Chemical Engineering - Heat Transfer - Discussion

Vaporization at the surface occurs in the case of film boiling, and in nucleate boiling bubbles are formed first slowly and then vigorously. So, this is a case of film boiling.

In a boiling curve, If we increase the temperature after natural convection, bubbles are started and went to the surface and then evaporated. From this region evaporation starts. So we considered as the vaporization takes place directly at the heating surface, it is called nucleate boiling.

Nucleate boiling:
Is characterized by the growth of bubbles or pops on a heated surface, which rise from discrete points on a surface, whose temperature is only slightly above the liquid's. In general, the number of nucleation sites are increased by an increasing surface temperature.

If a surface heating the liquid is significantly hotter than the liquid then film boiling will occur, where a thin layer of vapor, which has low thermal conductivity, insulates the surface. This condition of a vapor film insulating the surface from the liquid characterizes film boiling.