ABSTRACT
The impact of the in-flow characteristics inside the injection nozzle on atomization has been experimentally and computationally studied. Measurements are carried out using a transparent glass nozzle. Pulsed laser sheet with a synchronized charge-coupled device (CCD) camera and image processing, together with a particle image velocimetry (PIV) setup have been used as measuring techniques. Images and relevant image processing are used to visualize and quantify the rate of generation of cavitation bubbles inside the nozzle, the spray particle size distribution, and cone angle. Velocities inside and outside the injection nozzle are measured using PIV. The experimental investigation has been extended to include a wider range of the injection nozzle geometrical aspect ratios and working parameters. The computational model is a three-dimensional, two-phase, turbulent model to solve both the in- and out-nozzle flows. A novel coupling mathematical model is proposed for the definition of the probability density function of the issuing droplet size distribution, based on the in-flow developed conditions. A good agreement between both the experimental and computational results has been found under all conditions. According to both the experimental and computational results, it has been found that the onset of cavitation inside the injection nozzle, its location, collapse, and consequently the issuing spray configurations depend on the flow cavitation number, the nozzle geometrical characteristics, the liquid temperature, and the injection and back pressures. According to the quality of the obtained results from the model, it can be used to extend the study to cover a wider range of spray applications.
ABSTRACT
In the present paper, the formation and development of cavitation inside the nozzle of an atomizer with different geometrical characteristics have been studied numerically. Different shapes of inlet nozzles and different nozzle-length-to-diameter ratios have been investigated. The developed model has been built as a three-dimensional (3D) one, where the turbulence is modeled considering large eddy simulation. The obtained computational results showed good agreement with the reported experimental results. It has been found that the occurrence of cavitation depends on the amount of energy needed to overcome the viscosity and friction between the liquid layers. The mass flowing through the nozzle decreases with increasing cavitation. The intensity of cavitation depends on the nozzle entrance shape. Sharp edges cause cavitation to occur early in the nozzle, followed by an inclined shape, and then the curved entrance. The dissipative energy in the cavitation and bubble collapse result in an increase in the turbulent kinetic energy of the issuing liquid. This causes more liquid disintegration, leading to larger spray volume and smaller droplet size. The obtained results for spray droplet size distribution have been compared with experimental data developed by other researchers, and a good agreement has also been found.
