 So thanks a lot and welcome to the workshop. So I'm happy to introduce Professor P.R. Narayan to you. He's an associate professor of Chemical Engineering Department, School of Chemical and Biotechnology of Sastra Deemtobi University. And sir, over to you. We can begin the session now. Good morning and I'm happy to meet so many students, so many interested participants for a three day workshop and open form. I also thank the opportunity of FOSI. I mean, not only for this workshop, we have a long interaction with the FOSI team and I should openly admit it is through FOSI. I had first started using Sylab when I joined Sastra. And ever since from there, I think there's no turning back. So from Sylab, it went to DWCIM, then OpenModelica, and then OpenForm. So it's almost like my journey at Sastra has been continuously along with also FOSI team. And I'm also happy that many of my students have got eventually benefited in their career because of learning this open source tool. So this why I'm telling is not just for gratitude, but all the participants where there are like about 138 now. If you have any doubt that whether this free and open source tool will build your career, you can take it for granted. Yes, it does build. If you put in the right efforts, if you sustain your efforts, if you really learn well, and if you can demonstrate your learning, certainly it helps in a career. I can give specific examples where somebody have learned the free and open source tool, got admission for their masters or in their PhD, they were specifically asked whether you know this tool. And given evidence, and they have given an evidence of textbook companion project. So I'm not sure whether those were stored, but I think maybe in the validation session, I think you can get it clarified from the FOSI team on the textbook companion. Payal, was that told in the first session? Yeah, I mean, they have got a general introduction of what we do, but to today after the session ends, I mean, after all the talk ends, I'll go into the details of participation and how can they participate in our project and how can they get involved in it? There are also specific examples where I can say, with respect to open form, where one of my students was asked, show how you know open form, and by knowing open form, he got a very good opportunity in terms of his PhD. And not only this is in research and PhD, there are also cases where people have got industrial jobs because they know, let's say, open modelica. So take it for granted, knowing free and open source, becoming yourself expertise in a free and open source is certainly going to make a way for a very good career. Now, with this gratitude note to the entire FOSI team, I wish to begin this lecture. We will make this a more interactive one. Now, I hope you have seen the link that I have put in the chat window. If you have not joined, you can clearly join the 20 meter, and in your screen, you must be seeing now a thumbs up icon and which you have to click. Now, if we can see now, there are about 58 people who have clicked it. So we have about 145 people in the Zoom, but I think about 64, 68. Now you can see in the lower bottom of the screen, which I'm sharing in the Zoom, you can see the count. So I'll just wait till this number reach something around about 100 so that we can carry on. So those who are on mobile, you can prefer to use the app, which might be very easier to use. And if you're joining the Zoom on a laptop, then I think just on another browser tab, you can open it. Okay, so we will just go to the next slide in this. Now, you will see a question, I have just asked, what is your affiliation? I just want you to pick a choice based on your affiliation, whether you are a current undergraduate student or a post graduate, or if you're industry or any other entrepreneur. So that's the last option. So this is just fairly to know the audience to whom I should talk. So I should be knowing a fair knowledge of the community in which I'm going to address. So we have about more post graduate participants, so that's nice. So eventually that means that this session is going to be more of a very comfortable for me. Yeah, no option for research scholar, probably you can put industry and others. So sorry, I didn't think that there was also a research scholar. So maybe you can put on industry and others. Sorry, I think that is from one Mr. Bharuti Prasanna. Industry and others, I'll just take it as also as industry and research scholars, whoever is not there in the other options. So fairly there are also second year students who have shown interest. Like we have about 13 second year students and about 11 third year students and quite a number of post graduate students. So that's fairly nice. That makes also the session going to be more lighter for me. Okay, so we will go. So now you've got an idea of how to operate on the Mentimeter, right? And you understood what is the Mentimeter. Your name was never captured, right? So when you responded, your name was never captured in the Mentimeter. That means eventually I do not know who is putting it. So it's only you will know what choice you have put. So in a way, so I hope this will break your stigma of asking questions. You don't need to worry what questions you are asking or what options you are choosing. As long as you choose it appropriately to your conscience, it's fine. So there are still about 156 participants, but I think we have got only 87. So I'm not sure there is any issue for the other participants. Nevertheless, I'm