 In today's lecture, we start off with discussion on axial flow turbines. Now, in a aircraft engine or any other kind of gas turbine engine, axial flow turbine of course, is the heart of the engine and it is the one that supplies power to the axial flow compressor, which we have been talking about in the last few lectures. So, the gas turbine engine actually derives its name from the axial turbine or the turbine that is embedded within the engine for supply of power. Now, axial flow turbine is one of the possible turbines. It is possible to have radial flow turbines, which we will talk about later on in this lecture series. So, today we start off with axial flow turbine. Now, as the name suggests, as in case of axial flow compressor, the flow is supposed to be essentially axial in nature. Now, which as we know in case of axial flow compressor, it does not quite fully go axial, it kind of zigzags through the blades. In case of axial flow turbine, that zigzagging is actually even more, mainly because as we shall see as we go along that the turbine blades are far more cambered or curved and as a result of which the flow has to take very large turnings through the turbine blades eventually exiting again axially. So, the flow generally in a very approximate sense remains sort of axial and we shall see very soon that even the word axial, which essentially means that it should be parallel to the axis of the machine is also not quite satisfied, because at various parts of the axial flow turbine, we shall see that flow actually becomes at an angle to the axial you know axis of the machine. So, the name axial turbine was given you know something like 70, 80 years back and that name has remained unless you change it with a very complex name, it is not possible to describe exactly what is happening. So, axial flow turbine is a reasonably accepted name for turbine just like in case of axial compressor. So, everybody understand what is meant even though everybody knows that the flow is not exactly axial in the strict physical or mathematical sense. Now, axial flow turbine essentially comes after the combustion chamber in a gas turbine arrangement. So, before the combustion chamber comes axial flow compressor, which we talked about or any compressor for that matter centrifugal compressor, which we shall do in this lecture series later on. Now, compressors come before the combustion chamber and turbines come after the combustion chamber. Now, which is an indication that the flow going into the turbines is hot coming out of the combustion chamber, which is what goes into the turbines. Now, this is a very old concept that the flow going into turbines should have very high potential energy. Now, the now the terminiculature potential energy essentially indicates that the flow has very high levels of internal energy. Now, in the old fashion turbines for example, in water turbines or for that matter in hydraulic turbines as used in hydraulic power plants, what is normally done is that the water is indeed released on to the turbine with very high potential energy and that potential energy sometimes is artificially created by dropping the water from a height. So, the height of a water fall essentially creates the high energy level, because that converts the potential energy to kinetic energy and we shall see exactly same thing needs to be done in case of axial turbines also in a gas turbine engine with reference to aircraft engine that you need to have that potential energy, because very soon even before it hits the actual rotating turbine that potential energy would have to be converted to kinetic energy and that high kinetic energy or high velocity jet when it hits the turbine the turbine works. So, the basic principle of working of the turbine depends on the fact that you have a high momentum jet whether it is water or air or gas as in case of gas turbine engines that high velocity jet must be created when it hits the turbine which is free to rotate. So, the principle is when this high momentum jet hits the free turbine the turbine by virtue of allowing that jet to pass through it and through certain let us say clever gas dynamic aerodynamic manipulation extracts a lot of work out of that high energy gas or fluid and this extraction process is what the turbine essentially does by extracting energy from that fluid the turbine rotates and in the process of rotating it creates mechanical energy and that mechanical energy is then transmitted through the shafts to compressor or to any other thing in case of a land based power plant for electrical power generation the shaft essentially transmits power to the electric power grid for creation of electric power and transmission to the grid. So, that is the mechanism by which fundamentally a turbine is essentially conceived. Now, let us take a look at what are the various issues related to the turbine that we would need to really get into because what we need to get into is the aerodynamics or the gas dynamics of turbine and we need to get into the fact that it has being a part of a heat engine. You know we are talking about a heat engine and a turbine is very much component of a heat engine a major probably the fundamental component of the heat engine that we are talking about and as a result of which it has to satisfy or conform to very closely as closely and accurately as possible to many of the laws of thermodynamics. So, those laws