Turboshaft expander for free piston gasifier

[blunteversmoke]

Hello everyone!

I really like this forum - loads of people with cool ideas and hands of gold Especially the turboshaft bikes everyone seems to be building are wycced kewl. Keep up the good work!

I am less a GT fanatic than FPE one, free piston designs being capable of far higher combustion efficiency than both conventional piston engines (no crankshaft to worry about) and gas turbines (no continuous extreme temperatures), all the while being simpler than both. They produce hot and high pressure working gas, much like gas generator part of a two-turbine turboshaft engine. Usually, a turboshaft expander is utilized for power extraction from that gas (although positive displacement expanders are also thinkable); everything put together, it looks something like this:




Now, I am a complete newbie myself, so before even thinking about building anything, I am seeking information on what type of power turbine to choose, how to size it, what to convert or build it from, shaft RPM reduction types and so on.
What I am envisioning:


Working gas parameters 4 to 6 bar, around 500°C. With that, what flow is needed for what power rating on what turbine type at what RPM? Are there any online calculators?

Power turbine type can be axial or radial, power can be anything from 5 to 50 kW, depending on what type and size can be made cheapest and most simple when the complete package is considered - not least the reduction gear.

I would love to put it to practical use, as well - a bike or car installation comes to mind - but that is a thing of distant future, as I see it.



Loads'o'thanks in advance! Take care

Dmitry

[racket]

Hi Dimitry

An interesting ambition :-)

With a potential pressure ratio of 4 to 6 :1 across the freepower stage you'll be needing a couple of stages of radial wheels or probably 3 stages of axial wheels to do the job efficiently.

As for mass flow , you won't be needing much for 50 Kw -65 hp, with a 6:1 PR you'll have say a 200C drop through the turbine , so a flow of ~0.5 lbs/sec will be enough , even less flow if your temperatures were higher .

Freepower rpm will be very high due to the small flow and high pressure ratio/gas velocities , so think >60,000 rpm , which will require expensive/precision gearing.

Cheers
John

[blunteversmoke]

Thanks for that bit of info!

What do you mean by multiple stages - multiple wheels on one axle or actually a compound of multiple power turbines, each with its own reduction gearing? Also, I read that radial turbines can expand up to 9:1 PR in one stage - or would it not apply for the gas parameters I'll have to work with?..

re 50 kW, I'd be content with anywhere from five to fifty; hell, I'd even be absolutely content with two or three
I just figured that there's some point where cost of the power turbine breaks even with the cost of reduction gear for smaller, high-revving turbines, so gave power range specifications as wide as realistically possible for me

Temperature of 500°C I've given is the maximum realistically achievable on a homemade FP gasifier, it's far more likely to be 400-425°C, as the world's experience with FPGs suggests.


Yikes! Over 60000 RPM don't sound good, is there no way around it? A bigger power turbine? Or would that be inefficient? Also, are axials not spinning slower than radials for same size and gas pressure, could that be a way?..
Because getting working gas pressure as low as 4 bar could probably be achieved at the expense of some efficiency, but that is the absolute lower limit, again, as the world's experience with FPGs suggests. By the way, can I just rule-of-thumb the RPM in proportion to PR? Going back from RP of 6 down to 4, that would land me in the ballpark of 40000 RPM which is a speed some few homemade turboshafts run at, is that realistic?..


Sorry to bombard you with n00bie questions, it's just that I'd would like to know where to start, what to look and what to watch out for. Because the insane 60000 RPMs are a bit hard to chew on when compared to 15000-25000 of the homemade turboshafts that you see on the net, and that are far easier to gear down...




Again, thanks a lot!

[racket]

Hi

Multiple turbine wheels on a single shaft , the lower pressure drop per stage means lower gas speeds and lower rpm .

LOL, 60,000 rpm was being conservative , more like 120,000 rpm if you try and have all your expansion through a single stage , your mass flow is pretty small so wheel size will be small hence the high rpm , you could have partial entry on a large wheel to reduce rpm a tad . .

Our larger freepower engines have 30,000 rpm speeds with 5 times your flow rate whilst only using a ~1.8 :1 PR across the stage .

For small engines its not practical to use such high pressure ratios , OK on a large setup with several axial turb stages .

Cheers
John