Namaste. My name is S P Sukhatme and along
with my colleague professor U N Goitonade, I will be giving you a series of lectures
on the subject of heat and mass transfer. We are from the department of mechanical engineering
at the IIT Bombay. Now the subject matter which we will be covering under these lectures
is the syllabus as is prescribed for the subject 'heat and mass transfer' in most universities
in India and we will be doing this through about thirty odd lectures. We are from the
department of mechanical engineering as I said. So, the matter that we will cover will
be primarily from the point of view of mechanical engineering students. However, heat and mass
transfer is an important subject also in the chemical engineering curriculum, in the aeronautical
engineering curriculum and also taught, parts of it are also taught in other disciplines.
So, although we will be covering the subject from the point of view of what is the syllabus
in mechanical, what we have to say would, I think, be of interest also to students from
other disciplines. We will proceed something like this. I will give you an outline of the
lectures which we are going to give. First of all, we will have an introduction to the
subject covering a few lectures to cover the laws - the basic laws - that govern the subject. Then, we will move on to the topic of heat
conduction in solids, then thermal radiation, then the mode of heat transfer by convection
and in this we will talk first of forced convection, then natural convection, then we will go on
to change of phase. Change of phase means: either during the heat transfer process a
liquid gets converted into vapor because it receives heat - latent heat - which converts
it from liquid to vapor or heat is taken out of it and therefore it condenses and from
the vapor state it becomes liquid. Now, during this heat transfer process what is the rate
at which heat transfer occurs forms the subject matter of the topic condensation and boiling.
Then we move on to the topic of heat exchangers. Heat exchangers are devices which are widely
used for a variety of purposes in many applications to transfer heat from one fluid to another
- one fluid at a higher temperature, one fluid at a lower temperature. And we will talk about
the thermal design and the working of such heat exchangers and then finally we will go
to the topic of mass transfer and introduce the elements of mass transfer. Now, you may ask me the question why is mass
transfer taught alongside heat transfer when really we are covering heat transfer through
most of these lectures. And the answer is something like this. The process of mass transfer
has many similarities with the process of heat transfer. Heat transfer occurs when there
is a temperature difference. Mass transfer occurs when there is a concentration difference.
The equations describing these are very similar or analogous and therefore when we derive
a relation - an equation - for a particular heat transfer situation it is very often true
to say that that relation - with some modification - is also valid for a corresponding analogous
mass transfer situation. So, the purpose of introducing you to mass transfer is to point
out the similarity so that you can use heat transfer relations for studying certain types
of mass transfer problems. As far as the books for this subject are concerned, there are
a variety of books; there are many books written and are available to cover the syllabus. The
two books which I am putting down in front of you - the first is which I have written
- 'A Textbook on Heat Transfer' - the fourth edition of the book. It is by Universities Press. This is a book
which we will be following to a large extent but not all of it because it goes much beyond
the syllabus that is normally prescribed in the undergraduate curriculum. The book which
we will also be referring to is the book by Incropera and Dewitt on fundamentals of heat
and mass transfer. It is widely used in India, widely used in the US, has been used for the
last twenty years - an excellent book with a lot of practice oriented problems. So, these
are two books which would be useful to you to refer to but there are many more. And the
important thing is while you are going through these lectures, we will be also doing certain
numerical problems for you. Now you will have to do some problems on your own also and that
is why you will automatically need to refer to certain text books or reference books to
do further problems on your own. Now let us begin with the introduction. The first thing we ask ourselves is - what
does the subject of heat transfer deal with? What is it all about? Why is it important?
