 So, the idea of next may be 15-20 minutes is that I will just tell what we are proposing but we would like to take the feedback from you may be informally now itself. But we are going to have an interaction session in evening, you would have seen the schedule, you just take the schedule, so we let us take recap of the schedule so that we know what is our big picture. Yeah, if you see here we have preliminary discussions, it is not edited but the edited version is with you preliminary discussions that is what we are going to cover is I am going to tell in next 10-15 minutes and maybe you can tell this is okay, this is not okay, this is not covered, this is covered, you can tell that now but in a detailed fashion we will take that up at 4 o'clock because our idea of not putting this in fact honestly this should have been that discussion should have been preceded all the lectures but I thought that or we thought that we should not be wasting our quality time on that discussion only because that is endless that can go on. I put that in the late evening portion that is between 4 to 6 because that is very important in the sense that we can calibrate ourselves for our end workshop which is slated in December. So what is the idea is we will get started with conduction 1, conduction 2, conduction 3, I am sure what does this conduction 1, conduction 2, conduction 3 is there so essentially as I said we are going to cover today, today is conduction, we will be in conduction mode, tomorrow and day after tomorrow we will be in convection mode and 15th will be radiation thoroughly and 16th in the morning session, radiation 1 hour lecture followed by heat exchangers 3 hour lecture. Of course in the afternoon is thoroughly planning what to do and what not to do for December session that is the overall plan and coming back to topics or before topics I want to submit to you couple of things that is this is not teaching program I would not agree that this is a teaching program I would like to call it as learning program okay and you are all teachers and you have taught several times it is not that we are going to teach you as professor Arun said it is not monologue, it is mutual learning, it is mutual learning cannot happen one way it is mutual learning so you can interact or you can stop us at any point you can stop us at any point and ask us the question may or may not be we will be in a position to answer it immediately I hope you will appreciate what I say that because you are all teachers but we will definitely carry over those questions back home we will think about that we will discuss over tea, coffee, lunch, dinner okay and then definitely we will figure out the answer many a times the answer is there here but it is not coming out properly on the other day just on the other day I was just taking with sake compressible flow CD nozzle experiment I just because we had taught that in the theory but just to prime them I just started just half an hour lecture because professor Arun was supposed to take but I filled in for him on that day because he was out so I just taught him for taught the guys for 20 minutes compressible flow one guy asked I said downstream because we know that in supersonic flow I said whatever I change in the downstream it does not change in the upstream it does not upstream does not know what I do in the downstream one question one student asked very nicely why does it why does it happen immediately I could not recollect and tell but after I came out in the late evening it occurred to me it is all because the fluid interacts one fluid particle interacts with the other fluid particle in terms of acoustic wave if it is a supersonic flow my fluid particle is moving much faster than the acoustic there is no way that my acoustic wave interacts because my fluid particle velocity is very high so that is the reason why downstream so point is we will not be able to answer all the questions instantaneously okay never the less we will take every question very seriously and as I keep saying in my class no question is trivial as long as it is cleared in my mind it might be very obvious to you but may not be obvious to me but as long as it is not obvious to me it is a doubtful and I cannot put it under the carpet I have to take it glaringly open admit that I do not know it no problem as long as I learn to admit that I do not know I cannot learn I think that I guess you will all appreciate that as teachers that we need to appreciate what we do not know then only we can learn so that is what I want to submit let us be very honest to each other about what we know and what we do not know if you do not know something no problem we will write down that question properly we have enough people to discuss also okay luckily there is a gradient see flow happens only when there is gradient I guess knowledge also flows from higher gradient to lower we have stalwarts here we can always ask them if it does not occur okay so we do not have to think that we know everything that is that is my honest submission let us be honest to each other and try to learn mutual it is not teaching it is learning mutual that is what I would say so another thing is that I do not know because I believe that you are all teachers we will all be serious nevertheless we have been asked to take attendance I guess now we are 100% attendance any absence no so we are asked to take attendance session by session so but I guess that is not required but let us see how do we go about so yeah how many this show of hand how many have taught the course once or second time just please raise your hand how many such people are there and somebody is teaching it for the first time next year there was one hand so everybody else has taught it at least two times this is the second time that was the first show of hand others are all more than two so these one third people who are taught it first time or you know doing it the second time so in case some things are not clear please stop ask us etc because most of us learn by teaching experience only so the pace might be little bit faster whatever we will modulate no too many questions happen we will say that okay now we will take this question a little later or over tea we will take it or over coffee we will take but do ask the question or if you feel that you do not want to ask the question you write it down we will again discuss we will again discuss later evening we will discuss that is not an issue but I would think that we should be interacting as much as possible so coming back to the syllabus so what we are going to cover is in conduction one you have the hard copies with you please do not write until and unless I insist that because few derivations I would like to like you to write but do not write as long as otherwise you may feel if you are feeling sleepy you write okay that is what I insist in the class because physically if you are physically alive only you can be alert otherwise if you are feeling very sleepy you are not slept