Pool boiling and flow boiling are two important heat transfer mechanisms that occur in
multiphase systems.
Pool boiling is a heat transfer phenomenon that occurs when a liquid is heated to a
temperature above its saturation temperature at a solid surface. The liquid adjacent to
the surface vaporizes, forming bubbles that grow, detach from the surface, and rise into
the bulk liquid. The heat transfer rate in pool boiling is strongly dependent on the
surface temperature, the liquid properties, and the surface characteristics.
Flow boiling is a heat transfer phenomenon that occurs when a liquid is heated to a
temperature above its saturation temperature while it is flowing over a solid surface. The
liquid vaporizes, forming bubbles that move along the surface with the flow. The heat
transfer rate in flow boiling is affected by the flow velocity, the liquid properties, the
surface characteristics, and the pressure.
Comparison of pool boiling and flow boiling:
Feature
Pool Boiling
Flow Boiling
Fluid motion
Stagnant
Flowing
Bubble
formation
Nucleate boiling
Nucleate boiling and
convective boiling
Heat transfer
rate
Lower
Higher
Applications
Heat exchangers,
steam generators
Power plants, nuclear
reactors
Nucleate boiling is the most common type of boiling and is characterized by the
formation of vapor bubbles at discrete sites on the heated surface. These sites, which
are typically microscopic imperfections on the surface, are called nucleation sites. As
the liquid is heated, it becomes superheated, meaning that its temperature is above its
saturation temperature. This superheated liquid can spontaneously form vapor bubbles
at the nucleation sites. The vapor bubbles grow as they absorb more heat from the
liquid, and eventually they detach from the surface and rise into the bulk liquid. The
detachment of the bubbles creates a circulation of liquid around the surface, which
further enhances the heat transfer rate.
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Convective boiling is a type of boiling that occurs when the flow of the liquid over the
heated surface helps to transfer heat from the surface to the bulk liquid. This type of
boiling is often referred to as forced convection boiling because the flow of the liquid is
being forced by external means, such as a pump or a fan. In contrast to nucleate
boiling, convective boiling does not require nucleation sites, and bubbles can form
anywhere on the heated surface. However, the heat transfer rate in convective boiling is
typically lower than that in nucleate boiling.
Comparison of nucleate boiling and convective boiling:
Feature
Nucleate Boiling
Convective Boiling
Mechanism
Bubbles form at
nucleation sites
Bubbles form
anywhere on the
surface
Heat transfer rate
Higher
Lower
Need for nucleation
sites
Yes
No
Enhancement of
heat transfer
Liquid circulation
Forced flow
Applications of nucleate boiling and convective boiling:


