Gas Turbines - Combustion Chambers — Page 254, Lesson 337

We're starting a new topic now: the combustion chamber of the gas turbine. Let's look at the early multiple combustion chamber system, because it teaches us the fundamental jobs every chamber must do. First, the big picture. In an early multiple system, you have several separate chambers arranged around the engine, behind the compressor. Each chamber has a flame tube inside an individual air casing. The flame tube is where the burning happens; the air casing is the outer shell. Between them, airflow is managed carefully. Now, the key feature I want you to understand first is transpiration cooling. In these early designs, the air casing wall is made of laminations—thin layers. A film of air flows between these laminations. That's transpiration cooling: the air itself becomes the coolant, flowing through the wall structure to keep the casing from overheating. Modern designs use a different method, but this is the classic one. Next, ignition. Most gas turbine engines have only two igniters—the spark sources. In fact, the engine would probably start quite readily with just one working. But because there are only two, you need a way to pass the starting flame from the ignited chambers to all the others. That's the inter-connector—a passage linking one chamber to the next. Here's the sequence. Immediately after light-up, the flame in the chamber with the igniter causes pressure to rise inside that chamber. That creates a pressure differential between that chamber and the one adjoining it. That pressure difference drives the burning gases through the inter-connector, where they ignite the mixture in the next chamber. This process continues around the engine until all chambers are burning. Then the pressures equalize, and the flow through the inter-connectors ceases. So the inter-connector is only active during the start sequence. Now, thermal expansion. The chamber is bolted to the compressor at the front end—it cannot expand in that direction. At the turbine end, there's a sealing ring. That ring allows the chamber to elongate due to expansion, letting it expand into the nozzle box—the portion of the engine immediately preceding the nozzle guide vanes—while maintaining a gas-tight seal. So the chamber grows backward, not forward, and the seal stays intact. Finally, the corrugated joints. These are the wavy connections on the flame tube. They allow tertiary air to bleed into the flame tube. That causes a gradual drop in the temperature of the gases before they exit into the nozzle guide vanes. So the corrugations aren't just structural—they're the entry point for cooling air that protects the turbine. Now, the system itself. The straight-through flow multiple combustion chamber system was developed from Sir Frank Whittle's original design. It was used on some earlier axial flow engines and is still in use on centrifugal compressor engines like the Rolls Royce Dart. It consists of eight or more chambers disposed around the engine, to the rear of the compressor section. Each chamber is a flame tube with its own individual air casing. A similar system was used on the Rolls Royce Avon—a powerful axial flow compressor engine for its time, used on many military and commercial aircraft for many years. So, to tie it together: the chamber must contain combustion, manage cooling through transpiration and tertiary air, distribute the starting flame via inter-connectors, and accommodate thermal growth through the sealing ring. Those are the core functions.