Gas Turbines - The Turbine Assembly — Page 261, Lesson 350

I want to walk you through the turbine assembly of a gas turbine engine. This is the part of the engine that actually extracts the energy from the hot gases and turns it into the mechanical power that drives everything else. Let's start with the task of the turbine assembly. The simplest way to picture it is to think of the axial flow compressor running in reverse. In the compressor, you put mechanical energy in and you get compressed air out. In the turbine, you put hot, high-energy gas in and you get mechanical energy out. The construction mirrors that. Initially, a stator section — and I want you to remember the proper name, the nozzle guide vane — directs the air axially onto a rotor section. So the nozzle guide vanes are the stationary part that aims the flow, and the rotor is the spinning part that the flow strikes. The turbine's job is to extract energy from the hot gases flowing through it and convert it into mechanical energy. That mechanical energy is then used to drive the compressor and the gearboxes. Those gearboxes can operate accessories, or, in engines that don't rely predominantly on jet propulsion, they can power propellers or rotors. So the turbine is the heart of the whole power output chain. Now, the energy available in the gases takes three forms. There's heat energy, there's potential energy — which here means pressure energy — and there's kinetic energy, which is velocity energy. When the turbine converts all of these into mechanical energy, the value of all three is reduced as the gases pass through. That's the fundamental trade — you're taking energy out of the flow, so the flow loses heat, pressure, and velocity. But here's an important contrast to hold onto: the velocity of the gas in the combustion chamber is lower than the velocity of the gas in the exhaust unit. So even though the turbine is extracting kinetic energy, the gas is still moving faster at the exhaust than it was in the combustion chamber. The combustion chamber slows the flow down to allow burning, and the turbine accelerates it again as it expands. Now let's talk about the stresses in the turbine, because this is where the engineering gets serious. During normal operation, the rotational speed can be such that the blade tips travel at over 1500 feet per second. At the same time, the temperature of the gases driving the turbine can, in a modern engine, reach as high as 1700 degrees Celsius. And the speed of those gases themselves can be as high as 2500 feet per second — which, at those temperatures, is close to the speed of sound. Let me put those numbers in perspective. A small turbine blade that weighs only 2 ounces when stationary can exert a load of two tons while working at top speed. That's the centrifugal force from spinning that tiny blade at enormous speed. Now, that tensile loading — the pulling force on the blade — coupled with the tremendous heat, causes a phenomenon called creep. Creep is the stretching of the metal of the blade beyond its ability to reform back to its original length. So the blade literally grows permanently under the combined effect of heat and tension. That's a critical failure consideration, because once a blade has crept, it's permanently deformed. Whatever materials have been used to produce the turbine — and that's where the excerpt cuts off, so we'll pick that thought up next. But hold onto the key ideas: the turbine is the compressor in reverse, it extracts heat, pressure, and velocity energy to drive the compressor and gearboxes, and it operates under extreme conditions of speed and temperature that cause creep in the blades.