Understanding Heat and Mass Transfer

Lecture - 1 Introduction on Heat and Mass Transfer

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Summary

In this introductory lecture on heat and mass transfer, Professor SP Sukhatme outlines the course's scope and objectives. The subject is covered from a mechanical engineering perspective but is also relevant to chemical and aeronautical engineering. The lecture introduces the basic laws of heat transfer, including conduction, convection, and radiation, and how they relate to mass transfer. Key concepts are illustrated with practical examples, such as heat exchangers and car radiators. The session concludes by touching on solar energy as a multifaceted heat transfer example, highlighting the industry's relevance to everyday technology and energy efficiency.

Highlights

The course is taught by Professors SP Sukhatme and Yuan Gaitund from IIT Bombay 🎓.
This lecture serves as an overview of the heat and mass transfer course, set across 30 lectures 📝.
Key discussions include the basic laws governing heat transfer: conduction, convection, and radiation 📖.
Mass transfer is introduced as having similarities with heat transfer, enabling cross-disciplinary applications 🔄.
Real-world examples, such as the car radiator and solar energy, are used to illustrate the concepts 🚀.

Key Takeaways

Heat and mass transfer are vital subjects across multiple engineering disciplines 🔥.
The course is designed from a mechanical engineering perspective but is applicable to other fields like chemical and aeronautical engineering ✈️.
Understanding heat transfer helps in designing equipment like car radiators and steam condensers 🚗.
The principles of conduction, convection, and radiation are fundamental to mastering heat transfer concepts 📚.
Solar energy is an important application that uses multiple modes of heat transfer 🌞.

Overview

In this foundational lecture, Professor SP Sukhatme sets the stage for a comprehensive understanding of heat and mass transfer. Covering the scope of the course from a mechanical engineering perspective, he underscores its importance in other fields like chemical and aeronautical engineering, making it a universal study within various technical curricula.

Heat transfer principles such as conduction, convection, and radiation are extensively explored. The lecture highlights how these core concepts apply to real-world challenges, such as designing effective heat exchangers and managing temperature in chemical processes. This grounding ensures students can apply theoretical principles to practical engineering problems.

The lectures also delve into the realm of renewable energy applications, with a specific focus on solar energy as a case study for understanding multiple heat transfer modes. By examining a solar flat plate collector, Professor Sukhatme illustrates the simultaneous operation of conduction, convection, and radiation within a single system, stirring interest in sustainable engineering solutions.

Chapters

00:00 - 05:30: Introduction The chapter titled 'Introduction' begins with musical notes and crowd applause. It seems to set an energetic and engaging tone for what is to come.
05:30 - 11:00: Course Outline The chapter titled 'Course Outline' begins with a greeting in a musical tone, followed by the speaker introducing themselves. The speaker, SP Sukatme, is mentioned to be collaborating with a colleague named Professor Yuan. The introduction suggests that both will play a role in the outline of the course. The excerpt provides a brief, informal start to the presentation of the course structure.
11:00 - 20:00: Introduction to Heat and Mass Transfer This chapter serves as an introductory overview of the course on heat and mass transfer. It is the first in a series of lectures on the subject provided by Gaitund from the department of mechanical engineering at IIT Bombay. The lectures will cover the standard syllabus prescribed in most Indian universities for heat and mass transfer.
20:00 - 33:00: Problems in Heat Transfer The chapter 'Problems in Heat Transfer' discusses the subject matter relevant to mechanical engineering, focusing on the principles and problems associated with heat and mass transfer. Although it is tailored for mechanical engineering students, it acknowledges the importance of these topics in chemical and aeronautical engineering curriculums as well, suggesting that components of this subject are covered in those disciplines too.
33:00 - 46:30: Modes of Heat Transfer The chapter on 'Modes of Heat Transfer' is introduced with an emphasis on its relevance not only to mechanical engineering students but also to those from other disciplines. The instructor plans to cover the subject material from the syllabus and offers a broad outline of the lectures that will follow, indicating a structured approach to the exploration of various modes of heat transfer.

