 Good morning everybody and it is also see how the interaction happens. So, we are going over to Hyderabad, there was a question. What is the difference between static pressure and total pressure? What is the difference between static pressure and total pressure? Sometimes I wonder we are teaching fluid mechanics or heat transfer. So, that is what professor Gaitande was saying we should have actually had first fluid mechanics rather than heat transfer. Anyway the question is what is static pressure and what is total pressure? What is static pressure? The name static itself suggests that something is static. So, this is what professor Anderson gives in his text book fluid mechanics text book not fluid mechanics aerodynamics. If I sit on a fluid particle and keep moving along with the fluid particle whatever pressure I feel that is static pressure, but while I am moving I am going to have the kinetic component also along with me that is dynamic pressure that is rho v squared by 2 that is dynamic component. If I add this static component of the pressure that is the static pressure and the dynamic component that is dynamic pressure then I am going to get the total pressure. So, this is static and dynamic pressure. Static pressure if you still are not convinced with my answer I had told on the other day itself how do we feel pressure? Feeling pressure is to go to its extreme end where there is no pressure. When do you do not have pressure in vacuum? You evacuate a chamber nothing is there. Why no pressure is there in the vacuum? Because there are no molecules. If there are molecules there is intermolecular collision because of the intermolecular collision there is pressure. So, I always feel the pressure as the collision of the molecules. When there is no molecule then there is no pressure because they cannot collide with anyone because there is no molecule. So, that is static pressure. Thermodynamic pressure whatever pressure Gaithundey has taught us is thermodynamic pressure that is this static pressure. There is no difference between thermodynamic pressure and static pressure. Thermodynamic pressure and static pressure are same only when we add the kinetic component to it that is the dynamic component to it that is rho v squared by 2 I get the total pressure to the static pressure. Is that ok? Thank you sir. Thank you. We will go over to NIT in Tiruchirapalli now. Sir, in the case of flow through the duct what is the correlation for nostril number, for laminar and turbulent flow, for entry length other than circular duct such as rectangular channel. Ok, good question. Actually in case of a duct in developing flow what is the correlation for friction factor and nostril number? Yes, we have not covered this in non-circular geometries. Why non-circular? Even circular we have not covered because this is a developing length usually we cover this portion in PG that is post graduate advanced heat transfer course. It is a good question, but we cannot there is analytical solution for that it is not that there is only experimental result. There are correlations available please put up this this is what is called as grades problem g r a e t z problem grades problem we have not covered this in our course please put up this question in the moodle we will come up with the answer for that. There are correlations as a function of x by d in fact if you if you want to see the correlation directly it will look something like this nostril number equal to some constant plus some constant upon r e into x by d. So and next is again constant upon r e squared into x by d whole squared. So the correlation is essentially a function of Reynolds number and the location where we are interested in getting the heat transfer coefficient. So we will put up this result definitely, but usually for a u g e transfer we do not teach this this is beyond the scope of the u g heat transfer definitely if you put up this question we will put up this answer in the moodle ok. We will now go over to St. Joseph College Kerala. In the case of flow over a flat plate is it possible that right from the leading edge the turbulent boundary layer starts? Yes good question for a flow over a flat plate we always study that initially there is laminar boundary layer then we have transitional boundary layer then we have turbulent boundary layer is it possible that can I get the boundary layer becoming turbulent right from the leading edge? Yes it is possible how can I do that that is how people do that is they put a tripping wire initially itself so that my boundary layer becomes turbulent or I can if I take a rough plate right from the beginning it is going to become turbulent that is when I say rough you imagine that sand grains have been poured on the rough plate and then immediately that provided that sand grain roughness thickness is greater than the laminar sub layer thickness which is what we have been insisting right from the beginning if you have broken that viscous sub layer because of the presence of the laminar because of the presence of the sand grains then definitely my boundary layer is going to become turbulent right from the leading edge. Thank you very much let us go over to J. N. T. Sir in a pool boiling curve the curve declines from C to D and then it increases from D to E so why is it so can you explain sir and one more question yeah yeah can you explain the Wilson plot in heat exchangers thank you. Wilson plot okay Wilson plot I have explained during yesterday's heat exchangers but still if you think that we need to answer that please put this in moodle I will answer that but essentially Wilson plot is used to measure the heat transfer coefficient on the tube side keeping the annulus side heat transfer coefficient very very high that is the essence of Wilson plot but I will answer that in greater detail through your moodle question but what was your question will you please repeat we could not follow that for now I am flashing the boiling curve with reference to boiling curve will you please ask your question. Climbing from C to D and increasing from D to E okay so why is it so can you explain. When C to D there is a decrease in the heat transfer coefficient and D to E why is it it is increasing okay so what is happening. Word of caution please look at the scale it is a logarithmic scale x axis is delta T which is increasing from about 100 at leading first