yesterday because of your travel or something you better write something on the paper so that you are engaged you are engaged okay so coming back to syllabus this is conduction in conduction we will go on a very fast track what are the three modes of conduction but here we will be emphasizing more on energy balance full heat transfer is about energy balance because this is one thing we are going to apply throughout this course e dot in minus e dot out plus e dot g is equal to e dot s t this is the mother of all equations what I say in the class always so this is what we need to appreciate pretty well so this is what we will be emphasizing a lot of course we have thermo physical properties and t diffusion equation we intend to cover this conduction 1 I had written between 1030 to 1 I think conduction 1 if I am right conduction 1 and conduction 2 conduction 2 I guess fairly all of us all our courses we cover in a detailed fashion so this I guess we are really going to that is conduction 2 okay because here also we will cover everything good thing is that we have complete PPT power point presentation is there and we are going to share that with you okay so conduction 1 and conduction 2 will be between 11 to 1 or maybe after 10 minutes so conduction 1 I intend to cover let us see how do we go in the afternoon I we will be covering in the first one hour because we have 2 hours there 2 to 4 2 to 4 so 2 to 3 we will be spending time on fins so here again we can go we can tend to go a little bit fast on fins and transient conduction I am going to stop a little bit here and focus more on transient conduction focus more and more on lumped on why I say this because at least I have understood how to measure heat transfer coefficient by this lumped on just this lumped little lumped 1 equation theta by theta i equal to e to the power of minus e by tau we can do PhDs I mean my words in fact my PhD students are going to use this technique okay how do we exploit this relation to measure heat transfer coefficient and how do I choose a thermocouple how do I choose a thermocouple what is the meaning of this time constant all that I think we will be appreciating in transient conduction so my emphasis here although it is 1 hour I guess I will be emphasizing on lumped itself for 30 to 40 minutes or even up to 45th minute but last 15 minutes I guess I will go on a fast track which is based on charge that is plane wall long cylinder few semi infinite solids and when can I apply semi infinite so all that I will be covering maybe in 15 to 20 minutes so and of course at the end transient conduction in multi dimensional systems so that is about conduction convection is as I keep saying in first transparency also it is there fluid mechanics is indispensable for convective heat transfer until and unless I understand fluid mechanics I cannot understand convective heat transfer so we need to take a recap we need to recapitulate whatever we have studied in fluid mechanics so that is what we do here and we will understand that heat transfer coefficient is very similar to surface here so that that is what Randall told so that is what we are going to cover in convection one this is all basics but throughout the course we will try to emphasize the historical perspective I do not know how many of you have read the books of Anderson I guess fluid aerodynamics all books of Anderson have one specialty that is he gives for every topic he gives historical perspective whether it is Mac whether it is Prandtl whether it is Bernoulli so that is what I am copying we are copying here why because if you understand the historical perspective we will understand the subject better and perhaps we can enthuse the student much better so that the that is the contention so of course here and important thing that you have to tell me differential analysis is what we are going to emphasize that is conservation of mass momentum energy this is what we are going to emphasize more so I do not know whether this differential approach is being taught at your end yes it is taught it is great because that is very important I guess because if we take the differential equations and if you non-dimensionalize we realize what is the importance of dimensional similarity that is what made professor Nusselt a professor in fact he became professor because he derived that okay so that is what we are going to emphasize here in convection 2 and convection 3 so we will understand the importance of all these non-dimensional numbers which come through principle of similarity by non-dimensionalizing each of these equations and of course after this it becomes an external flow it is not so interesting because we will teach only flat plate and subsequent to that it is all correlation base so we cannot that is where it becomes little tricky to hold the interest of the student here but in internal flows again there is developing flow concept mixing cup temperature concept why it is called as bulk mean temperature or mixing cup temperature we will understand in convection 5 so that is about so basically after we fluid mechanics differential equations external internal and natural convection so natural convection will be really touch and go we will simply write the differential equations non-dimensionalize that and write come across Grashof number that is essentially what we do in natural convection so then of course radiation so radiation is little difficult because many of us do not work in radiation as much as we work in conduction and convection okay honestly I have also not worked much in radiation but since 2 3 years only I have been thrown into fire to work for fire so I realize that fire is full of thermal radiation so that has given me a little opportunity to understand little bit about emissivity something like that but still lot to learn but still it sounds abstract little bit let us make it interesting to the extent possible so here all fundamental definitions will be radiation 1 that is all intensity solid angle, emitted radiation, irradiation all that stuff and then in radiation 2 we will cover blackbody radiation and all sorts of definitions which you will perhaps see in Seigel and Hoven or Inkropera and David and other textbooks so of course absorptivity reflectivity and transmissivity followed by Kirchhoff's laws view factors and diffuse of course radiation between black surfaces this is basically enclosures and then we will take it to black surface of course shields and radiation effects in temperature measurement I would think this is the most important thing this I have learnt very hard way I cannot measure we believe usually that we insert a thermocouple anywhere and it is going to give us the temperature not so always not so always it is very important that the convective heat transfer coefficient is very high if I am taking a flow through a pipe if the convective heat transfer coefficient is very less let us say it is a natural convective domain I insert a thermocouple most probably I am not measuring