A PHOTOGRAPHIC STUDY OF FLASH-BOILING ATOMIZATION

To gain an insight into the mechanisms of flash-boiling atomizationheated water was injected from a single-hole orifice into heated air (steady injectionsliquid pressure 697 kPaair pressure ambienttest temperatures from 300 to 426 Korifice diameter 0.34 mmlength 1.37 mm). The breakup regime of interest in the study was that where the spray divergence starts at the nozzle exit. Short-duration backlit photographs and laser diffraction dropsize measurements showed that these flashing jets comprise an inner intact core which is surrounded by the diverging fine spray. These details about the spray structure are not visible in conventional photographs of flashing sprays that use scattered light illumination. The present results cast doubt on a previously proposed theory of flash-boiling atomization that attributes the divergence of the spray cone to the expansion processes that occur in an underex-panded compressible flowsince that theory implies that the liquid is already atomized upon leaving the nozzle. Insteadthe photographs show that drops are expelled from the unbroken liquid jet starting at the nozzle exit (presumably by rapid vapor bubble growth within the jet). The core region remains intact for some distance downstream of the nozzle exitand its breakup eventually produces relatively large drops. As the liquid temperature approaches boilingthe intact length and the core drop size decrease. Thus operation close to boiling is desirable for effective atomization. Howeverthe nozzle mass flow rate decreases and practical difficulties are found (owing to “vapor-lock”) as the liquid is heated near boiling. © 1990 Elsevier Science Publishing Co., Inc.