 Land-based gas turbine power generation units such as the one that is shown here typically operate at a constant speed and constant load. So they are more or less operate at the design operating conditions and departures from design operating conditions are very rare and usually not allowed. So consequently, these are very, very efficient. Now in contrast, in the case of an aircraft engine, departure from design operating condition is unavoidable because the engine typically produces the maximum amount of thrust during takeoff and takes in maximum amount of air also during takeoff, typically around 1200 kilogram per second. And the thrust requirement decreases as the altitude increases and that the cruising altitude of 35,000 feet or so. The thrust level also decreases and the mass flow rates typically would be around 600 kilogram per second or even less than that depending on the size of the engine. So because of the huge variation in the ambient conditions as well as the mass flow requirements and thrust requirements of the engine, off design operating conditions actually are inevitable in the case of an aircraft engine. So the engine needs to be able to handle this in a robust and efficient manner so that the fuel economy is good and other advantages accrue from that. Now when the mass flow rate requirement or the thrust requirement reduces, the rotational speed of the rotor also decreases. So all these factors tend to make changes in the operating condition. For instance, we said that under design operating condition the axial velocity remains constant. Now when the mass flow requirement or the rpm requirement of the engine changes or rpm of the engine changes, the axial velocity will also have to change. And how do we handle this change in axial velocity? Because typically the velocity triangle is such that when the air approaches rotor blade either here or here, the relative, the absolute velocity should be such that the relative velocity vector is tangential to the blade profile at entry and at exit also. So when the velocity changes, the axial velocity changes, the flow at entry will no longer be tangential to the blade surface. So for instance, at design operating condition, the inlet velocity vector, relative velocity vector is tangential to this. Because the axial component of the relative velocity Cx is the same as the axial component of the absolute velocity Vx, any change in Vx also causes a change in Cx. So design operating condition, the relative velocity vector is tangential at entry. But if there is a change, then the relative velocity vector will no longer look like this. It may perhaps look like this. If it looks like this, then as you can see the flow separates from a certain part of the upper portion of the blade. Now instead of being like this, if the relative velocity changes like this, then you can see that there is going to be separated flow in a certain part of the blade surface of the lower side. So both these change the relative velocity at the outlet also and this can cause a compressor stall resulting in very poor performance or unsteady performance of the compressor which is very dangerous from a mechanical perspective also. So we need to have an effective strategy by which we can handle changes in the axial component of the velocity. Since Cx equal to Vx, any change in axial component Vx due to a change in mass flow rate definitely translates to a change in the axial component of the relative velocity and can cause non-tangential entry into the rotor. So one way to do this in a multistage compressor is as we mentioned earlier, we can make these stationary vanes or guide vanes move. So when the absolute velocity changes or axial component of the absolute velocity changes, the absolute magnitude of the absolute velocity and the flow angle can also change. So the idea is to re-orient these state vanes in such a way that even with the reduced axial velocity, the flow can still be made to enter the following rotor stage with the relative velocity aligned with the direction of the blade tangent. So such a strategy is called variable guide vanes strategy and this is quite widely used in air carpentry. So we see the variable guide vanes here in a Rolls-Royce engine and it can be seen that these guide vanes are attached to the casing in such a manner that they can be pivoted about this point and they can be moved this way or that way depending upon the change in the axial component of velocity or mass flow rate. So as we said earlier, the mass flow rate requirement of the engine decreases as the aircraft climbs and once the aircraft starts descending, the mass flow rate requirement begins to increase as more thrust is required. So consequently, these variable guide vanes may be moved in both ways to make sure that the flow that enters the following rotor is always the relative component is always tangential to the blade at entry into the following rotor. So we can see how the variable state of vanes adjusts the absolute flow angle so that the relative velocity vector enters the following rotor passage tangentially. So this allows a considerable variation or considerable departure from design operating condition and still avoid compressor stalling under such circumstances. Another strategy that is used in the case of aircraft engines is the following. So here as we can see, this is a turbofan engine. There is a huge fan in the front which generates about 75 to nearly 85 percent of the thrust and the rest of the engine generates only the remaining thrust. So the fan being so large and having such a large outer diameter needs to turn as slowly as possible so that the centrifugal stresses in the blade are minimized. On the other hand, as the air flows through the subsequent compressor stages, when it reaches the tail end of the compressor, these blades become so small that they need to spin at a high speed just to get the flow through the blade passage. So there is a wide variation in the desired rpm of various parts of the compressor. So the fan needs to run as slowly as possible and high pressure stages need to run as fast as possible to get the flow going through the stages as smoothly as possible while keeping the absolute axial velocity constant. So there is a differing requirement here. So it suggests that the different parts of the compressor should then be mounted on different shafts or spools as they are called so that each one can turn at its own optimal speed and this is a strategy that is widely used in aircraft engine. This is called multi-spool. So the