just again posting the message in the Zoom. This is for our other participants who are ready to join. So I have posted the message in the Zoom chat window. You will see a link or you can go to menti.com and use that numeric code which you see on the screen, 8907-8131. There is a question that might appear. It will ask your affiliation. I agree the research scholar option I missed in the choices. So if you are a research scholar, you can probably put in industry and others. So of the 160 participants in the Zoom, so we have got about now 107, so that's nice. So I'm proceeding further, assuming that we have majority of them as research scholars and post-graduates, but nevertheless, a bunch of students are also there from second years. So that means the tone of the talk, the talk, I have to keep it at the post-graduate level. I will still be very mindful of the undergraduate population that is there in the talk and pace the talk appropriately. So the next question is just, I just want to know which field you are from. Now I have listed four, but you could be from other field also. So if you are from other field, you can just put there. Now when I say chemical engineering, it is all chemical technology related to chemical engineering. Similarly, mechanical is mechatronics, mechanical engineering, metallurgy production, all related field could be in mechanical itself. So these are like broad domains and not necessarily your degree. So it could be mechanical production, manufacturing, metallurgy, all I have clapped in one. Chemical engineering, chemical technology, petroleum, petrochemical, all clapped in chemical. Then you had civil, others is other branches. So predominantly we have, then that means it is very clear we have people from mechanical. So that's nice. So fairly now I have understood the audience to whom I'm talking to. Okay. Now you will now see a question where you have to enter a phrase or a sentence, right? Now I will hide the results from the Zoom. So first let you enter because I don't want you to read somebody's text and then enter. You can enter what is something that you think you have learned in the, like say you're confident. You can enter multiple times also. Multiple entries are allowed in this option. So you can do enter multiple times. I'll wait for a minute. I think they can unmute and respond, right? Professor Narin? No, they are actually texting it. I have actually written it. So I don't know. Actually, yeah, that's why one person was asking how should I respond? Oh, okay. So you should click on that link and in the link they should respond. If you see in the Zoom meet, you will see there are 64 people who have responded. So let me just unhide the results. Now you can see what people have texted. So Blender they have learned, right? Was Blender not? Yes, yesterday we had a session on GUI for open form block. Okay. So our team member as the other took the session. That's why they know this. Good. Laminar and turbulent flow, equation, discretization, fantastic, different types of meshing, running an open form. So that's good, right? So this is just for me to warm up so that you're interacting and you're not only looking at the screen, right? So nice. So the next is something little, you need to think and put what is your proficiency in these things? So on the right hand side is, I mean, one choice is that you know it very well, right? Let me just hide the results, okay? Let us have people and then we can put. So 13 people have entered actually now. Okay, there are about 90 people. So now you can see the, what is the average the people have responded, okay? So I should appreciate fairly everybody have learned and because we have a very good post-graduate population and I think all these are then eventually easy. And it's very surprising for me to know that, you know, more of the derivation of the Navier-Stokes than of the energy balance equation. At any given day, I would probably say that I know energy balance equation derivation very well than an Navier-Stokes. So this means really the credit goes to the session handled by the Navier-Stokes equation in this open form, which means you have learned, right? So that is actually one success I should attribute to the FOSSE team because that clearly reflects the people know now continuity and Navier-Stokes equation much better than the scalar equation which is an energy balance equation. That's nice. And I'm happy to see that most of you know what is the Eulerian and Lagrangian approaches also. So that's nice. Okay. So the next is something that, sorry, just it's not the POSSE, yeah. What should I talk in this session? Remember this session is on heat transfer. So this session is on heat transfer. So if you are asked to suggest a topic to me in heat transfer what you would like to suggest in heat transfer. So let us restrict to at least the domain of heat transfer within heat transfer what you think I should be talking about for the next half an hour or so. I'm just showing the results, convection, customization, convection, how to incorporate heat transfer, smooth horizontal pipe discretization of heat transfer, equation of lump heat transfer, convective, good energy equation, LMTD heat exchangers, good. So fairly many people have written something which I should say that I had thought of before coming to this talk. So that's nice. With this we'll go ahead. We are almost close to 1030. So I think we should start now a good business. So this is pass to open form. What I mean by pass to open form means