 What is the significance of the isentropic efficiency in steam turbine? Is that your question? I suppose your question is, what is the significance of isentropic efficiency of a steam turbine? The isentropic efficiency is defined for an adiabatic open system, which either produces power or delivers power in open system of the type turbine or compressor or nozzle. If you have, we can define a term called isentropic efficiency. Such devices are usually one inlet, one outlet, steady state. Hence from the first law, from the second law for such a device on a TS diagram, the entropy of the inlet state and the entropy of the exit state under ideal conditions would be the same. The ideal condition for this means adiabatic and reversible and hence isentropic. However, even if they remain adiabatic, this is a requirement, they cannot be reversible and hence the entropy at the exit will have to be higher than that at the inlet. Actual process will be something like this. For example, this is true for a turbine, we will have Se greater than Se star, which should be equal to Si and because of this, your He will be greater than He star, the enthalpy. In case of a gas, even temperature at exit E will be higher than temperature at He star and we define the isentropic efficiency as the actual power output divided by the ideal power output, which is given by W dot star. So the isentropic efficiency represents the non-reversible behavior of a turbine or a compressor or a nozzle. If the turbine or compressor or nozzle is adiabatic and reversible, that means isentropic, there will be no irreversible behavior shown and the isentropic efficiency would be exactly 100 percent or one over to you. Sir, hello. Yes, go ahead. Sir, just one question is, the state of water in atmospheric air is superheated condition, sir. How is it possible, sir? Please explain, sir. I think this was explained just now by Professor Bhandarkar that if you look at the typical condition, which is not 100 percent relative humidity, but something less than the saturation condition from the point of view of air, you will notice that if you look up the temperature of air, which would be the temperature of water vapor and if you look at only the water vapor part in air, so look at the partial pressure of water vapor, you will find that it is in the superheated state, except in the limiting case, where the vapor pressure and the concentration of water vapor is so large that we have 100 percent relative humidity, in which case in the limiting case it will be dry saturated vapor. Any increase in humidity will not increase the vapor content, it will only increase the liquid content. That means, any addition of vapor will tend to condense that moisture in the vapor, over. Sir, why internal combustion engine use higher temperature as compared to external combustion engine, sir? Can you tell me the answer? Okay. The reason for this is from the engineering and materials point of view, when we burn fuel, the fuel releases heat and can reach a certain temperature. You would have learnt about this release of heat as calorific related to calorific value and the temperature related to the flame temperature of that fluid. Now, when this is internal combustion, the energy is released within the fluid itself, you do not have to transfer it. So, if you can take the fluid almost near to its flame temperature, because you do not have to transfer it, there is no resistance to the heat transfer from the combustion products to the fluid, because the combustion is taking place in the fluid itself. Whereas, if you burn the fuel external to the working fluid, then the temperature reached will be in the outside the working fluid. You will have to transfer the heat across the wall which separates the working fluid from the combustion chamber and this heat transfer requires a certain amount of temperature difference. So, by that amount, depending on the area provided and type of intervening boundary, thickness of metal and its conductivity, the temperature of the working fluid reached will be that much lower. Over to you. Hello. Yes, go ahead. I have a doubt, which will be more efficiency, whether reheat cycle or regenerative cycle zone? Please repeat your question. What has more efficiency? Reheat cycle or regenerative cycle zone. So, the answer to your question, your question asks comparison between the efficiency when you do reheat, compared to the efficiency when you do reheat. I suppose this pertains to Rankine cycle or it may even pertain to Brayton cycle, but in any case, the change in efficiency because regeneration and because of reheat depends on the parameter. That means at what intermediate pressure you do the amount, you start the reheating and up to what temperature you take the reheating. Same thing depends on the regeneration. So, I cannot say that one is always better than the other. Over to you. Thank you, sir. Over and out. 1, 2, 4, 9, Sri Ramakrishna Institute, Coimbatore. Sir, I have a doubt related to entropy. Yes. The internal energy stepens upon the temperature only. That is true only for an ideal gas, only for an ideal gas the internal energy depends only on temperature. That is Joule's law. Sir, what about entropy, for