Spray morphology and droplet characteristics in gas centered swirl coaxial (GCSC) atomizers

Abstract

Studies on liquid atomization and spray formation in coaxial type atomizers are necessary to improve the performance of liquid propellant rocket engines. Recess ratio, expressed as the ratio of orifice recess length to inner orifice diameter, is one of the geometrical parameters that influences the characteristics of sprays discharging from coaxial atomizers. The present thesis reports an experimental investigation on the atomization and spray formation processes in recessed gas-centered swirl coaxial (GCSC) atomizers through the analysis of spray morphologies and measurements of size and velocity characteristics of spray droplets. A GCSC atomizer discharges a swirling liquid film and a gas jet via its annular and central orifices, respectively. Specific attention is given to elucidate the role of atomizer recess ratio of the GCSC atomizer on the characteristics of the spray that results from the breakup of the annular swirling liquid sheet by the central gas jet. The experiments are carried out in a spray test facility using water and air as the experimental fluids. The measurements of pressure drop across the atomizer orifices and fluid flow rates are made in the test facility. The images of sprays at different flow conditions are captured using the technique of backlighted shadowgraphy. Laser-based optical diagnostic systems (Phase Doppler Interferometry and Spraytec) are used to measure the size and velocities of spray droplets at different spatial locations of the spray below the atomizer exit. In the present study, the spray condition from the GCSC atomizers is expressed in terms of the gas-to-liquid momentum flux ratio, J. The quantitative variations of the spray cone angle and the streamwise breakup length of the annular liquid sheet, with the atomizer recess ratio, RR are presented for different GCSC atomizer flow conditions. An increase in RR of the GCSC atomizer results in a smaller spray cone angle and a longer sheet breakup length. A power law analysis of the experimental data shows that the sensitivity of the sheet breakup to J is more in case of higher recess ratio GCSC atomizers. The formation of a fully developed spray (spray free from ligament/droplet clusters and non-spherical droplets) in GCSC atomizers is quantified. For a given J, an increase in RR significantly reduces the cone angle of spray. The distance from the atomizer exit to the fully developed spray zone decreases with increase in J. Among the tested GCSC atomizers with varying RR (RR = 1, 2 and 3), a fully developed spray is rapidly seen for sprays discharging from the atomizer with RR = 2. The generation of more axially directed ligaments for the sprays discharging from higher RR (RR = 3) atomizer makes the fully developed spray to begin at a farther distance from the atomizer exit, compared to that from the atomizer with RR = 2. The spray from a recessed GCSC atomizer comprises two distinct spray morphologies: a central dense spray of finer droplets and an outer coarse spray of bigger droplets. The radial boundary between the two spray morphologies is marked by a jump in the mean drop size of the spray recorded along the radial direction. The mean drop size of the central spray decreases with increase in J, whereas that of the outer coarse spray is independent of J. The reduced spray cone angle and enhanced interaction in the sprays discharging from a high RR GCSC atomizer facilitate the migration of bigger droplets from the annular film breakup region to the central fine spray. The mean drop size recorded at the spray axis increases substantially with increase in RR from 2 to 3. The radial profiles of the mean velocities of spray droplets at different J are presented. The measured radial and tangential velocities of the spray droplets are two orders of magnitude smaller than that of their axial velocity, suggesting that the spray flow discharging from recessed GCSC atomizers is axially dominated. For the sprays with high J, the profile of the mean axial velocity of the spray droplets exhibits trends similar to that of a self-similar free gas jet.