Today we will discuss about boiling, boiling curve and types
of boiling. Boiling is a major concern for any thermal industry. It is a
phenomenon in which water is converted into steam or we can say it is a phase
change process from liquid state to gases state. When liquid is heated to its
saturation temperature (a temperature at which the liquid starts to convert
into vapor) its vapor pressure becomes higher than the surrounding pressure of
atmosphere. Due to this pressure difference the liquid particles start to move
into atmosphere. This process is known as boiling.
What is boiling?
Boiling can be easily understood by the following
experiment.
Let’s takes water into an open container at room
temperature. The container is open to
atmosphere so the pressure exerted by the surrounding to the water is
atmospheric pressure. Now put the container on fire which added heat and rises
the temperature as well as energy of water. Due to this energy rises vapor
pressure of water also increases. After continuous heating we reach a point at
which the vapor pressure of water becomes equal to atmospheric pressure. This point
is known as boiling point. Further increase in temperature causes to increase in
vapor pressure and it becomes more than atmospheric pressure which causes
movement of water particle into atmosphere. Further rise in temperature
converts whole water into steam. This phenomenon is known as boiling.
The temperature at which the boiling starts is known as
saturation temperature at constant pressure. The saturation temperature of
water at atmospheric pressure is 100 degree centigrade. If the pressure
increases the saturation temperature is also increases.
Boiling Curve and Types of Boiling:
Let’s take a look of the curve given below.
This curve is known as boiling curve. This curve is divided
into four regions according to heat transfer phenomenon. When we continuously
supply heat into water the phase change process takes place. This process
depends upon the surface temperature. These regions sometimes refer as
types of boiling.
Natural Convection Boiling:
AB is the first most regions in which the heat transfer
process takes place by free convection. The water particles associated with the
heating surface take heat from it and start moving at the top. Now the other
particles occupied blank space and start heated from heating surface. This
process continuously run until whole water is converted into steam.
Nucleate Boiling:
The next region BC is known as Nucleate region or Nucleate
type of boiling. This is the most desirable part of boiling which is used by
many industries. In this region, bubbles formation take place. The water
particles associated with the heating surface vaporized and converted into
bubbles. These bubbles move at top of the liquid and collapse. This region can
be further divided into two regions.
The first one is shown by BB’ in which bubbles formation
take place and the bubbles start to move at top side. These bubble passes form
water which is below its saturation temperature so the bubbles transfer its
heat to surrounded water particle and collapse into water.
The other region is B’C in which the whole water converted
at saturation temperature and the bubbles start to pass from surrounded water.
They do not transfer its heat into surrounding and they collapse at the top of the
liquid and release steam into surrounding. This phenomenon of boiling is known
as nucleate boiling. This region provides maximum heat transfer rate. The top most point at boiling curve in
nucleate boiling region is known as critical heat flux point.
Transition Boiling:
The third region is shown by CD which is known as transition
region. This region is nothing but the mediator region of nucleate boiling and
film boiling. This is very undesirable part of boiling curve due to its poor
heat transfer rate. In this region the water particles associated with the
heating surface converted into steam and forms a vapor blanket. This blanket
has poor heat transfer rate thus other liquid particles find hard to take heat
from source. This is the reason behind the low heat transfer rate and the undesirability. The lowest point of transition region is known as Leidenfrost Point.
Film Boiling:
The last but the least part of boiling curve or types of
boiling is known as film boiling. This region has huge heat transfer rate. It
is shown by DE in boiling curve. This region has highest temperature difference
between water and the heat source. When we increase this difference further the
heat transfer rate increases continuously. In this region the most of heat is
transferred by radiation. When we further increase the temperature of heat
source after transition region the density of vapor blanket start decrease and
form a radiation heat transfer region between liquid particles and heat source.
That’s why this region shows significant increase in heat transfer rate.
These are the most common boiling region of boiling curve or
types of boiling. If you have any query regarding this article, ask by
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