Some beginner questions about materials and others

[thebigjam]

I've been planning a turbojet build out of a generic T3/T4 ebay turbo, I'm just not sure on what the thickness and alloy to use for the flame tube and combuster chamber. My turbo has a inducer size of 65.5mm, which gets me a 393mm L x131mm D flame tube on JetSpecs. I know stainless is the best material but what thickness sheet should I go with? getting stainless pipe is pretty expensive so I'll have to weld it from some sheet stock. I've heard that somewhere around 1mm/19gauge is good but I'm not sure if that's too thin to tig weld for a beginner or not. How thick can I go? I would also prefer to use stainless for the combustor housing as it gets quite humid where I am, what thickness material should that be? I assume thicker as it is a pressure vessel? 

Finally is there cutaway diagram for the inside of a combustor housing and flame tube for propane? Can I just weld on a NPT fitting and let the propane flow through or do I need a distribution system? Thanks 

[racket]

Hi

Have a look here
jetandturbineowners.proboards.com/thread/680/diy-turbines

1mm is OK but 1.2 mm is easier to weld

For the outer can , maybe look for an old 7"dia stainless fire extinguisher, the air over water type , great pressure vessel but lightweight .

The propane MUST be sprayed radially within the flametube , NO axial injection , a short length of tube with the end closed and 12 X 1/16" dia holes in the side wall .

Cheers
Jophn

[thebigjam]

Thanks,
Didn't think of fire extinguishers, they'll definitely save a lot of money and effort

Do you know where I can find some pictures of how the fuel injection is built? I've tried digging around but a lot of the photos are broken.

[racket]

Hi

The fuel injector needs to spray out the propane towards the Primary air holes, the injector can be rather crudely made so just a length of metal tube with holes in the side , the main thing is to have a fair number of holes so as to produce good coverage of the FT cross sectional area , in the size flametube you require , 12 holes in 2 rows of 6 will provide good coverage , and if no bigger than 1/16" - 1.5mm dia , the fuel jets will be strong enough to flow across the flametube , too large a size and the fuel will be concentrated in the core of the FT .

Cheers
John

[thebigjam]

Does the spacing of the holes on the flame tube matter or can I just roughly place them in the front, center, and back?

[racket]

Yes it does matter , but as long as you follow Jetspecs the engine should work , so Primary holes opposite the propane injection , Secondary holes 75mm further along the flametube , and Tertiary holes half way between Secondary holes and the outlet of the flametube .

If you only wish to use propane you may find that it will be difficult to reach maximum power using the standard BBQ cylinder as the required draw off of fuel for your sized engine will be beyond the cylinders ability

Cheers
John

[thebigjam]

I'm looking more in the the combustion chamber design, is there a preferable way to fasten the flame tube on to the main combustion chamber? I've seem some designs use 5 or 6 larger bolts while others use multiple of what looks like M5 bolts with nuts tacked on the back side like in the design I'm following: jetandturbineowners.proboards.com/thread/1294/new-member-first-turbojet-build

Here's my understanding of the combustion chamber construction so far, does it sound right?:

- Main chamber with flanges welded on both, nuts tacked to the inside

- Flame tube welded on plate that bolts on to combustion housing flanges

- "funnel" welded to plate that bolts on the flanges on the other side

- Pipe that goes from compressor is welded so that air enters radially to the flame tube side of the combustion chamber


What material is good for the flanges and fasteners? I was thinking regular steel for the flanges and plates for cost and stainless fasteners, although I heard that stainless fasteners may not be great for high temps. Also, are gaskets necessary for the flanges?

[racket]

Hi

Flanges can be in steel , but make them thick to prevent warpage especially at the turbine scroll where a heatproof turbo gasket is required .

Theres no need to have that mid section flange joint , one at the cap is sufficient , but do fit a slipjoint inside for sealing the flametube during expansion .

Bolts can be plated steel .

Cheers
John

[thebigjam]

I ran the dimensions in jetspecs, are the default hole diameters of 6 10 and 16mm good? 

I saw another build using 5.5, 8, and 13mm holes although their flametube was smaller in diamater as they did theirs in inches, would those be better hole sizes? 




Here's the one I'm basing my build off of:
 

He has a turbo with the same dimensions as mine but the calculator gave out different flametube dimensions when in inches? The length is the same but the diameter is off by .94 inches.

[racket]

Hi

Your initial Post gave an inducer of 65.5 mm, whats changed ??

The hole sizes in the one you are copying are OK

Theres no need to be too pedantic with a Jetspecs combustor , its made to generous proportions so that it will work , and the use of propane makes it even less problematic .

Cheers
John

[thebigjam]

I accidentally put in the turbine exducer diameter instead of the compressor.

I was just kind of worried that converting to inches resulted in a diameter nearly 1 inch smaller, but I think I'll just run with it. Some parts are coming soon and I'll update as I go, thanks.

[racket]

Hi

Thanks for the clarification .

Jetspecs varies the flametube dia to inducer dia ratio at 2"-50mm , the <50 mm sizes are "bigger"due to the lower pressure ratios that are generally generated by them .

Cheers
John

[thebigjam]

What size diameter,thickness of tubing should I use for propane injection, and how far should it stick in to the flame tube? From what I found they look to be between 15 and 20mm in diameter. I assume that I'll have to weld the end shut so maybe slightly thicker is better?

[racket]

Hi

It only needs to be 1/4" OD tubing , but something a tad larger like 3/8 - 1/2", 10 mm -12 mm will give more room to drill the radial holes in , it needs the radial injection holes positioned so as to spray towards the flametube primary wall holes, yep , axial end must be closed , only radial injection

Cheers
John

[The Lazy Destroyer]

I'm no expert but below are  a couple pictures of my injector. Tubing was 1/2" diameter and 1/16" thick. Mostly for convenience as that is what I have a lot of here, and drilling in tube smaller than 1/2" sounds like a pain.
I think it was 8 holes, 1/16" diameter possibly? Located where my primary holes are. I left some extra length on the tube in case I needed to modify it, and the end is just welded shut. 

My jet also uses the $100 ebay T3/T4 turbo, FYI.