now I will change the screen and I'll come back. I'll stop sharing the screen and I will share the screen which I'm going to use now. Now what we will do is as people are interred we will see about the equations pertaining to heat transfer. Rather I should say that we will see the equation pertaining to energy, energy balance in CFD. Okay, how is energy balance is being done in CFD? So with this let us go. Now what we know as of now in CFD is CFD is all about solving flow. Primarily it only solves flow. But we don't have only flow, along with flow we need to solve for energy. So this flow is nothing but momentum. Now why we solve this momentum equation because we want to get the velocity pattern. We want to get the velocity pattern. If you ask the question why we need to get the velocity pattern because this velocity pattern affects the performance. Performance typically that means we will say as engineering characteristics. Engineering characteristics of that particular equipment. Now the equipment that we are talking of could be a heat exchanger. Let's say if it is a heat transfer it could be an evaporator. It could be a reactor. It could be a turbine, right? So this is heat transfer of an equipment. Equipment that is used in a process. It's not only this velocity pattern. This velocity pattern also has an effect. It also has an effect or rather this energy will also have the temperature distribution also has an effect on this velocity pattern. And that is why it is not about only solving flow it is also about solving energy. The other equation which we solve in CFP is the species which we actually call as mass balance. Now this is different from the continuity equation. The continuity equation is something that we call as overall mass balance, right? All of these and there is no choice over and above this continuity equation has to be solved. So there is no choice in this. This is must, this is must. These two are optional. These two are optional. It is not in all the CFP problems you will solve energy and species. Now here you might have a doubt. If you take a typical textbook problem on heat transfer let's take a typical textbook problem on heat transfer which will be like a slab. This would be a slab, the slab. So that means this is a solid slab. This is a solid slab. And they will typically tell that this end is kept at a cold point. So the cold point would be 30 degrees because 30 degrees is cold with respect to another surface. And this could be at like say 80 degrees. Now what is the temperature distribution in this solid slab? Now, can we solve this? Can we solve this in CFP if you ask this question? Can you solve this in a CFP tool? Can I solve in CFP tool? The answer is S. It is not whether you can solve or not in CFP tool but if I reverse the question slightly is it something necessarily a problem of CFP? No, if I ask something like that then it is not. It is solvable in a CFP tool. Why it is solvable in a CFP tool? Because already we have seen that energy equation is there in the tool so you can solve. But if I ask the question just because it is there in the tool is will this become a CFP problem? No, typically CFP problem means it should solve the flow it should have the velocity pattern captured over and beyond capturing the velocity pattern if it also captures the temperature pattern and the species pattern that is an added goal in the problem statement. Pure conduction. This is pure conduction. Or rather in CFD terminology this is called as pure diffusion. Pure diffusion problem. You necessarily don't require CFD as a tool because these have clear analytical solutions or they can be easily solved by a mathematical tool as well. I mean by writing boards also. So before even going to CFD if I ask me what context I'm now going to look when I say I'm solving heat transfer is I have a flow there is certainly a flow. Along with flow there should be now energy. So this is the context I'm looking into. This is the context I'm looking into. So somebody has asked engineering KTCS what is the shortcut you have used? Characteristics. Okay, so that is not characteristics. I think it is characteristics you are talking. Okay, now what I mean by I'll just answer the question and then we will proceed. What I meant is engineering characteristics which means let's say for example your heat transfer coefficient. This is one characteristics. This is like a global characteristics which you require because this will affect the heat transfer area and this heat transfer area is what you are expected to design. When you say you design an equipment you are actually arriving at the value of a heat transfer area. It is not directly that area that is important. Through this area you are actually going to get the dimension it could be length, breadth, height, diameter or anything else configuration and whatever it is. All these dimensions particularly is important depending on which kind of equipment you have. It is a shell and tube tray or something else. So this is one engineering parameter or an engineering I would say parameter and this parameter eventually is governed by how you solve and this is what is important in design and that is linked to also the velocity pattern. Now, because it is heat transfer I'm talking about heat transfer characteristics but it is not only heat transfer like say for example if you might have seen terms something like conversion, yield, efficiency. Now, efficiency could be energy efficiency. All these are parameters which are required which are required