point to about 1000 so heat transfer coefficient is not increasing what we are saying is that the the the wall temperature has increased suddenly and this transition region which we have shown so nicely by C to D line you cannot capture it so nicely you will in fact it is a region of oscillatory flow so you will have heat transfer very good at an instant very poor heat transfer when vapor is in contact at an extension so this line is just drawn for convenience in fact you will not be able to get such a nice curve at all okay so it is heat transfer is not heat transfer is not increasing or anything like that what we are saying is this part of the curve D to E has been obtained by this concept of this film boiling as you are making the film as the film is becoming thicker and thicker your heat transfer is going to become poorer and poorer okay but when the film becomes thicker the temperature of the gas or temperature of this vapor inside the film will also be higher so radiation heat transfer will become higher so that is a cause for heat transfer and increase there so it is not two phase anymore it will become single phase but radiation can contribute to even up to 60% of the heat transfer because of the high temperature okay remember for radiation it is to you all to the power of 4 so its contribution is very high okay thank you very much we will move over to PhD College Coimbatore which has to ask a question sir in turbulent sea measurements already told you animometer used to measure the turbulent sea in heating conditions in no heating conditions whether you use the animometer the question is for turbulence measurements we have always mentioned that we can use hot wire animometer but can I use hot wire animometer for heated conditions what all examples have given is all for unheated condition that is what you mean perhaps is that if I am having fluid flowing it is a cold fluid that is it is just maintained at ambient condition and we are doing the experiments under adiabatic conditions that is there is no heat transfer yes that's a good question under heated condition what will happen the fluid temperature is varying from one location to another location so it is yes it is difficult to measure under heated condition because the fluid temperature is varying from one location to another location why the basic principle over which this hot wire animometer is working is CTA that is constant temperature animometer it is called as why because the temperature of the wire is maintained constant that means what we are worried about is the temperature between temperature difference between the wire and the fluid but from location to location if the temperature difference between the wire and the fluid is varying then my calibration characteristics of the wire are going to vary why because my calibration I have done between wall temperature of let us say 400 degrees wire temperature of 400 degree Celsius and a fluid temperature of 30 degree Celsius if the fluid temperature is continuously varying my calibration characteristics are going to vary so if the fluid temperature is varying that is under heated condition it is difficult to use constant temperature animometer for that there are high high end techniques that is either particle image velocity symmetry or laser interferometry we have to employ is that okay okay so next question is conductivity based so conductivity varies from hardness material so we know do you have any relationship between the that's a hardness conductivity okay question is whether thermal hardness thermal conductivity varies with the hardness are you aware of any correlation for thermal conductivity with hardness honestly I don't know I never even thought that hardness would matter the thermal conductivity so far but you please put up this question we will try to see that and come back to you I don't know really let me think allowed why do you think that hardness would affect my thermal conductivity thermal conductivity is the arrangement of the molecules inside my body so why should hardness affect the thermal conductivity anyway you put this question let me answer by thinking or searching the material can I add something as a as a completely non thermal or unthermal expert wouldn't hardness also depend to a large extent on the arrangements of molecule anyway so in which case whatever arrangement of molecules you think is responsible for conductivity by implication at least the hardness should have some role to play because hardness and the molecular arrangement should somehow be related I do not know I'm just speculating would my friend from remote center like to comment on this observation and then I will ask him again to reflect back over to you okay sir so next next question is in heat transfer and thermodynamic subjects lots of assumption okay sir but in real and practical case not like that once a minute sir real and practical case is not like that so how can blue or how can what is the reliability of the subject see one thing is regarding your first question diamond and graphite are both carbon okay but I'm sure both of both of them have different hardness I don't know the numbers but diamond is used to cut metals so diamond is harder than graph graphite so there the structure is going to be different not that I'm an expert in all these things but thermal conductivity also is going to be different so if you go to from a point of view of lattice structure etc I think we will be able to understand this we can post it to our colleagues in metallurgy department and try to get some answers for it second thing related to assumptions I feel it is actually very philosophical okay see not being able to solve a problem is not an option so you have to solve a problem what is the accuracy with which you want to solve a problem that is what is going to drive our solution methodology now to have a solution methodology we need to have some tools and those tools are what we are getting to see from basic courses and even applied courses so that to that matter so what we teach in colleges though it has a lot of inherent assumptions it is actually developing the thought process more than the solution technique okay so the thought process how do I how do I put this problem in a mathematical form so that I can make it tractable otherwise no problem will have any solution okay so with that that is let me add