Many bubbles are formed and grow at the wall, and detach from the wall under the effect of buoyancy.
This heat transfer regime is called nucleate boiling. The bubbles create a lot of agitation and mixing near
the wall, which enhances heat transfer. As a result, nucleate boiling is a much more effective
mechanism than free convection, as suggested by the higher slope of the boiling curve in this region.
B→C
At a critical (high) value of the heat flux, there are so many bubbles crowding the wall that they can
merge and form a continuous stable vapor film. Thus, there is a sudden transition from nucleate boiling
to film boiling. This sudden transition is called Departure from Nucleate Boiling (DNB), or Critical Heat
Flux (CHF), or burnout, or boiling crisis, and is associated with a drastic reduction of the heat transfer
coefficient because vapor is a poor conductor of heat. Consequently, the wall temperature increases
abruptly and dramatically (to >1000C), which may result in physical destruction of the heater.
C and beyond. This is the film boiling region. Heat transfer from the wall occurs mostly by convection
within the vapor film and radiation across the vapor film.