which condition increases or decreases? See entropy in general, even for an ideal gas is a function of temperature and pressure. And the entropy, if you have either a constant volume process or a constant pressure process, the entropy increases with temperature. And if the temperature is maintained constant, the entropy increases as you go to lower pressures and entropy increases as you go to higher volumes. Over to you. Sir, in Prasila thermodynamics we are using one word, arbitrary. What is the meaning for arbitrary? Arbitrary means anything which you select. Where did I use that word? Because the word arbitrary is used in many places in my lectures. So, can you refer me to the context? For integration, we are using cyclic integration. The arbitrary constant. Okay, okay, okay. The arbitrary constant means a constant of your choice. Nothing special about it. I think I made a statement that the energy of a system or internal energy of a system, the absolute value of that number of that internal energy is of no use to us. Because it is defined as a difference and we will always be using it as a difference in any exercise, any problem in any situation. Thank you. No problem. That is the difference. 1116, HG, SI, TS, Indore. Over to you. So, I know I have asked this before and some of this has been covered earlier also. But again, quickly the entire thermodynamics is rooted in the ideas of energy, space, time, matter and this very enigmatic term called entropy. And very little has been done to cover this idea in detail any further than that. Exactly, the fundamental idea of entropy is related with say improving the performance of any of these work producing devices that we in mechanical engineering come across. That's been the question for me earlier also. Okay, I don't know to what extent and in what branch of mechanical engineering your endeavour is this, your endeavours exist. But for thermal engineers like us who work with nozzles, turbines, compressors and their components, entropy is a very important property because we use it day in and day out for analysing the performance of a nozzle, performance of other components of turbines. You can't solve a single problem in the detailed analysis of a turbine without using entropy because that's the anchoring property out there for all, not only the overall process but even individual processes. So, as you work and start applying it, entropy doesn't remain such an abstract concept anymore. Over to you. Sorry to interrupt but it's not been, so is there one common definition say amongst all say physicists, chemists or chemical engineers as well as mechanical engineers the way it's understood and defined fundamentally. Because if it's not or at least that's not something that has been covered so far here and unless we are absolutely clear about that idea relating any further with improving the performance of these work producing devices that we are coming across is really not relatable. They are really not making a very definite link between the two. See you are entitled to your opinion but the fact remains that the common definition of entropy among engineers, physicists and chemists it's simply ds is dq by t for a reversible process. Nobody will take objection to that definition of entropy. So, if you still say that it is ill defined or not defined, you are welcome to your opinion. I can't say anything more. Over to you. Some of us may be interested in working on one of this ideas energy or matter dispersal leading to the idea of entropy again within the realm of chemical engineering or chemical thermodynamics. Would this be of any interest to you or any of the faculties with you if you want to consult any further on this? You will have to talk to members in IIT Bombay who work in the chemistry department or chemical engineering department. In mechanical engineering we are working in plant of the power producers and refrigeration kind where except for the combustion process which is a specialized chemical reaction. We don't really have any reactions going on. Over to you. Thank you. Second question would be what would be a good definition of a spontaneous process and a good example to support that? Well, spontaneous process is one which can take place without any trigger being applied. For example, mass left at a height without any support comes down. That's a spontaneous process. You can give any many illustrations of a natural process like that. You don't have to hunt out for example. Water at a height. Water fall. Water at a height. It's a spontaneous process. Without any external agency, you have a system containing hot water just left by itself unless you insulate it and prevent the heat transfer. You just leave it spontaneously. It starts cooling, starts transferring heat to the surroundings which are at a lower temperature. Finally again, we have enjoyed your course very thoroughly all along. This presentation has been excellent. Thank you. My final question would be again, all these entities or quantities for example, matter, time, space, do you feel or do you think can be reduced to one common source? For example, the current cosmological model of big bang for instance? Well, I am not that deep into physics so I can't really answer your question. Over to you. All right. Again, thank you for all your time and all your explanations. Over and out. There are only a few minutes to 5.30. Let me try one more thing. 