Effect of the propellant mass fraction in a binary mixture on the spray characteristics as generated by homogeneous flash boiling

When a binary mixture of a high vapor pressure propellant and a low vapor pressure component is injected through a simple atomizer at high enough pressure, flash boiling of the propellant may occur. Due to the sudden depressurization, propellant-vapor nuclei are developed to a rapid evaporation, resulting in break-up of the liquid mixture into a fine spray that is characterized by tiny and fairly uniform droplets. This method is presently used in quite a number of applications. Depending on the initial conditions we distinguish between heterogeneous and homogeneous nucleation. Flash-boiling process within the aperture leads to highly efficient atomization. Finer sprays are achieved when increasing the superheating degree; thus, homogeneous nucleation is apparently the preferred regime for atomization. Nevertheless, experimental studies involving atomization under homogeneous nucleation regime are rather scarce. In the present work we study the effect of the propellant mass fraction in a binary mixture on the spray characteristics as generated by a homogeneous flash-boiling process. We have selected Chlorodifluoromethane (CHClF₂ or R-22), and PMX-200 silicon oil as the two components of the mixture and studied the effect of the mixing ratio on the droplets' velocity and size distribution and the radial distribution of the droplets' velocity and size. We used a TSI's Phase Doppler Particle Analyzer (PDPA) to characterize the spray, and a controlled 3D positioning system to measure the droplets characteristics at accurate and specific positions. We show that lowering the mass fraction of the propellant results in a progressively higher value of the mean velocity at any radial distance, a higher mean droplets' size, and a higher standard deviation. For the present homogeneous nucleation flash-boiling atomization system, we found that the break-up efficiency, as defined by Sher and Zeigerson-Katz [Atomization Sprays, vol. 6, no. 4 (1996)], is rather low, on the order of 10^-6, while a higher propellant mass fraction yields a lower process efficiency. The lower efficiency is attributed to the different mechanisms of spray formation in homogeneous and heterogeneous nucleation.