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 steam is generated because of the heat addition. As a result,
the flow regime goes from bubbly flow (for relatively low values of the 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.