Gas Turbines - The Exhaust System — Page 279, Lesson 365

Let’s start with the exhaust system as a whole, because it’s easy to underestimate it. The exhaust system is an often underrated part of the propulsion unit, yet its design exerts a considerable influence on the performance of the engine. So we’re not just talking about a pipe that lets gas out — the shape and behaviour of that pipe directly affect how much thrust the engine produces. First, the purpose. The gases that discharge from the turbine must exit in the correct direction and at the optimum velocity to provide the thrust of the turbojet engine. In a turbojet, thrust comes from accelerating that gas rearward. But in a turboprop engine, the job is different — there, the turbine gas temperature and the back pressure at the turbine are, to a large extent, dictated by the design of the outlet nozzle. So the exhaust system isn’t just about thrust in a turboprop; it controls the conditions at the turbine itself. Now, the temperatures we’re dealing with. The temperature of the gases entering the exhaust system can be between 550°C and 850°C. That’s already very hot. But this can rise to as high as 1500°C if afterburners are used — and afterburners are also called reheat. So if you see the term reheat, that’s the same thing as an afterburner. Here’s the critical safety point. If the aircraft has the exhaust system running through the fuselage — which it does on many jet aircraft — the fuselage must be protected from these temperatures. There are two methods used together. First, a clearance is allowed between the jet pipe and the aircraft skin, and air is allowed to circulate through that gap. Second, the jet pipe itself is insulated with some form of fibrous material, and that fibrous material is sandwiched between thin layers of stainless steel. So you have a stainless steel sandwich with fibrous insulation in the middle, plus an air gap around the pipe. That’s how the airframe survives those temperatures. Let me show you the basic layout. Now let’s talk about jet pipe design, because this is where the clever part is. The gas velocity leaving the turbine can be between 750 and 1250 feet per second. That’s somewhere around Mach 0.5 — Mach 0.5 is half the speed of sound. Now, if that gas has to negotiate a long jet pipe before being ejected into the atmosphere to provide thrust, a great deal of turbulence will be caused within the pipe. And turbulence is bad — it lowers the efficiency of the engine and reduces its thrust. So the design goal is to manage that high-velocity gas so it doesn’t churn around and waste energy. Here’s the key design feature. Look at the outer casing of the jet pipe — its shape appears to be convergent, meaning it seems to taper inward. But at the point where the gas leaves the turbine, the shape of the volume within the casing is in fact divergent — it actually expands. How is that possible? It’s made possible by the insertion of the exhaust cone. The exhaust cone is a conical shaped device positioned close up to the turbine disc rear face. So the cone sits right behind the turbine disc, and it fills the centre of the pipe. Because the cone is there, the annular space between the cone and the outer casing is what the gas actually flows through — and that space is divergent, it widens out. And what does that divergence do? As well as helping to reduce the velocity of the gases — that’s the start of the sentence, and it’s the key effect. The divergent shape slows the gas down. That’s exactly what we want before the gas is ejected, because we’re managing that 750 to 1250 feet per second flow to avoid turbulence and to set up the conditions for efficient thrust production. So to summarise what we have: the exhaust cone creates a divergent flow path inside a convergent-looking casing, which reduces gas velocity, and the whole assembly is protected by an air gap plus fibrous stainless-steel sandwich insulation, with temperatures ranging from 550°C to 850°C normally, up to 1500°C with reheat. That’s the foundation of the jet pipe.