my temperature or in a furnace where it is still essentially inside everything is natural convection of course radiation between surfaces if I assume that the medium inside the furnace is not participating temperatures can be off by as much as 100 to 200 degree Celsius so that is what we are going to appreciate radiation effects in temperature measurements okay if I am measuring temperatures in rarefied gas dynamics let us say there are no molecules so how do I measure it is almost impossible to measure temperature in a rarefied atmosphere because there are no molecules there is no movement why so we will appreciate when we go to radiation effects in temperature measurement so and of course at the end radiation heat transfer in enclosures and shields and heat exchangers can be a course by itself I am sure many of you might be teaching heat exchangers course also so but then here we have the constraint that we need to cover it in short time but we will try to cover LMTD and epsilon NTU approach for various types of heat exchangers of course we will be covering the analysis wise parallel and counter flow and the design approach by and large we will cover for three hours so that is the thing so maybe I will give five minutes for you to respond to say is this okay or something is left out maybe couple of maybe I will give five minutes for you guys to respond yeah yes please do not stand up feel free yeah boiling and condensation consciously we are not good but you think that should it be there many goes to the field we will be supposed to deal with boilers and in refrigeration site condensers evaporator I do not disagree at all the importance of but only thing is that within the time frame which you have in university is it is it standard is it enclosed it encompasses in the standard syllabus okay and also mass transfer some aspect can be touched like the fix law and Stefan's law so those basics could be touched upon okay may not be in depth but they should at least have some concept about the okay because I do not know whether I can we have we have the material my material for boiling and condensation is handwritten but I have scanned that and it is there and boiling curve boiling curve complete PPD is there I can but I I will try to see whether I can enclose I mean include that within our course let us see whether at what pace we will go but that is a good suggestion may not be at this level but at the final workshop maybe yes definitely we can know even now I can include material is there it just that I have to cut past I have to delete few things because I have taught that in two phase flow portion of heat exchanger so I can cut past that is a good suggestion mass transfer yes in convective heat transfer when I am teaching convective heat transfer anyway I am teaching Reynolds analogy while I am teaching Reynolds analogy I can include that we can calibrate that but I will not promise that mass transfer thing during this course because I need to add that that I have that material which I have taught in combustion course but I can yes that is a good suggestion yes sir any other same thing you have consciously another thing which we have done is we have not added numerical conduction which you would have seen in many books but now when I was just talking with Professor Vedula over coffee he was just saying no you need to add numerical conduction because it is pretty simple so in fact he was because we are not teaching here to our own guys in fact Professor Arun left because our midsum is happening so he has just gone to distribute question papers and he will be back within 15 minutes the point is do you think numerical conduction also has to be added but whether we will be able to handle that definitely not now in this workshop but in December yes we can make that because I do not have the PPT ready for that but we can make it essentially as it is an extension of conservation of energy we will not go with FDM method not with finite difference method but with control volume if we take it then it becomes easy but do you all of you agree or do not agree I need to know that's true professor I do agree with that but the point is within our curriculum do you have time number one number two whether it is it is there in the syllabus or not that is do you have in this then if you have in this syllabus we better do it so we will add so numerical conduction has to be added any other suggestion feel free yes I knew that this will be broached up because consciously I had deleted this but yes we will add it no issues yes anyone else so can I move ahead is that okay so summary is the feedback what I have got is boiling and condensation mass transfer numerical so in boiling and condensation to what extent we can cover is I guess condensation we can take nusselt condensation that is condensation over a flat plate vertical flat plate and give correlations for all other configurations to that extent we can cover number one and on the boiling side we can cover the boiling Karu Nuki Amas boiling Karu and that's it we will not get into maybe we can cover Rosenhoes equation that's where we will stop because above that it would be I am sure you will not have critical heat flux and all that do you have relations for critical heat flux do you do have fine that is Rosenhoes but other than that other correlations do you have cooper's correlation okay no actually okay okay please finish it off okay okay in the workshop what we are aiming is for teaching UG so let us keep the PG teaching in the back of our mind but let us focus more on UG teaching UG teaching but then yes to the extent possible we will try to cover okay but we have the material that's true so I am afraid see professor Arun teaches two phase flow course that is full of boiling and condensation okay that's for the lecture UG teaching should be limited to topics very thoroughly that's like conduction, convection and radiation we should focus or we should emphasis more only on the basic that conduction, convection and radiation apart from this that boiling condensation and what else has been just now mass transfer and numerical conduction that can I think this will be helpful to the chemical engineering students they are extensively using the mass transfer phenomena in the chemical engineering but so far as concerned to mechanical we are merely focusing on the heat transfer engineering so I don't think that mass transfer should be included in the workshop okay the second thing is what numerical analysis I think this kind of analysis will be taken care by these comparison flow dynamic itself so when the somebody is teaching this finite difference matter and finite volume they are basically taking aid of this heat diffusion equations first and because the question is there is it is in the syllabus I think not in most of the universities mass transfer is what I will do is I will add these questions in the feedback form let me see how each one of you will respond based on that I will react I