We are starting a brand-new chapter today: Gas Turbines – Combustion Chambers — Page 247, Lesson 331

We are starting a brand-new chapter today: Gas Turbines – Combustion Chambers. This is where the gas turbine engine actually makes its power, so I want to walk you through the whole task of this component, from the airflow split to the fuel systems. First, let's establish the fundamental job. The combustion chamber has to take the air coming out of the compressor, mix it with fuel, burn it, and deliver a high-energy gas stream to the turbine. But here's the critical engineering problem: the air leaving the compressor is already hot and at high pressure, and the turbine blades downstream can only tolerate a certain temperature. So the chamber has to raise the gas temperature enough to produce thrust, but not so much that it melts the turbine. That brings us to the first big concept: the temperature increase allowed versus the temperature increase required. The temperature increase allowed is limited by the metallurgical limits of the turbine blades — you simply cannot exceed that or the engine destroys itself. The temperature increase required is what you need to generate the power output for the engine's operating condition. These two are in constant tension, and the whole design of the chamber is about managing that gap. Now, the fuel itself. We're burning kerosene, and it has a specific property called the flame rate. This is the speed at which the flame front propagates through the fuel-air mixture. Kerosene's flame rate is relatively slow, which means if you just dumped all the fuel and all the air together and tried to burn it in one go, the flame would be unstable and could blow out. So the chamber is designed to burn only a small portion of the air with the fuel, and then mix the rest of the air in afterwards. This is where the airflow division comes in, and I want you to remember these three terms precisely: primary air, secondary air, and tertiary air. Primary air is the portion that is actually mixed with the fuel and burned — this is the combustion zone. Secondary air is introduced downstream of the primary zone to cool the combustion products and complete the burning process. Tertiary air is the final portion, used mainly for cooling and dilution to bring the gas temperature down to what the turbine can accept. So you have a staged process: burn hot with primary air, then progressively cool and dilute with secondary and tertiary air. Now let's look at the physical components. The combustion chamber has several distinct parts, and I'll walk you through them in order. There's the casing, which is the outer pressure shell. Inside that, you have the flame tube — this is where the actual combustion happens. The flame tube has a series of holes and louvers that admit the secondary and tertiary air. At the front, you have the fuel spray nozzle, which atomizes the kerosene into a fine mist so it can mix with the primary air. And at the rear, the hot gases exit through the turbine entry. There are three main system configurations, and I want you to understand the difference between them. The first is the multiple combustion chamber system. This is the early design, where you have a number of separate flame tubes arranged around the engine, each with its own casing and its own fuel nozzle. They're all connected by interconnecting tubes so that the flame can propagate from one to the other during starting. Now, a critical detail in this multiple system: the fuel drain system. Because the flame tubes are separate, any fuel that doesn't burn during shutdown can collect in the bottom of the chamber. So there's a drain valve that opens when the engine is shut down, allowing this unburned fuel to drain out. This prevents a dangerous pool of fuel from igniting on the next start. When the engine is running, the pressure inside the chamber keeps the drain valve closed. The second configuration is the tubo-annular system. This is a hybrid — it uses a number of separate flame tubes, like the multiple system, but they're all contained within a single annular casing that surrounds the engine. So you get the individual flame tubes for combustion stability, but a common outer casing for simplicity and weight saving. The third is the annular system. Here, there's a single continuous flame tube that forms a complete ring around the engine, with no individual tubes. This is the most modern and efficient design — it gives the best airflow distribution and the shortest overall length, but it's more difficult to cool evenly. Now, let's talk about the air/fuel ratio, and I want to give you the exact term: the stoichiometric ratio. This is the chemically correct ratio of air to fuel where there is exactly enough oxygen to burn all the fuel completely — no excess air, no excess fuel. For kerosene, that's roughly 15 parts of air to 1 part of fuel by mass. But here's the key point: the combustion chamber does not run at the stoichiometric ratio. Because of the temperature limits we discussed, the overall ratio is much weaker — maybe 45 to 60 parts of air to 1 part of fuel. Only in the primary zone, where the actual burning happens, do you approach the stoichiometric ratio. The rest of the air is the secondary and tertiary dilution air. Next, we have pressure losses in the chamber. This is unavoidable — any time you have to slow the air down, mix it, burn it, and accelerate it again, you lose some total pressure. This loss is a direct penalty on engine efficiency, so the designer tries to minimize it, but it can never be zero. The pressure loss is a key performance parameter of the chamber. Then we have combustion stability. This is the ability of the flame to stay lit and steady across the entire operating range of the engine — from idle to full power, and through rapid throttle changes. If the flame blows out, that's a flameout, and that's a serious event. The design of the primary zone, the recirculation of hot gases, and the flame rate of the fuel all contribute to stability. Related to that is relighting. This is the ability to re-establish combustion after a flameout, particularly at altitude. If the engine flames out at high altitude, the pilot needs to be able to relight it. The chamber must be designed so that the igniter can re-ignite the fuel-air mixture even in the rarefied air at altitude. Finally, we have combustion efficiency. This is a measure of how completely the fuel is burned. A high efficiency means nearly all the fuel's energy is released as heat; a low efficiency means unburned fuel is wasted, which reduces thrust and increases fuel consumption. Efficiency is highest at cruise conditions and drops off at idle and at very high power. Now, the fuel delivery systems. There are three main types of fuel spray nozzles, and I want you to know each one. The first is the airspray system. Here, the fuel is atomized by a stream of air — the air is used to break the fuel into fine droplets. This gives good atomization at low fuel flows. The second is the duplex system. This uses two separate fuel circuits — a primary and a secondary. At low power, only the primary circuit operates, giving a small, well-atomized spray. At high power, the secondary circuit opens up as well, increasing the fuel flow. This gives good atomization across the whole operating range. The third is the vaporizing tube system. Here, the fuel is sprayed into a tube, and the hot gases from the combustion zone vaporize the fuel before it's mixed with the primary air and burned. This gives very good mixing and a stable flame, but the tubes can be prone to carbon buildup. So, to tie it all together: the combustion chamber takes high-pressure air from the compressor, burns a small portion of it with kerosene at near-stoichiometric conditions in the primary zone, then dilutes and cools the products with secondary and tertiary air to a temperature the turbine can survive. The whole design is a balance between the temperature increase allowed and required, managing the flame rate of kerosene, minimizing pressure loss, ensuring stability and relight capability, and maximizing combustion efficiency. And the fuel is delivered through one of three nozzle systems: airspray, duplex, or vaporizing tube.