Nucleate boiling is used in a variety of applications, including:
o
Heat exchangers
o
Steam generators
o
Evaporators
o
Refrigerators
Convective boiling is used in a variety of applications, including:
o
Power plants
o
Nuclear reactors
o
Heat pipes
o
Spray cooling
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Subcooled boiling and saturated boiling are two distinct stages of boiling that occur
when a liquid is heated to a temperature above its saturation temperature.
Subcooled boiling occurs when a liquid is heated to a temperature above its saturation
temperature while the liquid is still in the liquid phase. This means that the temperature
of the liquid is higher than the temperature at which it would start to vaporize. In
subcooled boiling, the heat transfer rate is relatively low because the liquid is not yet
boiling. However, as the liquid is heated further, it reaches the saturation temperature
and begins to vaporize.
Saturated boiling occurs when a liquid is heated to a temperature above its saturation
temperature and the liquid starts to vaporize. In saturated boiling, the heat transfer rate
is much higher than in subcooled boiling because the vapor bubbles are constantly
forming, growing, and detaching from the surface. This creates a circulation of liquid
around the surface, which further enhances the heat transfer rate.
Subcooled boiling is typically characterized by three distinct stages:
1. Initial subcooled boiling: In this stage, the heat transfer rate is relatively low
because the liquid is not yet boiling. The liquid may experience some thermal
expansion and turbulence, but there is no significant vaporization.
2. Transition boiling: In this stage, the liquid begins to vaporize, and the heat
transfer rate increases rapidly. The vapor bubbles form and detach from the
surface, creating a circulation of liquid around the surface.
3. Fully developed nucleate boiling: In this stage, the heat transfer rate reaches its
maximum value. The vapor bubbles are constantly forming, growing, and
detaching from the surface, and there is a significant circulation of liquid around
the surface.
Saturated boiling is typically characterized by four distinct stages:
1. Bubble formation: Vapor bubbles form at nucleation sites on the heated surface.
2. Bubble growth: The vapor bubbles grow as they absorb more heat from the
liquid.
3. Bubble detachment: The vapor bubbles detach from the surface and rise into the
bulk liquid.
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4. Wake entrainment: The wake of the rising vapor bubbles entrains liquid from the
bulk liquid, which enhances the heat transfer rate.
The transition from subcooled boiling to saturated boiling is a complex process that is
affected by a number of factors, including the liquid properties, the surface temperature,
and the surface characteristics.
Here is a table that summarizes the key differences between subcooled boiling and
saturated boiling:
Feature
Subcooled Boiling Saturated Boiling
Liquid Phase
Liquid
Liquid-vapor mixture
Heat Transfer
Rate
Low
High
Bubble
Formation
At nucleation
sites
At nucleation sites and
anywhere on the surface
Bubble Growth
Slow
Fast
Bubble
Detachment
Occasional
Frequent
Wake
Entrainment
Minimal
Significant
Subcooled boiling and saturated boiling are important heat transfer mechanisms that
are used in a variety of applications, including:

Heat exchangers

Steam generators

Evaporators

Refrigerators

Power plants

Nuclear reactors
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Burnout
Burnout, also known as a boiling crisis or departure from nucleate boiling (DNB), is a
sudden and dramatic decrease in the heat transfer rate from a heated surface to a
boiling liquid. This occurs when the liquid film on the surface is completely vaporized,
leaving the surface exposed to a vapor blanket. This vapor blanket insulates the
surface, preventing heat transfer from the surface to the liquid. As a result, the surface
temperature can rise rapidly, leading to damage or melting of the material.
Boiling crisis
Boiling crisis is a more general term for the sudden and dramatic decrease in heat
transfer rate that occurs when a boiling system reaches a critical point. The critical point
is the point at which the heat transfer mechanism changes from nucleate boiling to film
boiling.
Departure from nucleate boiling (DNB)
Departure from nucleate boiling (DNB) is a more specific term for the boiling crisis that
occurs when a boiling system reaches a critical heat flux (CHF). The critical heat flux is
the maximum heat flux that can be transferred from a heated surface by nucleate
boiling. If the heat flux exceeds the critical heat flux, the boiling system will enter the film
boiling regime and the heat transfer rate will drop dramatically.
Dryout
Dryout is a specific type of boiling crisis that occurs when the liquid film on a heated
surface is completely vaporized. This can happen if the heat flux is too high, the
pressure is too low, or the surface temperature is too high.
Boiling transition
Boiling transition is the process that occurs when a boiling system changes from one
boiling regime to another. The most common boiling transitions are the transition from
subcooled boiling to saturated boiling, and the transition from nucleate boiling to film
boiling.
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Here is a table that summarizes the key differences between these terms:
Term
Definition
Burnout
Sudden and dramatic decrease in heat
transfer rate from a boiling surface
Boiling crisis
More general term for the sudden and
dramatic decrease in heat transfer rate that
occurs in a boiling system
Departure from
nucleate boiling
(DNB)
Specific type of boiling crisis that occurs
when a boiling system reaches a critical heat
flux
Dryout
Specific type of boiling crisis that occurs
when the liquid film on a heated surface is
completely vaporized
Boiling transition
Process that occurs when a boiling system
changes from one boiling regime to another
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