 Now, professor has taught about condensation. Now, one actually it is good to begin with questions that is why. So, what is the, what do you think will be the issues or problems when you are dealing with the presence of the second phase of the same substance or in case of air water systems are two two substance, but two phases. Let us take water and steam either boiling or condensation. What will be the issues that one will have to deal with in terms of analysis for say formulating a problem or for coming up with number of equations number of unknowns. So, what will be the issues presence of interface presence of interface? So, what does the interface do to you? What does the interface do? Why is it a complicated thing? It continuously changes, ok. Interface profile interface thickness or dimension characteristic dimension whatever you want to call it changes. Nature of the interface ok. Now, I think instead of asking questions let us deal with example. All of us have made tea at some point of time. How many who have not made tea ever? Everybody has, ok. So, you have made tea. We also have used geyser for getting hot water for bath. Both are heat exchangers. Some kind of heat transfer is happening. So, when what is the difference between the two? Water for making tea and also you are heating the water from tap water temperature to whatever 70, 80, 100 and in your geyser also you are getting hot water. What is the difference between the two? So, one is a flow. So, in case of geyser or hot water for bath it is a flow. It can be a pool in olden days when you had this brass boiler it would be a stagnant pool of water or storage type water heater also stagnant pool of water heated. And what you are making for tea that water is essentially you are doing what we call as pool boiling because it is a stagnant pool. Stagnant in the sense what there is no bulk motion of the fluid. Bulk motion it is not flowing. So, between these two itself there is some difference. We have we do not know what it is at present, but we appreciate that there is some difference. Now, why does two phases become suddenly very complicated as opposed to single phase? That is that is one question if you are able to answer then we will be able to appreciate where we are getting actually. So what is Reynolds number single phase? I will use this subscript one phase means single phase. What is the definition of Reynolds number? Rho v d by mu let us say it is pipe flow of water d is the diameter v is the average or mean velocity. Now, I heat the pipe I have an electrical coil which is surrounding the pipe. So, we have done internal flow all of us. This is constant wall heat flux and it so happens we also have assigned such problems for heat exchangers. So, water is coming in let us say at 85 degree centigrade and it is going out as some amount of steam. I just got the answer actually what Professor Bajan says is that it becomes two phase flow. If I take vapor also, if I take liquid also it becomes two phase flow that is why we go ahead to make our life easy. We go ahead and make an assumption that the shear stress is 0. We assume that as if there is no vapor that is why and it is not moving that is the that is the that is an assumption otherwise we land into two phase. So, whatever we have studied in single phase flow is what we need to adapt. So, now if you have such a flow situation let us say somewhere here you get a two phase mixture. All of us have studied thermodynamics and if I want to draw this process on a p v or a T v diagram p v diagram let us say. So, what is the line of constant temperature look like or let me draw T v I like T v diagram. So, T v diagram line of constant pressure is like this right no no no that is what sorry sorry it is like this sorry this is p is equal to constant. So, we have water at some temperature getting heated and this is your exit condition 1 and 2 have been marked T s thick at T s constant over on thick does not matter. So, what is the state condition at 2 let us say p is equal to 1 bar what is saturation temperature of water at 1 bar approximately 100 degree centigrade state 1 is approximately say 80 degree whatever it may be I do not care T 2 is 100 2 can be here 2 can be here a b whatever what is the difference between state point a state point 2 and state point b quality thermodynamic quality what do we define that as mass of vapor we will hold on to that definition I do not want to define it that way why we will see in a minute there is a flow. So, what mass are we talking about it will be a problem, but we know a relationship this unfortunately we will involve a little bit of writing. So, I urge all of you to write also h enthalpy of state 2 is nothing, but h f plus quality at state 2 times h g minus h f at pressure p is that right. So, I can write x 2 is h 2 minus h f divided by h g at that given pressure. So, if I know the enthalpy of the fluid that how do I know the enthalpy of the fluid by the amount of heat that I have supplied. So, h is known h f f h f g are properties at that pressure I know that I can get the quality. So, what why why am I bringing all this. So, in any flow situation one of the basic calculations that we will end up doing is delta p pressure drop. So, single phase what is the pressure drop f l I will write it in terms of head I like that form f l v squared by 2 g d then what is f a function of Reynolds number and some roughness which is what is v given by m dot divided by rho cross sectional flow area. These definitions are correct it does not we do not even know whether it is these velocity and friction factor all these things we have studied from fundamental there is nothing new about it. Now, you tell me between date a and 2 here a is having lesser quality than 2 all of us have observed this think of this thought experiment you have gone out of town for a few days tap has been all taps have been