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Pool Boiling

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by

Mostafa Sobhy

on 25 July 2015

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Transcript of Pool Boiling

POOL BOILING HEAT TRANSFER
Presentation Content
THEORY
APPLICATIONS
WHAT IS BOILING
Boiling is a liquid-to-vapor phase change process
So, Is it same as Evaporation ?
So, What is the different ?
No, It is not
Classification of Boiling
Pool Boiling
Pool Boiling
Boiling is called pool boiling in the absence of bulk fluid flow.
Any motion of the fluid is due to natural convection currents
the motion of the bubbles under the influence of buoyancy.
Pool Boiling Stages
Bubbles do not form on the heating surface



The fluid motion is governed by natural convection currents



natural convection boiling ends at an excess temperature of about 5°C.
The bubbles form at an increasing rate
The heat flux at end of th stage is called the critical (or maximum) heat flux
(5°C < T < 10°C), isolated bubbles are formed
(10°C < T < 30°C), bubbles form at such great rates columns of vapor in the liquid
When (T) is increased , the heat flux decreases
Why?
This is because a large fraction of the heater surface is covered by a vapor film, which acts as an insulation
the heater surface is completely covered by a continuous stable vapor film.
The presence of a vapor film is responsible for the low heat transfer rates in the film boiling
The heat transfer rate increases with increasing excess temperature due to radiation to the liquid
Enhancements of Heat Transfer in Pool Boiling
The rate of heat transfer strongly depends on the number of active nucleation sites on the surface, and the rate of bubble formation at each site.
Therefore, any modification that enhances nucleation on the heating surface will also enhance heat transfer in nucleate boiling.

It is observed that irregularities on the heating surface, including roughness and dirt, serve as additional nucleation sites during boiling,
Pool Boiling Applications
Cooling Applications
Evaporators
In ACs

Pool boiling is of interest in
high heat flux applications
because of its potential for removing large amount of heat

resulting from the latent heat of evaporation
and little pressure drop penalty for circulating coolant through the system.
Enhancements
Flow Boiling
Subcooled Boiling
Saturated Boiling
Berensen has shown that heat flux in the nucleate boiling regime can be increased by a factor of 10 by roughening the heating surface.

the effect of surface roughness is observed to decay with time, and the heat flux to drop eventually to values encountered on smooth surfaces
Surfaces that provide enhanced heat transfer in nucleate boiling permanently are being manufactured and are available in the market.
Special surfaces is achieved either by coating the surface with a thin layer (much less than 1 mm) of very porous material or by forming cavities on the surface mechanically
Such surfaces are reported to enhance heat transfer in the nucleate boiling regime by a factor of up to 10,
Evaporator
Boiler
Kattle
In the transition boiling regime, both nucleate and film boiling partially occur
where the heat flux reaches a minimum is called the
Leidenfrost
point.
THEORY
Leidenfrost
Boilers
Boilers are enclosed vessels that provide heat to water until become Super heated or staurated Steam.


Boilers are used in "Electric Power planet - Distilation Process...etc"


Most common boilers used pool boiling theory
Waste Heat Boilers "HRSG"
Nuclear Steam Generating Systems
Fire Tube Boilers
Distilation Process
An air separation plant separates atmospheric air into its primary components, typically nitrogen and oxygen,
The most common method for air separation is cryogenic distillation.
Transform crude oil into useful products such as liquefied petroleum gas (LPG), gasoline or petrol, kerosene, jet fuel, diesel oil and fuel oils.

The crude oil distillation unit (CDU) is the first processing unit in virtually all petroleum refineries.
Cooling of nuclear Reactor by Coolant Boiling

Cooling of electronic equipment

Regenerative Cooling of rocket motor

When the liquid refrigerant reaches the evaporator its pressure has been reduced, dissipating its heat content and making it much cooler.
This causes the refrigerant to absorb heat from the warm air and reach its low boiling point rapidly.

References
Wikipedia
SciFri YouTube Channel
Nucleate Pool Boiling Experiment By NASA


Prepared By Pool Boiling Team Members
Under Supervision of Dr H.Elsawy

Mohamed Roshedy Saber
(Searcher)
Mostafa Sobhy Ramadan
(Presenter)
Hamed Yousry
(Searcher)
Full transcript