UNSTEADINESS IN EFFERVESCENT SPRAYS

Abstract

The ideal spray theory of Edwards and Marx was used to investigate the dependence of effervescent spray unsteadiness on operating conditions, spatial location, and fluid physical properties. Droplet size, velocity, and arrival time at a particular spray location were measured using a Phase/Doppler Particle Analyzer. The droplet arrival times were used in calculations of interparticle arrival time gaps and interparticle time distribution functions. The spray was determined to be steady (interparticle time distribution function obeying inhomogeneous Poisson statistics) or unsteady (interparticle time distribution function not obeying inhomogeneous Poisson statistics) by comparing experimental and theoretical (steady) interparticle time distribution functions with results reported in terms of the number of deviations between the two. Since the spray was assumed to be a Poisson process, the expected deviation is the inverse of the square root of the number of interparticle events. A chi-square analysis was performed on the discrepancy. Results demonstrate that all droplet size classes, which range from diameters of 3.2 to 60.4 μm, exhibit unsteady behavior. Stokes number calculations show that the largest droplets are incapable of following the turbulent flow field motions. Gas-phase turbulence can therefore be eliminated as a cause of unsteadiness for those drops. Chi-square calculations demonstrate that the probability for obtaining such results from random fluctuations is less than 0.001. Hence, it is concluded that effervescent atomization is an inherently unsteady process. Results also indicate that spray unsteadiness is influenced by the air-to-liquid ratio by mass (ALR) and the liquid mass flow rate, depending on the properties of the liquid used in the spray, and that fluid viscosity and surface tension can affect the level of spray unsteadiness only when the spray is operating in the bubbly or intermittent slug regime. For such conditions, the spray is more unsteady when a lower-viscosity or higher-surface-tension fluid is utilized. When using a liquid that has a low viscosity (0.03 kg/m-s) and high surface tension (0.065 kg/s2), a decrease in ALR or liquid mass flow rate causes the spray to be more unsteady. The use of a high-viscosity (0.124 kg/m-s) liquid lessens the effect of operating conditions on spray unsteadiness. Finally, it was found that the spray is more unsteady at its edge, as well as at greater downstream distances.