of thermodynamics are extremely important with reference to the turbines probably they were a little less important when we were talking about axial flow compressors because the heat energy was not of a great importance there, but in turbine the energy transaction involves a lot of heat energy and hence the thermodynamics is quite an important issue. So, in today's class and may be in the next lecture also we will be talking a little about thermodynamics alongside aerodynamics. So, that we have a full understanding of the entire aerothermodynamics of the course the coinage of the word aerothermodynamics has been done quite some time back and many people prefer to use that terminology aerothermodynamics when we are talking about heat engine components like turbines. So, let us get into axial flow turbines and let us try to understand the basic introduction of how an axial flow turbine works and how it produces work. Now, an axial flow turbine basically can be an unit that is somewhere in the middle of the gas turbine engine and this if it is used as a part of the gas turbine engine and this is something which is conceived right at the beginning when the engine is designed or configured and as I mentioned it comes immediately after the combustion chamber. Combustion chamber is indeed of course the stomach of the engine because that is where the energy is put in fuel is burned and it takes the working medium that is air to high energy level to very high temperature. So, energy is put in there compressor had already compressed the air to high pressure and now that high pressure high temperature gas is going into turbine. So, turbine gets a feed of high pressure high temperature gas air is compressed by the compressor and then fuel is burnt and the air fuel mixture is being released to the turbine at very high temperature and pressure. So, this combination of high pressure and temperature when released on the turbine essentially is carrying itself with it a very high potential energy when this potential energy high potential energy is available. So, one of the first things that the turbine would like to do is convert this potential energy to jet. So, what it does is this large potential energy is converted to kinetic energy and when this kinetic energy is released on the rotating free rotating set of blades which is turbine blades it actually can extract certain amount of energy from this passing fluid by virtue of the aerodynamics or aerothermodynamics which we are talking about and which we shall be talking about in this few lectures from now onwards. So, basically we are talking about conversion of energy to mechanical work. So, energy has been created fuel has been burnt and that energy is now being harnessed to do mechanical work which is what we want to begin with. The turbine essentially produces mechanical work if you are talking about an aircraft engine or aero engine. Remember turbine does not produce thrust it produces work how we use that work to produce thrust that is separate issue. So, in actual turbine we are not bothered about you know creation of thrust we are essentially bothered about creating mechanical work and as we know little that part of that mechanical work actually goes into running the compressor which compresses the air which is coming into turbine. So, it has gone into a loop energy loop between turbine and compressor. So, in typical aircraft engine a good amount of energy actually taken out by the turbine and created into mechanical energy actually goes into running of the compressor and that is the loop that takes away energy and the remainder of the energy is used in creation of thrust. So, creation of thrust cannot be done with all the energy that is released in the combustion chamber only part of the energy would be eventually available for creation of thrust rest of it goes into the turbine compressor energy loop or work loop. So, let us see how the turbine now or what kind of turbines are normally used in extraction of energy from the passing hot high pressure gas mainly air. What happens is as I was just mentioning the turbine takes out work it passes on to the compressor, but there are two other possibilities. The one possibility in modern most of the modern civil air even military engines that they are turbo fans. So, much of the energy specially in the civil aircraft application much of the energy goes into from the turbine to the fan or a big fan which is part of the turbo fan. Now, this big fan creates a thrust of its own it does not create thrust through the nozzle of the main hot engine it creates what is known as cold thrust. So, much of the turbo fan thrust is created by that a cold thrust of the fan itself in some of the turbo fans that is the major thrust much more than that of the hot thrust created by the hot nozzle. So, that work is supplied by the turbine. So, turbine directly supplies work to the fan and fan by virtue of rotation creates a thrust very much like a propeller as it used to be in the earlier days for 50 years aircraft were flown with propellers there were no turbo fan engines or turbo jet engines. So, those 50 years the propellers used to get power from the turbines. Turbines was always there of course, in the in the beginning for many years it was piston engines, but as soon as some version of the gas turbine engine came gas turbines were initially running the propellers and it is the propellers