Lecture - 1 Introduction on Heat and Mass Transfer Transcription

00:00 - 00:30 [Music] Heat. Heat. [Applause]
00:30 - 01:00 [Music] And my eyes namaste. Uh my name is SP Sukatme and along with my colleague professor Yuan
01:00 - 01:30 Gaitund I'll 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'll 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'll be doing
01:30 - 02:00 this through about 30 odd lectures. We are from the department of mechanical engineering as I said. So the matter that we'll 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
02:00 - 02:30 taught in other disciplines. So although we'll 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'll give you an outline of the lectures which we are going to give.
02:30 - 03:00 First of all, we'll have an introduction to the subject covering a few lectures to cover the laws, the basic laws that govern the subject. Then we'll 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'll talk first of convection, then natural convection. Then we'll go on to change of phase. Change of phase means either
03:00 - 03:30 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
03:30 - 04:00 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'll talk about the thermal design and the working of such heat exchangers. And then finally we'll go to the topic of mass transfer and introduce the elements of mass transfer. Now you may ask me the
04:00 - 04:30 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's a temperature difference. Mass transfer occurs when there's a concentration difference. The equations describing these are very similar or analogous. And therefore when we derive a relation an equation for a
04:30 - 05:00 particular heat transfer situation, it 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 this similarity so that you can use heat transfer relations for studying certain types of mass transfer
05:00 - 05:30 problems. As far as the books for this subject are concerned there are a variety of books uh there are many books written and available to cover the syllabus. The two books which I'm 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's by university's press. This is a book which we'll be following to a large extent but not all of it because it goes much beyond the
05:30 - 06:00 syllabus that is normally prescribed in the undergraduate curriculum. The other book which we'll also be referring to is the book by Incroper and Devit on fundamentals of heat and mass transfer. It is widely used in India, widely used in the US. It's been used for the last 20 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 uh the important thing is while you are going
06:00 - 06:30 through these lectures, we will be also doing certain numerical problems for you. Now you'll have to do some problems on your own also. And that is why you will automatically need to refer to certain textbooks or reference books to do further problems on your own. Now let us begin with the introduction. The first thing we ask ourselves
06:30 - 07:00 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's take up heat transfer first. Now first of all when does heat transfer occur? Now whenever there are
07:00 - 07:30 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 in. The
07:30 - 08:00 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
08:00 - 08:30 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 under goes 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
08:30 - 09:00 equilibrium state because of the heat and work interactions, the system goes attains a certain set of v a certain state which it is described by temperature, pressure etc etc. Now in heat transfer in thermodynamics sorry 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
09:00 - 09:30 the other hand, in heat transfer, we say because there's a temperature difference, heat flow occurs. What is the rate at which that heat flow is occur? And at a certain point, if I want only a certain temperature to be attained, how much time would it take? That's the kind of question we'll ask. So, it's a subject which is dealing with the rate at which heat flow occurs. That's the distinction between what we study in thermodynamics and what we
09:30 - 10:00 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 give an example so that you know you'll understand what I mean. All of you have sat in a car. All
10:00 - 10:30 of you have seen a car radiator sitting in the front of the car. If you open the bonnet right in front there, there's a very small rectangular uh uh 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° centigrade. And that hot water is cooled by air which flows over
10:30 - 11:00 that radiator cooled by 5 or 10° and then again circulated around 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
11:00 - 11:30 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's 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
11:30 - 12:00 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 sub the whole object of study 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 cars which drive us all over the
12:00 - 12:30 world. So that's just one example to illustrate why this subject is of importance. So during this course we'll 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 we would be able to size heat exchange equipment and be able to do elementary design. That's
12:30 - 13:00 the whole object of this 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 the region of low concentration. And let me
13:00 - 13:30 again take an example. Take this room. This room contains air, oxygen and nitrogen in a proportion of say 4 is to 1 typically nitrogen oxygen in the ratio of 4 is to1 approximately. All right. Let us say in the corner there in that room at the top there I hold a cylinder of compressed gas of nitrogen and let some nitrogen out. Obviously in that corner top corner there the concentration of nitrogen in
13:30 - 14:00 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 a 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's a high concentration of nitrogen there. there's a lower concentration here. Nitrogen will move in this direction, will diffuse in this
14:00 - 14:30 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,
14:30 - 15:00 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 place 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'll be able to get the required film thickness totally removed by the
15:00 - 15:30 time the air moves over a certain region over that surface. These are the kind of questions we have to answer in order to size mass transfer equipment for a particular purpose. So when we study mass transfer, we'll be able to design equipment in which mass transfer occurs. Now to further our understanding let us look at some problems of heat interest in heat transfer. It is important that you try to appreciate the width of the subject
15:30 - 16:00 the breadth of the subject before we take up really get into the equations which govern the laws etc. I'm going to look at three problems. First of all I'm going to look at a problem of heat loss through thermal insulation on a steam pipe. That's the first problem. Then I'm going to look at the uh a problem of heat transfer to water flowing through a tube. And then I'm going to look at heat
16:00 - 16:30 transfer in an electric furnace. These are three problems of interest in heat transfer. And we'll look at them one by one. The first one, 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