And then we ask the same question for ourselves about mass transfer. What does the subject
of mass transfer deal with? What is it all about? Why is it important? So, let us take
up heat transfer first. Now, first of all, when does heat transfer
occur? Now, whenever there are temperature differences in a body, we know from experience
that these temperature differences are reduced in magnitude in the course of time by heat
flowing from the regions of high temperature to the regions of low temperature. The body
under consideration may be in the solid state, it may be a liquid or it may be in the gaseous
state. It doesn't matter which state it is. The point is when there are temperature differences,
we know from experience, heat flows from the region of high temperature to the region of
low temperature. The subject dealing with the rate at which this heat flow occurs, I
emphasize again 'rate', the subject dealing with the rate at which the heat flow process
occurs is called heat transfer. Now, it is important straight away to distinguish the
subject of heat transfer from the subject of thermodynamics which all of you must have
studied a little earlier. You must have studied the first law, the second law, certain power
cycles and so on. In thermodynamics, normally when we have a
system in a certain state and that system undergoes certain heat and work interactions;
because of those heat and work interactions, the system goes from one equilibrium state
to another equilibrium state and during that shift from one equilibrium state to another
equilibrium state, because of the heat and work interactions the system goes - attains
a certain state which it is described by temperature, pressure, etcetera, etcetera. Now in heat
transfer, in thermodynamics, we are not ever generally asking the question how much time
goes in that process. We never concern ourselves with the rate at which that heat interaction
takes place. On the other hand, in heat transfer we say because there is a temperature difference
heat flow occurs; what is the rate at which that heat flow is occurring? And at a certain
point if I want only a certain temperature to be attained, how much time would it take
- that is the kind of questions we will ask. So, it is a subject which is dealing with
the rate at which heat flow occurs. That is the distinction between what we study in thermodynamics
and what we study in heat transfer. Now, why is it important? Why is it important
to study heat transfer? It is important because once we have these laws which govern the process
of heat transfer, we will be in a position to design equipment - size equipment - in
which the heat transfer process occurs. Now, let me given an example so that, you know,
you will understand what I mean. All of you have sat in a car; all of you have seen a
car radiator sitting in the front of the car. If you open the bonnet, right in front there
is a very small rectangular box-like structure which is the car radiator. What does the car
radiator do? The car radiator receives hot water which has come from the engine cylinder
walls; that hot water typically is at about say 95 degrees centigrade and that hot water
is cooled by air which flows over that radiator - cooled by 5 or 10 degrees - and then again
circulated round the cylinder walls. So, the hot water picks up heat in the cylinder walls
and gives up heat in the radiator and this way it maintains the cylinder walls at a particular
temperature - safe temperature. In the radiator, it gives up heat to the air which flows over
it; that air is pulled by a fan which is behind the radiator. So, the purpose of the radiator
is to take away heat from the hot water and give it to air which is the environmental
air, the surrounding air. That device - the car radiator - is a heat exchanger. To be able to design that heat exchanger,
you need to understand the process of heat transfer - the convective process of heat
transfer occurring on the air side, occurring on the water side, the conduction process
that is occurring in the fins and the tubes which make up the radiator and then only can
you design that car radiator. So, the whole object of studying heat transfer is to be
able to design size devices in which heat transfer takes place and the car radiator
is a good example because as you well know, millions of car radiators are made every year
for the millions of car which drive us all over the world. So, that's just one example
to illustrate why this subject is of importance. So, during this course we will be deriving
such equations in convection, in radiation, in conduction so that we can design heat transfer
equipment; by design, I mean find the appropriate size for a given end state that you are desiring
in your fluid. We would be able to size heat exchange equipment and be able to do elementary
design - that is the whole object of teaching this subject. Now, let me move on to mass transfer. Just
like I said in heat transfer, if there is a temperature difference, we know that heat
flows from the region of high temperature to the region of low temperature. Similarly,
if there is a concentration difference, we know that mass moves from the region of high
concentration to a region of low concentration. Let me again take an example. Take this room.
This room contains air - oxygen and nitrogen in a proportion of say four is to one typically.
Nitrogen, oxygen in the ratio four is to one approximately, all right. Let us say in the
corner there in that room at the top there, I hold a cylinder of a compressed gas of nitrogen
and let some nitrogen out. Obviously, in that corner - top corner - there the concentration
of nitrogen in the air is going to increase compared and going to be higher compared to
the concentration elsewhere in this room. What will be the net result if I let out the
certain amount of nitrogen? Immediately, that nitrogen will move and diffuse in this room
so that eventually the concentration will again be uniform in this room. There is a
high concentration of nitrogen there; there is a lower concentration here. Nitrogen will
move in this direction, will diffuse in this direction so that that concentration difference
is reduced. So, mass transfer occurs. In this case, the mass being transferred is nitrogen.
The species being moving is nitrogen. Mass transfer occurs when there is a concentration
difference. Heat transfer occurs when there is a temperature difference. All right, now
mass transfer is important also in a variety of processes. For example, to give you an
example, let us say we have to dry a particular surface - a film on a particular surface.
