Technical challenges of small gas turbines 

 Part 2

 

 Note: If you are un-familiar with the small gas turbines, they are a niche sector of small to medium size jet engines for missiles and drones. For more information, see this post

 In continuation of our rundown of the technical challenges that were faced by the designers SGT's, next in line are the clearance and smoothness (tolerance).

 The clearance problem is pretty easy to understand. You use a compressor to increase the pressure of the air going in to the combustion chamber. Just behind the compressor, there is high pressure air. In front of the compressor, there is ambient air, at a pleasant 0.9 bar pressure. And there is a tiny gap between the rotating compressor and the stationary casing of the compressor. This gap is there so that the compressor rotor doesn't rub the casing when it is rotating like crazy. But the high pressure air, which doesn't know the real purpose of the clearance gap,  sees it as a way to get out of the high pressure situation. It goes through this clearance, back into the lower pressure area in front of compressor. 

A simple schematic showing possible reverse flow location 

 All this means that whatever fuel you spent in compressing the leaked air is a waste of money.The problem is more severe for SGT's than for large gas turbines. This is because the leakage losses depend on the ratio of the clearance gap to blade height. SGT compressor blades, are small. This  means that for the same clearance gap, losses faced by SGT may be too much, while the losses of a large gas turbine may be tolerable.

For the same clearance, the clearance to blade height ratio is much higher for SGT. This means more loss :(

 This brings us to the next issue. The one of smoothness: aka manufacturing tolerances. The manufacturing tolerances are no where more critical than the compressor air foils and the seals. These tolerances are specified as a fraction of the major dimension of the part.For a wide chord fan blade of a large turbofan such as Trent 800, the tolerances can be relatively high , in the order of 100 microns or so. Tolerances of trailing edges of air foils are of the order of 3 hundredths of an inch(Leyes and Fleming, Page 22). This is OK, because the major dimension, the chord of the blade, is large and the ratio of tolerance to chord is still small enough that you can live with it. However, try maintaining the same ratio in a SGT compressor airfoil and suddenly you are in need of machinery and skilled labour who can give very close tolerances , sometimes of optical grade quality. It is not that these equipment are not available. It is just that when you are making an engine that is going to blow up in one hour, investing so much in such high precision components is difficult to swallow. It is usually termed in gentlemanly terms in books as "undue costs". 

Compressor blades and seals have very low tolerances of manufacture (image from power-technology.com)

The next major problem faces by the pioneers has to do with the combustor. Let us look at that next.