A visual experimental investigation was carried out to study the flow boiling heat transfer in a circular channel with diameter of 6.78 mm and two transparent sections (100 mm) at the two ends of the channel. A high speed camera was used to record the bubble behavior and the flow patterns at the exit of the channel with 5,000 FPS (Frame Per Second). The working fluid was deionized water, and the subcooling at the inlet was 10 K with constant system pressure of 0.101MPa. The mass flux ranged between 300 kg/m^2s and 700 kg/m^2s with different heat fluxes. It was found that the heat transfer coefficient increased with increasing vapor quality for different mass fluxes since the heat flux increased with other identical working conditions. Also, the heat transfer coefficient was inversely proportional to the liquid film thickness in annular flow. With increasing heat flux and a constant mass flux, the heat transfer coefficient of annular flow increased and fluctuated more intensely, because the liquid film thickness decreased and fluctuated intensely.