16:30 - 17:00 those sheds. Typically one may have a central facility where the 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've gone to a lot of trouble to generate that steam in a
17:00 - 17:30 steam generator. You know we'll have 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 superheated 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 uh steam to uh condense or lose heat. So we put insulation around it. Insulation may be in the form of
17:30 - 18:00 some fibrous insulation like mineral wool, glass wool etc. which is put round 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'll understand things better. Let us say uh let me draw a graph let us
18:00 - 18:30 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 centm and on the y-axis I have the heat
18:30 - 19:00 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
19:00 - 19:30 is as indicated by the cross here. That's the amount of heat being lost. Obviously, if I put insulation on this pipe, the heat loss rate is going to decrease. Typically I'll 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 to design
19:30 - 20:00 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'm marking 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's using he'll go horizontally then go vertically and say this is the thickness of insulation
20:00 - 20:30 needed. Or the problem may be in reverse. He'll be told we are going to put 3 cm of insulation or 5 cm thickness of insulation. What will be the heat loss rate as a consequence? In which case given a certain thickness he'll go up like this having got this graph to this graph then draw a horizontal line like this and say this is the heat loss rate and therefore from here to here that is from point this point which is
20:30 - 21:00 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 you follow. 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.
21:00 - 21:30 Now let's go to the next problem that I mentioned that is heat transfer to water flowing through a tube. Okay. Now here is a tube. Here is a tube. The diameter I've given is 2.5 cm for this tube. Water is entering it at 30° centigrade. On the outside of this tube, we have steam, low pressure steam at 50° centigrade condensing on the outside of
21:30 - 22:00 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 along the length of the tube. Now where does this situation occur? This is a situation which will typically occur in a steam condenser. You have a power plant. Say in a power plant from the turbine exit you have low pressure steam. You need to condense
22:00 - 22:30 that steam with the help of cooling water and then raise the pressure of that condensed water. Then put it back into the steam generator. That's the rank 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'm showing one as an example. So let us say coming back to the single tube now what is the problem
22:30 - 23:00 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 m two taken as an example. What would be the exit temperature to0 of the water leaving this tube or vice versa? Suppose I specify that I want an exit temperature of say 35° centigrade.
23:00 - 23:30 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 50 and that would be attained with an infinitely long tube. So the heat transfer engineer who's designing for this situation will typically have to tell what would be the length of the tube for a given exit temperature or what is the reverse
23:30 - 24:00 problem that means given a length what would be the exit temperature. Now let's 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
24:00 - 24:30 millm in thickness. It would be maybe a millimeter in thickness and maybe a few cm 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 properties like ductility, malleability etc etc. We want it to regain these properties and typically for that one does some kind of an analing heat treatment process. The
24:30 - 25:00 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 centrade of that order.
25:00 - 25:30 So the problem before the heat transfer engineer would be the following. Now he'll say we are to he told here is an electric furnace. The temperature in this furnace is say 1,200° centigrade or 1,000° centigrade. It is required that the temperature of the steel strip at the exit be 720 centrade or some temperature like that. The steel strip is entering at room temperature 30° centigrade.
25:30 - 26:00 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 analing temperature for that particular steel so that when it goes out it's just
26:00 - 26:30 above that temperature then it cools down 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 which an engineer would face. Now I could go on of course giving you examples but I think you get the idea but uh let me just show you one more example of a problem which is of current interest.
26:30 - 27:00 We are in a situation nowadays in which electronic circuits or electronics plays a key part in our lives and miniaturization in electronics is the key word today. VSI for instance or VVLSI whatever it is you know we go on making transistors thousands and hundreds of transistors over very small areas compared to the old days when a 100 transistors might occupy a whole tabletop for instance like this. Now
27:00 - 27:30 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 temperatures its life that uh its period for which it will work consistently
27:30 - 28:00 reduces. We would like say design specification maybe 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 uh this is a what you call a a
28:00 - 28:30 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. One 2 I'm just drawing three. If I had a plan view actually there would be there may be an array like this. One would be like this in the plan view two like this and so on. So it
28:30 - 29:00 could be a rectangular array of of chips. Typically each of these may be let us say a cm by cm by maybe 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.
29:00 - 29:30 The requirement before the heat transfer engineer would be that the surface temperature the surface temperature here TS or T wall whatever we want to call it should be or should not exceed a value like say 50° centigrade or 60° centigrade. That's a typical
29:30 - 30:00 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 do we do? One is to hold down the temperature is we may have say for instance air blown over these trans these electronics chips. We may blow air over these chips. The air may be say entering at 30°
30:00 - 30:30 centigrade. All right. And we ensure therefore that 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's a typical convective heat transfer problem for which you need equations, data, the flow uh the flow around the the electronic
30:30 - 31:00 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 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 uh heard of these.
31:00 - 31:30 You've done 12th, you've done heat, studied heat. The three modes are conduction, convection and radiation. So let's 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
31:30 - 32:00 energy and molecules having less energy. The it's 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
