Turbine noob surge question

[barnyard]

Hi guys, this is my first post, and my first jet engine! I've build a jet engine using a turbo from a large catapillar generator. Currently the engine will start up and build rpm' and boost, about 2psi, then it blows out and shuts right down. I start it with a leaf blower, and I've been running it on gasoline , yeah I know crazy and dangerous, but it hasn't burn down the place yet. I do plan to run diesel, but for right now my fuel system consists of a EFI fuel pump and a 4.00Gph hollow furnace oil nozzle.

The turbo inlet inducer is 70mm, and the exhaust side exducer is 94mm. My combustion chamber 6" diameter and about 13" long. The flame tube is 4" diameter, and I laid out the holes using jet spec as best I could. I did have the inlet tube to the combustor going in at a tangent, but after reading lots of advise against that I have since changed it a more centered approach.
I really don't know why the turbo blows out, I've tried putting a larger fuel nozzle in and the turbo will spool a lot higher, say 10psi, but still flutters and then blows out. The flame tube is pretty black with soot though which seems like it's rich, so I'm stumped. Any advice is welcome:) thanks, James

[racket]

Hi James

Welcome to the Forum , this shouldn't be too hard to sort out .

Your flametube is too skinny , a 70 mm inducer has ~3850 sq mms of inlet area , if we multiply by my "standard" 3 times , it gives us ~11,550 sq mms , which represents a 121 mm diameter flametube bore as a minimum.

Jetspecs would have recommended at least 140 mm ,twice inducer diameter , the cross sectional area of the flametube has to be large enough to produce relatively slow air speeds to maintain combustion, your 4"-100 mm dia FT has only ~7850 sq mms of area , Jetspecs 140 mm dia would be ~15,400 or some 96% greater , even my minimum 121 mm dia , is some 46% greater.

Also gasolene doesn't have the same burning characteristics as kero or diesel , its less "stable", you could try a 90% diesel 10% gasolene mix, this should improve things .

Now for the EFI pump , lets assume its pumping at 75 psi , take off a say the 5 psi for P2 and any other losses and we have a 70 psi pressure drop across the fuel nozzle , a 4 GPH nozzle will only be flowing ~3.5 GPH , plenty for modest power .

Your turbo has a rather large/oversized turb wheel exducer for a 70 mm comp inducer at some 80% greater area , generally 25% or so is all thats needed , as a consequence, depending on the turb scroll A/R you could be flowing a bit more air than what would normally be expected from a 70 mm inducer which will exacerbate the undersized flametube issues.

Hope this helps

Cheers
John

[jetjeff]

Hi James,

Surging is a deep growling sound caused by flow break away from the compressor diffusor. This condition is usually brought on by a mismatch of the turbine and compressor. When you say 'flutter', I'm not convinced it's surge.

I'd try the 90% kerosene and 10% gasoline as John stated.

Jeff

[barnyard]

Ok diesel it is! Yah I figured someone would say the flame tube was to small, I built it 3 years ago, before I knew about Jetspec:(. So I will change combustor and flame tube size to match jet spec then, and try some diesel. What's you thoughts on the fuel spray nozzle, should I change to a evaporator style?
Also, I do have a smaller turbo, 2.71" inducer, 3.36" exducer, would that be a better suited turbo sizing? And if so, do you think I could use the larger turbo ( 3.76" exducer) as a free power wheel?!
Thanks a lot for the help,
James

[barnyard]

Thanks Jeff for the clarification on surging, good to know it doesn't sound like that. I'm not a fan of damaging turbo's.
James

[racket]

Hi James

Stick with your spray nozzle , ideally use a hollow cone spray pattern nozzle , the variflo nozzles will help with atomisation at lower power settings www.delavaninc.com/variflo.htm

Using the larger turbo for a freepower would be possible , though you may need to "wastegate/dump" some of the gases exiting from the smaller turbo gas producer to prevent it surging .

Cheers
John

[barnyard]

Oct 11, 2016 15:19:14 GMT -5 racket said:

Hi James

Stick with your spray nozzle , ideally use a hollow cone spray pattern nozzle , the variflo nozzles will help with atomisation at lower power settings www.delavaninc.com/variflo.htm

Using the larger turbo for a freepower would be possible , though you may need to "wastegate/dump" some of the gases exiting from the smaller turbo gas producer to prevent it surging .

