First engine complete! 62mm turbo, ~20lbf

[dieselworks]

Hi everyone,

This is my first real post on the forum. I'm currently wrapping up my bachelors in aerospace engineering and finally got around to building a turbojet engine. Here are some pictures and thoughts on the build. 

I used a VSRacing dual ball bearing turbo. The compressor is 62x76mm and the turbine is 84x62mm. In retrospect, this wasn't the best choice, but it does work okay. 

The engine makes around 20-22 lbf of thrust at 1300 deg K turbine inlet temperature at around 28-30 psig chamber pressure. I have pushed it really hard for short periods of time making around 32 lbf at 1550 deg K and 40 psig. That temperature will undoubtedly cause the inconel wheel to creep pretty quickly, so just a short term test. It did make the cast iron housing glow, which was fun to look at. Due to the low friction ball bearings, the engine will run at only 2-3 psig and 1000-1100 deg K making for a really mild idle. 

I would be curious to hear the thoughts of some of the experienced people on these numbers. My impression is that this is really hot for a given pressure and thrust level. I suspect this is due to the compressor wheel being too large for the turbine. Could the smaller than ideal turbine be causing extra flow resistance? Could the inlet to the turbine be choked? Maybe the smaller turbine doesn't generate enough power so the system needs additional enthalpy rise in the combustor for steady state operation? Am I on the right track to think that having a larger turbine relative to the compressor would reduce temps for a given pressure and thrust level?

I am using glow plugs for ignition which has been working quite well. It lit kerosene the second I cracked open the fuel shutoff valve for the first time, which was quite exciting. I have since switched to using 100% diesel which works great. (hence dieselworks) Kerosene is pricey and inconvenient to source in Alabama. 

I am using an atomizing spray nozzle rated at 5 gph at 100 psi (probably water, but the flow rate isn't dramatically different with kerosene or diesel). It has a 90 deg full cone spray pattern. I calculated I would need 14.7 gph (based on 1.6 lbm/s of air at 1/60 fuel to air ratio). However, even with the 5 gph nozzle I am able to reach 1400 deg K, which is hotter than I calculated I would get with the 15 gph nozzle. This supports my hypothesis that the flow is choked, or at least dramatically less than the advertised 1.6 lbm/s. I would expect a little exaggeration, but its off by a lottt....

I thought about getting a new journal bearing turbo with a 75x105mm compressor and 96x88mm turbine. I'm not sure if this would work well, as the flame tube I used was already a touch small at 4.88 inches ID. JetSpecs recommended 5.15 inches ID for the 62mm turbo and 5.90 inches ID for the 75mm. That's 46% more area than my current flametube. As such, it may not be worth messing with further. I am kinda leaning towards building something bigger and more optimized. 

Anyways, I think it turned out pretty good everything considered. It was my first build and I had a lot of fun. Needless to say, I am hooked. 


Throttling demo:
www.youtube.com/watch?v=jn1Ly-1R7EY


Another test, with view into the chamber:
www.youtube.com/shorts/JzS62zqHVCA


The build:



Big flames on the first test run:



Glowing housing, after running at 2330 deg F for a bit... 



I'm excited to hear yall's thoughts and be a part of this cool people club. 

Best,
Brandon 

[racket]

Hi Brandon

Nice build :-)

Yep , theres a few inconsistencies , a 62mm inducer will probably flow ~50 lbs/min - 0.83 lbs/sec , at the higher efficiency island on the map that we need to run at , so that will cut your fuel usage back :-)

Then theres the turb scroll A/R , if its a tight one then your flow will be restricted even further , you'd be needing a scroll A/R that at least 1.0+ rather than 0.6 .with that relatively small exducer size

Thirdly the jet nozzle sizing might also be contributing to your high temps if its a tad undersized , though from the pics it looks a fair size .

Your flametube cross sectional area is ~4 times inducer area so it'd be OK , 3 times is plenty if decent fuel presentation .

Your temps were certainly high :-( ................far too high , you were lucky something didn't decide to exit the engine ...............though from the colour of the scroll housing and jet nozzle I doubt you were at 2330F - 1276C- 1550K , it would be glowing yellow not red , so you may need to check your thermometer

Have you done any running without the jetnozzle to see if temps moderate ??

