Gas Turbines - The Turbine Assembly — Page 272, Lesson 357

Let’s start with the blade shape, because that’s where the turbine’s real work happens. The nozzle guide vanes — the stationary aerofoils just upstream of the turbine — are shaped to form convergent ducts. As the hot gas accelerates through those narrowing passages, some of its potential energy, which is pressure energy, is converted into kinetic energy, which is velocity energy. So by the time the gas hits the turbine blades, it’s moving fast. Now, the turbine blades themselves come in three types. First, the impulse type, which works like a water wheel — the gas strikes the blade and pushes it around by direct impact. Second, the reaction type, which rotates as a reaction to the lift the blade creates, much like an aerofoil generating lift. And third, a mixture of the two, called the impulse/reaction blade. That combination type is the one shown in Figure 17.4. Here’s the clever part about the combination blade. Figure 17.5 shows an end-on view of how its shape changes from the base to the tip. The blade has a greater angle at the tip than at its base, and that gives it a twist. That twist ensures the gas flow does equal work along the entire length of the blade, and it enables the gas to enter the exhaust system with a uniform axial velocity — meaning the flow is even and straight, not skewed. Now, a practical note: gas turbine engines normally do not use pure impulse or pure reaction blading. The proportion of each type used depends on the engine’s design requirements, but in general the combination impulse/reaction is the most common. Pure impulse blades are used in air starter motors — that’s the little turbine that spins the engine up to starting speed. And pure reaction blading is very rare. If it is used, the nozzle guide vanes are designed to divert the gas flow direction without altering the pressure of the gas. Next, let’s talk about how the blade is held onto the disc — turbine blade fixing. This matters enormously, because when the engine is rotating at working speed, the turbine blade and the disc are under considerable stress. The method of fixing the blade to the disc is therefore extremely important. The most common method on modern engines is the fir tree fixing. The serrations that form the fir tree are very accurately machined, and that accuracy ensures the enormous centrifugal load is shared equally between all the serrations. Here’s the key behaviour: while the engine is not rotating, the blade is free in the serrations. But during operation, the centrifugal force imposed by rotation holds it firmly in place. Figure 17.7 shows both the fir tree fixing and the turbine blade shroud, which we mentioned earlier. Finally, let’s look at the losses in the turbine. The turbine is a very efficient mechanical device, but it still suffers losses during operation. On average, these total about 8%. That 8% breaks down like this: approximately 3.5% comes from aerodynamic losses in the turbine blades, and 1.5% from aerodynamic losses in the nozzle guide vanes. The rest — the remaining 3% — is divided fairly equally between gas leakage over the blade tips and exhaust system losses. So to tie it together: the nozzle guide vanes convert pressure energy into velocity, the blades extract that energy as rotation — either by impulse, reaction, or the common combination — the fir tree fixing holds the blade against centrifugal load, and about 8% of the energy is lost to aerodynamics, leakage, and the exhaust. That’s the turbine assembly in a nutshell.