to be not designed. I mean you need a certain value for a certain application or you might be looking for a certain range of values as part of your process. Now, how will you arrive at these values? There are design procedures. Now, if you ask to actually calculate the heat transfer coefficient are you doing CFP? If you ask the question it is no, right? So that means for example, I think many of you would know how to calculate shell and tube heat exchanger, right? Where the heat transfer rate is given as overall heat transfer coefficient times area times temperature gradient. And this is linked to the diameter, the length and the number of tubes and so on which is a configuration of the heat transfer equipment in this case, shell and tube. Are you by doing CFP and arriving this? If you ask this question, no, that's a simple no, right? We don't use CFP to arrive at this but now the question is not that. One assumption that we make in shell and tube like say for example, if these are two tubes, I'm just taking two tubes in a shell, right? I'm taking only two tubes in a shell, right? I'm not at all drawing the other parts. I'm just making it a very gross diagram. Now, you are assuming the fluid equally splits in both and then comes out equally from both. Is this assumption valid? If you ask now this question, is this assumption valid? You are assuming the temperature drop in both the tubes are same. Let's say for example, if you send it at like say 30 degree and you are heating it something, if this comes to 80 degree Celsius, you are assuming this 30 degree also will come to 80 degree Celsius. This is what you do when you are doing the conventional heat transfer calculation. But is this assumption valid, right? I'm not questioning the method, but can you assume this if I take number of tubes to be 40, if I like say I have 150 tubes so on and so forth. Will you still think that this is going to same? Probably now you will start thinking no, there might be some variation. I'll just think if there is some variation will not the tubes now bend differently because there is a thermal stress with respect to the tubes and the tubes will bend. What if the first tubes bend like this, right? And the second tube bend like this, right? Then your heat exchanger will break. Is it clear? So that means you need to assess you have calculated still the heat transfer area but the performance of that equipment needs to be not only judged by that parameter, it is also judged by whether all the tubes in the heat transfer equipment are working perfectly. Now this is assessed by trial and error also or by periodic check also. So before CFD, if you ask me like say some hundred years before how people were doing people were actually checking people who used to check for every week there are systems where people used to check for every month and every month they have to change the heat transfer tubes due to corrosion, bending and whatever it is. Now we have techniques, now we know to up really assess. Right? Now that is facilitated by actually solving and then seeing whether the temperature distribution is same. So I hope I have made some impact I mean some clarification on why is this engineering characteristics is all important. Now let me go to the rigorous equation. Now all the transport equation what we call as transport equation in CFD it is a transport of a quantity. Let us assume that it is transport of a quantity. The quantity can be either mass or momentum or energy and we follow phenomenological model but that means we will conserve we will conserve this quantity. We have to preserve this quantity. This quantity cannot be lost, right? That is the very fundamental. So what is the equation that we have whether it is mass or energy we can say is whatever is going in, in a control volume so I am taking a control volume so I am not going to talk on why control volume approach or what is a control mass approach or something I am directly assuming that there is a fair understanding now among all the participants who are here on a control volume and remember this control volume it could be a tank and this control volume could be a very, very small fictitious which is here. So if you integrate the control volume you will get this tank. Okay we are writing on a fictitious very, very small volume on an equipment. This control volume can be as part of a tube also. So this means on a pipe flow this small part can be this control volume. It can be on a part of heat exchangers also. So if you have a heat exchanger and let's say the shell and tube heat exchangers suppose if you have this control volume could be something over here. It could be anywhere here or here. It is only when you integrate this control volume you get the equipment. So that's our understanding. So let us so all equations or in CFP can be written as in plus generation is equal to out plus consumption plus accumulation. This is the equation for all. This is the equation for all the three equations. The only difference is when you talk about continuity you will not have the generation and consumption term. So now we have to talk about what is the in and out in terms of energy. How the fluid energy how the energy comes in or goes out in a control volume how the energy comes inside the control volume and goes outside the control volume. So the energy can come in and go out in two ways. Now I'm not doing the complete derivation I'm just giving the snapshot for you to understand the equation. One it can come by flow another it can come by