on to this I don't definitely I don't agree that what all we are what all we have studied yesterday also I made this statement in the evening one of these students asked me this we I made a statement that what all we studied should we be throwing it to garbage because it is not not nearing to real life no I don't agree with you at all see my student had sat here and attended my PhD student sat here and attended at least for the convection portion because he was little jittery about convection the question for him is if I take a body and put it in a fire what is the what are the modes of the heat transfer important all this while you don't believe all this while we were considering radiation and convection what was I considering for convection I was considering forced convection when I when he saw this Grashof upon Reynolds number whole squared here and midnight day before yesterday we got a paper which was published by BRC Mysore which had taken natural convection so then we asked ourselves the question whether I should take the natural convection or forced convection yesterday evening we sat down and calculated gr by re squared to our horror we found that actually it is natural convection had I not known all these fundamentals about Grashof number Reynolds number and the velocity boundary layer thermal boundary layer and convective heat transfer I would not have designed in modeling how much amount of heat is received by my body because of fire so to identify which mode of heat transfer is dominant over the other I can only decide that if I am strong in fundamental if I am not strong in fundamentals I am not strong in applied research applied research feeds into fundamental research fundamental research feeds into applied research these two talk each other that is these two are complementary to each other they are not separate we are separating them because we cannot spend lifetime for applied research or fundamental research so applied research leads into fundamental research fundamental research leads into applied research okay so I will now go over to the K. K. Wag Institute Nasek My question is for the heat exchanger where counter flow is taking place and if you are changing the diameter of inner pipe so then how to analyze such heat transfer means what will be the effect on heat transfer then conventional piping pipe heat exchanger see what we are doing is what the question is in a counter flow heat exchanger tube in tube heat exchanger if I increase the diameter of the inner pipe what will be its influence of its performance first thing what is happening if I increase the diameter of the inner pipe what is happening the pressure drop of the inner side is decreasing number one if I assume that the mass flow rate is same what will happen to my Reynolds number 4 m dot divided by pi d mu my Reynolds number will come down if my Reynolds number will come down the Nasek number that is the heat transfer coefficient on the inner side will come down but let us see what happens in the outer side on the outer side what is happening imagining that your outer diameter is constant and I have increased the outer diameter of the inner pipe what is happening my flow area on the annulus has decreased so velocity has increased for the same mass flow rate Reynolds number has now increased because of which the heat transfer coefficient on the outer side will increase perhaps pressure drop will definitely decrease so pressure drop will increase so what is happening what is whatever positive thing was happening on the inner side now that has got shifted to the annular side so perhaps the effect will be more or less same but I will not conclude why because you will have to sit down and calculate this to get the exact feel of the numbers but by and large overall feel you get is that not much effect would be there on the area only one thing is if the if the circuit is thermally imbalanced if one side is very high heat transfer coefficient and the other was very low your overall heat transfer coefficient would be lower than the lower of the two so now if you have increased one of them and reduce the other the imbalance has gotten reduced so your overall heat transfer coefficient probably will increase a little bit and that would help in reduction of the area but that is again problem specific situation it cannot be generalized why don't you sit down and calculate this because it's not out of the world already one example is there just change the diameter put this in excel sheet and plot all the graphs you will get the area of the heat exchanger how it is varying so now you start thinking how are the parametrics affecting the performance of the heat exchange it may be a good idea to put a similar question in the test that is what we do often in IIT by the way some discussion takes place in the last lecture of the course and two days later in the end same you will find a large 10 mark question only on that particular discussion and those who did not attend the last lecture would suffer horrible there is a government college Salem let's go over to government college Salem sir in studying radiation through electromagnetic waves so whether what type of wave motion we can able to study about it whether longitudinal or axisymmetric or flexural torsional whether something can be connected to that see you the waves general question is the electromagnetic waves which are studying in radiation are they longitudinal or various other cases you have taken let me cut short that question let me say that generally these waves are transverse wave generally these waves are transverse okay that would be the answer shortest possible answer I can think of but whatever I taught yesterday was all through modest there is a initial discussion in thermal radiative heat transfer radiative heat transfer by modest about this longitudinality and the transverseness itself initially people were thinking that these waves are longitudinal but later on they realize that they are transverse okay okay sir one more question sir okay as thermal thermal radiation is being discussed throughout sir whether can you just flash some points about atomic or nuclear radiation question is you have only focused on thermal radiation you are not focused on nuclear radiation professor first thing I don't know what is nuclear radiation okay so that is not the answer the answer is what are