think that way we will put this let me keep in the back of my mind most of the time should be spent on the conduction, convection and radiation and we will be having the three to four hours which will couple all those things. So with that I guess we are just five minutes ahead of time so we will get started with conduction yeah we will get started with conduction one okay so what is this conduction one what is that we are going to cover as I said we are just going to answer questions what is heat transfer how is it transferred and why is it important to study it and how is it transferred different from thermodynamics and as I said I am going to emphasize more on energy balance and there are two problems for energy balance I am going to follow that okay so that is what we will try to do in next maybe half an hour to 45 minutes so I am going to go consciously fast for the simple reason that you have taught this several times number one I will only emphasize where I need to emphasize more and I will not emphasize few things which I think that they are I can gloss over but you can always stop me and slow me down wherever you feel okay. So what is heat transfer as I keep saying it is very difficult to define heat transfer all that we can say that when heat transfer can take place we cannot define heat transfer so all that we can say that it is the energy when there is a temperature difference so that is all we can say about heat transfer it is not a definition so we are not really answered what is heat transfer we have answered only when does heat transfer take place and there are three modes of heat transfer as we say conduction convection radiation but few people disagree on this few people disagree on this for example guy 20 doesn't say that there are three modes of heat transfer you always insist that there are only two modes of heat transfer that is conduction and radiation why because it is conduction within the boundary layer what we are studying it as convection so people people do argue that there are only two modes of heat transfer in fact we can emphasize that the students also why because they indeed it gets ingrained in them that convection is nothing but conduction within the thermal boundary layer so as I said there are three modes now what we will do is we will just we will just take each mode what it is how does it work how do I quite okay so let me start off with conduction so conduction takes place through solid or a stationary fluid because this is the question whenever I ask students in the class does conduction take place from that wall to this wall which is filled with air and I heat for one of the walls usually the answer is no because the feeling is that usually the feeling is that for all of us when we are beginning heat transfer we feel that the heat transfer takes place only through solids so heat transfer indeed take place through liquids and gases but it is not as effective as solids that is very true but nevertheless conduction does take place through fluid that is liquids and gases so but then how do I quantify this how do I quantify this that is as I said conduction is because of there is no bulk motion there is no movement if I take a plate if I take a plate if the conduction is happening within from one wall to the another wall this plate itself is not moving it is stationary but then what is moving the molecules within my body are interacting with each other are interacting with each other that is what creates the conduction that is what that is what not doesn't create that is what is the reason for conduction to happen okay so the molecules it is the interaction of the molecules that is what this figure shows so red here indicates high temperature and blue here indicates low temperature so the high temperature molecules talk with the low temperature molecules and that is how the conduction takes place from high temperature to low temperature now how do I quantify conduction how do I quantify it's it's it's quantified by 4 years law of conduction is it derived first of all it's an empirical that's what we need to emphasize it's empirical when I say empirical what do I mean empirical based on experiments it's not derived from fundamental principles it is empirical you would have used the terms these terms are very common because whenever one thing I realized while teaching is that the words what I use my students should be knowing those for example I use the word upstream downstream if I don't know what is upstream and downstream if I keep on telling him I am telling I'm changing the downstream what will happen to upstream when he doesn't know what is upstream and downstream how will he answer so first I need to answer what is upstream what is down why did I say this huh I I landed up into this because I need to use these words very carefully empirical semi empirical analytical closed form solution these are the words I use left and right especially I see many of you are PhDs and m techs you are very convergent with these words but when I'm starting as a beta guy or a BE guy I will not be knowing all these words so I need to tell empirical means that which is coming through thoroughly experiments I have z as a function of x and y I will keep x constant very why I will keep y constant very x and then I fit a curve whatever curve I get that's what I am going to use that is empirical what is semi empirical then some aid of theory for example we have taken the aid of for example Cole Brooks correlation friction factor correlation of course I am not teaching that Cole Brooks friction factor correlation we will come from fundamentals but the last constants I am just going to fudge them little bit based on experiment so I am coming from theory but the constants I am fudging why I have to fudge also I have to tell my student why I may be fudging because there are some assumptions which I am stating which I am not actually following in so that's the reason I need to fudge that so that makes my equation semi empirical equation analytical what is analytical analytical solution is using pen and paper you have a differential equation or a boundary condition you can solve that with pen and paper you don't need even calculator perhaps okay that is analytical solution okay another thing is closed form solution same whatever you solve analytically whatever solutions you get that is closed form that is tractable that is tractable means mathematically it is amenable it yields to mathematics paper and pen mathematics I don't need to as you are saying I don't need to go to CFD give a set of equations solve it I don't have to do that because in these courses whatever we teach are all essentially analytical empirical or maybe semi empirical closed form solutions we don't try to teach CFD type we don't take a set of equations and try to solve them right so these words have to be ingrained in our students very properly in the very first class yeah what I mean is when I say analytical means what I say is whether it is