This is done all the time in many pieces of equipment. We have a film of liquid. We need
to dry it. In order to dry that film, you pass air over that film and the air picks
up. The film of water or liquid - whatever it is - evaporates into the air and mass transfer
takes and it dries. What is the rate at which that mass transfer will occur? What should
be the size of that surface so that I will be able to get the required film thickness
totally removed by the time the air moves over a certain region over that surface? These
are the kind of question we have to answer in order to size mass transfer equipment for
a particular purpose. So, when we study mass transfer, we will be able to design equipment
in which mass transfer occurs. Now, to further our understanding let us look at some problems
of interest in heat transfer. It is important that you try to appreciate
the width of the subject, the breadth of the subject before we take up - really get into
the equations which govern the laws, etcetera. I am going to look at three problems. First
of all, I am going to look at the problem of heat loss through thermal insulation on
a steam pipe. That is the first problem. Then, I am going to look at the problem of heat
transfer to water flowing through a tube and then I am going to look at heat transfer in
an electric furnace. These are three problems of interest in heat transfer and we look at
them one by one. The first one now: heat loss through thermal insulation on a steam pipe. Steam is widely used in manufacturing process
industries. All of you know that. Suppose, let us say I have a big plant with a lot of
workshops - sheds all over - and steam is needed in each of those sheds. Typically,
one may have a central facility where a steam generator is located. Steam is generated and
there will be pipes - pipelines - which carry that steam to the various sheds. Even within
the shed, there will be long pipelines along the walls which carry the steam all along
the length or the breadth of that shed so that it can be delivered at a particular point
where it is required. Now, we have gone to a lot of trouble to generate
that steam in a steam generator, you know, with a burnt oil or something like that, given
heat to water and raise that steam at the pressure required. So, here in this example
as you can see, we have talking of steam at 5 bar and 170 centigrade - super heated steam
at this condition - and we are passing it through a pipe. Having gone to all the trouble to generate
that steam, obviously we don't want that steam to condense or loose heat so we put insulation
around it. Insulation may be in the form of some fibrous insulation like mineral wool,
glass wool, etcetera which is put around and a certain thickness of insulation is put on
the pipe. The question before the heat transfer engineer is what should be the thickness of
insulation to put? Now let me just draw a sketch so that, you know, we will understand
things better. Let us say, let me draw a graph, let us say in this graph on the x-axis, I
have the thickness of insulation. On the x-axis I have the thickness of insulation which is
put around the pipe. Let us say thickness of insulation in centimeters and on the y-axis
I have the heat loss rate in watts per meter that is heat loss rate
per unit length of the pipe. Let us say, when I have no thickness of insulation,
there is no insulation on this pipe. The amount of heat being lost is as indicated by the
cross here. That is the amount of heat being lost. Obviously if I put insulation on this
pipe the heat loss rate is going to decrease. Typically, I will get a graph something like
this - as the more and more insulation is put, the heat loss rate watts per meter will
go on decreasing. Now, the problem before the heat transfer engineer who is to design
this and decide what thickness to put is the following. He is told - restrict the heat
loss rate to a particular value - let us say given by this as I mark in here. So, find
the thickness of insulation to restrict the heat loss rate to the value specified here.
In which case, having done this calculation and got this graph for a particular insulation
that he is using, he will go horizontally, then go vertically and say this is the thickness
of insulation needed. Or the problem may be in reverse. He will
be told - we are going to put 3 centimeters of insulation or 5 centimeters thickness of
insulation. What will be the heat loss rate as a consequence? In which case, given a certain
thickness, he will go up like this. Having got this graph to this graph, then draw horizontal
line like this and say this is the heat loss rate and therefore, from here to here that
is from this point which is the heat loss rate without insulation to the heat loss rate
because of the insulation, this is the amount of reduction in the heat loss rate. So, because
of his knowledge of heat transfer and heat conduction occurring in the insulation, the
heat transfer engineer will be able to either decide on the thickness of insulation to put
or find the heat loss rate for a given thickness of insulation. This is a typical problem for
calculation which we encounter. Now, let us go to the next problem that I mentioned - that
is heat transfer to water flowing through a tube. Now, here is a tube. The diameter I have given
is 2.5 centimeters for this tube. Water is entering it at 30 degrees centigrade. On the
outside of this tube, we have steam - low pressure steam - at 50 degree centigrade condensing
on the outside of this tube. So, obviously heat is going to flow from the outside to
the inside and this water which is flowing through the tube is going to get heated up
and going to go on increasing in temperature as it moves around the length of the tube. Now, where does this situation occur? This
is the situation which will typically occur in a steam condenser. You have power plants.
