the coal dust in the air. Consequently, the method to produce tiny water particles and the diameter of the water particles are important for reducing coal dust. If the diameter of water is near the diameter of coal dust, according to [PERSON] [4], better dust removal can be achieved. [PERSON] et al. [5] had measured the coal dust particle size. The results showed that the distribution scope of the major coal dust particle size is about 10-100 \(\upmu\)m. There are three main kinds of methods to produce tiny water droplets to capture coal dust: rotary spray [6], ultrasonic atomization [7; 8; 9], and spray nozzle [10; 11]. The rotary spray uses a rotating disk to produce tiny water particles. This method has been widely used in industry, such as in the fields of dust suppression [6] and for cutting tool cooling [12]. In recent years, many studies on the rotary atomized dust collector have been launched from the shear fracture of liquid membrane, but ideal particle sizes are hard to obtain [13]. [PERSON] et al. [14] experimentally investigated the mechanism of ligament formation on a spinning wheel using photographs taken by a high-speed camera. Three different liquids with Newtonian properties were used at various flow rates, and the wheel rotational speed was varied in a wide range. [PERSON] et al. [15] investigated liquid disintegration in ligament and sheet formation mode and developed a mathematical model of unstable wave growth for the spinning disk. [PERSON] et al. [16] used an infrared thermography-basedtechnique for the characterization and visualization of liquid sprays. The Sauter Mean Diameter (SMD) is about 20-55 um, but the speed rotary bell atomizer is very high, above 20,000 rpm. [PERSON] [17] used high-speed shadow graph imaging to visualize the edge of a serrated rotary bell at speeds varying between 5000 and 12,000 rpm. The droplet (ranging from 40 to 300 um) diameters formed a normal distribution. Ultrasonic atomization can also be used to produce water particles to reduce coal dust [18]. Ultrasonic atomization has also been widely used in many fields of science, such as household humidifiers and medical science [19], so many peoples have studied it. [PERSON] et al. [7] investigated the effect of the frequency on the size distribution of ultrasonic mist. A bimodal distribution was obtained for the mist generated by ultrasonic atomization with a wide-range particle spectrometer. The peak diameter decreased with increasing frequency, and the concentration of the mist particles increased in the smaller range. [PERSON] et al. [20] tried by experimentation to understand the mechanism of ultrasonic vibration at the gas liquid interface causing the atomization of liquid. It has been found that the average droplet size produced by the pseudo-plastic liquid is less than that produced by the viscous Newtonian liquid having viscosity equal to zero-shear rate viscosity of the shear thinning liquid. [PERSON] et al. [21] performed an experimental study to improve the understanding of the characteristics of ultrasonic water atomization when excited with waves in the MHz range. It was confirmed that the droplet size distributions are relatively independent of the forcing voltage. For the oscillation frequency in the present measurements, 1.65 MHz, the droplet size distribution function presents a main peak between 3 and 5 um. The supersonic swirling separation is a new technology for gas processing. [PERSON] and [PERSON] [22; 23] used the Discrete Particle Method to study the particle motion in supersonic flows with a strong swirl.