This document discusses different types of boiling phenomena including pool boiling and flow boiling. It explains that pool boiling occurs when a stagnant pool of liquid is heated from below, while flow boiling occurs under bulk motion of the fluid. The document also describes the different boiling regimes that can occur during pool boiling, including nucleate boiling, boiling crisis/departure from nucleate boiling, and film boiling. It discusses bubble growth and collapse during boiling and provides diagrams to illustrate boiling heat flux versus temperature difference.

2. Boiling is a phenomenon that occurs at a solid-liquid interface
when a liquid is brought in contact with a surface maintained
at a temperature sufficiently above the saturation temperature
of the liquid. As the heat is conducted to the liquid vapour
interface, bubbles are created by the expansion of entrapped
gas or vapour at small cavities in the surface. The bubbles
grow to a certain size, depending on the surface tension at the
liquid-vapour interface and temperature and pressure.

3.  POOL BOILING
 FLOW BOILING

4. In this case a stagnant pool of liquid is heated and boiling
occurs in the liquid at the bottom of the bulk liquid pool (This
overall process is called pool boiling. To form the vapour phase
within the continuous liquid phase one must heat the liquid to a
temperature above its saturation temperature, Tsat (Tsat is that
temperature at which the liquid exerts a vapor pressure equal
to the ambient pressure

5. HOT SPRING

6. This is the form of boiling which takes place under the
bulk motion of the fluid i.e. there is a multiphase flow
formed inside the container or the conduit where the fluid
is present. It is also called Forced convection boiling
because the bulk flow is obtained via an external device.
One example of such boiling is boiling inside an oil/gas
pipeline.

8. Bubble growth takes place when heat is conducted to the liquid-vapour
interface from the liquid. Evaporation then takes place at the interface,
thereby increasing the total vapour volume.
Force balance for a bubble
Where
=Pressure difference between the liquid and inside of
the bubble
=Surface Tension
r= radius of the bubble

9. For liquid pressure to remain constant, the above equation
requires pressure inside the bubble must be reduced.
Corresponding to a reduction in pressure inside the bubble
will be a reduction in the vapour temperature and a larger
temperature difference between the liquid and inside the
bubble would cause the bubble to conduct the heat to the fluid
and collapse at the site. However, the bubble will likely rise
from the heated surface and the farther away it moves, the
lower the temperature of the liquid becomes and then the
bubble would conduct heat at some other location inside the
bulk of the liquid and would collapse there. But, if the liquid is
superheated i.e. Tliquid >Tsaturation , then the bubbles would rise
up without collapsing, while given that the liquid is
superheated enough ,the bubbles may rise to the surface
without collapsing.

10. The different regimes of boiling in a typical case of pool boiling
in water at atmospheric pressure in a graph, which is the
conventional log-log representation of heat flux vs. superheat.

12.  In the range A-B, the liquid near the wall is
superheated and tends to evaporate forming
bubbles wherever there are nucleation sites such
as tiny pits or scratches on the surface. This
mechanism is called NUCLEATED BOILING. The
bubbles transport the latent heat of the phase
change and also increase the convective heat by
agitating the liquid near the heating surface.
Nucleated Boiling is characterized by a very high
heat transfer rates for very small difference in
temperature.

13. When the population of bubbles become too high at some value
of flux C, the bubbles may obstruct the path of the incoming
fluid. The vapor thus forms an insulating blanket covering the
heating surface and thereby raises the surface temperature. This
is called BOILING CRISIS. The Maximum heat flux just before
reaching the boiling crisis is called CRITICAL HEAT FLUX
(C.H.F.). This situation is commonly referred to as
DEPARTURE FROM NUCLEATED BOILING (D.N.B).

14.  In the range C-D, immediately after the critical flux
has been reached, boiling becomes unstable and
the mechanism is called Vapor film boiling or
Transient Boiling. It has the very unusual
characteristic of a negative slope, i.e., as the
driving temperature difference increases, the heat
transfer rate worsens.

15.  In the range D-E, a stable vapour film is
formed on the heating surface and the heat
transfer rate reaches a minimum value. This
stage is called Stable Film Boiling. The Point D
is called as the point of minimum heat flux or
Leidenfrost Point.

16. Vapor Phase Boiling Around A Hydrophilic Metal
Sphere

17.  It was observed by Leidenfrost in 1756, that during vapor
boiling, the vapor layer thus formed prevents the interaction of
the heating surface and the water droplets. Water droplets are
yet present in the flow. When the temperature exceeds the
Leidenfrost point, the water droplets, as they touch the heating
element, starts evaporating at the bottom and the vapor layer
are formed between the droplet and the heating element. The
droplet does not evaporate completely, but instead starts
shooting off in all distances over the heating element before
slowly evaporating. This vapor layer acts as insulation and
drops the heat transfer rates massively. This effect was called
the Leidenfrost Effect.