integration differentiation Fourier transfer anything I should be able to solve it with simple paper and pen maybe you can take your calculator to that extent only but you are not taking your computer you are not taking excel sheets you are not taking any you are not reducing the differential equation into set of linear algebraic equations and solve it simple mathematics can be adapted whatever we study in engineering and I should be able to solve that's all I mean by analytical solution whatever solutions I am putting in our course in the heat transfer course in all the textbooks whatever we are seeing they are all analytical solutions they are all mathematically that's okay that is okay that is okay but still I am using mathematics fine so Fourier's law of conduction is an empirical thing which he has done through experiments okay so if we see that here we see here heat flux is a function of conductivity dt by dx okay so here one thing we need to realize is here I keep saying in the class but it becomes very easy if you think like this so what I keep saying is see we know voltage and current whenever there is a drop in the voltage whenever there is a gradient in the voltage what can happen current can flow so current can flow only when there is a differential voltage or then when high from high voltage to low voltage high potential to low potential we have studied from high school next thing is fluid mechanics what is the analogy in fluid mechanics for this from higher pressure to lower pressure higher pressure to lower pressure what flows fluid flows that is volumetric flow rate meter cube per second volumetric analogically in heat transfer will you please tell me what is this voltage or pressure temperature gradient temperature gradient temperature gradient is what is there for what will what will it create what is heat flux for heat transfer that is current for electrical and all electrical parlance what is temperature gradient for heat transfer that is voltage for electrical parlance I always think because for me it becomes very easy to think in terms of fluid mechanics rather than heat flux and temperature analogically it becomes very easy few guys are very good in electrical then for them voltage and current is very easy we start feeling this heat flux and temperature I started feeling heat flux and temperature gradient only when I started thinking in terms of volumetric flow rate and high pressure to low pressure or high head to low head so this is one thing which is very important this analogy to keep in the back of our mind so coming back here so if you see here heat flux is equal to minus k dT by dx and dd by dx is T2 minus T1 and here what is that I need to emphasize what is that I need to emphasize is that there is minus sign why am I putting that minus sign because I want to make my heat flux positive in the direction of the decrease of temperature or the temperature this most of the students do not register this fact but we need to emphasize so that is very important rest all one can manage okay so this is Fourier's law of conduction is the quantification law for conduction similarly what is the quantification law for convection Newton's law of cooling and for radiation Stefan Boltzmann law so now let us go back to convection so convection for convection to take place we need to have a surface and on that there should be a moving but this moving fluid can be either created by a real fan or a pump or in case of natural convection it can be because of the temperature gradient which cause density gradients and in the presence of gravity only it can come alive in space there cannot be natural convection okay there has to be G and also density gradient then only body force can be created so you have the moving fluid and the heat flux is there of course this in this transparency it has been shown that T s is greater than T infinity it is the usual notation that we either use T s or T wall for the wall temperature T infinity or T f is the notation what we use for fluid okay notations are very important it is told that in the history that Newton used the notation y prime and y double prime for differentiation sorry Newton used dy by dx and d squared y by dx squared Lipnitz used y prime and y double prime and Lipnitz could not go as much as Newton went ahead people say that is because of the notation the notation is as important as the concept itself so we should be consistently throughout the course you should using one notation it gets ingrained if you see for example if you are reading radiation in modest and if you switch over to cycle and oval you will feel very discomfort why because the notations are so different and you go crazy by the time you get used to the notations you are lost in the concept okay that is the reason why we want to get attached why we start liking one book because you are used to that okay it becomes it becomes like orthodox you become orthodox because you get used to that so the point here is let us use consistently the symbols throughout the course whichever course it is so that people start attaching with each symbol meaning okay so next is radiation of course here we need to before going to radiation let us try to quantify convection okay so let me go to convection and try to see what is Newton law of course here convection as I said takes place because of the as I said convection is nothing but conduction conduction is what what is conduction here I have used the word the diffusion diffusion means what what do we mean by the word the diffusion today's world I take this example in the class also in today's world news gets diffused very fast because of news channels another is fasting could diffuse much faster than Gandhi is faster because that at that time the news could not diffuse because there were no news channels but in today's world news can diffuse very fast because of news channel here diffusion means heat is getting diffused that is one molecule is going and telling the other molecule to carry the energy so here diffusion is happening within the convection within the thermal boundary that's what we mean by diffusion and of course advection let's not bother about advection is outside the boundary layer that is because of the main fluid let's not bother about that when we come to convection we will handle that but main point here is when I say boundary layer there are several issues very difficult to touch the student because one problem we face while teaching convection is that we believe that they have been taught in fluid mechanics either students say that they have not been taught which is the usual answer which is not true which is not true but they would have been taught but they would have forgotten but it's our responsibility to prime them and on a fast track get all the fluid mechanics fundamental essential for convection to bring to their ramp that's what I keep saying I use these words and another thing I try to do this and that's what I have learned from professor vedula