closed. Now, you have come back and you are trying to fill water and your tap is very far away from where the bathroom is. So, you have connected a hose pipe only one tap is there. So, you have connected a hose pipe hose means a pipe with tubing and it is transparent. You open the tap and you observe what is happening what do you observe initially air comes out after that what happens intermittently air water everything comes out correct and then after a while water is going to come out. Now, if the tube was almost horizontal why am I talking in air we will draw it if the tube was almost horizontal if this is your tap and this is your bucket and this is the tube which is connecting it. If the tube was almost horizontal everywhere in this time when you have seen the air water flow you would have seen air occupying spaces on top right everybody has observed this nothing no rocket science here these are all observations. Now, if the tap is like this and the bucket is close by tube is very long. So, you have to have a vertical portion and then some horizontal part what do you observe in this case do you observe something like this air here like that you observe here what I will see probably initially is intermittent chunks of liquid separated by some air and then finally it will become fully water same tap you have been out for the same number of days same tube same bucket same opening also of the tap, but a different nature of flow acceptable what does this tell me m dot is the same I have kept the tap open same orientation of the geometry is very important first thing. So, let us write down what are the issues at the cost of being repetitive, but because in 2 hours we probably will not be able to derive or study anything from the point of view of derivation, but at least the conceptual understanding you should be able to present to students one is orientation. Now, second exercise thought exercise again this we also have observed this when you turn on the geyser for hot water for bath when it is only water flowing it is at a nice velocity some velocity which we can relate to we can measure if need be etcetera when it is only water, but if the water flow rate or if the valve is open very small what do you see you have water and then steam is coming out what is that velocity of steam is the same as air or higher than higher than why it is the same water it has gotten converted to steam why is it higher why is the velocity higher density is how many orders of magnitude 3 orders of magnitude higher or lower than water lower than water for steam roughly if you are talking about atmospheric pressure. So, now when the same m dot is completely in liquid phase that is if instead of state a I am withdrawing water as saturated liquid for example, state 1 prime state 1 prime is nothing, but water at 100 degree centigrade state a is say x is equal to 0.2 let us say like that just some numbers 1 prime to a it is the same m dot conservation of mass has to be valid otherwise you have thrown the problem out of the window. So, m dot is fixed what is change velocity now if I want to define my dear Reynolds number what velocity should I use same water some of it has converted to steam same as that of water why steam is going to flow at a much faster rate than water. So, in the pipe also for example, steam will flow at a much faster rate if I have you did this thought experiment with air and water coming out of a tap you hold on if you put your hand there that that kind of thing when you see the water velocity is it will trickle down actually, but the air will flow out faster. In the same cross section depending on the relative mass flow rates of the two phases two fluids in case of air water or same fluid two phases in case of water and steam we are going to have almost similar or depends on the quality you will have very very large difference in the velocity. So, then orientation was one issue second issue is the difference in velocity. Let us say some good friend of ours gives us a number average velocity some number or we say let us take liquid velocity fine. Now other for other thing in the Reynolds number rho mu. What do you take what is viscosity of gas versus liquid which is higher I am not as convergent with numbers you know the way he is, but which units banana, apple, orange what is the unit. So, water was 10 to the power minus 3 you said air is 1.8 minus 5. So, two orders of magnitude lower air density is how much 3 orders of magnitude. So, what Reynolds number I should calculate then another issue which I just quietly remembered now m dot by rho AC for each of that particular phase let us say we are dealing with steam water or air water who will give me m dot if it is that pipe flow which is where it is heated total m dot I know total m dot I know. So, probably by getting thermodynamic quality probably energy balance I will get a m dot associated with each phase. Everybody with me I am I am still not gone into anything related to two phase. I am just telling you how different how it becomes so complicated suddenly because of the presence of a second phase of the same substance m dot may be I can get x times m dot total or 1 minus x times m dot total I will get that rho also I will get that 1 over V F or 1 over V G who will give me this what area do you take area occupied by vapor portion area occupied by vapor portion and area occupied by liquid portion. So, if I look at you know it you know it you know it fine. So, let us say we are looking at this kind of situation horizontal pipe air is occupying only this part. So, this area if I know and this is water I will be able to get the actual phase velocity. So, U F and U G will be the actual phase velocity very very hard to get because areas are unknown. Other way of doing it is probably using the whole area of cross section of flow and I will get A C of pipe is equal to pi R C square that I