1118 3 Jaya Chama Rajendra College Mysore. Over to you. Hello. Hello. Hello. Good evening sir. Good evening. Hello. Sir, my question is when, in a throttling calorimeter, when wet steam is passed through the throttle, how it is going to convert to superheated steam, sir? It doesn't always convert to superheated steam. If you, I think that was one of the questions in the exercises. If you look up the Mollier diagram, H s, the x equal to 1, the saturated vapor line goes something like this. And critical point is somewhere here. And these are the lines of constant dryness fractions. So, if you have in the wetness somewhere here, after, if it ends up in the superheated zone, these are various pressure lines. This is T at this, this is T inlet and this is the inlet state I. This is the process. I am showing it by continuous line, but actually it should be a dotted line because the throttling relation, the throttling process, when first law is applied to it, we get just H i equals H e. And we know that as pressure reduces, the entropy at constant enthalpy goes on increasing. So, if the inlet dryness fraction is not very low, you will end up with, which is usually P ambient at exit, you will end up with superheated steam. But if you are inlet, say I prime, at a much lower dryness fraction, even after the throttling, you will end up with P prime, which is still in the wet zone. In this particular case, you cannot determine the dryness fraction using purely a throttling calorimeter. You will have to use, throttling comes separating calorimeter or you will have to use a heated, heating throttling calorimeter over to you. Thank you, sir. One more question, sir. Sir, in throttling process, when the steam is passed through the throttling, the steam is get into a superheated steam. But in the refrigeration system, when passing through the throttling, we get into the liquid. What is the difference between here throttling in that refrigeration system in throttling, sir? Please clarify that one, sir. The basic idea is simply this. The basic difference is this. In throttling calorimeter, usually we have vapor, which dryness fraction near one. Whereas in refrigeration system, the throttling process begins with saturated liquid, a very low enthalpy liquid. So, when you throttle it, well, at lower pressure, the dryness fraction is higher. So, it is higher than zero, but it is nowhere near one. So, that is why it ends up being a liquid vapor mixture with a low dryness fraction over to you. Actually, I confused over the shock waves. Actually, the question is, what are the benefits of shock waves? Okay, this is a question pertaining to Professor Puranik, who has gone away for a meeting at our R and D office. So, he will be back here at 9 o'clock tomorrow morning. We will note down the center 1118. And please raise your hand at 9.10 or 9.15 tomorrow morning. And we will pass this question on to him. Over to you. Sir, one more question. Sir, what is the difference between detonation and knocking in engine? Detonation and knocking in an IC engine. I passed it on to Professor Bhandarkar and he says this is much more detailed in combustion and combustion aerodynamics. He does not want to answer that just now. We will discuss this later. Ask it on Moodle. We will do our homework and get back to you. Usually, we use nitrous oxide in an engine. So, how does the performance increase by using nitrous oxide? In fact, this is the first time I am hearing that we are using nitrous oxide in an engine. In fact, nitrogen oxides are the products of high temperature combustion in presence of nitrogen. So, we will not be using it. Engine will be producing it and it is a very significantly bad pollutant. So, we go to extra lengths to operate our engine. We design the combustion chambers so that they are not produced. So, we do not use them in an engine. Over to you. Sir, usually in supercars, like race cars and race bikes, we use it. Well, I do not know for what purpose they are used. We will have to check that out. But in our normal engines, we do not use it. Over to you. One more question. Sir, usually we use nitrogen gas filled in like tires of vehicles right now, like recently. So, why do we do it? Any specific use? I have been thinking about it. There does not seem to be any thermodynamic reason. There are some reasons, some possible reasons are perhaps the permeability of nitrogen through the rubber or tube or the tire may be lower than that of oxygen. So, perhaps the deflation rate is much lower. But remember that air, which we generally fill our tires with, is 80 percent nitrogen. So, replacing that 20 percent oxygen, 20, 21 percent oxygen by an equivalent nitrogen, I do not think makes any significant difference. Over to you. I know many people who have gone over to nitrogen tires and then have come back to normal air inflation. I did not find any difference except that I have to pay more money to fill up the nitrogen. Over to you. Thank you, sir. Over and out, sir. That brings us to the end of the day.