which were producing thrust earlier with piston engines. So, the thrust making was created by the propeller and when the turbines came turbine was supplying power instead of the array of piston engines. So, turbines had the job of supplying mechanical power shaft power to propellers to fans to compressors. So, that is a basic job that the turbine has. Now, let us see what kind of turbines people have been using over the years the fundamental nature of the turbines. If you look at this you will see that there are two groups of turbine that has been identified here and it is called a two spool axial flow turbine. The basic premise is that one group of turbine actually does work from the hot gas coming from the combustion chamber and then having finished its work extraction whatever energy is still left over is passed on to the another group of turbine which is called LP or low pressure turbine because of the work done by the high pressure turbine. The pressure level has gone down the potential energy level has gone down and with the comparatively low potential energy it has gone into the LP turbine which then extracts a lot of work and then that is passed on to the compressors or the fans. In this particular engine for example, this LP turbine would be supplying a lot of power directly to the fan or big fan of a big turbofan engine. Whereas, each P turbine or high pressure turbine would essentially be supplying power to the compressor which compress the air which was going into the combustion chamber before the fuel was burned. So, many of the modern engines the turbine like compressors as we have seen before is often split up in two groups one is called the high pressure turbine and other is called the low pressure turbine and high pressure turbine runs the high pressure compressor low pressure turbine runs the low pressure compressor and or the big fan. In some of the engines as you may have done before in some of the courses there may be three groups of turbine. So, we may have a HP turbine we may have the intermediate pressure IP turbine and then we may have LP turbine or low pressure turbine. So, it is possible that many of the modern turbofan engines have three groups of turbine to run three groups of compressors and fans. Now, this is a split which is normally done by the engine designer. So, engine designer configures the whole engine and then hands over to the compressor designers for designing the compressors and the turbine designers for designing the turbines which requires a lot of analysis in the process of design which we shall talk about a little later in this lecture series. So, turbines essentially can be essentially in number of groups and of course, all the groups together we call them multistage turbine. So, typically an aircraft engine today invariably has multistage turbine. Long back in the early days when the gas turbine was used for powering aircraft and it was running essentially let us say a propeller or a simple jet engine only one stage of turbine was often used to run the entire set of compressors. So, when the engines were small quite often a single stage of turbine was often sufficient to run all the compressors together and then later on they were only two stages and the split between LP and HP all the splits were not there. So, a single spool set of turbines may be one, may be two were sufficient to run the entire set of compressors which could have been 8, 10, 12 stages of multistage compressor. So, this multistaging started very early, but the multispool came a little later and now as I mentioned you could have up to three spools of multistage turbines. So, we have to design we have to analyze them stage by stage. We have to understand how the turbine works stage by stage rotor stator or other stator rotor put together in turbine the stator comes first. So, those other steps by which we have to understand the working of the turbine and then finally analyze the turbines which we shall be doing from next lecture onwards in detail and then later on the design of the turbines. So, let us take a look at that one single stage which is the fundamental unit of turbines or a multistage turbine. If you look at a single stage typically it has a stator which often sometimes people call a nozzle and the reason is as we shall see very soon the shapes of the blades and the passage between the blades is that of a nozzle and hence quite often it is called a nozzle. The good reason why it is called nozzle is because as we have just discussed the job of the stator is to convert high temperature high pressure gas C T 0 1 P 0 1 of very high and it is coming with some actual velocity which is not very high at this moment. Convert this high potential energy gas to high kinetic energy gas as it passes through this stator nozzle and that is another reason why it is called often just a nozzle because it actually is a de facto doing the job of a nozzle by converting potential energy to kinetic energy because that is what nozzle actually does. So, many of the stators unit or binary essentially in many books and literature are simply referred to as nozzle. The rotor of course is the free to rotate set of blades which then extracts the energy. So, as the flow comes through the stator nozzle it acquires very high kinetic energy with that high kinetic energy it impinges on the rotor and the rotors made of certain aerodynamic shapes, airfoil shapes