will use for both I will get one velocity which is called as as he rightly said superficial velocity that is some kind of a apparent velocity superficial velocity this I can calculate this I cannot calculate unless I know the areas occupied by the two phases I cannot calculate this quantity. This I can calculate if I know the amount of the mass flow rate of each of the phases properties I will know total cross sectional area of flow I will know. So, this I can calculate. So, which of these velocities actual velocity J U or V whatever you want to call and J which of this will be larger for a given phase U versus J which is larger for the same phase. Let us say U G versus J G which will be larger what is the definition m dot G divided by rho G A G m dot G divided by A times rho G full area full area is a larger number compared to actual phase area. So, this quantity is going to be. So, this usage of superficial velocity uses a apparent or smaller value of quantities that you are going to calculate whenever it occurs in the calculation. You have still not answered one last thing what properties I am going to take for pressure drop I am still not gone beyond delta P friction factor I need Reynolds number. So, should I take density I can do V F G is equal to whatever way viscosity something else I will do. So, some kind of weighted. So, just the fact that you have introduced a second phase has made my calculation of something which was so mundane half a page you know formula we used to write we used to get everything suddenly become so difficult I have to worry about the fluid I have to worry whether the pipe is horizontal or vertical I have to worry whether it is what is the actual velocity. If I do not get the actual velocity then I am doing superficial velocity how correct is that I do not know. And what is the set of properties that I am going to use should I use for example, I will just tell you people use 1 by mu is equal to x by x by mu F plus 1 minus x by mu G others will use x just as you had for this one x times V G mu G plus 1 minus x times mu F each of this is going to give you a different mu which is correct each of this will give you a different Reynolds number. So, point I am trying to make is life becomes very difficult all of a sudden and then we have to deal with so called empirical relationship for getting an idea of what we are doing. Now, coming to fundamentals also what equations did we write in fluid mechanics continuity how many equations were there 1 momentum 3 energy how many 1 equation this is single phase 2 phase what will I write how many continuity equation 2 continuity equation very good how many momentum equation 6 momentum equations how many energy equation 1 only if I write I can write 1, but when I write 1 I will lose all the information what is happening at the interface right what is happening let us maybe we are going a little ahead. So, if this is the interface this is the diverging pipe or whatever let us say water is getting converted to steam if I write a complete equation for this pipe versus writing a phase equation what am I losing by writing the complete equation what is there in this black control volume which is not there in this blue control volume what is what are these arrows that I have shown liquid getting converted to vapor what what happens because what is happening because of that conversion some utilization of latent heat then what else some part I do not know how much what else specific specific volume density what density what density change why mass is getting transferred is only mass getting transferred what does it carry with it enthalpy we like enthalpy so we answer it momentum also is getting transferred. So, mass transfer including momentum transfer and energy transfer what else now we come somebody told first thing interface here something somebody told that is precisely why we neglected the interfacial shear stress when we took condensation because we did not want to get into trouble by taking two phase because we have to handle interfacial shear stress so that is why we made the interfacial shear stress 0 we assumed as if there is no vapor that is there is no vapor moving around so that is that is that why we got to do away with all these complications. So, if the flow is going from right to left to right what is the which of the fluid is faster by logic vapor is going to be faster liquid is slower what will be the nature of the stresses if I expand this diagram let us say this is the liquid and this is the vapor and the flow is from left to right and this is the wall of the pipe and we just put arrows to show the nature of the shear and the interfacial stresses shear stress at the wall let us see how it is which direction flow is from left to right right to left. So, I will call this tau wall F liquid same direction tau wall G are the magnitude same no why what is tau wall mu du by dy at the wall mu is not same 99 percent velocity distribution is not going to be the same why on earth will be the same they will never be the same. So, shear force now if I have to calculate what does it depend on shear force area what area surface area how will give me the surface area for liquid and vapor flow if it is a nice circular pipe and if I if the interface is very smooth and beautiful I can do geometry and get some kind of circumference 60 degrees means 1 6th of the circumference 90 degrees that accordingly life is not so good. So, a surface of liquid a surface of vapor I am just writing all unknowns only unfortunately I have not made life easy for anybody now tell me about interface what happens at interface what is the direction of shear stress listen carefully what is the direction of shear stress in the liquid and in the gas. So, tell me first for liquid shear stress for liquid opposite which direction right to left which is faster vapor is faster liquid is slower. So, what