32:00 - 32:30 this situation. Now let me just draw a sketch or two to illustrate ideas what we mean by conduction. First of all let us consider a solid a solid structure. Say a solid typically has molecules or atoms. Let's say the molecules like this. I'm just drawing a set of molecules. They may not be in a square array. Whatever be the pattern set of
32:30 - 33:00 molecules making up the solid and typically these solids which form a latice will be vibrating in that solid more vibrations means a higher temperature. So when we say that this 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
33:00 - 33:30 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 latice vibrations and these vibrations are more because of a higher temperature region than in a lower temperature region. So one in a solid one mode of mechanism by which the transfer of energy takes place is latis
33:30 - 34:00 vibrations. Latis 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 solids. electricity flows it's the electrons that are flowing. In the same way those electrons also carry energy which is heat transfer by conduction. So
34:00 - 34:30 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 not in nonmetals 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
34:30 - 35:00 general, non-metallic solids. Now, unlike a solid, in a liquid or a gas, 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'll 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's a set of
35:00 - 35:30 molecules making up a liquid or 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
35:30 - 36:00 there's 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. All right. 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 is what we call as conduction in a liquid or a gas. So in a liquid or gas the transfer of energy occurs due to
36:00 - 36:30 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's 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 it is the next
36:30 - 37:00 mode. We 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
37:00 - 37:30 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
37:30 - 38:00 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 microscopic motion. Then
38:00 - 38:30 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
38:30 - 39:00 then it is called force 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's called as natural convection. So convection is of two types. Force 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
39:00 - 39:30 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
39:30 - 40:00 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 a 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
40:00 - 40:30 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 number one the rate of emission. The characteristics of this radiation are let me read that uh write that out. Number one the radiation increases with
40:30 - 41:00 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
41:00 - 41:30 emission increases with temperature level. That's one characteristic to note. A second characteristic to note is we do not require any material medium for the energy transfer to occur. It's 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. No material medium required.
41:30 - 42:00 required for energy transfer to occur. These are two characteristics to note. radiation the thermal radiation in the form of electromagnetic waves does
42:00 - 42:30 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 moment earlier, you'll 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
42:30 - 43:00 primarily a problem of heat transfer by convection and that too forced convection and the third problem of analing 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 force convection and the third thermal radiation but I don't want you to get
43:00 - 43:30 the impression that this is so always in general problems in heat transfer are not amanable 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
43:30 - 44:00 they 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 them. Now I'm going to give an example of a a situation in which all these modes are occurring. We're going to take up one example like that. All of you must have heard the word energy crisis. Now energy is on the
44:00 - 44:30 is a dominant part. You read the newspaper every day the talk of the price of oil. The price of oil is increasing. We have to import oil in India lots of it. So we have to pay for it. So any increase in the price is a 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
44:30 - 45:00 countries is met by fossil fuels. Fossil fuels means coal, oil and natural gas. And all these are depleting because they're not renewable. they're 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's look one way of using
45:00 - 45:30 solar energy which to illustrate different modes of heat transfer. I'm going to show you and you will uh see it here. I'm showing you here on the screen a an array of what are called a solar flat plate collectors. You're seeing four collectors here. Here each of them typically is about 2 m by 1 m and they are used for heating water. An array like this would
45:30 - 46:00 heat water typically to a temperature like 60 70 80° centigrade with ease. And we need hot water for so many applications at home in hotels, hospitals, cantens etc etc. So this is one application of solar energy which is a very viable application which pays for itself fairly soon is being widely used in India. Now now let's look at one such solar collector. There are four in the picture. I'm going to now look at one.
46:00 - 46:30 This is one collector so that we see the inside of it. What it looks like a solar collector. This is 2 m in length, 1 m in width as I mentioned. Typically on top there's a glass cover which you had seen in the picture. And below that there is an absorber plate. This is the absorber plate. It's a thin copper sheet a millimeter or so in thickness. on that absorber plate or on the back
46:30 - 47:00 side 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'm pointing out here then gets distributed in these flow tubes and then goes out through the outlet header. Right? That's the position. So the water the solar energy falls on the
47:00 - 47:30 flat plate collector and the water as it flows through gets heated up and typically as I said you can get hot water at temperatures like 60 70 80° centigrade in this collector. Now let let's 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
47:30 - 48:00 tube. The the absorber plate. These are absorber tubes below it. And this is the glazing the glass cover on top. This is how the a 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
48:00 - 48:30 tubes. Now today we will stop here but next time I'll look at the different modes of heat transfer that are occurring in this device. So we'll take off from this point in the next lecture. We've 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
48:30 - 49:00 components that make up the flat plate collector. Next time in the next lecture we'll 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've done today so that we get a feel again for what we've achieved. Today for instance we've done the following. First of all, I've
49:00 - 49:30 outlined the subject that we are going to cover in 30 lectures. Given an outline of it, what are the different topics that we are going to cover? Secondly, I've 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
49:30 - 50:00 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'll be taking it up continuing that next time. So today we'll stop at this stage. Thank you.