Gas Turbines - Bleed Air — Page 396, Lesson 497

I want to walk you through turbine cooling, because this is where the gas turbine engine really starts to test the limits of its own materials. Let's start with the fundamental logic of bleed air. Air has a lot of work done on it to raise its pressure as it passes through the engine. So it's logical to extract that air from as early a stage in the compressor as possible, as long as it can still perform its function. The earlier you take it, the less work has been invested in it, but you need enough pressure for whatever job it has to do. Once the air has done its job, it's either dumped overboard, or ejected back into the main gas stream at the highest possible pressure. That second option gives you a small performance recovery, because you're putting that air back to work. Now, the main parts of the engine that need cooling are the combustion chamber and the turbine section. We've already covered combustion chamber cooling in a previous chapter, so let's focus on the turbine. The gas turbine is a heat engine. High thermal efficiency depends on high turbine entry temperatures. But there's a limit to how much heat can be released into the turbine from combustion, and that limit is imposed by the materials from which the turbine blades and nozzle guide vanes are manufactured. If these components are continuously cooled, then the temperature of their operating environment can actually exceed the melting point of the material they're made from. That's the key idea — cooling lets the gas be hotter than the metal could survive on its own. The turbine discs are also heated, by conduction from the turbine blades. So they need cooling too, otherwise you risk disintegration from continued thermal stress. Some modern turbofan engines use cooling air to control turbine blade tip clearance — that's active clearance control — by controlling turbine casing temperature. There's also a feature on some engines of selective cooling of the compressor rotor using bleed air. That controls thermal growth of the compressor blades to improve compressor efficiency. Now let's get into turbine blade cooling itself. Figure 27.2 shows the development of turbine blade cooling since its inception. Originally, it was considered sufficient to pass low pressure compressor air through the blade — that's single pass internal cooling — and in doing so, retain the blade's temperature below the critical level at which excessive creep would occur. Creep, remember, is the slow deformation of metal under heat and stress. But the requirement for greater engine power and efficiency meant higher gas temperatures were necessary. Low pressure compressor air was no longer able to provide enough cooling on its own, so a supplementary source was needed. Research showed that by passing high pressure compressor air through the blade as well as the low pressure air — that's called multi-feed — a reasonable increase in gas temperature could be achieved before blade failure was experienced. An additional increase was attained by creating a boundary layer effect, which is film cooling. That's done by passing air through small holes in the leading and trailing edge of the blade. To some extent, this boundary layer protected the turbine blade from the onslaught of the hot gases coming from the combustion chamber. So you have three stages of development: single pass internal cooling with low pressure air, then multi-feed adding high pressure air, then film cooling with those small holes. This was the type of blade engines used for the following decade. Eventually, though, events dictated that further advances in blade technology had to be made. That's where we'll pick up next.