The behavior of near-wall bubbles in subcooled flow boiling has been investigated photographically for water flow in vertical, one-side heated and rectangular channels at mass fluxes of 500, 1500, 2000 kg/m2s under atmospheric pressure and for R134a in channels of the same kind at mass fluxes of 1000, 2000 kg/m2s under 7 bar. Digital photographic techniques are used for the visualization, which are rapidly advanced in recent. Primary attention is given to the bubble coalescence phenomenon and the structure of the near-wall bubble layer. At subcooled and low-quality conditions of both fluids, discrete attached bubbles, sliding bubbles, small coalesced bubbles and large coalesced bubbles or vapor clots are observed on the heated surface as the heat flux is increased from a low value. Particularly for R134a, vapor remnants below discrete bubble on the heating surface are observed. Nucleation site density increases with the increases in heat flux and channel-averaged enthalpy, while discrete bubbles coalesce and form large bubbles, resulting in large vapor clots. Waves formed on the surface of the vapor clots are closely related to Helmholtz instability. At sufficiently high heat fluxes, three characteristic layers were observed in the heated channel: (a) a superheated liquid layer with small bubbles attached on the heated wall, (b) a flowing bubble layer consisting of large coalesced bubbles over the superheated liquid layer, and (c) the liquid core over the flowing bubble layer.

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