is that try to use technical terms in your general language for example I use the term choking in the gate if too many guys are there and not many guys can get out I use the word choking so by using that those words regularly and I use the word entropy don't create high entropy I keep using this so that it's disorder I don't have to tell much because professor UNJ has taught you entropy too much but here I am not using entropy in thermodynamic sense I am using in terms of disorder the point is if we try to use technical words in our colloquial language we start feeling those words much better than otherwise okay why did I use this term here yeah the point here I wanted to strike is that fluid mechanics is indispensable so fluid mechanics we need to bring to their ramp we have to prime them and then only start fluid mechanics start convective heat because here there are several issues very difficult to make the student appreciate the boundary layer is very thin how thin it's less than an mm can I see it can I not see it is thermal boundary layer going to be larger than the hydrodynamic boundary layer when can it be when can it not be all these questions we need to be answering properly so that can be understood only through proper understanding of fluid mechanics okay so now there are various types of convection in fact boiling and condensation is part of convection only it's not a separate chapter and here here the important thing is the heat transfer coefficient let me finish off this and come to heat transfer coefficient then forced convection we know there is a pump or a blower and in natural convection there are temperature gradients along with acceleration due to gravity and boiling and condensation there has to be essentially there has to be essentially two phases two phases now before we get to again boiling and condensation let us try to take Newton's law of cooling see the contributions of Newton Newton is there everywhere okay Newton's law of cooling is q double dash equal to h into T infinity minus T s very innocuously sounding equation very harmless very harmless equation now I keep asking this question all of my students all the time is h dependent on because as a student this is how I used to think if I increase q double dash keep T infinity minus T s h will increase or otherwise keep q double dash constant vary my delta T h will change is it true h is dependent on the class of the problem what I had it is this equation is only a quantifying parameter quantifying equation in professor aw that is words who has taught me convective heat transfer h finding h for each class of a problem for a problem is a phd why is to take an example of this because it is so involved if I have to take heat transfer coefficient flow around this cylinder it's there but let us say I take half insulated half either maintained at constant wall temperature or constant heat flux that's a phd or there is a body force let us say this fellow is rotated there is a body force so that's a phd the point here is finding h involves effort time and it depends on class of a problem when I say class of a problem what do I mean by class of a problem there are three classes of a problem let us say it can be internal internal flow what is internal flow flow through pipes what is external flow flow outside the surfaces I took cylinder flow around a building flow around bluff body that is external there is another class of problem very interesting class of problem which we don't usually touch upon in the class that is jets we call them as free shear flows why they are called free shear there is boundary layer there but we don't call that as boundary layer free shear flows because there the boundary layer is called a shear layer why is it called a free because there is no boundary it is free see every word has a meaning we have to look at it I keep telling this to the student we have to go to the depth of each word I keep taking the example see saw why is this word see saw called see saw you have seen him down you have saw up see saw see saw you see beauty life boy why is it called life boy if you apply that soap your life is buoyant borna vita why is it called borna vita it is borna with vitamins why is flow called incompressible anything not I use the word in not compressible every word has a laminar it is laminar it is going in laminar turbulent it's mixed I cannot handle it it's turbulent weather is turbulent crowd is turbulent every word has a meaning we have to have the patience to stop and think and mull over that word in my mind that's all it is if you do that we are done we will be efficient in the class that's what I have realized if I go underprepared if I don't many a times sometimes I think okay I have PPT I will just walk into the class and deliver usually I goof up usually I go for I might have taught 10 times 20 times 25 it doesn't matter I need to prepare my mind before going it's not that I have got it by hearted and gone gone there it's just that I have brought things to my ram it's just that brought that things to my ram that's that's what another thing I am digressing but it's introductory thing so that's what I am digressing a little bit that is I keep telling another thing to my students also that's what I learned from Professor Vedula in the initial days whenever I used to discuss with him papers used to say whenever you are sleeping and not getting sleep you think about this problem how does it work what he meant I used to wonder why is this guy saying that whenever I am not getting sleep you think what he meant I understood after several years what he meant was we need to do while studying we need to do Patan Chinthan Manan that's what everyone says that is reading or studying observation and reflection most of the times we just study when we go to the exam students when they come to the exam then they realize when should I take plus when should I take minus why because he has started thinking there Manan is perhaps Chinthan is perhaps happening there so Chinthan and Manan can happen only when I am not in front of the book and I am not in front of the computer so we need to when I use the word mull over I need to mull over means I need to think a lot about every concept okay sorry for digressing coming back H is the heat transfer coefficient it depends on the class of the problem so I need to measure this H for turbulent flow you have no option you have to measure the Hs you cannot compute them most of the times but for laminar flows yes for any type of the problem usually one can get the analytical solutions of the closed form solution with assumptions okay but there is a catch here yes heat transfer coefficient is dependent on temperature gradient where in natural condition this statement that H is independent of heat flux and temperature grade temperature difference is valid only for forced connection it is not valid for natural condition we have to be careful when we make these statements when we make these statements because