All you need to know about Boiling (for ESE)

Dear readers,

Boiling heat transfer is a common phenomenon, from the boiling of refrigerant in evaporator coils of household refrigerator to the boiler of water in steam power plants.

How boiling differs from evaporation?

Evaporation occurs at liquid-vapor interface when the vapor pressure is less than the saturated pressure of the liquid at a given temperature while Boiling occurs at the solid-liquid interface when a liquid is brought into contact with a surface maintained at a temperature T_s sufficiently above the saturation temperature (T_sat) of the liquid.

Boiling is classified into two types:-

(A) Pool Boiling

It means the boiling of stationary fluids. In pool-boiling, the fluid is not forced to flow by any external source, any motion of fluid is due to natural convection.

Boiling curve in Pool-boiling

We know,

Convection heat transfer= hA (T_s – T_sat)

where T_s – T_sat =   d T_excess

Depending upon the value of d T_excess , four different boiling regimes are observed.

Various Regimes in Pool Boiling:-

i) Natural-Convection Boiling: The liquid is heated above the saturation temperature in this region.So,the liquid is slightly superheated  in this case and evaporates when it rises to the free surface.The heat transfer from heating surface to the fluid is by natural convection.

ii) Nucleate-Boiling: The first bubble start forming at point A of boiling curve at various preferential sites on the heating surface.

This regime can be separated into two regions.In region A-B,isolated bubbles are formed at various preferential nucleation sites on the heating surface.But those bubbles are dissipated in the liquid shortly after they separate from the surface.

In region B-C, the heater temperature is further increased and bubbles formed at very high rate.These bubbles move up to the free surface and break up releasing their vapor content.

iii) Transition Boiling:  As d T_excess is further increased,the heat flux starts decreasing because  a large fraction of the heater surface is covered by  vapor film,which acts as an insulation due to the low thermal conductivity of the vapor relative to that of liquid.This is also called unstable film boiling regime.

iv)Film Boiling:  In this region,the heater surface is completely covered by a continuous stable vapor film.The 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 high temperature.

(B) Flow-Boiling

In Flow boiling,the fluid is moved,by an external source such as pump as it undergoes phase change process. The flow boiling is also classified as either external or internal.

i) External flow boiling:  Boiler over a flat plate is called external flow boiling. It is similar to pool boiling, but the added motion increases both,the nucleate boiling heat flux and critical heat flux.

ii) Internal flow boiling:  It is much complicated in nature because there is no free surface for the vapor to escape and thus both the liquid and vapor are forced to flow together.The different stages are shown in figure.

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