In a power plant, from the turbine exit you have low pressure steam. You need to condense
that steam with the help of cooling water and then raise the pressure of that condensed
water, then put it back into this steam generator. That is the Rankin power cycle. So, this tube
which I am showing you would actually not be by itself, but would be one tube in a large
bundle of tubes on which steam would be condensing. I am showing one as an example. So let us
say coming back to the single tube - what is the problem before the heat transfer engineer?
The problem is the following - steam condensing on the outside at 50, cooling water entering
the inside of the tube at 30. The problem before the engineer is if the length of the
tube is 2 meters - 2 I have taken as an example - what would be the exit temperature To of
the water leaving this tube or vice versa? Suppose I specify that I want an exit temperature
of say 35 degrees centigrade. Then what should be the length L of the tube in order to have
an exit temperature of 35 or 40, whatever it is. Obviously, the highest temperature
that this steam, this water, can attain-because steam is condensing at 50 - is fifty and that
would be attained with an infinitely long tube. So, the heat transfer engineer who is
designing for the situation will typically have to tell what would be the length of the
tube for a given exit temperature or what is the reverse problem. That means, given
a length, what would be the exit temperature? Now, let us go on to the third problem which
I mentioned. The third problem is heat transfer in an electric furnace. Here we have steel strip which has been rolled
in a steel mill undergoing a heat treatment process. Now, typically a steel strip would
be a few millimeters in thickness. It would be may be a millimeter in thickness and may
be a few centimeters wide. It has been rolled by some rolling process and it has been formed
into a bundle after the rolling process. Now during the rolling process, because of
the deformation that has taken place, the steel loses certain properties - certain desirable
prosperities like ductility, malleability, etcetera, etcetera. We want it to regain these
properties and typically for that, one does some kind of annealing heat treatment process.
The heat treatment process here consists in heating that steel strip up to a temperature
which is specified for steel heating it just above that specified temperature and then
allowing the steel strip to cool down slowly. So, the job of this furnace is to heat the
steel strip up to a temperature required for that heat treatment process to occur, some
temperature usually around 600-700 centigrade of that order. So, the problem before the heat transfer engineer
would be the following now. He is told here is an electric furnace. The temperature in
this furnace is say 1200 degrees centigrade or 1000 degrees centigrade. It is required
that the temperature of the steel strip at the exit be 720 centigrade or some temperature
like that. The steel strip is entering at room temperature - 30 degree centigrade - and
is flowing, moving with a certain velocity which is specified. What should be the length
of this furnace in order that you get this desired exit temperature or the reverse problem.
Given a certain length, what should be the velocity with which that steel strip should
move so that the required exit temperature is attained. That exit temperature must be
just above the annealing temperature for that particular steel so that when it goes out
it is just above the temperature. Then, it cools downs slowly and during that cooling
process which is at a slow rate, after that cooling process it acquires the desirable
properties - mechanical properties - that we are looking for. So, this is a typical
heat transfer problem an engineer would face Now, I could go on, of course, giving you
examples but I think you get the idea. But let me just show you one more example of a
problem which is of current interest. We are in a situation now-a-days in which electronic
circuits or electronics plays a key part in our lives and miniaturization in electronics
is the key word today, VLSI for instance or VVLSI, whatever it is. Now, we go on making
transistors, thousands and hundreds of transistors over very small areas compared to the old
days when hundred transistors might occupy a whole table top, for instance like this.
Now, transistors generate heat, remember that, and it is a requirement that if the transistor
is to operate well, its temperature inside must not exceed some safe value. In fact,
there are two temperatures - one temperature is a temperature which if exceeded will completely
stop the transistor from functioning. That should obviously never be attained. But also,
it is known that if the temperature works at high temperature, its life - its period
- for which it will work consistently reduces. We would like say design specification, may
be you want the transistor to work for a few thousand hours. In which case, it will be
specified that the surface temperature should never exceed a particular value - typically
50 centigrade, 55 centigrade, typical values that we have. Now let me just draw for you
a situation. Let us say, this is what you call a board.
