Gas Turbines - Compressors — Page 242, Lesson 319

I want to walk you through the rest of the compressor chapter, and we're picking up right where the material left off — with the fan blades of a high bypass ratio engine. So, the high bypass ratio engine's low pressure compressor blades are more commonly known as the fan blades. They were initially manufactured from solid titanium, and titanium was chosen because it has the properties of strength with lightness. Now, a low blade weight is absolutely essential if the fan is to be able to withstand the out of balance forces which would occur if a blade failed. Think about that — if a blade fails, the whole fan becomes unbalanced, and a heavy blade would create enormous destructive forces. So keeping the blade light is critical. But here's the problem. Even though titanium is enormously strong, the blades still had to have incorporated into their design something called a snubber. Let me define that precisely: a snubber was a support fitted at mid-span which prevented aerodynamic instability. So it's a structural support at the middle of the blade that stops the blade from vibrating or flexing in an unstable way. Unfortunately, it also added weight, and particularly when two of them were required, it interfered with the supersonic flow characteristics of the air at the extremities of the blade. So the snubber solved one problem but created another — it disrupted the airflow near the blade tips. Experiments were then carried out with new materials, particularly carbon fibre. But its flexibility greatly reduced its effectiveness, and its use has largely been discontinued. So carbon fibre was tried and rejected because it was too flexible. Now, the greatest advancement was achieved by fabricating the blade from a honeycomb core sandwiched between two outer skins of titanium. Let me make sure you've got that construction clearly: you have a honeycomb core in the middle, and on either side of that core, you have an outer skin of titanium. So it's a sandwich — titanium skin, honeycomb core, titanium skin. This method gives added strength with less weight, and that enabled the introduction of what's called the wide chord fan blade. The stability of the blade is ensured as a result of its wider chord — the chord being the distance from the leading edge to the trailing edge of the blade — and therefore the snubber is no longer necessary. So by making the blade wider, you get the stability you need without the weight penalty of the snubber. Now let's move on to a completely different topic: compressor and turbine contamination. Accumulation of contaminants in both the compressor and the turbine section of the engine reduces the efficiency of the unit and can seriously affect its performance. So this is a real operational concern. The contaminants on the compressor, which are mostly salt and pollution from industrial areas, reduce the aerodynamic efficiency of the blades. So salt and industrial pollution build up on the compressor blades and degrade their aerodynamic performance. In the turbine, the contamination takes a different form. It's called sulphidation — that's a build up of sulphur deposits from the burning fuel. This sulphidation destroys the aerodynamic shape of the turbine blades and the nozzle guide vanes, and it will, over a period of time, erode their surface finish. So in the turbine, it's not just a performance loss — it's actual physical destruction of the blade shape and surface. Now, how do we deal with this? If the major cause of contamination is salt ingestion, then a timely rinsing of the compressor with fresh water can avoid the harsher treatment which otherwise will be required. This can be carried out either while motoring the engine over on the starter, or while running the engine at idle speed. This procedure is known as a desalination wash. So a desalination wash is a fresh water rinse of the compressor, done either on the starter or at idle, to remove salt contamination. If the contamination has reached the stage where a desalination wash is not sufficient, then the application of an emulsion type surface cleaner may be necessary. This is sprayed into the engine intake under the same conditions as the desalination wash — so again, either motoring on the starter or at idle. This procedure is known as a performance recovery wash. So the performance recovery wash is a stronger chemical treatment, using an emulsion type surface cleaner, for when salt isn't the only problem or the contamination is more severe. And the turbine also benefits from this treatment — so both the compressor and the turbine benefit from these wash procedures. Let me just make sure you've got the key contrasts clear. First, the fan blade evolution: solid titanium with a snubber, then carbon fibre which failed due to flexibility, then the honeycomb core sandwiched between titanium skins, which gave us the wide chord blade and eliminated the need for the snubber. Second, the contamination: compressor contamination is salt and industrial pollution reducing aerodynamic efficiency; turbine contamination is sulphidation, sulphur deposits from burning fuel, which destroys blade shape and erodes surface finish. And the two wash procedures: desalination wash with fresh water for salt, and performance recovery wash with an emulsion type surface cleaner for more severe contamination — both done either motoring on the starter or at idle. That's the full picture of this section.