Atomization of Superheated Water

An experimental study of disperse composition of an air-droplet flow under atomization of water superheated relative to the saturation temperature is conducted. Atomization was accomplished with the use of centrifugal (with a nozzle diameter of 0.6 mm) and once-through jet (with a nozzle diameter of 0.3 mm) atomizers. Water pressure at the inlet to the atomizer was 4 and 8 MPa, with a temperature being varied from 20 to 270°C. The disperse composition of the atomized water was determined from a distribution of the intensity of the scattered laser beam with a wavelength of 532 nm over a wide range of the scattering angles. This technique was combined with measurements of light extinction, filming of the droplet spray, and temperature measurements in the flow to determine a degree of droplets evaporation. The experiments showed that at a water temperature of more than 170°C flashing of water discharging from the atomizers takes place, due to which considerably finer atomization of water is obtained as compared to that with the use of mechanical and pneumatic atomizers. At a temperature of injected water of 220–240°C, 70% of droplets (by mass) have a value of d₃₂ diameter of 0.2 to 3 μm. While atomizing superheated water, the size distribution of droplets, has a bimodal pattern, i.e., along with the existence of fine droplets, certain amount of droplets of 6–15 μm in diameter (up to 30%) is retained in the flow. When atomizing superheated water, centrifugal and jet atomizers provide approximately the same distribution of droplet sizes. This attests that flashing plays the determining role in the atomization process. Saturation of water to be atomized by air or carbon dioxide does not noticeably change disperse composition of the air-droplet mixture, while atomizing water at a temperature of more than 170°C. Due to the impact of the air flow, fine droplets form a spatially localized spray extended along the flow with a semi-angle of expansion of 10–12 deg. This fact should be accounted for while arranging water injection systems. The fine atomization achieved significantly increases a rate of droplet evaporation in the carrying flow, prevents their deposition on the articles of equipment, and provides more efficient cooling of the working medium.