This is a board on which we have a number of electronic chips. Let me just draw a few
as an example. This is a board and let us say on this board I have a number of electronic
chips like this -1 2, I am just drawing 3. If I had a plan view actually, there would
be, there may be an array like this. 1 would be like this in the plan view, 2 like this
and so on. So, it could be a rectangular array of chips. Typically, each of these may be,
let us say a centimeter by centimeter by may be a millimeter high and within it they have
all the electronics that we are talking about. Now a requirement would be, the heat that
is being generated from these chips would be flowing out like this. This is how the
heat is flowing - shown by the red arrows. The requirement before the heat transfer engineer
would be that the surface temperature- the surface temperature here Ts or Twall - whatever
we want to call it should be or should not exceed a value like say 50 degrees centigrade
or 60 degrees centigrade. That is a typical specification. So the heat must flow out,
which is flowing out of this will obviously raise the temperature of the surface but that
temperature must never go above a temperature which is specified, typically 50-60 or something
like that. So what we do? One is to hold down the temperature is, we may have say, for instance,
air blown over these electronic chips. We may blow air over these chips. The air may
be say entering at 30 degree centigrade. We ensure therefore, this surface temperature
never exceeds this specified value. Now the problem before the heat transfer engineer
is at what velocity should this air be blown so that this surface temperature is not exceeded?
That is a typical convective heat transfer problem for which you need equations, data,
the flow around the electronic chips and so on to be analyzed. So here is another example
- the cooling of electronic chips as an example which would be of interest to us as we go
along. Now that we have a feeling of the types of problems that we are going to handle, let
us discuss the modes of heat transfer. There are three modes and all of you must
have heard of these. You have done twelfth, you have done heat, studied heat. The three
modes are conduction, convection and radiation. Let us take them up one by one. Conduction
first. First of all the definition, then let me explain what we mean by, what the definition. Conduction is the flow of heat in a substance
due to exchange of energy between molecules having more energy and molecules having less
energy. It is the flow of heat in a substance - the substance may be a solid, it may be
a liquid or it may be a gas, doesn't matter what is the state - due to the exchange of
energy between molecules which have more energy and molecules which have less energy. Molecules
having more energy are at a higher temperature, molecules having less energy are at a lower
temperature. All right? That is conduction, the flow of heat because of this situation.
Now, let me just draw a sketch or to two to illustrate ideas, what we mean by conduction.
First of all let us consider a solid, a solid structure. A solid typically has molecules or atoms.
Let us say the molecules like this. I am just drawing a set of molecules, they may not be
in a square array, whatever be the pattern. A set of molecules making up this solid and
typically these solids which form a lattice will be vibrating in that solid. More vibration
means a higher temperature. So, when we say that - let us say the molecules here, these
molecules are at higher temperature, these molecules are at a lower temperature which
are at some distance away. Then the vibrations here will be higher than the vibrations here.
There will be more energy associated with these molecules than with these and because
of these vibrations they will tend to interact with the next set and the next set and the
next set and transfer that energy from one molecule to the next to the next. So, the
transfer of energy by conduction is because of the lattice vibrations and these vibrations
are more because of the higher temperature region than in the lower temperature region. So, one - in a solid - one mode of mechanism
by which the transfer of energy takes place is lattice vibrations, lattice vibrations.
Now, solids if they are in the metallic form also have free electrons. That is the structure
of metallic solids. Free electrons are the basis on which electricity flows in solid.
When electricity flows, it is these electrons that are flowing. In the same way, those electrons
also carry energy which is heat transfer by conduction. So, the free electrons in a solid,
when there is a temperature difference, also form the basis for transfer of heat by conduction.
So, a second mode by which conduction takes place in a solid - if it is a metal mind you,
non-metals there are no free electrons - is the motion of free electrons. These are the
two modes, the second for metallic solids, the first for solids in general - non metallic
solids. Now unlike a solid, in a liquid or a gas that
is a fluid, in a liquid or a gas, molecules have freedom of movement. In a solid, they
are restricted to a particular point where they will be vibrating or something like that.