we looking at the thermal aspects this course is all about heat transfer so that's why we said always when I used radiation I use the term thermal radiation the qualitative sorry the objective what I need to use for radiation is thermal radiation definitely we are not covering nuclear radiation at all because nuclear nuclear radiation doesn't come under the purview of thermal radiation that's what I emphasized when we studied Planck's distribution the Planck's distribution is all about thermal radiation by virtue of temperature whatever radiation is taking place is thermal radiation nuclear radiation that is x-rays and gamma rays are not under our purview okay I have a question again as a mutant observer not associated with this I would submit that whenever there is a nuclear radiation there has to be an accompanying thermal radiation because of nuclear radiation so it may perhaps not be completely correct to say that nuclear radiation is outside our purview but perhaps the right thing would be to say that while nuclear radiation itself may not be in our purview any thermal radiation resulting out of nuclear radiation is certainly of concern this is a good point or what Prasafatek has told in fact always we said no we cannot actually differentiate things we need to take them in an integral sense yes actually that's what in nuclear replication this is called as Neutronics and Thermal Hydraulic Aspects Neutronics and Thermal Hydraulic Aspects cannot be separated actually they have to take they have to be taken together in fact what was happening in Fukushima Fukushima what was happening neutron activity was going up what was they trying to do they were trying to take the water from the sea and pump in water into the Fukushima nuclear reactor containment what was happening it is actually both neutron activity and the thermal hydraulics neutron because of neutron activity there is increase in the temperature someone has to take out that heat transfer that high temperature who is trying to take it out the water the least possible thing one can do is to pour water actually I need to spray water but I cannot do because it is an emergency I have to just take water from the ship and pour it because I don't have motors and pumps because there is no electricity there I am just using my ship which is sitting far away and pouring water on how effective it was only time will tell because we don't know people are indeed sitting and studying the thermal hydraulic and the neutronic aspects they feed into each other that we cannot separate them out professor is right but we are not taught you neutronics that's all I want to thank you very much there is a question from SVNIT this would be the last question that we would take over to you SVNIT I am always wondering about the term that NTU number of transfer unit so can you please elaborate this term NTU because as you see the NTU definition it includes overall resistance surface area and semen so if we eliminate the fluid properties and the wall thickness the only term which is representing the NTU is the surface area so this term was introduced because of the number of heat transfer units will be incorporated to increase the heat transfer okay the question is what is to put it short what is the physical significance of NTU him and just one clarifications NTU is a name given to something which refers to the dimension size so okay number of transfer units probably the name is what is bothering it just refers to the surface area associated with heat transfer so whether you call it number of transfer units or area term it doesn't matter now you said that you take away the conduction etc what is NT what is that NTU what is that we are looking in fact why did we get to NTU let me ask him and you always wonder I know but what is that we are doing for NTU why did we introduce NTU why did we introduce NTU NTU because we wanted to get the relations for the heat transfer load in terms of area remember in NTU method unlike in LMTD approach we know the area in LMTD approach we didn't know the area we had to figure out the heat exchanger area but in NTU method we know the area that's why the area is there in NTU so having known the area what would be my heat exchanger performance if I have to quantify that that has to be done in terms of the known parameter which is NTU that is area NTU for all practical purposes we can think that as area that is why we say if NTU is large size of my heat exchanger is large if NTU is less size of the heat exchanger is less provided as you rightly said U that is the resistance is same and m dot cp that is the mass flow rate into specific heat of the fluid is same ok. In fact that is why see you have a given heat exchanger of a given dimension you are calculating the performance how good it is and suppose you decrease the length by half in fact in that convection 5 the slides it was told double the length you could get almost 99 percent of performance. So, what we are trying to say is this increase in the surface area does not translate to a corresponding similar increase in the performance. So, performance evaluation is possible because of this non-dimensional term called NTU which is nothing but a representative area I would call it you probably usefulness of this exchanger or something may be that is what it is. Sorry in addition to my question is it based on experimental or pure proposed theory? NTU is it obtained experimentally or see NTU is not coming out experimentally what is that we are looking for we are writing an equation for effectiveness. Effectiveness is a function of NTU and ratio of the specific heat why ratio of the specific heat that decides my temperature gradient and NTU decides my area. So, professor Arun has been insisting whether it is heat exchangers or fins or constant wall temperature constant heat flux boundary condition in case of convection all the time the driving force the driving potential in heat transfer is the temperature difference. So, the effectiveness is a function of NTU and ratio of the specific heat how did we get this we have derived this from fundamental and these derived things have been found over experience, but that they are going to be near nearer to the experimental results they are not very much of within the experimental uncertainty is that ok Hemant. Yeah, yes sir. Thank you very good thank you thank you so much.