is the nature of shear stress on the interface on the vapor side and on the liquid side vapor side will be higher will the first question will the magnitudes be the same why not shears may be I have to rephrase sorry shear stress at the interface with liquid vapor shear stress at the interface will the magnitude of the shear I am just talking about value where is the magnitude same when I do it on the liquid side and on the vapor side or different it has to be the same because shear balance has to happen shear force has to be balanced correct. So, now direction has to be opposite because that has to cancel off. So, in which side is it going to be left to right and which side it is right to left liquid it is towards left how do you decide why I have marked I have not written whether it is right or wrong. Yes go ahead it will oppose the motion. So, what is this direction correct gas is trying to go fast if this were a stationary wall all of us like will be able to answer this just because this is moving we are having a difficulty treat this as a stationary wall this is trying to go faster liquid is withholding. So, now am I right tau interface only I will call I will not do notations tau interface on the gas side on the liquid side who gives me this value nobody only thing I know is that these 2 have to be equal. So, energy equation have to be 2 1 for the liquid phase 1 for the vapor phase when I add the interface when I add the equations I lose all information about the interface. So, then I will write for some m dot which is m f m dot f plus m dot g tau and so on. So, when interface conditions get lost completely momentum transfer momentum is transferred from here to here. So, this would be a positive quantity when it enters negative quantity for the liquid when it is lost when I add the 2 all the interface terms when you derive the equation it is a good idea it will be good idea to check whether the signs are opposite for the liquid and vapor phase. So, that when I do the summation I have to get an equation which is similar to what you had in the Navier-Stokes equation with some additional terms because of m dot f m dot g etcetera. So, life suddenly has become very complicated why are we studying all this because we have nothing better things to do not like that because it has vast areas of application all this was just blah blah blah to get you prime. So, now let us just go to proper subject. So, essentially whatever introduction is there is here what is the difference between boiling and evaporation? I am your grandmother I do not know vapor pressure partial pressure I do not I have still not understood those concepts very well. So, I am your grandmother tell me in a language with example that I understood saturation I am grandmother I am not studying engineering boiling is as usual I as usual cannot tell me with respect to temperature. Evaporation. Evaporation. Partial pressure. Partial pressure, but it evaporation can happen even when the water is kept at 30 degree centigrade if the air is considerably dry if the air is considerably wet like how it is evaporation will take place very slowly clothes will not dry that fast in Bombay as opposed to how it dries in Delhi. Why? Evaporation is slower in Bombay. So, this is how you have to convey kranbanas do not know engineering students also are like those people because they are seeing topics for the first time. So, education has to come through simple. So, partial pressure partial pressure normal people only do not understand what will happen. So, evaporation can happen at all temperature is there bubble formation in evaporation? No. Boiling everybody has made tea everybody knows there is bubble formation that is the primary difference partial pressure etcetera are the causes and effects of it and the rates of evaporation are determined by those things. Condensation of course, he has covered in great detail then latent heat of vaporization surface tension this is something which we have not touched at all, but surface tension is probably one of the most important properties of the fluid which determine boiling and properties of the fluid in each phase and typical values of the heat transfer coefficient. This is boiling addition of heat such that generation of vapor occurs evaporation occurs at the liquid vapor interface evaporation is a surface phenomena boiling is a bulk these are all from single. So, nothing whatever it is q is nothing but h times T wall minus T sat I will come to this T wall minus T sat business just take it as opposed to free and forced convection is that heat transfer coefficient was function of Reynolds number and Prandtl number that means it was dependent only on thermo physical properties, but here in boiling the important thing would be latent heat of vaporization and also as professor said surface tension surface tension it will become more apparent as we go along, but surface tension is one of the most important property variables which is very important in boiling and condensation in generally in two phase flows. So, that is let us see if you are operating even droplet vaporization because all of you are seem to be going gaga over IC engines. So, there also there is a spray whenever spray is there again two phases that is air and my liquid whichever it is getting sprayed. So, air is getting entrained. So, there also surface tension please significant. So, bubbles what is a bubble all of us have seen bubbles being formed etcetera. Why is it stable and why it breaks sometime collapse. So, let us just do a little bit about this bubble business bubbles exist because of surface tension at the liquid vapor interface due to the attraction of the liquid molecules to the interface and surface tension decreases with increase in temperature these are thermodynamics fluid mechanics that all of us know surface tension is 0 at critical pressure and no bubbles