Technical challenges of small gas turbines 

 Part 3

 Note: If you are un familiar with the small gas turbines, they are a niche sector of small to medium size jet engines. For an introdduction, see this post.

The combustor sits behind the compressor and heats up the compressed air so that the energy content of the air rises, of which some will be extracted back by the turbine. The combustor is second only to the compressor in terms of design complexity and the SGT's minuscule size compounds the problems even higher.  The first problem is in getting the fuel inside the combustor. 

Like in ordinary Kerosene fueled stoves, complete combustion happens when the fuel stream is broken down in to fine droplets. This is so that for a given volume of fuel, smaller and more the number of droplets, more will be the surface area. More the surface area, more is the area of contact with Oxygen and the droplets vaporize faster and finally the vaporized fuel burns better. 

 Moreover, this atomized fuel has to be introduced into the combustor at multiple locations. This is so that the fuel distribution in the combustor is uniform. A spray bar, with multiple nozzles for injection is usually used in for injection in this manner. 

Multiple fuel injection nozzles on a spray bar.

For an SGT, the fuel flow rate in to the combustor is already low. Divide that into multiple nozzles, and the fuel flow through each nozzle becomes even lower. But in order to get good atomization, the nozzle orifice has to be very small so that the fuel breaks up into fine drops as it leaves the nozzle. Designing an orifice for such low fuel flow rates as encountered in SGT's result in very delicate orifices that get clogged with carbon deposits easily. 

Fuel breaking up into tiny drops as it is pushed out of the tiny orifice. Image from jrthomson

This major problem precluded the use of fuel injecting nozzles in small gas turbines and a very innovative way of injecting fuel into the combustor was developed by engineers of Teledyne CAE. 

Lets look at slinger combustors next.

Edit: I couldn't find any publicly available resources that describe the operation of the slinger combustor and I can't give out any propreitary knowledge that is not in open domain.

So let us conclude this post on technical challenges regarding the combustor problems of SGTs by noting that slinger combustors offer solution to the problem.