Technical challenges of small gas turbines 

Part 1

The Williams WR24 samll gas turbine,  (image from blueyonder.co.uk)

Commercially, the main challenge face by gas turbines was the competition from the cheaper and more reliable reciprocating engines. These Internal Combustion(IC) engines were well capable of producing the power required for unmanned drones and target practice vehicles. It was the small gas turbines, (SGT's) that were encroaching into the territory long held by IC engines. This being said, it is not that the challenges faced by fore-runners of the SGT industry (such as Westinghouse and Fladder inc.) are only from the market.

They faced quite a few challenges on the technical front as well. As Hans Von Ohain rightly put,

 ”.. the small gas turbine could not be considered simply as a scaled down version of a large gas turbine”.

The first problem the SGT faces is against physics itself. To understand what begrudge physics has against the SGT, we need to look at the Reynolds's number. 

Reynolds's number and Mach number are the primary parameters that provide an idea of the state of the flow. While Mach number is quite common, Reynolds's number is usually an  unfamiliar term to non-mechanical engineers. However this parameter is very much intrinsic to all fluid flow. It gives an idea of the dominance of the momentum of the fluid, that wants to keep going, over the sticky viscous forces that don't want the fluid to flow freely. If a blob of water is flowing very fast, or if that blob is very big, its momentum is high and consequently it will be very hard to stop that blob of water from moving ahead. It will have a large Reynolds's number. Consequently, if our blob was made of molten tar (am referring to the stuff used in laying roads) instead of water, it would move or should I say ooze, very slowly, at a leisurely pace. Its movement is retarded by the sticky forces, that only let it move slowly, but in a highly ordered fashion. 

Flowing tar, highly ordered flow. (from www.pavingexpert.com)

What this Reynolds's number has to do with the success of the SGT? Everything actually. It turns out that small gas turbines, with the small fluid flows through them, usually have low Reynolds's number. And lower the Reynolds's number, lower will be your efficiency (Don't ask me why. Ask the people at NASA. They figured this out way back in 1949). The engine will still produce as much thrust as you ask out of it, but it will gulp down more fuel in doing so. More fuel you need for thrust, more fuel you need to carry and the bigger your engine needs to be to carry the more fuel and more fuel for the bigger engine.. you get the idea. So the lower efficiency for smaller gas turbines mean that the odds are usually in favor of the large engines.

This is not the only problem. There are much more technical hurdles that need to be overcome. Most prominent among them are the leakage effects and smoothness requirement. Let's see them in detail in the next post.