essentially managed to extract a lot of energy as that high energy gas is passing through the rotor and in the process of passing through various laws of physics and thermodynamics it manages to give up a lot of its energy to the rotor. And so the rotor acquires the energy in mechanical form of work by virtue of rotation and then it passes on to the shaft where the rotor blades are very firmly fixed with the help of a very well known inverted fir tree root attachment and then this rotor is of course running the shaft which powers the compressor or anything else that you would like to send power or work to. So, that is the basic mechanism by which the axial flow turbine essentially works that you have a stator in which essentially high kinetic energy is created from high potential energy and then the high kinetic energy jet is made to impinge on the rotor in a certain aerodynamic manner which we shall see very soon and if you can do that correctly then with very high efficiency the rotor can extract a lot of energy of that high energy internal energy gas. So, then that high internal energy is been converted now to external energy kinetic energy and then this is now taken away by virtue of rotation of the turbine in the form of mechanical work. So, that is the mechanism by which a single set of axial flow turbines work the flow as I mentioned is generally axial as shown in the picture if you take the median path right through the turbine you will find it is more or less axial which is parallel to the axis of the engine. But in many turbines in many cases as we just saw here for example, the flow here is more or less axial in the H P even though near the casing and near the hub it may not be actually axial it may have curvilinear path. But afterwards as we can see the flow becomes very clearly non axial and it goes into at an angle may be in a curvilinear path and then goes out again more or less axial. So, it has come in axial going out axial, but in between it is entirely possible that the flow may be non axial as it is passing through the turbines. So, similarly in a single stage it is quite often approximately axial, but in reality the flow may be somewhat non axial passing through the blades. Now let us as I mentioned let us take a quick look at some basic thermodynamics that is inevitable in a turbine discussion because it is part of a heat engine. Now what happens is as I mentioned the flow comes in from the combustion chamber. So, the starting point shown here is 0 3 typical of a gas turbine engine and it is come from the combustion chamber. Now this high energy gas is now being released to the turbine and in the process of releasing through the turbine it drops in energy level from 0 3 to 0 4. So, it has dropped from the pressure of P 0 3 which is the total pressure to total pressure of P 0 4 and in the form of static pressure it is dropped from P 3 to P 4. So, the drop of energy and we can see it is also dropped from T 0 3 temperature to P 0 T 0 4 temperature and this drop in temperature and this drop in pressure is actually representative of its giving up the energy or loss of energy which is gone into the turbine work or creation of work through the turbines. Typically to complete the picture we have the total drop here from 0 3 to 0 4 the static drop from 3 to 4 along the dotted line and of course, as we know the different between 3 and 4 is the dynamic head which is the entry dynamic head of C 3 square and the exit dynamic head of C 4 square. Now in this diagram which is the temperature entropy diagram the line 0 3 0 4 or 3 4 are the real turbine working lines. If thermodynamically the process through the turbine is entirely ideal then the flow would be actually thermodynamically dropping all the way vertically from 0 3 to 0 4 dashed 0 4 prime over here and static pressure and temperature would be dropping from 3 to 4 prime. So, this these are vertical drops or in terms of thermodynamic term it is the isentropic drop during which S or entropy change would be 0 in a real process there is entropy change. So, that process is not an isentropic process one may call it polytropic process or definitely a non isentropic process. Now, as we can see here if we go by the drop let us say the temperature drop of 0 3 to 0 4 prime as we can see in this diagram very clearly the 0 through 3 to 0 4 prime drop is indeed far higher than 0 3 to 0 4 drop which is H 0 t which is the real work and H 0 t prime is the ideal work. So, inner turbine realistically the work extracted or work done is often somewhat less than the ideal work that would have been possible if the work was thermodynamically and gas dynamically absolutely ideal as per the theory. As a result of which one can say that there is a efficiency attached to working of the turbine which is typically expressed in terms of the ratio between the real work and the ideal work and this ratio is typically referred to as efficiency of turbine for the sake of clarity quite often it is simply referred to as isentropic efficiency. As also we see here the real dynamic head at the exit is C 4 square the ideal dynamic head is C 4 prime square for the sake of certain amount of simplification quite often it is assumed that this two values of C 4 that is C 4 prime ideal and C 4 that is real are equal to each other which means the different between 4 prime to 0 4 prime and 4 to 0 4 are equal to each other that is for