In a liquid that is not so or in a gas. Molecules have some freedom. Let us say there is a set
of molecules making up a liquid or a gas. Well, those molecules have some freedom of
moving around in some fashion or the other. You follow, and they move over short distances
in the solid or liquid in a random fashion. Now in an overall sense, the liquid or gas
may be stationary in a macroscopic sense - MA - macroscopic sense. The solid or liquid may
be stationary but in a stationary liquid or gas, there is always this random motion taking
place over short distances. In a gas, those distances are a little longer, in a liquid,
those distances are a little smaller. Now the transfer of energy which occurs due to
collision between molecules when they move in this random fashion over short distances
within a liquid or a gas is also what we call as conduction in a liquid or a gas. So, in
a liquid or a gas the transfer of energy occurs due to collision of molecules. Molecules at
a higher temperature giving their energy to molecules at a lower temperature but this
is, remember, the random motion that is taking place. In an overall sense the liquid or the
gas is stationary. So this is how the conduction mode occurs in various substances - solids,
liquids and gases. Now, in convection which is the next mode
- we just now mentioned that in a liquid or gas there is this random motion of the molecules
taking place - now in addition to this random motion, fluids can be made to move on a macroscopic
scale in a fluid or a gas. They can be made to move by forcing them to move. Say I have
air in a pipe, I blow that air through the pipe so I can make the air move because by
forcing it to move or I can make the air move because of creating temperature differences.
Temperature difference causes a density difference which causes a movement of the air. You understand?
So, fluids can be made to move in a microscopic sense, in a fluid which may be in the liquid
or in the gaseous state. The transfer of energy - let me read this out. The transfer of energy from one region to
another due to macroscopic motion in a fluid, added on to the energy transfer by conduction
- which I have described earlier - is called heat transfer by convection. That is the meaning
of convection. We already have conduction taking place if we have temperature differences.
In addition if I have a certain movement in that liquid or gas - the movement may be caused
by a temperature difference or it may be forced, some macroscopic motion - then energy being
moved from one point to another because of the movement of molecules. So, the transfer
of energy due to this microscopic motion added on to the energy transfer by conduction which
is occurring because of random motion, the two together are what we call - the sum of
the two together - is what we call as heat transfer by convection. And as I mentioned
a moment ago and let me repeat that, if I force the flow to move in a particular manner
then it is called forced convection - that is the fluid motion is caused by an external
agency. And on the other hand if the fluid motion
occurs due to density variations caused by temperature differences, then that is called
as natural convection. So, convection is of two types: forced convection, when I cause
the fluid motion to be caused the fluid motion is caused by an external agency and natural
convection, when the fluid motion occurs due to density variations which are caused by
temperature differences. Now the third mode is radiation which is quite different and
let me read out the definition. All physical matter emits thermal radiation
in the form of electromagnetic waves because of vibrational and rotational movements of
the molecules and atoms which make up the matter. This is known from physics. The matter
may be again in any state - it could be a solid, it could be liquid, it could be a gas,
it could be a plasma - it doesn't matter what it is. If it is at a certain temperature level,
whatever be the temperature level - it is known it emits radiation in the form of electromagnetic
waves and this is emitted because of the vibrational and the rotational movements of the molecules
and atoms which make up that matter in solid, liquid or gaseous state. This is known. What
are the characteristics of the radiation? The characteristics are, the characteristics
of this radiation are: No.1 - the rate of emission, the characteristics of this radiation
are, let me read that, write that out. Number 1: the radiation increases with temperature
level. Let me write that out. Number one characteristic, rate of emission
rate of emission increases with temperature level - the higher the temperature, the more
the radiation emission by thermal radiation. Rate of emission increases with temperature
level. That is one characteristic to note. A second characteristic to note is - we do
not require any material medium for the energy transfer to occur. It is in the form of electromagnetic
waves. Electromagnetic waves can go through a vacuum. They don't need any material medium
like air or a gas or a liquid for transfer of energy. So, we do not require - no material
medium required for energy transfer to occur. These
are two characteristics to note. Radiation - thermal radiation - in the form
of electromagnetic waves does not require any material medium for its transfer. So,
now we have discussed the three modes of heat transfer - conduction, convection and radiation.
And now if you go back to the problems that we described a movement earlier, you will
recognize immediately that the first problem - thermal insulation around a pipe - is primarily
a problem of heat conduction primarily, the second problem that we talked about - flow
of water in a tube - primarily a problem of heat transfer by convection and that too forced
convection and the third problem of annealing steel strip primarily a problem of radiative
heat transfer - radiation. So, these were three problems and you will
notice these problems were of each of these three modes, the first of conduction - primarily
of conduction, second - forced convention and the third - thermal radiation. But I don't want you to get the impression
that this is so always. In general, problems in heat transfer are not amenable to this
kind of separation. Very often - more often than not - the problem that occurs or a situation
that one faces is one in which all the modes of heat transfer occur together and one must
be able to tackle the problem by understanding these modes and analyzing them together. So
I don't want you to give the impression that the situations always occur in which one can
separate and say this is the mode or that is the mode. There are very often situations
in which the modes will occur - all will occur simultaneously and one needs to take account
of. Now, I am going to give an example of a situation
in which all these modes are occurring. We are going to take up one example like that.
