Gas Turbines - Combustion Chambers — Page 254, Lesson 341

We're starting a new topic now: the combustion chamber of the gas turbine. We've already covered the compressor and the turbine, so now we're looking at the heart of the engine where the fuel is actually burnt. I want to begin with the annular combustion chamber, because the text gives us a clear list of its advantages over the other two systems it was developed from. Let's go through them one by one. First, for the same power output, the length of the annular chamber is only 75% that of a tubo-annular system of the same diameter. So it's a quarter shorter, which saves space and weight. Second, there are no flame propagation problems. In a can-annular or tubo-annular system, the flame has to spread from one chamber to the next through connecting tubes. In an annular chamber, it's one continuous ring, so the flame doesn't have to travel anywhere. Third, compared to a tubo-annular system, the air casing area is less, and consequently less cooling air is required. Less surface area means less heat to manage, so you don't need to bleed off as much air for cooling. Fourth, the combustion efficiency is raised to the point where unburnt fuel is virtually eliminated. This allows the oxidation of carbon monoxide to non-toxic carbon dioxide. So instead of poisonous carbon monoxide going out the exhaust, you get carbon dioxide. And fifth, there is a much better pressure distribution of the gases impinging on the turbine, so it has a more even load placed upon it. That means the turbine blades aren't being hit by uneven pressure pulses, which is better for their life and for smooth operation. Now, let's talk about the air/fuel ratio. To obtain the maximum heat release, the chemically correct air/fuel ratio of 15:1 must be used. That's fifteen units of air to one unit of fuel, by weight. This is called the stoichiometric ratio. Now here's an interesting contrast. In a piston engine, using this ratio would cause detonation and dissociation. But in the gas turbine engine, it poses no such problem, because there are no peaks of pressure to assist in their generation. The piston engine has pressure spikes; the gas turbine burns at a steadier pressure, so it can safely run at this chemically correct ratio. The fuel and air are mixed and burnt in the primary zone in this 15:1 ratio. But then, the addition of secondary and tertiary air will dilute the mixture. The overall ratio may vary between 45:1 to as weak as 130:1. So the primary zone is rich, and then you add more air downstream to cool the gases down before they hit the turbine. Now, about pressure losses. It has been stated that combustion theoretically occurs at a constant pressure. In fact, there is a small loss in pressure throughout the combustion chamber. This is caused by having to provide adequate turbulence and mixing. The losses vary from 3% to 8% of the pressure at the entrance to the combustion chamber. So you lose a little pressure, but that's the price you pay for good mixing. Finally, let's look at combustion stability. During normal engine running conditions, combustion is self-supporting. Effectively, the ignition system can be switched off as soon as the engine has attained self-sustaining speed. That's the speed at which, after start, it can accelerate without the assistance of the starter motor. But there are certain engine operating conditions which do require ignition. For instance, following a flame out. That's the extinction of the flame due to various unusual occurrences, such as the ingestion of large amounts of water during take off from contaminated runways. Another condition which can cause flame extinction is when the air/fuel ratio becomes too weak. This is most likely to occur when the engine is throttled back during descent, when a low fuel flow and a high air mass flow will coincide. So you're coming down, you pull the throttle back, the fuel flow drops, but the air mass flow stays high, and the mixture gets too lean to burn. So, combustion stability means smooth burning, coupled with the ability to remain alight over a large range of air/fuel ratios and air mass flows. That's the definition you need to hold onto. The engine has to stay lit across a wide operating envelope, not just at one perfect setting. That figure shows the division of airflow through the combustion chamber, with the primary air. And this next one shows an early combustion chamber, one of several that would have been used in a multiple system. And here's the multiple combustion chamber system itself. So to tie it together: the annular chamber is shorter, has no flame propagation issues, needs less cooling air, burns more completely, and loads the turbine more evenly. It runs rich in the primary zone at 15:1, then dilutes to as weak as 130:1 overall. It loses 3% to 8% of inlet pressure to turbulence, and it must stay alight across a wide range of conditions, which is what we call combustion stability.