simplification of many thermodynamic analysis. So, if we now look at various kinds of turbines the way this work is extracted there are essentially two kinds of turbines which will be talked about in some detail from next lecture onwards. One is simply called the impulse turbine now the name comes from very old fashion notion that you have a turbine on which jet is impinged and as the jet is impinged the turbine works by virtue of the impulse that is created by the impingement of the jet and as the flow is made to take a large amount of turn through the turbine we shall see the turning very soon it simply gives up a lot of work by virtue of turning. So, the essentially it is the impulse of the jet when it hits the turbine which creates the rotation of the free rotor blades and that produces or you know helps the extraction of the work and hence it is simply called impulse turbine because the work seems to be done by impulse force that is the jet impinging on the free rotor. The other kind of turbine that developed later on much later actually and have been used now in gas turbine engines in a craft engines it is called reaction turbine. Now, in a reaction turbine as the word must be familiar to you there is a certain amount of reaction force that is coming into picture now where does it come from it comes from the fact that as the flow is made to go through the turbine blades the rotor blades we have seen the flow going through the stator blades you know goes through a nozzle kind of action and which creates the jet. If we do the same thing let us say for the rotor that means it is allowed to go through a certain amount of nozzle effect then flow coming out of the nozzle of the rotor would have a nozzle effect. Now the nozzle effect essentially is captured by the fact that a nozzle produces a reaction effect or reaction force this is how we get our thrust of a aircraft engine now supposing we do the same thing in a very small way in a turbine and allow a jet to come out of the turbine with a certain small amount of reaction which then produces a force backwards on to the freely rotating blades. So, the reaction force adds to the working of the turbine in addition to the impulse of the impinging jet. So, a typical reaction turbine has both the impulse effect as well as this additional reaction effect which is been created by the design of the turbines and as a result of which we have two effects through which the work is extracted from the passing high energy gas. And this is called reaction turbine a reaction turbine by basic understanding is a impulse plus reaction turbine it is simply called a reaction turbine, but it is indeed an impulse plus reaction turbine. So, we have two kinds of turbines you can have impulse turbines which is pure impulse only due to the impingement effect and the other is where that reaction is coming in a little more subtle way and adding to the working capability of the turbine. So, let us take a look at a simple you know pressure distribution what is done in a turbine is as we see it is using aerofoils or some aerofoil shapes kind of things to do the work that we are talking about. Now, typically if you have a flow over the aerofoil you are invariably going to have one surface which is more curved and the other surface which is less curved. So, the one surface which is more curved is typically called the suction surface which is the more convex surface and this is what we had seen in case of axial flow compressors also. The other surface which is a concave surface is actually called often the pressure surface and on the suction surface you have acceleration and then deceleration or diffusion on the pressure surface you have very mild continuous suction. So, on one surface you have very sharp lot of suction and then a little bit of diffusion. So, the net effect would be a suction or acceleration whereas, on the pressure surface you have continuous mild suction or acceleration. So, that when the two flows from suction and pressure surface meet up again at the trailing edge the net effect would be a net acceleration through the blades. So, typically axial flow turbines effectively create a net acceleration through the blades whereas, in case of compressor as we have seen a similar pressure distribution would have shown that the flow has net deceleration through the blades. So, the flow through the compressors when net diffusive flows through the turbines they would be net accelerating flows or expanding flows. We have a quick look at what these blades would be look like if you take a cut through the blade any blade if you take just a cut you would simply get this kind of aerofoil looking shapes. Now, what happens is flow as we were discussing when it comes through the stator or a nozzle it goes through this passage and this passage is a curved accelerating passage hugely curved accelerating passage as you can see and this curved passage produces this jet high velocity jet which is shown here as C 2 and then if it impinges and is and this rotor blade starts moving it will have a motion. So, this air which is coming through will also experience motion of the rotor which is u that means the rotor would seem to be moving away from the jet that is coming. And as a result the resultant velocity would be v or v 2 coming into this moving blade. So, that is the velocity with which the flow is entering the moving blade. So, the flow was coming