therefore, sudden change of phase you will see you will not see any bubble formation at critical pressure ok. Pool boiling making of tea what do you observe this thought experiment all of us are going to do now together I have a pool and mind you the gas is not kept at high it is kept at low to medium low heat flux and there is water here what do we observe water is at tap water 30 degrees initially you want to make it boil immediately bubble forms at 30 degrees first initially what do you see natural convection. So, let us write down one by one natural convection what happens during natural convection yes. So, because of buoyancy or density difference then. So, I will have some kind of a convective current pattern like this what else next after that now surface is started to get hotter and hotter fluid is also getting hotter and hotter bubbles very small bubbles and they collapse everybody is seen this those how many of you have observed this not able to relate try doing this when you go back home turn the gas to a very low value and just stand there and you will see one bubble will come small one collapse immediately nothing it will not even leave it will collapse immediately. So, if I take circle cross sectional area lot of the same size unfortunately because of space let us say first bubble was formed at this location so small that before you see it is collapse little later some other surface will give a give out a bubble we are not doing any measurement we are just observing that also will collapse then what will happen few places such small small bubbles will form and this will collapse why is this collapse happening it will give heat to the above liquid that is there always why does it collapse. Why should it lose? Because of the temperature gradient in the fluid what happens when the it is like a balloon so balloon is trying to grow and I am applying force there is no physical force there is a cooler temperature or cooler layer of fluid which is touching a slightly hotter layer of fluid. Now for that is this phenomena to occur right bubble formation what is the temperature of the fluid that you would observe is it equal to 100 degree centigrade or less than 100 at atmospheric pressure what would be the temperature? Slightly less than 100 surface would have just cross 100 then what will happen I am still noting I should not touch the gas what happens many such for sometime this activity happens then what will happen number of sites available you are using very technical terms so I should not use sites number of places at which these kind of activities are there are going to increase ok. Now let us take one site one location this bubble was small it collapsed after sometime what happens to that site it will the bubble probably will become larger will be able to sustain itself for a larger amount of time will probably become bigger in size may or may not leave the surface I do not know ok that is again a function of the temperature distribution right ok still more time you do it probably will leave and collapse somewhere in between the here so it has left it has come here and then disappeared collapse finally at one after sometime it will just leave and come to the surface and burst off when this is happening some other surface would be undergoing some face lack for the same process meaning it would also be growing and collapsing at some point etcetera one question I want to ask do these say site A and site B will the size of the bubble be the same no why fluid and surface temperatures are the same I do not see any difference perception where is this bubble form how is this bubble form microscopically any surface will have such kind of pits and cavities or an exaggerated scale let us say this is one kind of a this is way too complicated to explain in two minutes but let us try what will happen is this bubble will get formed here all cavities just because there is a cavity does not mean a bubble will be formed first thing to note just because I have a cavity so every cavity will not necessarily support bubble nucleation nucleation means birth or starting so bubble nucleation every cavity will not allow bubble to form some cavities will get completely flooded some cavities will not allow a bubble to be formed etcetera we are not going into the details of it so let us say this cavity the good cavity it will allow bubble to form what will happen this as I am supplying the heat how does the bubble grow why should a bubble be formed in the first place in a cavity heat trap what do you mean by heat trap but if this so called cavity is there a prerequisite apart from the let us say this cavity is going to support bubble formation it has satisfied various other conditions what is the most important prerequisite for a bubble to be formed entrapped air entrapped air has to be entrapped in the cavity so if the air is entrapped in the cavity only then will it first give rise to a once a bubble is initiated then it can grow because of you know balance of surface buoyancy inertial forces etcetera etcetera and this bubble will with if air packet is there only bubble can form that is why surface tension is important see dissolved air can come out also so that is why cavities which that is why do you not see all all sides becoming active at the same time size of the cavity some cavity will be like this some cavity will be like this some cavity will be like this each of each cavity even though all of these may be active potential for tile nucleation sites they will not start producing bubbles at the same time that is because of presence of air is essential and sites in which air was not there and that is if the site is flooded with water and all the air is gone still because of the increase in temperature solubility of dissolved gases will decrease they will start to come out that will cause a bubble to form in an originally infertile site. We