Cheers
John

Thanks John for the nozzle info. I see the charts show up to 300psi, is that what most people are running? Seems pretty high pressures to attain DIY. What kind of pump would I need for that?
James

[racket]

Hi James

With a generously sized flametube we can use fuel that hasn't been as finely atomised as would be required in a smaller dimensioned flametube where fine atomisation is a must .

But , you probably need a higher pressure than the usual EFI pump , a small gearpump is whats needed , I used a 1 cc/rev hydraulic pump initially for my Garrett TV84 engine but pressures were in the 750 psi range .

The Variflo nozzles allow more modest pressures , if you can provide 100 psi above engine P2 you should be OK .

Cheers
John

[barnyard]

Yay I figured out how to post pictures! Here is the engine in its past and present form. A friend gave me a furnace blower that I tried hooking up as a power wheel, running on the smaller turbo I have. The engine would run up to about 5psi, I think fuel being the limitation. The power wheel actually turned pretty good! I could still stall it out, but was a fun experiment.

My hopes are to make a jet engine powered kart, ideally with a power wheel, not just thrust.

[racket]

Hi James

Have you checked out my 2 shaft kart for some ideas jetandturbineowners.proboards.com/thread/40/2-shaft-turbine-kart-build
Your gas producer would be an ideal size to flow through the Cummins freepower wheel

Cheers
John

[barnyard]

Hey John, I have seen your kart, and read most of that thread, very inspiring! I like your idea of a fire extinguisher for a combustor can, and plan to borrow the idea.

[racket]

Hi James

Excellent :-)

The setup I used is about as simple as it can be for a freepower kart , the chain drive is proven karting gear so relatively cheap and reliable, the Cummins ST-50 VT-50 turb wheel is very common .

The fire extinguisher was purposely left long to act as a "torque arm" to balance the weight of the turbo hanging off the other side of its mounting , the triangulated stays at the top end of the extinguished produced a reasonably rigid lightweight mounting.

I noticed your turb scroll is fairly "tight" , can you find the A/R , its generally cast into the scroll near the rectangular inlet .

All the best with the build :-)

Cheers
John

[barnyard]

Hi guys, I'm building a new combustion chamber for the engine. I've got a 178mm ID fire extinguisher that I want to use for the combustor. I would like to use a 153mm OD flame tube, but that only gives me 12.5mm clearance between the combustor and flame tube. Is 12.5mm enough of a gap to allow air to circulate all the way around the flame tube?
Thanks,
James

[racket]

Hi James

I'd be inclined to go a tad smaller on the flametube diameter , the FT I used in my kart jetandturbineowners.proboards.com/attachment/download/67 was big enough to cope with a 3.5"- 89 mm inducered comp , and was only ~ 140 mm dia .

You'll notice that the FT has a long snout where it goes into the turb scroll , this was deliberate so that the air entering the combustor discharged into a plenum surrounding the snout and "funnel" jetandturbineowners.proboards.com/attachment/download/68 this minimised any restriction to entry as the delivery tube also had a diffusing funnel to increase the flow area at the junction of tube and can , its important not to restrict flow at this point as compressor surge can result .

Once the air has discharged into the plenum it can spread out and work its way up the clearance between FT and outer can , if you must use the 153 mm OD tube for the FT , then its imperative that you feed your air into a plenum at the snout/funnel so that it can spread out and enter the half inch clearance space , 12.5 mm clearance would be sufficient , but I'd prefer to see a tad more , something >15 mm .

Cheers
John

[barnyard]

Ok John I will shrink down the flame tube piping I have then to give more clearance to outer wall.

I'm still not sure what turbo I should use. I have a smaller twin scroll turbo, 70mm compressor inducer, 85mm turbine exducer. Compressor is .75 AR. Turbine is 1.6 AR.
The second turbo I have is a single scroll, compressor is 72mm inducer, and turbine is 93mm. I don't know the AR's though. centre section is tagged CAT part number OR-6100, serial number 4mf-047.