Gas Turbines - Bleed Air — Page 396, Lesson 499

I want to walk you through turbine blade cooling, and I want to start with the idea that drives the whole design. Designers reasoned that if passing air through a blade once could lower its temperature, then passing it through more than once would lower it even more. That proved true, and they eventually found the optimum number of passes is five — that's the quintuple pass. So the state of the art in turbine blade manufacture today is quintuple pass, multi-feed internal cooling, combined with extensive film cooling. Let me unpack those terms. Multi-feed means the cooling air is fed into the blade at several points, not just one. Internal cooling means the air travels through passages inside the blade itself. And film cooling means a thin layer of cooling air is ejected onto the outer surface of the blade, forming a protective film between the hot gas and the metal. So you have air going through the blade five times internally, plus a film on the outside — that combination is the current benchmark. Now, the nozzle guide vanes are cooled in a similar way to the turbine blades, but with one major difference: only high pressure compressor air is used. That's HP air. So while the blades might use other sources, the nozzle guide vanes rely exclusively on high pressure compressor air, and that HP air is supplemented by film cooling, just like the blades. Next, the turbine disc cooling. In the vast majority of gas turbine engines, the turbine blades are fixed to turbine discs. Heat conducts from the blades into the disc, so the discs must be cooled and prevented from suffering thermal fatigue — that's the damage caused by uncontrolled expansion and contraction. The front and rear faces of each turbine disc are cooled by high pressure compressor air, and the actual pressure in each disc cavity is controlled by interstage seals. So the seals aren't just keeping things in place; they're actively regulating the pressure that governs how much cooling air reaches each disc face. Now let's talk about sealing itself. To prevent oil or air leaking into spaces where it shouldn't go, several types of seal are in use, and most work on the principle of the labyrinth — a maze. The labyrinth seal consists of fins that rotate within an annulus. An annulus is a ring-shaped space. Now, that annulus can contain oil, or — where the exterior of the seal is static — it can be a soft abradable material, or a honeycomb structure. With those latter two, during initial engine running the fins rub against the annulus material and cut into it, giving the minimum clearance. That's the key: the seal wears itself in to the tightest possible gap. During operation, there's a pressure drop across each fin, which results in a restricted flow of air from one side of the seal to the other. So each fin acts like a barrier that the air has to squeeze past, losing pressure each time. When these seals are used to seal bearing chambers, the air pressure prevents oil leakage by flowing from the outside of the seal to the inside. That flow direction is crucial — the air pushes inward, keeping oil from escaping outward. And it has an additional benefit: it induces a positive pressure that assists the oil return to scavenge. Scavenge is the system that collects and returns oil to the tank, so that positive pressure helps push the oil back. One more point on placement. Where seals have to be placed between two rotating shafts, there's a risk of friction between the fins and the abradable material, because the shafts can flex during operation. So that's a design consideration — flexing shafts can cause the fins to rub against the abradable material, which is exactly what the initial wear-in is meant to manage, but here it's a continuous operational concern. So to tie it together: blade cooling uses quintuple pass multi-feed internal cooling plus film cooling; nozzle guide vanes use only HP compressor air plus film cooling; turbine discs are cooled on both faces by HP air with pressure controlled by interstage seals; and the labyrinth seal is the workhorse — fins rotating in an annulus, creating pressure drops that restrict flow, and in bearing chambers that air pressure keeps oil in and helps scavenge it back.