New project advice

[racket]

Hi Wingers

If you want to reduce the volume of the flametube , shortening it is the preferred route , but it probably needs to be at least 1.5 times the diameter , and diameter needs to have an area of 3 times compressor wheel inducer area for a reasonable cross section to keep air/gas velocities low enough for kero to burn...............propane combustors with their fast burn rates can get by with restricted volumes but kero is a little slow to burn by comparison .

When designing the combustors for my "micro" engines I use the 3 times inducer area , but because they're annular flametubes the distance between outer and inner walls is only 2-3 inches so the length only needs to be 4 or 5 inches long even for a 98mm inducered engine , BUT , the fuel presentation needs to be broken up into a multitude of "segments" , 18 mini co-joined "wall less" flametubes within the one annular flametube in my case .

jetandturbineowners.proboards.com/thread/19/10-98-engine

The whole point of Jetspecs is simplicity , keep it big , keep it simple , and it'll work, smaller flametube with higher heat release rates require more sophistication, be prepared for extra work getting it right..............theres no such thing as an oversized combustor, but there is such a thing as an undersized one .

Cheers
John

[wingers]

OK, thanks - I'll bear that in mind. When quoting FT length I'm assuming that includes the length of any adaptor to convert the FT to turbine inlet? i.e. the length is from fuel injector to turbine blade. Is that right?

Wingers

[racket]

Hi Wingers

With a turbo engine theres a requirement for a smooth transition from the Primary/Secondary cross section to the scroll inlet which is generally a lot smaller in area , this will necessitate a fair bit of axial length in some cases if we don't want to incur losses from a sudden reduction in area , a ~30-45 degree included angle for the "funnel" will be OK .

We need to have full cross sectional area in the Primary and Secondary zones to make sure "slow" combustion is safely accomplished , after that things can be "speeded up".

As with any flametube theres a fair bit of guess work involved along with development time if you want a high heat release rate and a fairly uniform temperature profile entering the turbine scroll , ..............thats why Jetspecs was created with generous proportions, too many guys thought they could create their own compact high heat release combustors without enough idea of whats required , the end result was engines spewing flames from their turbine wheels .

I wouldn't be including any of the scroll volume/length into the combustor calculations, too many unknowns with that.

Cheers
John

[wingers]

OK, I guess that makes sense. It's obviously a key requirement that the combustion finishes before the turbine blade so that's something to stick simple with. I'll keep full size until after the tertiary holes and then start a gentle reduction. I haven't yet designed the reduction tube but I will keep it as smooth as possible.

Sorry for all the questions but I've thought of another one for you...
I've seen a lot of spark/glow plugs mounted to the combustor end cap for propane combustors but is that also reasonable for kerosene? I'm concerned that the ignition point will be too close to the injector to be reliable, as the kerosene won't have atomised fully in time. I can obviously try to find a long-reach spark plug but will that be enough? In fact, thinking about the ignition circuit it may be that a glow plug is a simpler option. I've never really messed about with disease-als so I'll have to see how they compare with regards to plug reach. They might be even longer which could be a good thing...

Wingers

[racket]

Hi Wingers

The end cap positioning of the ignitor is a "simplistic" approach for ease of manufacture , its better positioned on the side wall where theres a more predictable flow of air and atomised fuel mix passing by, end cap positioning for propane preheat/pilot light, before feeding in kero, works OK as the propane will "recirculate" past the ignition source , or it can be directed towards the ignitor from a suitably positioned pilot light injector .

Glow plugs will work.

Cheers
John

[wingers]

I can see how the end cap mounted plug makes the assembly/design easier but I'm looking to fireup on kerosene if at all possible. I think that means I'll have to do some more thinking and find a way to move the plug further down the FT...

Wingers

[racket]

Hi Wingers

I've always used sidewall plugs ,its not difficult .

General method, .............with flametube in its normal running position do a rough positioning of the plug position on the can's outer wall so that its clear of any of the flametube holes, attach the plug thread boss to the sidewall of the combustor and using an old spark plug with the earth removed and the centre electrode ground to a sharp point, screw in the plug and "centre pop" the position on the flametube wall , remove "slave" plug, remove flametube and drill a 12mm hole (for 14mm plug) in the flametube wall .

Ideally we don't want any air getting in around the plug at the flametube wall as the air blast will blow the fuel/air mix away from the ignition source , so "adjust" plug depth etc etc to achieve this end , we want the electrode projecting into the flametube with enough radial clearance around the electrode so that it isn't damaged by thermal expansion/movement of the flametube during operation of the engine .

Cheers
John