Gas Turbines - The Exhaust System — Page 279, Lesson 367

I want to walk you through the exhaust system of a gas turbine engine, and I'll start right where the gases leave the turbine. This is the part of the engine that takes the hot, high-energy gas stream and turns it into the high-velocity jet that produces thrust. First, let's look at the exhaust cone. As the gases leave the turbine, they're moving at very high speed — between 750 and 1250 feet per second. The exhaust cone is the central, cone-shaped structure that the gas flows around. Its job is to smooth out the gas flow as it passes down the length of the jet pipe, which minimizes turbulence. It also prevents the hot gases from flowing directly across the face of the turbine disc, which reduces disturbance and, importantly, prevents overheating of that disc. Now, inside that exhaust cone, we also support the rear turbine bearing. It's held in place by turbine rear support struts. These struts are streamlined by fairings — those are the smooth, aerodynamic covers over the struts. And here's a clever detail: those fairings also straighten out any residual whirl in the gas stream as it exits the turbine. Residual whirl is the swirling motion left in the gas after it passes the turbine blades. If that whirl is allowed to continue into the jet pipe, it causes additional losses, so the fairings help eliminate it. From there, the exhaust gases travel down the jet pipe to the atmosphere through the convergent propelling nozzle. This is the nozzle that narrows toward the exit. Its shape accelerates the gas. In a turbojet engine, this nozzle increases the gas velocity to Mach 1 — that's the speed of sound in relation to the temperature of the gases — at virtually all throttle openings above idle. When the gas reaches that sonic speed, the nozzle is said to be choked. Let me make sure you understand what "choked" means, because it's a critical concept. When the nozzle is choked, no further increase in velocity can be obtained unless the gas stream temperature is increased. In other words, the nozzle has reached its limit — the gas is already moving at the local speed of sound, and you can't push it any faster just by changing pressure. The only way to get more velocity is to raise the temperature of the gas, for instance with the assistance of reheat, which is the afterburner system. Now, let's look at the convergent-divergent nozzle, which is a more advanced design. When the gas enters the convergent section — the part that narrows — its velocity increases, and there's a corresponding fall in static pressure. The gas velocity at this point reaches the local speed of sound, Mach 1. Then, as the gas flows into the divergent section — the part that widens again — it progressively accelerates toward the open exit. Here's the key: the reaction to this increase in momentum is a pressure force acting on the inside wall of the nozzle. A component of that force acts parallel to the longitudinal axis of the nozzle, and that component produces the further increase in thrust. So in a convergent-divergent nozzle, you get supersonic flow in the divergent section, and that's what gives you the extra thrust beyond what a simple convergent nozzle can provide. Finally, let's talk about the low ratio bypass engine exhaust system. This is more complex because you have two gas streams to pass to atmosphere — the bypass air and the hot exhaust gases. The low ratio bypass engine exhaust combines these two streams in a mixer unit. That mixer ensures thorough mixing of the two streams before they're ejected into the atmosphere. This is important because mixing the cooler bypass air with the hot exhaust gases improves efficiency and reduces noise. Let me also mention the danger areas. The inlet and exhaust of a jet engine are hazardous zones — the inlet can suck in objects and people, and the exhaust is a high-velocity, high-temperature blast. These are areas you must be aware of on the ground, around the aircraft. So to summarize the whole flow: gases leave the turbine at high speed, pass around the exhaust cone which smooths the flow and protects the turbine disc, the rear bearing is supported by struts with fairings that also remove residual whirl, the gases travel down the jet pipe, and then through a convergent nozzle that accelerates them to Mach 1 — choked — or through a convergent-divergent nozzle that can accelerate them beyond that for extra thrust. And in a bypass engine, the two streams are mixed before ejection. That's the complete exhaust system. Take a moment to let that sink in, and when you're ready, we can move on to the next part of the powerplant.