diffusion. What do you mean by it can come by flow because the fluid is flowing because there is a mass flow rate and this mass flow rate is associated with an energy that we call as specific enthalpy H. Every matter is associated with energy. Now if a mass is coming in then the enthalpy associated with that mass can also come in. Now what is diffusion? Diffusion is without motion. So this is something that we call as convection or rather in CFD language we should call these as advection because if you ask fluid physics people they will call about advection and they will say convection as sum of advection and diffusion. Advection is of motion. Advection is what is because of motion. It is what it because of velocity rather flow. So that means how to write convection. So we know that convection can be written in terms of mass flow rate. So this is always rate. You should remember everything is rate. So rate means quantity any quantity per unit time and what quantity we are focusing now? We are focusing on energy rate. So that means energy rate which I put as E dot if I write in SI unit it will become joules per second, right? Joules per second. So if I know mass flow rate and if I multiply that with the specific enthalpy and I will get this energy rate. For example, if this is the mass flow rate at inlet multiplied by its specific enthalpy. Now mass flow rate, let's say in SI unit is kgs per second. If specific enthalpy is joules per kg eventually you will get joules per second which is the rate at which the energy flows in. Here we are considering only the enthalpy form of energy. We are not considering the kinetic and the other forms of energy. Similarly, the out also. Out can also have diffusion and out can also have it is not only in but both in and out can have energy leaving it because of flow happening and flow going out. Now what about diffusion? Diffusion is something. Diffusion mode of energy transfer. Now this is what we call it as conduction where we do not really require the fluid to flow but it is actually the submolecular or the molecular means of transporting a quantity. So this is how, I mean like typically you tell an example if you just stand here at hot place you don't need the fluid to flow towards you. You can just feel the hotness just because of conduction because the molecules hit you and the molecules hitting you, it conveys the energy. Now how is a conduction mode of energy rate is given? This is what is given by the transport loss that you might have studied. For example, for energy transfer it is the Fourier's law. The Fourier's law of heat conduction. That primarily tells the diffusional mode of transfer. So here again the energy rate or you can also write that as Q is equal to a diffusion coefficient times a gradient of temperature. Any transport law can be written like this but this is a very generic way of writing. What is the form in which that you know? You know it as lambda times gradient. Lambda is what you call as thermal conductivity. We can also write it in another form which is like lambda times rho Cp and times del of rho Cpt. Eventually if you write in this form this term is called thermal diffusivity. Like how you have momentum diffusivity this is also called thermal diffusivity. So now you know that in and out can be written in terms of either convection or diffusion. Now let's come about generation. Generation or consumption. So they are almost equal to one other just with a sign. So if you put plus sign it let's say become generation. If you put minus sign let's say it becomes the consumption or the loss. Or the loss. One is by heat transfer. So that means from one system to another. Or one control volume to another control volume to transfer a tank click base. This is nothing but heat exchange. And it is for this heat exchange we know the equation Q is equal to HA delta T. Heat transfer coefficient times the area of contact between the two systems times the temperature. There's another way or for another means also by which this generation and consumption can happen that is because of reaction. If you think of combustion chamber, if you think of engine coal combustion there is a reaction happening. And by virtue of reaction there can be a generation of heat or there can be a quenching of heat. There can be quenching of energy or there can be generation of the enthalpy. That can also be written. Now this is usually written as how much energy change how much enthalpy change is produced is resulted per unit of the reaction. Per quantum of the mass that is getting reacted times how much is your reaction taking place. So the quantum of reaction times how much the enthalpy associated with this reaction is what is this heat exchange. This is primarily heat exchange. If you have this system, how much heat loss is there through the surface of the system to another system which is around it. I mean there's nothing like called a system and a surrounding is also an equally a thermodynamic system. So that is something that you need to know. There's nothing like system and surrounding is something different. Surrounding is another thermodynamic system. The system is something that you focus on which you write the equation. But the surrounding is something in another system for which also the same thermodynamic loss or value. Now I should actually thank, I mean if I am not sure if how many of you have heard the thermodynamic lectures of Professor Sukatme again from IIT Bombay it is available in YouTube and also as part of I think the NME ICT