All of you must have heard the word 'energy crises'. Now energy is on the - is a dominant
part, read the news paper. Every day, there is talk of the price of oil that price of
oil is increasing. We have to import oil in India - lots of it. We have to pay for it.
So any increase in the price is something which we feel very much in India. So, energy
is dominant in our thinking and is going to be dominant in our thinking because much of
our energy requirement - much of our commercial energy requirement - in India and in many
countries is met by fossil fuels. Fossil fuels means coal, oil and natural gas and all these
are depleting because they are not renewable, they are decreasing. So, we need to, therefore,
look for other ways of getting energy. Solar energy is one such option. The solar energy
option, particularly the direct form is one such option. Now let us look one way of using
solar energy which, to illustrate different modes of heat transfer. I am going to show
you and you will see it here. I am showing you here on the screen an array
of what are called as solar flat plate collectors. You are seeing four collectors here, here
each of them typically is about 2 meters by 1 meter and they are used for heating water.
An array like this would heat water typically to a temperature like 60 70 80 degrees centigrade
with ease and we need hot water for so many applications at home and hotels, hospitals,
canteens, etcetera, etcetera. So, this is one application of solar energy which is a
very viable application which pays for itself fairly soon. It is being widely used in India.
Now, let us look at one such solar collector. There are four in the picture. I am going to now look at one - this is one
collector - so that we see the inside of it, what it looks like. A solar collector - this
is 2 meters in length, 1 meter in width as I mentioned. Typically on top there is a glass
cover which you had seen in the picture and below that there is an absorber plate. This
is the absorber plate. It is a thin copper sheet, a millimeter or so in thickness. On
that absorber plate or on the backside of it, a number of flow tubes are soldered and
the water which is to be heated flows in these tubes. So it enters here at the point which
is called the inlet - flows in this header which I have shown here, which I am pointing
out here - then gets distributed in these flow tubes and then goes out through the outlet
header. That is the position. So the water, the solar energy falls on the flat plate collector
and the water as it flows through, gets heated and typically as I said you can get hot water
at temperatures like 60 70 80 degrees centigrade in this collector. Now, let us look at a cross
section of this collector. What would it look like? A cross section of this collector would
look like this. This is the casing of the collector. This
is the absorber tube, the absorber plate, these are absorber tubes below it and this
is the glazing, the glass cover on top. This is how the cross section would look. Solar
energy falls on this, is falling on this solar flat plate collector and heating up the water
which is flowing through these tubes. Now, today we will stop here but next time I will
look at the different modes of heat transfer that are occurring in this device. So, we
will take off from this point in the next lecture. We have described a solar flat plate collector.
You know what are the components that make it up - an absorber plate, tubes, a glass
cover on top and on the back side here. There is insulation to prevent heat loss to the
sides and the bottom. These are the components that make up the plate collector. Next time,
in the next lecture, we will look at the different modes of heat transfer that occur within this
device and write down a simple energy balance equation for it. So today, we are going to
stop here but let me recapitulate for you what we have done today so that we get a feel
again for what we have achieved. Today for instance, we have done the following. First of all, I have outlined the subject
that we are going to cover in thirty lectures - given an outline of it, what are the different
topics that we are going to cover. Secondly, I have talked about what the subject is all
about - the subject of heat transfer and the subject of mass transfer. Then I described
for you a number of problems of interest in heat transfer. Then, I took up the different
modes of heat transfer - conduction, convection and radiation and towards the end I started
talking about the example of a solar flat plate collector to illustrate how different
modes of heat transfer exist within a particular device. We haven't completed that; we will
be taking it up, continuing that next time. So today, we will stop at this stage.
I found this set of lectures over the summer when I was taking heat transfer. My professor couldn't teach worth crap and I relied on these to pass the course. I highly recommend them.
thank you!
You may be a life saver! I'm in Heat Transfer right now and the professor can't speak English and can't write legibly. This may just rescue me from the pits of "barely-passing" to "passing with flying colors," otherwise known as anything higher than a C+.
Why don't the foreign professors at my uni speak coherent English like this!?