Gas Turbines - Combustion Chambers — Page 261, Lesson 345

I want to walk you through the different fuel injection systems used in gas turbine combustion chambers. We've been looking at how fuel gets into the flame tube, and we've hit a fundamental problem: a fixed-size orifice, a simple hole, can only give you a finely atomized spray — a fine mist of fuel droplets — when the fuel pressure is high. At low pressures, the spray is coarse and poor. Let me give you the numbers. At low engine speeds, the fuel pump is simply incapable of providing the high pressures — we're talking 1500 to 2000 psi — that are required to give a good spray pattern. That's the core problem: a fixed orifice needs high pressure to atomize fuel well, but at start, when the engine is spooling up, fuel pressures are low. So we need another method to get sufficient atomization at start. The first solution is the Airspray System. The principle here is to use a high-velocity airstream to break up the fuel flow. Instead of relying on high fuel pressure to shatter the fuel, we let a fast-moving stream of air do the work. Because of this, the airspray system needs relatively low fuel pressures. And that has a nice knock-on effect: it can operate using a gear type pump, which is much lighter than the more sophisticated plunger type pump. So we trade a heavy, complex high-pressure pump for a light gear pump, and let the air do the atomization. The second solution is the Duplex System. This one effectively uses an orifice of variable size — it's clever. At low fuel pressures, a pressurizing valve closes off the main fuel feed to the nozzle. So the only fuel supply comes from the primary fuel line. That primary line feeds the primary orifice, which is a much smaller hole. A small hole is capable of providing a fine spray at lower pressures, because the same pressure across a smaller opening gives you better atomization. Then, when the engine accelerates during start, fuel pressure builds up until the pressurizing valve opens. Once it opens, fuel flows to the main orifice, supplementing the flow from the primary orifice. So at low pressure you run on the small primary orifice alone; at high pressure, both orifices feed together. The third method is the Vaporizing Tube System. Here, the fuel is sprayed from feed tubes into vaporizing tubes, and those vaporizing tubes are positioned inside the flame tube. Primary air is fed into the flame tube through the fuel feed tube opening, and also through holes in the flame tube entry section. Now here's the key: the fuel is turned through 180 degrees inside those vaporizing tubes. As the tubes are heated by the combustion process, the fuel is vaporized — turned into vapour — before it passes into the flame tube. So instead of atomizing the fuel into droplets, we vaporize it, and then it enters the flame tube as a vapour. So to summarise the contrast: the airspray system uses a high-velocity airstream to break up the fuel, needing low pressure and a light gear pump. The duplex system uses a variable-size orifice — a small primary orifice for low pressure, supplemented by a main orifice once pressure builds. And the vaporizing tube system turns the fuel through 180 degrees in heated tubes inside the flame tube, vaporizing it before it enters the combustion zone. Each one solves the same problem — poor atomization at low fuel pressures at start — but in a different way.