06-09-2012, 05:27 PM
Boiling
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Boiling: the process of addition of heat to a liquid such a way that generation of vapor occurs.
Evaporationoccurs at the liquid-vapor interface
Pv < Psat of the liquid at a given temp
evaporation involves no bubble formation or bubble motion
Natural convection Boiling
Governed by natural convection currents
Heat transfer from the heating surface to the fluid is by natural convection
Nucleate Boiling
The stirring and agitation caused by the entrainment of the liquid to the heater surface is primarily responsible for the increased heat transfer coefficient and heat flux in the region of nucleate boiling.
High heat transfer rates are achieved in nucleate boiling
Transition Boiling (Unstable film boiling)
Heat flux decreases because of larger fraction of heater surface is covered by a vapor film which acts as a insulation because of the low thermal conductivity of the vapor relative to the liquid
Film Boiling
The presence of a vapor film between the heater surface and the liquid is responsible for the low heat transfer rates
Heat transfer rate increases with increasing excess temperature as a result of heat transfer from the heated surface to the liquid through the vapor film by radiation, which becomes significant at higher temperatures.