from the stator nozzle with C 2 the blade is moving away with a velocity u and the resultant velocity v or v 2 is now going into this rotor blades and then the rotor blades give out the velocity v 3 that which is related to the moving blade and then if you subtract the motion of the blade itself u again you are you get the residual that is C 3 which is the exit velocity of the flow coming out of this turbine stage. These details would be done you know again and again starting with the next lecture and you would get to use this more and more as we go along in this lecture series. Now, this is just to tell you what is an impulse what is a reaction when the velocity v 2 and v 3 as shown here are equal to each other that is when we call it a impulse turbine. On the other hand the reaction turbine would have v 3 clearly more than v 2. So, in this diagram it simply means that v 3 more than v 2 would be a reaction turbine if they are equal it is an impulse turbine. As we shall see later on as we go along the values of u and v 3 and v 2 and even C 3 would vary from root to the tip of a turbine. Now, those are the details you would do in the following lectures. So, to complete the picture of axial flow turbine this is a diagram which we have put together details of this also would be done in more detail in the following lectures. Just to give you a quick idea of what is happening from 0 1 to 0 2 the flow is going through the stator that is the stator flow. So, this is the thermodynamic depiction of what is going through stator and rotor. So, that is the flow that is going through the stator and 0 2 to 0 3 is the flow that is going through the rotor and then of course, the static temperature drop is 1 2 and 3 corresponding pressure drop p 1 p 2 and p 3 and the corresponding dynamic heads the ideal and the real ones all of that is captured in this T S diagram. So, the flow from 0 to 1 to 2 station is the stator nozzle 2 to 3 is the rotor in which as we have seen it is possible that some amount of velocity acceleration has also taken place which means v 3 as shown here is more than v 2 in impulse they would be equal to each other. So, these are some of the things that comes out of the details of axial flow turbine and these details would be talked about more and more as we go along in this lecture series. Just to complete today's lecture I will just tell you what we are talking about in terms of efficiency and as I mentioned they are often called isentropic efficiencies because the relative comparative reference point is the isentropic change of parameters. So, typically total to total efficiency is often given in terms of ratio of the work done of the real and the ideal. If the C p is taken to be constant in both real and ideal frames then it is essentially the temperature differential or the work or the temperature change across the turbine in real and in ideal that the ratio of the two gives you the efficiency. Similarly, the static efficiency is the static temperature change real to static temperature change ideal that gives you the static to static efficiency. In the denominator the change can be split up in stator and rotor. There is a third efficiency which again is defined in terms of total temperature change to static temperature change across the stator and this is because as we see here the static temperature change is indeed often of a higher order than total temperature change and if you take total to static change you are talking about change all the way from 0, 1, 2 all the way down to 3. So, some of those things are factored into the total to static efficiency definitions and you would be doing details of these things and connecting these thermodynamic definitions to the aerodynamics in the following lectures. So, to complete the picture it is possible to even define a rotor efficiency which is essentially work done by the rotor in terms of temperature compared to that with the ideal or isentropic temperature change and then at the end of the day you can say that the work done by this turbine is captured in terms of the Euler's equation that we have done before and that is u into C w 2 plus C w 3 in case of turbine as we have seen in this particular diagram the C w 2 and C w 3 are on to opposite sides of the axis or actual direction as a result they sort of add up. So, C w 2 shown here and C w 3 shown here kind of add up. So, these are the things that would be coming up in the next few lectures in more and more greater detail. So, this is how the work is done in axial flow turbine. So, today we have just introduced what is axial flow turbine, what kind of turbines are used typically in many gas turbine engines some of the modern versions of them, what they look like, what kind of blade sections are used there and then we have just started off with the theory of axial flow turbines. They are aerothermodynamic theories of axial flow turbines which we shall carry on in the next few lectures in more and more detail. Firstly, the two dimensional theories and later on as I mentioned the flow becomes three dimensional and we shall do the three dimensional theories to understand how full axial flow turbine works. So, in the next lecture you would be indeed doing the detail two dimensional turbine aerodynamics and a little bit of thermodynamics may be. So, that is what you would be doing in the next class two dimensional flow cascade flow in axial flow turbines.