Application Of The Phase Doppler Measurement Technique For The Characterization Of Supersonic Gas Atomization
N. Apell, C. Tropea, I. V. Roisman, J. Hussong
Institute for Fluid Mechanics and Aerodynamics, Technische Universität Darmstadt, Germany
A dual-mode phase Doppler measurement system has been set up for the characterization of the complex spray produced through supersonic gas atomization. The atomizer design has been based on a generic close-coupled configuration operated with water and air, which leads to high local Mach numbers and complex patterns of compression and expansion waves within the gas flow field. As a result, the dense spray is characterized by high particle velocities and a wide range of particle sizes, which proves challenging for experimental investigations. Measurements of local particle size and velocity distributions have been performed in various radial positions within the spray downstream of the atomizer, where the spray is fully developed. The set point of operation, which is defined by the gas stagnation pressure and the liquid mass flow rate, has been varied systematically, covering a wide range. By decoupling both operational parameters, their influence on the atomization result has been investigated independently. Furthermore, the atomizer geometry has been varied in terms of liquid nozzle diameter and liquid nozzle protrusion length. In accordance with data from the literature, the gas stagnation pressure has been observed to lead to decreasing median particle size and dispersion of the particle size, when increased. However, as a novel finding, the atomization result has been shown to be more sensitive to changes in the liquid mass flow rate. Thus, increasing the liquid mass flow rate results in an increase in both the median particle size and the dispersion of the particle size. The relationship between the atomization result and the gas-to-liquid ratio has been clearly shown to be ambiguous, that is, a function of the gas stagnation pressure and the liquid mass flow rate. Neither the liquid nozzle diameter nor the liquid nozzle protrusion length have been found to have an influence on the median particle size. However, the latter clearly influences the dispersion of the particle size in a complicated way, while the effect of the liquid nozzle diameter is more subtle. It has been concluded that the complex coupling between operational parameters and the specific atomizer design evident in supersonic gas atomization requires a deeper understanding of the local gas flow field and its interaction with the liquid flow.