Boiling occurs when there
is a phase change from a liquid to a gas, this is achieved when the liquid is
heated to its boiling point 1. The boiling process itself is only a physical change;
there is no chemical change in the molecules themselves 1. The boiling flow
consists of two phases. A phase is a region in a thermodynamic system where all
physical properties are uniform. In a boiling flow, the two phases are liquid
and gas. These two phases interact with each other but this interaction has
proven difficult to accurately model. In order to model this interaction,
empirical correlations are to be used along with validation through software’s
such as CFD 2.
It is necessary to model
this behaviour of the two phases in order to develop a better understanding of
the process. The results can then be used for design improvements and
performing system analyses.
The study of subcooled
boiling have applications in various industries but most notably, in the
nuclear industry. An example of this would be in pressurised water reactors.
3 The void fraction affects the reactivity of the reactor core. As the void
fraction increases, so does the reactivity of the core. Other examples include:
· Gas lift pumps
· Oil wells
The void fraction and other parameters involved in a
subcooled boiling flow can be modelled using different methods. Using CFD is
one of these methods and will be the one used in this project. Therefore the
main objective of this project is to provide the
most updated method in order to model this subcooled behaviour. This will be
done by using calculations and correlations found in research and then to
compare them to the CFD model
Boiling is a very effective heat transfer
method due to the high performance level as a result of the latent heat
transport 4. A high heat transfer affects the size of heat exchanger devise
and the efficiency and performance of any industrial process. Therefore, it is
clear that boiling heat transfer is important to understand, as there are many
applications such as those in industry and even in daily life.
The high heat transfer that
comes as a result of the boiling process is due to the vapour bubbles that form
during the boiling process. The high heat transfer rates comes form the process
of convection vaporization. There is more to be understood about the mechanisms
behind subcooled boiling but it is clear that it is effective when transferring
large amounts of heat.
Two-phase flow is a type of
flow where two distinct phases exist at the same time and interact with each
other. Single-phase flow can be identified by the external geometry of the flow
channel as well as the type of the flow; these include laminar, transitional or
turbulent flow. Two phase flow, however, is more complicated and requires to be
classified according to the internal distributions or flow patterns. 5
Boiling is identified
through bubble formation. These bubbles occur on nucleation sites. These sites
aid in the separation of solids, liquids and gases. In order to achieve this,
the temperature of the surface being heated has to exceed the saturation temperature
of the liquid at a certain pressure. When the heat flux being applied to the
surface of a pipe is too high to transfer the heat to the core of the liquid,
this results in subcooled boiling. The process by which it operates is single-phase
The point at which the wall
temperature reaches the saturation temperature is identified as the onset of
subcooled boiling. As mentioned previously, bubbles form on the heated surface
at nucleation sites. The individual molecules in the liquid are not able to
physically separate; this is because they do not have the weight in order to
overcome the viscosity of the fluid surrounding them. Nucleation sites help
these molecules to separate. The individual molecules coalesce to form bubbles,
these bubbles now have the necessary weight to overcome the viscosity and so
they can now separate. This is typically seen further downstream in the pipe.
The bubbles continue to grow and finally
are able to detach from the surface of the wall after achieving a certain critical
size. This critical size depends on the surface tension and the flow regime of
the surrounding fluid 7
To get a better understanding
of what is going on in the subcooled region, the whole boiling process needs to
be understood. The following diagram shows the different phases involved in the
whole boiling process:
The first phase is called the single phase liquid
region. At this point, simple convective heat transfer is the main process
taking place. The heat applied is able to flow through the liquid; this is done
by means of convection. The heat transfer rate depends on a number of factors,
Type of fluid
Physical aspects may also be altered. The surface
area through which heat is being transferred, thickness of the material through
which heat is being transferred and other surface characteristics of the
conducting solid body. 8
The next flow regime is bubbly
flow. Here bubbles have started to form, they are transported and carried in a
continues liquid phase.
In the slug flow region, the
bubbles that had formed previously are now growing larger as they coalesce.
They have now formed “slugs” of gas. The slugs of gas are also, as a result,
separated by slugs of liquid.
After this comes the wavy flow
region. The slugs are grower larger and gaining more energy. The gas now flows
in a disorganised way. The gas is mainly concentrated through the centre of the
pipe whereas the liquid is situated around the walls.
Finally, annular flow occurs when the liquid
now forms around the wall of the pipe like a film, gas then flows through the
central core and can contain liquid droplets 9Points
A-B show the single phase region. Subcooled liquid flows into the pipe from the
inlet at point A. Here the temperature of the pipe surface is lower than the
saturation temperature so the heat transfer that takes place is single phase.
The wall temperature continues to rise and the subcooled boiling region beings
to develop. At
point C, the onset of nucleate boiling starts. This is the point at which
bubbles start to form. After this point, the wall temperature starts to even.
As more bubbles start to develop, most of the heat transfer comes from the
nucleate boiling and the effect of single phase convective boiling decreases. From
points E-G, the entire surface of the pipe wall is now covered with bubbles.
There is no convective heat transfer at this point. The subcooled liquid is now
nearly at saturation state. Point G is where the bubbles begin to detach from
the wall and join the bulk liquid. Here the void fraction value is of significance.
from G-H, the flow is now in the two-phase region. Here two phase heat transfer
takes place. At point H, the liquid temperature is equal to the liquid
saturation temperature 10