if you remember in Grashof now there is delta g beta delta t l cube divided by alpha nu so indeed my heat transfer coefficient is dependent on Grashof number means it is dependent on temperature okay so that is about Newton's law of code now we need to get the feel of this H so usually usually natural convection means heat transfer coefficients are of the order of 10s if it is forced convection heat transfer coefficients of the order of 100 but then I cannot make the statement always it depends on the fluid also it depends on the Prandtl number also if the Prandtl number is high that is for air for forced convection H can be of the order of 100 it can be flow over a flat plate or it can be flow in a circular pipe or a square pipe but if it is turbulent sorry if it is water what will happen to my H? H will go high because Prandtl number is high Prandtl number of air is 0.71 and for water it depends on the temperature but it can vary as high as 6 at room temperature it is 6 so having the feel of this heat transfer coefficient number is very important you can see here heat transfer coefficients for free convection are less for gases of course for liquids here again they are high because Prandtl number and here again they are of the order of 10s for forced convection in gases and 100s and 1000s even in liquids if the Reynolds numbers are high and for boiling and condensation they can run into 10s of 1000s and lakhs okay your definition of heat transfer coefficient itself is tricky why what is the difference between the heat transfer coefficient definition in forced and free convection and boiling and condensation H is defined as q double dash upon T infinity minus Ts in external and internal forced and natural convection but in boiling and condensation what do I take for T infinity I take T saturation T saturation so one can question whether should I be defining my heat transfer coefficient perhaps no because there heat transfer coefficient is dependent on q double dash because you are taking T sat okay very tricky I know there are some blank faces I have not reached so heat transfer coefficient is not independent of heat flux in case of boiling and condensation because I am I do know what reference temperature take so we take saturation at that pressure whatever I am operating okay so now that is about heat transfer coefficient and Newton's law of cooling yes mungesh in convection yes but within the but within the boundary layer it is again diffusion only you know but even the boundary layer is forming okay there is the actual motion of a particle from the surface of the plate and up to the boundary layer they are exchanging the heat and likewise the flow takes place but then it is diffusion through molecules only yes you are right you are right you can say that it is because of convection because it is not always diffusion because let me answer this question little cool I will slow down see it's not always diffusion the way I told why because my boundary layer can be laminar and it can be he as long as my boundary layer is laminar only it is because of the diffusion but once my boundary layer becomes turbulent my turbulent fluctuations will take over that means there there is it's not just on the molecular level there is the velocity which is taking care that is advection that's why I had postponed but that's fine what you said so it is although people say it may not be if you ask me I don't think it is appropriate to say convection as conduction it's not truly diffusion within the boundary layer because the boundary layer is again either laminar or turbulent even if I take a turbulent boundary layer we know that we are going to see that there are three layers again within the turbulent boundary layer your laminar sublayer your buffer layer your turbulent boundary layer within the buffer layer and the turbulent boundary layer you have random motion not just molecule but also slight moment of the flow there is flow one cannot purely say that conduction only we have conduction in particular solid and we are thinking of that there is a no relative movement between the molecules no molecules are indeed moving energy is transversed because of the energization of the molecules because high temperature molecules will have high energy we are not allowing the position of the molecule to leave its space unless it is a pure matter in pure metal we are having the electrons which is eventually carry over the energy from one part but do we really have the random motion what you have you wrote over here that random motion maybe it may not be right in the sense that the molecule is literally moving from here to here yes what you said is right unless we have what he is trying to say is that molecule is going to vibrate yes while vibration is taking place it is going to hit the adjacent molecule which is also vibrating maybe not vibrating as extensively as this molecule because this molecule is sitting at a higher temperature once it hits the other molecule which is sitting at a lower temperature yes my question is how it hits the other molecules by hitting only by hitting only by hitting only there is no other no other by collisions it is because of the very close pack of yes they are dense the molecules or that's why we thermal conductivity of solid is higher than compared to that of liquid compared to that of yeah because the molecules are closer closer so when it is vibrating it can reach the next molecule much easier we can think of only three dimensional objects and it particularly depends upon in which direction you are hitting so in that direction it will vibrate more and by that way we are having the correct so that you are telling the isotropic whether it is in which direction if it is let us say it is one dimensional heat transfer but see although I say one dimensional heat transfer but the conduction might be happening in all directions no the vibration can be happening on all directions I cannot say that the vibration is happening only in one direction that is not possible but yes what you said is right random motion of the molecules gives me a feel that it is moving all over the place that's right he is very right it is not literally moving from one location to another location it is for us just vibrating there only and it is just going to diffuse that heat while it is vibrating only the adjacent molecules then that adjacent molecules will capture the yes you are right yeah I will come actually I have it thermal conductivity can be both the things we will postpone this it can be because of the crystalline structure or because of the random molecule or random molecular motion yes so I will come to that let us just postpone this because I have a whole lot of transparency for that we will handle that when we come but that's true what you said we will have to correct ourselves in our world if we use the word random motion fine so then radiation so radiation is one thing which we need to appreciate that there is no this is one mode of