We're starting a brand-new chapter now: Chapter 17, Gas Turbines — The… — Page 261, Lesson 349

We're starting a brand-new chapter now: Chapter 17, Gas Turbines — The Turbine Assembly. This is where we look at the part of the engine that actually extracts the energy from the hot gas to drive the compressor and, in some cases, produce the shaft power. Let me give you the roadmap first, because this chapter is dense. We'll cover the task of the turbine assembly, the stresses it endures, the materials used for the blades, the anatomy of a turbine stage, the free or power turbine, multi-spool engines, blade shape, how blades are fixed, the losses in the turbine, and finally temperature measurement. That's a lot, so let's take it step by step. First, the task of the turbine assembly. Its job is to extract energy from the hot, high-pressure gas coming out of the combustion chamber and convert it into mechanical power to drive the compressor. In a turboprop or turboshaft, it also drives the propeller or rotor. So it's the engine's power take-off point. Now, the stresses in the turbine. This is critical. The turbine operates in an incredibly hostile environment. It has to withstand high temperatures, because it sits right after the combustion chamber. It also has to handle high centrifugal stresses from spinning at very high speeds. And there's thermal stress from the rapid temperature changes. So the turbine is one of the most highly stressed components in the whole engine. That brings us to turbine blade materials. Because of those extreme conditions, the blades are made from special high-temperature alloys, often nickel-based superalloys. These materials retain their strength at high temperatures where ordinary metals would soften or creep. Some advanced blades are even made as single crystals to eliminate grain boundaries, which are weak points. The goal is to maintain strength and resist creep at operating temperatures. Next, the turbine stage. A stage consists of a row of stationary nozzle guide vanes followed by a row of rotating turbine blades. The nozzle guide vanes accelerate the gas and direct it onto the turbine blades at the optimum angle. The rotating blades then extract the energy. So one stage is one set of stators and one set of rotors. Then we have the free, or power, turbine. This is a turbine that is not mechanically connected to the compressor. It's driven by the gas flow, and it drives the output shaft — the propeller or rotor. This is common in turboprop and turboshaft engines. The advantage is that the power turbine can run at its own optimum speed, independent of the compressor speed. That leads to multi-spool engines. In a multi-spool engine, the compressor and turbine are divided into two or three independent rotating assemblies, each on its own shaft, rotating at different speeds. For example, a low-pressure spool and a high-pressure spool. This improves efficiency and surge margin because each spool can operate at its optimum speed. Now, blade shape. The turbine blades are shaped like airfoils, twisted along their length. The twist is important because the blade speed varies from root to tip — the tip moves faster than the root. So the blade angle must change along its length to match the gas flow angle at each radius. This is called twist, and it ensures the gas enters the blade at the correct angle all along the blade. Turbine blade fixing. The blades have to be securely attached to the turbine disc. They typically use a fir-tree or dovetail root, which fits into a matching slot in the disc. This shape spreads the load and allows for thermal expansion. The blades are often held loosely in the slots so they can expand and contract without cracking. Then, losses in the turbine. There are several sources of loss. There's profile loss from the boundary layer on the blade surfaces. There's secondary flow loss from the gas flowing around the blade tips and the end walls. And there's tip clearance loss, where gas leaks over the blade tips without doing work. These losses reduce the efficiency of the turbine. Finally, temperature measurement. Because the turbine operates at such high temperatures, it's essential to monitor the temperature. This is done using thermocouples, which measure the exhaust gas temperature, or EGT. The EGT is a critical parameter for monitoring engine health and ensuring the turbine isn't overheated. So that's the overview of the whole chapter. Now let's dive into the details, starting with the task of the turbine assembly and the stresses it faces.