fan being large in size and having a large outer diameter should run at the lowest possible rpm so that the centrifugal stresses are minimized plus the tip speed also as minimized as much as possible. The fan tip speed in most modern turbofan actually or such that the relative velocity approaches the blade at supersonic speeds or at least transonic speeds. So reducing this as much as possible is always desirable and because the power requirement of this fan is so high, we need about 5 to 7 turbine stages just to run the fan. Now the intermediate and high pressure compressors are located downstream of the fan. So as the gases compress its volume decreases, the height of the blade also decreases. So the rpm consequently has to go up for maintaining a constant axial velocity because we designed this with constant axial velocity. The rpm has to go up for maintaining a constant axial velocity. The power requirement here is lesser so only 1 or 2 turbine stages are required to drive this. So because of the widely differing rpm requirements, the different parts of the engine like the low pressure, intermediate pressure and high pressure compressors are mounted on different shafts. So each combination, the driving turbine and the driven compressor operates at an optimum speed. So for example, this is the Rolls Royce 3 spool engine and you can see that the fan is mounted on a separate shaft which is driven by about 5 turbine stages. Typically this runs at the speed of around 3000 rpm. The next one which is the intermediate pressure compressor, several Rolls Royce 8 compressor stages for the intermediate pressure compressor are run by 1 turbine stage and this runs at higher speed of about 7500 rpm. Now the high pressure compressor which has about 6 stages is run by again 1 turbine stage, this usually runs at 10,000 rpm or so. So this is usually the outermost shaft. So this shaft is hollow. So the IP shaft, IP stage shaft runs inside this, it is concentric and runs inside this shaft and the fan shaft runs inside these two shafts. So these are three shafts mounted coaxially and concentrically and because they each unit, the compressor plus turbine unit can run at an optimal speed, it is possible to operate at off design operating conditions in a very robust manner while retaining the highest possible efficiency that we see at design operating conditions. So this allows aircraft, modern aircraft engines to be very very efficient and give excellent fuel economy, making possible for widespread air travel that we are that we are witnessing today. Another strategy that is used is counter rotating turbines. So here what is done is instead of making these individual spools all turn in the same or rotate in the same direction, we can actually make one set of spools run in the opposite direction to the following set of spools. So this may spin in this direction, this may spin in this direction. The advantage in doing it like this is that we can eliminate one set of guide vanes in between the spools which reduces the weight and also increases the aerodynamic efficiency of the entire turbine compressor system. So you can see that here this turbine spins in this direction, this turbine spins in this direction. So the corresponding compressor blades which are mounted on the same spool also run in opposite directions. So this allows a set of guide vanes to be removed in between these holes thereby reducing the weight and increasing the aerodynamic efficiency. That's other beneficial effects because the weight can be reduced then the power to weight ratio improves and in an aircraft engine application, the power to weight ratio of the engine is the most critical parameter from an operational perspective. So again, I urge the students to actually go and look at this YouTube video that I suggested earlier. This is illustrated there in a very nice manner. So far, we have seen the challenges that are faced in an axial compressor when it has to operate under off-design operating conditions. And as we said this happens more in the aircraft engines. So let us now look at the challenges that are faced in the turbine side of aircraft engines and what steps are usually taken to overcome these challenges. The most important challenge from the turbine side lies in designing the high pressure turbine stage. As we just saw the high pressure turbine stage spins at the highest possible rpm and it faces the highest pressure and highest temperature in the cycle because it is just downstream of the combustor. So consequently the high pressure turbine blades are the most vulnerable from a design perspective. And the challenges faced here are more from a metallurgical and material perspective rather than from an aerodynamic perspective. We have already seen that since the pressure gradient is favorable in a turbine, we can have very high pressure drops or enthalpy drops without any problem. Unlike a compressor where there is always a danger of separation even at design conditions if the pressure ratio is too high. So there is no, in that sense, the aerodynamic challenges on the turbine side are not perhaps as severe as they are on the compressor side. But the metallurgical and material challenges are quite severe in the case of the high pressure turbine. So this operates at the highest pressure, temperature and rpm. So it is a very challenging environment to operate in because the operating temperature 1700 Kelvin is about 400 degrees higher than the blade metal melting temperature, the turbine blade metal melting temperature is 400 degrees less than the temperature at which it operates. So the blade needs to be cooled very well, otherwise it will melt. So very effective blade cooling strategies are required. And because of these operating conditions, fatigue and creep are major concerns. So we need innovative materials for the manufacture of the turbine blade itself and manufacturing techniques such as single crystal blades and so on are required. And coatings such as thermal barrier coating made of ceramics are also used to prevent the blade from melting. So blade cooling, better materials, better manufacturing techniques, all these are challenges from a metallurgical and material perspective in the case of turbines. And aerodynamic challenges probably are not, as I said, as severe as they are in the case of an axial compressor. But all these challenges have been overcome or addressed very effectively in modern aircraft engines, which consequently are extremely efficient in terms of fuel consumption at the highest possible levels.