initiative. So you can go back and look into the lecture where he would have explained very clearly the ideologies of the first law, the second law of thermodynamics and how heat has to be conceptualized. So now let us just write what is the final equation would look like. So we say in plus generation will be equal to out plus loss plus accumulation. Now all this with respect to a control volume in which the fluid is flowing. So you have a specific mass flow rate or fluid that is going in. So that means you have some velocity uvw of the fluid that is going in. Here again the mass flow rate will be same but the uvw will be still different. There could be some heat loss or the heat gain it could be this side arrow or the other side arrow whatever it is. And if there is a reaction then there could be a reaction also occurring within this control volume. All these are a conduct, all these are for a very general case. Now if this is the case you will end up in an equation which you clearly know now or you might have seen in books as in will come because of the flow. I think I will avoid writing the differential equation for want of time. I will just write first, yeah. So because of flow times enthalpy this is the inlet that is happening because of advection. Plus whatever is the flux that will happen because of diffusion. Now I am not putting the directional coordinates here it can be x, y or z. So this could be dx, dy or dz. So this is because of diffusion. This is again in. Plus you can have either the generation the transfer within the system into the system or out of the system. So you can have here h a and area times here you can have a delta t which is actually the heat transfer. So this could be generation or this could be loss. I mean generation in terms it may be a depending on whether it is plus or minus. Again you have m dot h which is out which is actually the advection out plus again a diffusion. This could be again dx, dy or dz depending on how you take diffusion plus an accumulation term. This is d by dt of rho c dt. This is primarily organ how much enthalpy is stored in the system. This is where the equation starts and if you see the final if you go back and see the book see the other steps of derivation you will see the final equation would end up something like this. So it is this term that's coming like this. The mass flow rate term is what is coming as rho into u minus let's say plus this could be plus or minus. This is again dt by dx. I am writing only with respect to one coordinate but this could be actually a partial differential equation if you write it in all the three coordinates. Equal to minus this is the thermal conductivity times dt by dx. Again this I am writing with respect to x and it could be with respect to any of the coordinates and if it is all then it would be like a partial equation plus d by dt of rho c dt. This is actually this enthalpy term. So this talks about the net advective energy flow. This is the net diffusional energy flow. This is for conductive heat transfer. This is accumulation and this is heat exchange. Now let's take I will take another five minutes just to give a preview like say if you want to solve for a flow through pipe in a heat transfer what you should do so that you can practice. Let us assume that I will take a very simple case of a fluid flowing through a circular cross-section pipe. So this is a circular cross-section pipe so you know the diameter let us assume that you know the length. So this is known diameter is known. There is a fluid now you can take it as water flowing or you can take it as air flowing. I mean you can make it as either a constant density system or an incompressible system based on the fluid that you take. So the fluid is flowing in. I hope now you know how to simulate what I mean by how to simulate is suppose if you give here a flat velocity profile a uniform velocity profile and if your condition is such that that towards the end you might generate either a laminar flow through profile or it could be a case where you will generate a turbulent flow through profile. It's not laminar will go to turbulent please don't take it like that way. It is like depending on the condition whether your Reynolds number is in the laminar regime or it is in the turbulent regime for a flow through pipe. So for a flow through pipe it is roughly about 2,100 is laminar and turbulent is you have to be little more than 10,000 right in between is like a intermediate region now you can generate this now in the same problem all we have to see is what will happen if there is a heat transferred what will happen if there is a heat transferred through the walls so that means we are more specific about what is the wall boundary condition right now why we are worried about boundary condition because you should know that all our transport equation all our transport equation they are differential we have written in Eulerian control volume approach we will end up as a differential equation now to solve differential equation you need a boundary condition like for example if it is a first order equation then you require one boundary condition if it is like a second order equation you require two boundary conditions for that variable right only then you can solve so that is what we are talking and what I mean by solving it we are actually trying to integrate this equation so that we will generate back the actual equipment we will generate back the actual system so you might have seen or heard in your previous lectures what it means by