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Atomization and Sprays

Impact factor: 1.235

ISSN Print: 1044-5110
ISSN Online: 1936-2684

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Atomization and Sprays

DOI: 10.1615/AtomizSpr.2016015626
pages 1259-1305

STATE OF THE ART REVIEW OF FLASH-BOILING ATOMIZATION

Tali Bar-Kohany
School of Mechanical Engineering, Tel Aviv University, Tel Aviv, 6997801, Israel
Moti Levy
Faculty of Aerospace Engineering, Technion - Israel Institute of Technology, Haifa, Israel

ABSTRACT

Flash boiling atomization is now a widespread practice for creating fine sprays. The present paper aims to review and analyze our current knowledge on flash atomization processes and applications. First, the fundamental physical processes of flash-boiling atomization, i.e., nucleation and bubble growth. Then, their role in creating optimal spray (small droplet diameters and short breakup length) is analyzed. Special attention is given to reviewing and comparing different transition criteria. The conclusions can be used by those who aim to avoid accidental scenarios, or, to minimize hazardous scenarios. New surfaces are created thanks to boiling (or cavitation). The relative magnitude of the new surfaces increases as the bubbles grow, and once the two-phase fluid discharges, due to the relative kinetic energies of the liquid and the gas. Better jet atomization is observed when flashing occurs within the aperture. Higher superheat degrees lead to finer spray; thereby homogeneous nucleation should be aspired to when designing an optimal injector. Twin orifice injector with an expansion chamber is preferable for multi-component liquid, especially since it enables ones to achieve the desired sprays for lower pressures and superheat degrees. While in a single orifice injector, the highest superheat degree should be aspired to, the desired superheat degree has an upper limit for injectors with an expansion chamber. Too high superheat degrees can lead to extensive bubble coalescence and to flow stratification, thereby damaging the spray quality. The optimal degree is the one that will lead to high nucleation rate, accompanied with the highest slip between the phases. There is a lack in data of depressurization rates and their relation to spray characteristics for different liquids. In addition, experiments and data are lacking with regard to multi-component liquids.