Gas Turbines - Combustion Chambers — Page 261, Lesson 343

Let’s pick up with the combustion chamber and look at what happens when we push air through it at different rates. I want to walk you through the stability limits first, because that governs everything else we do with the flame. Look at Figure 16.7, the typical combustion stability loop. The key idea is that combustion will only stay stable between a rich limit and a weak limit. Rich means there’s a lot of fuel relative to the air; weak means there’s very little fuel. Between those two boundaries, the flame holds. But here’s the critical part: as the air mass flow increases, those limits get narrower and narrower. The range between the rich and weak limits shrinks as mass flow goes up, and beyond a certain point the flame is simply extinguished. So at high airflow, you have a very tight window in which combustion can survive. Now, inside that stable region, the diagram shows an ignition loop. That loop tells you something important: it is more difficult to start the combustion than it is to sustain it once it has started. In other words, the conditions needed to light the flame are stricter than the conditions needed to keep it burning. A direct consequence of this is that if the engine flames out at high speed or high altitude, you may have to reduce both parameters — both the speed and the altitude — before a successful relight can be obtained. You can’t just hit the ignition and expect it to catch under those extreme conditions. That brings us to relighting. The ability of an engine to relight varies according to the height and the forward airspeed of the aircraft. Figure 16.8 shows a notional relight envelope — that’s the flight envelope, the range of height and speed conditions, under which a serviceable engine would be guaranteed to relight. So if you’re inside that envelope, you can count on the relight working. Here’s the mechanism. When the engine is windmilling — that is, the airflow through the engine causes the compressor and turbine to rotate on their own, without any fuel being burned — the compressor will supply sufficient air. So all that’s then required is the opening of the HP fuel cock and the operation of the ignition system. HP stands for high pressure. The HP fuel cock is the valve that admits fuel to the combustion chamber. You achieve this by selecting the relight switch, and that switch functions separately from the normal start circuit. So it’s a dedicated control, not part of the routine engine start sequence. Now let’s talk about combustion efficiency. This is the efficiency with which the combustor assembly extracts the potential heat actually contained in the fuel. In other words, of all the heat energy locked up in the fuel, how much of it actually gets released and used in the combustion process. Modern gas turbine engines have a very efficient combustion cycle. At high power operating conditions, combustion efficiencies as great as 99% are achievable. And even at idle, the systems will still give as much as 95%. Figure 16.9 illustrates this, and it also shows the overall air/fuel ratio throughout the normal operating range of the engine. So you can see how efficiency holds up across the whole operating band. That very high combustion efficiency is due in no small part to the fuel spray nozzles used in large, modern gas turbine engines. The nozzles have the task of atomizing or vaporizing the fuel to ensure that it is completely burnt. Atomizing means breaking the liquid fuel into very fine droplets; vaporizing means turning it into a gas. Either way, the goal is to get the fuel into a form where it can mix thoroughly with the air and burn completely. And this is no easy job, considering the velocity of the airstream coming from the compressor and the small distance allowed within the chamber for combustion to occur. You have a fast-moving air stream and a very short space, so the fuel has to be prepared almost instantly. There are other problems too, and they come from the relatively low pressures attainable by the engine-driven high pressure fuel pump at engine start. Those pumps are driven by the high speed gearbox, and upon start selection they are only rotating at a minimal speed. So at the moment of start, the pump isn’t spinning fast enough to generate high fuel pressure. That low pressure makes it harder to get a good spray pattern. Figure 16.10 shows the fuel spray patterns at various pressures — you can see how the spray shape changes as pressure changes. So to tie it all together: the stability limits narrow with increasing airflow, starting is harder than sustaining, relight may require reducing speed and altitude, the relight switch works independently of the normal start circuit, combustion efficiency runs at 99% at high power and 95% at idle, and the fuel spray nozzles are what make that efficiency possible — despite the high airstream velocity, the short combustion distance, and the low fuel pump pressure at start.