32 - Boiling

Heat transfer and flow regimes in a vertical heated channel.

Consider a vertical channel of arbitrary cross-sectional shape (i.e., not necessarily circular) uniformly heated (axially as well as circumferentially) by a heat flux \(q\). At axial locations below the onset of nucleate boiling, the flow regime is single-phase liquid. As the fluid marches up the channel, more and more vapor is generated because of heat addition. As a result, the flow regime goes from bubbly flow (for relatively low values of flow quality) to plug (intermediate quality) and annular (high quality). Eventually, the liquid film in contact with the wall dries out. In the region beyond the point of dryout, the flow regime is mist flow and finally, when all droplets have evaporated, single-phase vapor flow.