should not have any in fact that is probably one in this micro channels they do know. There are some other experiments in which they tried smooth smoothest possible surface and nucleation is just not possible. In fact one nucleation cannot occur same thing for casting also no for casting also it is it is opposite for the birth to happen you have to have the rough surface and you have to have the air pocket. In fact when you have a when you have a new heat exchanger or whatever steam generator or whatever it is performance is not that good when it is new little bit older it becomes when it has that fouling deposits which are there those fouling deposits are porous and as long as the fouling is not impeding because of conduction resistance the presence of fouling actually is helpful for some time because those active I mean those pores which are present will allow boiling to occur or heat transfer to occur better in steam generators. So, what will happen is new tubes in a steam generator will perform poorly as opposed to a slightly used tube or if it has been used for a year and then taken out for cleaning when you do it chemically cleaning it is not going to become like brand new there will be still some places where these deposits are there and some places where it has not been removed fully the performance would be seen to be marginally better than the new ok. So, this spaces are very very important entrapped air etcetera. So, finally this bubble will leave the surface yeah. It is not air but it is the vapor of the. Wait it will be the air it will be the air but for the growth to happen it will be vapor coming in correct. So, next time what will happen I am going little bit further away than what we are intending anyway. So, actually what will happen is when this site a nucleation site cannot infinitely continue to be an active nucleation that is because of this what you are saying this entrapped air when it is completely gone it will become deactivated that is one thing and that is why what will happen is if I have way beyond what we are intending to do if I have a long pipe length is not a criteria steam generator pipe or heat exchanger whatever pipe where I have the ability to some good information that you will carry. If this is coming in liquid is coming in a T saturated quality equal to 0 what is seen is that for typically these are all just ballpark numbers do not take it as you know Mach number 0.3 or no it is not like that typically qualities of around 30 percent for boiling nucleation dominance that means in flow boiling phase change is primarily occurring because of nucleation and bubble formation what is the other way it can happen force connection flow is also there right. So, H for nucleate boiling and H for force convection these two are going to contribute and what happens this is way way beyond what we should be doing H versus quality nucleate boiling component decreases with increasing quality force convection increases with increasing quality. Why because nucleation gets completely suppressed suppressed because one thing is the sides will start losing the entrapped air second thing as more and more vapor gets generated the velocity the so called average velocity will increase because vapor is lighter. So, more pipe dimension is fixed space occupied by vapor is much larger than liquid. So, the liquid is pushed to the walls as the liquid is pushed to the walls like a bullet flowing through the water. So, vapor velocity increases. So, it drags the liquid faster and faster along with it because mass of the liquid is also gotten reduced and it is being pulled by a faster moving liquid faster moving vapor. So, so called residence time of the liquid to get heated and have this bubble formation all those luxury is gone. What happens is essentially after a while your flow pattern or your nature of flow becomes what we call as a annular flow that means in cross sectional view this will be your liquid and vapor will be in the center annulus will be formed and evaporation will be the mode of heat transfer. Phase change will occur by evaporation or evaporation will be dominant. I am not telling anything about temperature is T side that is not an issue, but the method of phase change is evaporation. Whereas at lower qualities method of heat transfer to phase change is by nucleation. Phase change is by nucleation at lower qualities phase change is by evaporation at higher qualities. Reynolds number will increase force convective heat transfer coefficient will increase. So, H will become larger and one very simple thing which you can take home is that this H total is almost a constant independent I am not saying it is independent of quality, but if you do not have a number you can say it as a constant value which can be evaluated roughly at x is equal to 0 and x equal to 1 you can take it almost to be the same and do a calculation. So, this is some from a practical point of view also very useful way beyond what we should be doing at this point. So, pool boiling. So, the thought experiment is over now we will say with this idea flow boiling of course, we said when the fluid is moving in a heated channel we will classify boiling in both cases as pool as well as flow into two parts one we call as sub cooled boiling another is saturated boiling sub cooled boiling is what you guys told me you know bubble is form collapsing leaving and collapsing that means sub cooled boiling is when T of this temperature I am writing is again your bulk mean mixing cup average blah blah blah temperature T is less than sub cooled boiling T wall greater than T sat when T equal to T sat T wall greater than T sat saturated nucleate. So, this is what it is sub cooled saturated now we will come to what is this boiling curve boiling curve is essentially a representation of the heat transfer coefficient what is heat transfer coefficient heat flux by delta T.