Flow effects on bubble growth and collapse in surface boiling

Abstract High speed photography was used to investigate the effect of fluid velocity on the growth and collapse of vapor bubbles in slightly subcooled distilled water in an open loop. Profile pictures of vapor bubbles at a magnification of about 2× on the film were made at about 7000 frames per s, over a range of water velocities from 3 to 7.5 ft/s, and heat fluxes from 5.94 × 104 to 1.46 × 105 Btu/hft2, with subcooling of 3.5 F°. This corresponds to a Jakob number at (Tsat) of 5.5. The photographic results showed the bubbles being ejected from the nucleation site and collapsing inin the subcooled main stream. This study showed an increase in fluid velocity resulted in a decreased bubble size and life span, whereas an increase in the heat flux resulted in an increased bubble size and life span. The fluid velocity did not show any effect on the frequency of formation of bubbles but, up to a limiting value, an increase in the heat flux resulted in an increase in the bubble frequency at a given site.