Gas Turbines - The Turbine Assembly — Page 272, Lesson 352

All right, let's get into the turbine assembly. We're going to look at the materials first, because that's where this section starts, and it's the foundation for everything else. We've got two big material ideas here. First, single crystal material. Think about how a metal normally solidifies from a melt — it forms lots of tiny crystals, and the boundaries where those crystals meet are called grain boundaries. Those boundaries are a weakness in the structure. They're the most likely starting point for any failure. Now, single crystal material forms as only one grain in the mould. Because there are no grain boundaries, it eliminates corrosion and creates an extremely creep resistant blade. Creep, remember, is the slow deformation of a material under sustained stress and high temperature — exactly what a turbine blade faces. So a single crystal blade resists that stretching and sagging far better. Second, ceramic materials. These are also being used in turbine blade production. Originally, the ceramic was applied as a plasma spray — a coating. That coating gives very good protection against a corrosive condition. And here's the specific chemistry: that corrosion is caused by a reaction between the base metals of the blade, the sodium in the air, and the sulphur in the fuel. So the ceramic coating is a barrier that stops that reaction from eating into the blade metal. Now let's move to the turbine stage itself. We need to recall what the compressor does — it added energy to the air by increasing its pressure. The turbine does the opposite: it extracts energy by reducing the pressure of the gases flowing through it. And that pressure drop happens in two places. First, as the gas is converted to velocity in the nozzle guide vanes. Second, as it's converted into mechanical energy in the turbine blades. So the turbine stage consists of two elements: one row of stationary nozzle guide vanes, and one row of rotating turbine blades. The complete turbine assembly comprises one or more of these stages on one shaft. And if that shaft is coupled to a compressor, the whole thing forms a spool. Let me show you Figure 17.1 — that's a single shaft, three stage turbine, similar to the one used on the Rolls Royce Dart turboprop engine. There are a few features in that diagram worth special note. First, the divergent gas flow annulus. The word "divergent" means the flow area is getting larger as you move backwards through the turbine. That larger area allows longer blades to be fitted moving backwards in the turbine. Why? To enable velocity to be controlled as the gas expands into the larger area. As the gas expands, it wants to speed up or behave differently, and the longer blades manage that velocity. Second, the blade shroud. This is an attempt to minimize losses due to leakage across the turbine blade tips, and also to reduce vibration. So the shroud caps the blade tips to stop gas sneaking over them and to dampen blade movement. Now, the clearance between the blade tips and the turbine casing. That clearance varies because of the different rates of expansion and contraction of the materials involved. The casing and the blades heat up and cool down at different rates, so the gap changes through the flight cycle. One solution is an abradable lining in the casing area — a material that the blade tips can gently wear into, reducing gas leakage through that clearance. But there's a more effective method: active clearance control, like that used in a modern compressor. It's better at maintaining minimum tip clearance throughout the flight cycle. Figure 17.2 shows active clearance control used in turbine case cooling, on an American engine. The idea is you actively manage the casing temperature — cooling the case to shrink it slightly and close that tip gap, keeping it at the minimum throughout the flight. So to tie it together: the turbine extracts energy by dropping gas pressure, first in the nozzle guide vanes converting to velocity, then in the blades converting to mechanical energy. The stage is one row of each. Multiple stages on one shaft, coupled to a compressor, make a spool. And the design details — divergent annulus, shrouds, abradable linings, active clearance control — all exist to manage gas leakage and blade behaviour in that harsh environment.