heat transfer which enjoys no medium if there is no medium it is actually perfect if there is a medium it is not perfect why let us say I take this example always now if this is enclosure and air is usually non-participated here I use the words participating and non-participated when I say non-participating it is not to not going to get involved in my radiation let us say I heat that wall and I don't heat this wall wall so now if my air is not participating in the radiation radiative heat transfer is it is as good as vacuum let us say then radiative heat transfer takes place between that wall to okay so radiation enjoys perfect it is perfect in vacuum and if the medium gets participating the amount of the heat transfer taking place from that wall to this wall gets impeded because of this that's that's one important thing and of course it occurs through electromagnetic waves so we will handle this what is a wave what is the size of the electromagnetic wave in thermal radiation domain all that when we go there I don't want to get into that but all that I want to say is that this is contributed by Stefan Boltzmann law so Stefan actually it's very interesting to know that Stefan actually found this law by simple empirical whatever experimental data was available at that time he did the Karoo fitting and he found that it is indeed 3 to the power of 4 okay and he gave the constant it was not 5.67 into 10 to the power of minus 8 it was some some some other number little different 5.53 or something I don't remember but that is that's all the difference was it was again Boltzmann his own student who derived this from fundamental thermodynamics to at the molecular level he went and he derived the value of signal so that's why it is called as Stefan Boltzmann law and Stefan is an experimentalist and you know Boltzmann is a theoretician and I will touch upon Stefan Boltzmann contributions when we come to radiation okay so this I guess I will skip epsilon and all we will handle irradiation we will handle when we come to radiation okay so to summarize we have taken three modes of heat transfer conduction convection radiation and there are three laws which quantify Fourier's law of conduction Newton's law of cooling and Stefan Boltzmann law so we have quantified them in that process we have come across thermal conductivity heat transfer coefficient that's where and Stefan Boltzmann constant so that's where we stand and now of course this is just just to say that here we in thermodynamics the unit always we are bogged down is joules but in heat transfer we are always use the unit we never use the unit word I mean we rarely use the unit joules we use always joules per second that is because here we are always interested in heat transfer rate so that's what it makes it different from so people call can people can call this also as incorpora as told another thing is that I am following extensively incorpora and David and chungal chungal chungal book CHC and GEL okay so I personally like chungal book not that I don't like incorpora and David incorpora and David is also very good but I like chungal because of the simple reason that I think even if a teacher is not there chungal can reach the student armchair reading is possible I call it as armchair reading this word I am taking it from professor Mukunda IISC he has written a book on combustion for armchair readers what it means is that you don't have to take a special effort to understand what Arthur is trying to tell you okay so I personally recommend chungal you please have a copy of chungal now in this day and age you can download ebooks left and right so you can download but I don't suggest that on an official note but I think for a teacher it's a good idea to have a book so chungal for reading the concepts incorpora and David for problems for problem and incorpora and David for problems why I say is that his numbers are representative of reality that is he transfer coefficient he takes or we are going to take a problem of yaggara potato let us say in transient condition he takes the properties very appropriately see the human body if I have to take what properties I should take for k row cp I should the property should be close to water because my body is full of water so this is what he emphasizes all the numbers in his problems are truly if you are researching in that area you will see that the number is representative of the number what you are getting in your experiment that is the beauty of incorpora and David's problems and it takes long time you don't get impatient even if I start sitting and solving a problem of incorpora and David it may take sometimes few hours and few days also sometimes if I am unlucky you don't have to get disgusted with yourself I am not getting no it's okay because his problems are involved sometimes because they are thought for working so the point was incorpora and David is a good book for problems and chengal is a good book for reading for example fins he doesn't tell why I should take infinite length means ml equal to 2.5 we are going to see how why ml equal to 2.5 incorpora and David doesn't take this train of making us understand but chengal takes it so chengal and chengal is a good book for understanding the concepts especially for students I usually recommend chengal for my students although my students do not purchase any books but that's what I recommend that's what I recommend so at least UG's don't purchase but Phd's and Phd's yes they do they are interested my students are only interested in the PPD's what I upload in the movies that's it okay so coming by other books there are other books there are plenty of other books Uzisik, Holman and there are plenty of other books Mills, Fankrith and Bone but all are equally good all are equally good you cannot say that any book is superior or inferior but extensively we use incorpora and David and chengal chengal of course the best book which I love is Béjan Béjan I would think as a teacher before going to the class every chapter I should read Béjan after I have prepared my notes why because the kind of insights Béjan gives through scale analysis I will introduce that scale analysis in semi we will take eta equal to x upon square root of what is that alpha t squared or some such thing I will take how on earth I should be taking that I will not know until I do the scale analysis so that is what so Béjan I would think it is a teacher's book it is a teacher's book if you are reading if you are teaching for the first time who is teaching first or second time I would recommend that we involve ourselves with chengal and interopera and David of course the PPTX which we are going to make is all there in the website you can always take that there is no harm in taking that we have generated the material from those two books but Béjan will be a good reading material for yourself before going to the class that that is what I would suggest okay so let's move on to this is what is the summary I had already summarized