Gas Turbines - Combustion Chambers — Page 254, Lesson 339

Let’s start with the multiple combustion chamber system, because that’s where the snout, the inter-connectors, and the drain tubes all live. I want you to picture a ring of separate flame tubes arranged around the engine, each with its own air casing. In Figure 16.3 you can see three features clearly labelled: the snout, which is the primary air scoop at the front of each chamber; the inter-connectors, which link the chambers together; and the drain tubes, which run between the lowest points of adjacent chambers. Now, the drain tubes exist for one very specific reason: the unlikely event of a failure to start, more commonly called a wet start. A wet start happens when the mixture inside the combustion chamber fails to ignite during a start attempt. By that point, a considerable amount of fuel has already been fed into the engine. If that fuel is not removed before the next attempt to start, the result will be a very long, very hot, and very dangerous jet of flame from the rear of the engine. So the drain system is a safety measure, pure and simple. There are two ways to get rid of that leftover fuel. The first is the fuel drain system itself, which uses those drain tubes. The drain tubes connect the lowest part of each chamber with the next chamber below it. After a wet start, the remaining fuel will try to find its own level, so it flows from the top of the engine down to the bottom chamber. Once it reaches the bottom chamber, it exits through the drain valve, which is located at the six o’clock position — that is, at the very bottom of the chamber. That valve is spring-loaded towards open, meaning it wants to stay open. During normal engine operation, internal pressure inside the chamber keeps the valve shut. So the valve only opens when the engine is not pressurised, which is exactly when you need it to drain. The second method is evaporation. To evaporate any remaining traces of fuel from the chambers, the engine is motored over on a blow out cycle. Here’s how that works: using the starter motor, the engine is rotated for the time normally allocated to a full start cycle, but with the HP fuel cock shut and the ignition system automatically de-selected. The HP fuel cock is the high-pressure fuel shut-off valve, so no fuel is delivered. With the ignition off and no fuel, compressed air flows through the combustion chamber and assists in the evaporation of any fuel still remaining inside. So the blow out cycle is essentially a dry, un-ignited rotation that purges the chambers with airflow. Now let’s move to the next system: the tubo-annular combustion chamber system, which is also called the cannular or can-annular system. It differs from the multiple system in one key way: it does not have individual air casings for each flame tube. Instead, a number of flame tubes are fitted within one common air casing. That gives you a more compact unit. Figure 16.4 is one of the few illustrations that actually shows an igniter plug — that’s the component that provides the spark to ignite the fuel-air mixture. Finally, the annular combustion chamber system. This one has only one flame tube, and that single flame tube is contained by an inner and an outer air casing. So instead of a ring of separate cans, you have one continuous annular space around the engine. A typical example is shown in Figure 16.5, with further detail in Figure 16.6, which is based on an original Rolls-Royce drawing. So to summarise the progression: the multiple system has individual casings per flame tube, the tubo-annular system shares one common casing around several flame tubes, and the annular system has a single flame tube between inner and outer casings. Each step is more compact than the last. And remember, the drain system and the blow out cycle apply to the multiple system’s wet start protection — that’s the safety net that prevents a dangerous flame jet on the next start attempt.