Gas Turbines - Combustion Chambers — Page 254, Lesson 336

Let’s start with the big picture of what a combustion chamber is doing. The air coming into it is moving fast, and we need to burn fuel in that flow without blowing the flame out, and we need the gas leaving it to be at a temperature the turbine can survive. That’s the whole job, and the way we manage it is by splitting the airflow into three distinct streams: primary, secondary, and tertiary air. Look at Figure 16.1 with me. The air entering the primary zone passes through the snout first. The snout is the entry lip of the flame tube — the inner shell where combustion actually happens. After the snout, that air is divided: part of it goes through the perforated flare, and part goes through the swirl vanes. The perforated flare is a ring of holes that lets some air through, and the swirl vanes are angled vanes that spin the airflow. So the snout, the flare, and the swirl vanes are the three components that shape the primary airflow. Now, primary air. This is about 20% of the flow coming into the combustion chamber. It is basically the air that gets mixed with the fuel and burnt. So of every hundred units of air entering, twenty are dedicated to combustion itself. But here’s the clever part: by passing through the flare and the swirl vanes, the velocity of this air is reduced, and it also starts the recirculation that is required if the flame is not to be extinguished. Think about that — a flame needs a stable, slow-moving region to sit in. If the air just blasted straight through at full speed, it would blow the flame out. The swirl vanes create a recirculating flow, a rolling motion, that holds the flame in place. Now the secondary air. The air that was not picked up by the snout goes into the space between the flame tube and the air casing. The air casing is the outer shell of the chamber; the flame tube is the inner liner where the burning happens. So between those two walls, we have a passage of air. Some of this air is allowed into the flame tube through secondary air holes. Secondary air is about another 20% of the total. It reacts with the primary air flowing through the swirl vanes to form a toroidal vortex. A toroidal vortex is a region of low-velocity airflow that resembles a doughnut or a smoke ring — a ring-shaped rolling circulation. This vortex stabilizes and anchors the flame, and it prevents the flame from being dragged down the flame tube away from the fuel nozzle area. So the fuel nozzle is where fuel is sprayed in, and we need the flame to stay right there, not get carried downstream. The toroidal vortex is what holds it. Now, the temperature at the centre of the primary zone reaches about 2000°C. That is far too hot for the materials of the nozzle guide vanes and the turbine blades. The nozzle guide vanes are the stationary vanes that direct the gas onto the turbine, and the turbine blades are the rotating ones that extract the energy. Neither can survive 2000°C. So we need a further drop in temperature before the gases can be allowed to exit the combustion chamber. That brings us to tertiary air. The remaining 60% of the total airflow is tertiary air. It is progressively introduced into the flame tube to cool and dilute the gases before they are allowed to go into the turbine assembly. So tertiary air does two jobs: it cools the gas exiting the chamber, and it also cools the walls of the air casing. That’s why it’s introduced progressively — in stages along the length of the flame tube — rather than all at once, so the cooling is gradual and the walls are protected. So let me tie it together. Primary air, 20%, is the combustion air — it mixes with fuel and burns, and the swirl vanes slow it and create recirculation. Secondary air, another 20%, enters through the secondary holes and forms the toroidal vortex that anchors the flame. Tertiary air, the remaining 60%, is introduced progressively to cool and dilute the gas before it reaches the turbine. The whole design is about managing velocity and temperature: slow the air to hold the flame, then cool the gas so the turbine survives. That’s the division of airflow through the combustion chamber.