Influence of a Coaxial Gas Flow on a Flashing Liquid Jet: Implications for Flame Spray Synthesis of Nanoparticles

Flashing or thermodynamic breakup of a liquid jet occurs when a pressurized subcooled or saturated liquid is released to a lower pressure, resulting in violent vapor nucleation, expansion, and break up of the liquid phase. Flashing is known to produce very fine droplet atomization, often not possible by traditional mechanical means. In this work, flashing atomization is introduced in a spray burner used in flame spray pyrolysis (FSP). Traditional operation of the spray burner requires atomization of a liquid precursor jet by a coaxial gas flow that also functions as the oxidant and fuel. In this way, the atomization quality of the spray is coupled with the combustion characteristics, particularly flame length. This has made control of nanoparticle characteristics difficult as they are heavily dependent on initial droplet size, temperature profile and flame residence time. The flashing mode of atomization may be introduced by pressurizing and heating the precursor liquid. Under appropriate conditions, this allows for independent control of oxidant/fuel gas flow rate without affecting atomization quality. This study specifically examines the influence of a coaxial He gas flow on a flashing water jet. New external flashing modes were observed that have not yet been identified that explain the multimodal velocity distributions often measured by PDPA systems for flashing sprays. It was found that flashing under low precursor superheat produced better atomized sprays due to the wind shear by the coaxial gas flow. Higher precursor superheat increased its flow rate and thus decreased the momentum flux ratio between the gas and liquid streams. Hence, measured droplet sizes reverted back to flash-only values with increasing superheat. However, gas flow still had the effect of increasing droplet velocities. Introduction Flashing occurs when a pressurized supercritical, subcooled or saturated fluid is released to a lower pressure, resulting in expansion, violent vapor nucleation, and break up of the liquid phase due to thermodynamic instability. Flashing of liquid jets has been studied since the early 1960’s [1]. Early works were primarily qualitative visualization studies documenting the phenomenology of the flashing process [2-4]. Later, empirical and semi-theoretical correlations were developed to predict spray properties based on initial conditions, though applicable conditions for these relations were limited [5]. Modeling work of jet breakup and droplet dispersion has also been performed for limited situations [6-8]. Recently, due to advances in spray diagnostics, some quantitative spray characteristic measurements have been performed [9-11] though currently a lack of comprehensive measurements exists and more are needed to facilitate modeling. Interest in flashing continues due to applications in accidental releases of pressurized liquids [12], fuel injection [13], emulsifications [14], and spray cooling [15, 16]. With flashing, fine droplet atomization and narrow size distribution are possible. Flashing may also be applied to flame spray pyrolysis (FSP) of ceria particles for catalysis of soot from diesel engines [17]. Traditional operation of the spray burner requires atomization of a liquid precursor jet by a coaxial gas flow that also functions as the oxidant and fuel. In this way, the atomization quality of the spray is coupled with the combustion characteristics, particularly flame length. This has made control of nanoparticle characteristics difficult as they are heavily dependent on both initial droplet size and flame residence time. Existing research of FSP by others is largely empirical and focused on precursor and fuel choices, flame characteristics and other combustion aspects [18]. The flashing mode of atomization may be introduced to the existing coaxial air-blast atomizer by pressurizing and heating the precursor liquid prior to Figure 1. Spray burner geometry. PDPA hardware characteristics signal processor FSA 3500 photo detector PDM 1000 laser Argon ion Bragg cell frequency 40 MHz wavelength 514.5, 488 nm focal length of transmitting probe 250 mm focal length of receiving probe 300 mm laser power 150 mW slit aperture 25 um off-axis angle 30 degrees diameter range 0.59-212.28 um velocity range 0-235.53 m/s beam waist 95.13 um