no slip free slip boundary conditions which are all for solving the momentum balance equation now I am going to take you and then tell about what is the specific boundary condition that you will use it for your energy transfer primarily what is the first type that is actually the adiabatic wall what do you mean by adiabatic wall so if the surface is adiabatic the fluid can go through it but then this energy transfer is not possible so that means no energy transfer so when I say no energy transfer which means q is 0 but q is 0 you need to think of two terms it can be either because of flow or it can be either because of diffusion but in wall you always don't have flow there is no flow across the wall so this you need not worry because in any case there is no flow across wall because if the mass flow rate in the normal direction to the wall itself is 0 there is no way that the energy will get left the only thing that you have to now worry is about the diffusional transfer so what is the diffusional transport equation now you should remember that is actually a Fourier's law which is based on the gradient which is based on the gradient so when you want to make the wall as adiabatic it means you are making the gradient as 0 the gradient of temperature across the surface across the wall surface is 0 so that means on the normal direction to wall on the normal direction to wall dT by dN this dN transfer I am telling about normal direction to wall if this is your wall that means in this direction dT should be equal to 0 there is no transfer allowed permitted so we can write adiabatic wall either by writing the heat transfer rate as 0 or we can specifically clearly write it as the temperature gradient being 0 why we are not worried about the flow because we know that any ways flow is 0 across the wall what is the next known condition in heat transfer so that is an isothermal wall now that is fairly easy so if you have a system where there is a flow is happening and now I am saying this surface is isothermal what I mean by that this surface is isothermal that means the temperature of the surface is fixed so that means to specify something as isothermal wall this should be a known constant you should prefix this to a known value that is what it means by isothermal it is fairly constant I mean fairly simple the third one is diathermic wall or what is called as the heat exchange or we can also very simply know it as it is a non adiabatic surface that means you have a control volume on which there is a flow happening but then you are saying that there is also a heat transfer happening but that does not mean the temperature of this wall is fixed this temperature of this wall is not fixed so that means it is neither isothermal nor it is adiabatic but then it is allowing some heat exchange if this is the case then for that wall surface you need to specify this heat transfer rate or the heat exchange rate now in CFD specifically in CFD these two you don't need to specify one area is from geometry it is automatically known because you are discretizing you are drawing the geometry and everything so it knows this this delta t it will compute so this delta t is what the system can compute based on what is there in the system profile and what is the temperature that is given outside but this heat transfer coefficient is something that we need to provide now you cannot be without providing this heat transfer coefficient this heat transfer coefficient is something that we have to provide so that means essentially you have three boundary conditions one is adiabatic isothermal and another is diothermic so in adiabatic the heat flux or the gradient is specified as zero in isothermal the wall temperature has to be specified whereas in a diothermic or in a surface with heat exchange we have to actually specify the heat transfer coefficient okay so any other questions so I think I will stop here I do know that we didn't do elaborate derivation but that's fine I think in one hour we have to just understand appreciate what is the problem that you are going to probably do and what is heat transfer in one hour I hope fair amount of understanding you have got any other question in buoyancy driven flow or energy and moment coupled as velocity yes so now in your CFD formulation like you might have to specify something as constant thermo physical properties or like something like incompressible flow constant density flow now if you are taking air and if you know that the temperature distribution degrees and let's say you are heating till 220 in that case you cannot assume air to be now incompressible it is no longer a constant density system because the variation in density will eventually change the velocity pattern and eventually that is going to affect the distribution so that means if you are doing systems where there is a change in density then it is to be a couple couple the other question is some the best book I would always suggest is the book if you are a mechanical engineer you should be very proud of a book called Suhas Patankar he is an Indian now I mean he is an Indian studied in college of engineering Pune and then eventually now I think he is in Wisconsin for a long time he has the finest book on heat transfer and CFP so read the book on Patankar so are we done okay so I hope we can close this talk session now and proceed to the next one I would like to thank Professor Narayan a lot thanks a bunch sir this was a very interactive and informative session yeah thank you thank you for joining thank you