Gas Turbines - Combustion Chambers — Page 247, Lesson 334

I want to walk you through the combustion chamber of a gas turbine engine. This is where the real energy release happens, and it's a deceptively difficult job. Let's start with the task itself. The combustion chamber has to contain the burning mixture of air coming from the compressor and fuel coming from the fuel spray nozzles. Its purpose is to allow the maximum heat release at a substantially constant pressure. That's the key phrase — substantially constant pressure. The chamber isn't there to build pressure like a piston engine's cylinder; it's there to add heat. The result is that the turbine receives a uniformly expanded, heated, and accelerated stream of gas. So the chamber's whole job is to deliver a smooth, even, hot, fast-moving gas flow to the turbine. Why is efficient combustion so important now? Two reasons. First, the rise in the cost of the fuel itself. Second, the increasing public awareness of atmospheric pollution from exhaust smoke. So efficiency isn't just about economy anymore — it's about emissions. Now, there's a hard limit on how hot the gas can be. That limit is imposed by the materials from which the nozzle guide vanes and the turbine are manufactured. The nozzle guide vanes are the stationary vanes just upstream of the turbine that direct the gas onto the blades. If the temperature goes even slightly above that limit — the slightest excursion — you risk the possible disintegration of the turbine, with probably catastrophic results. So that temperature ceiling is a safety-critical number. But here's the tension. Modern materials will allow a gas temperature initially in the combustion chamber of 2000°C plus. That's the peak flame temperature inside the chamber. However, when the gas exits the combustion chamber, the temperature must be reduced to between 1000 and 1500°C. So the chamber has to take the air, burn it hot, and then cool the products down before they hit the turbine. And remember, the air may already have been heated to around 600°C just from compression. So sufficient fuel must be added to raise the temperature further from that 600°C baseline. That 1000 to 1500°C exit temperature is at full power. At lower power settings, you need lower fuel flows. So the combustion chamber has to be capable of maintaining stable and efficient combustion over a wide range of engine operating conditions — from idle right up to takeoff power. Now let's look at the flame rate of kerosene, because this is where the real engineering problem shows up. Air enters the combustion chamber at approximately the same rate at which it enters the intake of the engine. Speeds of up to 500 feet per second are not unusual. But the flame rate of kerosene — that's the speed at which the leading edge of the flame travels through the vapour — is only 1 to 2 feet per second. Think about that contrast. The air is moving at up to 500 feet per second, and the flame can only propagate at 1 to 2 feet per second. If burning kerosene was exposed to an airstream travelling at 500 feet per second, it would be extinguished immediately. The flame would simply blow out. So something must be done to slow down the airflow after it leaves the compressor and before it reaches the primary zone. The primary zone is the zone inside the combustion chamber where the air is mixed with the fuel and burnt. That's where the flame lives. The way the air is slowed down is shown in Figure 16.1. The air is slowed and its pressure is increased after it leaves the compressor and before it enters the combustion chamber. In fact, the pressure attained at this point is the highest in the whole of the engine. That's a remarkable fact — the peak pressure in the entire engine occurs right here, in the diffuser section before the combustion chamber, not in the compressor itself. But here's the catch. The reduction in velocity is still not enough. Further decreases must be achieved if the flame is not to blow out. So the diffuser alone can't do the whole job. The chamber itself has to manage the airflow internally to keep the flame stable. That's the fundamental problem the combustion chamber designer is solving: you have a flame that propagates at 1 to 2 feet per second, sitting in an engine where the air is moving hundreds of feet per second, and you have to keep that flame lit, stable, and efficient across the whole power range. So to summarise what we've covered: the chamber contains the burning air-fuel mixture at constant pressure to feed a uniform hot gas stream to the turbine. The temperature is capped by turbine and nozzle guide vane materials — exceed it and you risk turbine disintegration. The chamber must deliver 1000 to 1500°C at the exit, starting from air already at about 600°C after compression, and it must stay stable across all power settings. And the core aerodynamic problem is that the flame propagates at only 1 to 2 feet per second while the airflow can be 500 feet per second, so the air must be slowed — first in the diffuser, where pressure reaches its engine-wide peak, and then further inside the chamber itself to keep the flame from blowing out.