Yes , something bigger :-)

Cheers
John

[dieselworks]

Hi John,

The turbine housing is 1.01 A/R. I have tried using running it without the exhaust nozzle. The temperatures weren't very different but it made quite a bit less pressure and thrust. I only ran the engine for about 20-30 seconds at 1550 K. So the housing was just starting to glow at that point. I have run it for quite some time at 1300 K, but that's still much too hot. I would like around 1000-1100 K for steady operation at 3 to 3.5 pressure ratio...

Do you think I could bolt something like this on with decent luck? I made my engine quite modular, so it wouldn't be too difficult to fit this.

75x105mm compressor and 96x88mm. It's got a 1.32 A/R twin scroll T6 housing. (Twin scroll is all I can find for cheap) That would put the flame tube area at 2.7x the inducer area.

www.ebay.com/itm/144622281508?_skw=75mm+turbo&itmmeta=01J9ZMT1KW2Q9WMM45GB34CVHB&hash=item21ac28e324:g:PxIAAOSwYOdkXKcd&itmprp=enc%3AAQAJAAAAwHoV3kP08IDx%2BKZ9MfhVJKn0Sf5uxPwQN8FQVTCDnLH96AmTl9PCiTZfr9s2vVsxCIubZWh19LcHEcFhl3SCLGEe%2FPMHY9F6S%2B0EWQHQ4wTqhMRs%2BnMgv2JTfIRx1sz8a6%2FWoK2SEjQlYWikLtTRzNPN8KIcloy4SXd3f4HI%2FgDTVM7MQPnj9M9ivgqhanMxq6ucp3fHXxTDzdNk0wtzhTTqmsIGaiVyj6bLTaXRZDg7f6LvU8m4ffn1PLZGvtA8lQ%3D%3D%7Ctkp%3ABlBMUIKa6PTPZA

-Brandon

[racket]

Hi Brandon

At 2.7 inducers it should be OK if attention is paid to the fueling , I ran a 2.5 inducers on my 2 shaft kart .

That turbo in the link would be suitable , plenty of extra exducer area compared to comp inducer , 37% bigger and with a 1.32 A/R scroll it shouldn't be choking up :-)

Cheers
John

[richardm]

Great set up ! My first reaction was: Ouch way too hot!Actually hotter than what most real jet engine can tolerate...But I think Racket has the best explanation and advice you can get. But just curious about your flame tube..Is it providing enough tertiary ( cooling) air ?

I would only add one thing: From what I see on your pictures , the thermocouple probe is upstream the turbine. That could lead to a catastrophic failure of your turbine wheel if the probe breaks off specially at those high temps ..

You should place it downstream in the tail pipe. Your temp reading will be lower course but you can calculate the turbine inlet temp from the energy extracted to run the compressor. From that you'll get your temp limit

Expect a 100- 150 ° temp drop I dont have the equation on hand but someone on this forum probably does.



Cheers ! Richard.

[dieselworks]

Hi John,



Thank you for the good information. I may get the ebay turbo and compare the performance and characteristics. After all, I still have a lot of learning to do on this subject...



Hi Richard,



I think my flame tube is suitable, or at least not far off. The metal shows discoloration up to the point where the cooling air enters. It's got the flame tube originally had 30x 0.25 inch primary holes, 10x 0.375 inch secondary holes and 10x 0.625 inch tertiary holes. I added more primary holes with the addition of a diverging duct flame holder where the fuel is injected. I don't think the overall flow through the primary soon has been increased much, but the flame is stable and can be throttled up or down quite rapidly now. The flame sits nicely in the diverging duct flameholder near the end of the flameholder. 


Here is the flame tube:



I had to add this diverging duct flameholder near the fuel injector to have steady flames:



It fits inside the flame tube



Thanks for your thoughts. 

Best,
Brandon 

[racket]

Hi Brandon

Flametube :-( too many holes , the total area of holes need to be the same as the inducer area, and if your turb scroll and wheel size are limiting mass flow it will only exacerbate the FT hole area issue

Plus your air delivery tube is creating swirl around the flametube , this can create issues with combustion as well as how the air enters the FT wall holes .

One other thought , your flametube outlet , is there a "funnel" to guide the gases from the large FT cross section to the smaller elbow are ??

Yep , take Richards advice and move the thermocouple to downstream of the wheel , if you measure your compressor temp rise it'll be a bit more than the drop through the turbine , for a safe reading simply add on your T2 rise to the TOT .

Another safety issue , your lube plumbing ................metal tubing for the pressure supply near the hot turb scroll , the radiant heat will degrade anything else and a burst hose spraying oil onto a red hot scroll won't be nice .

Cheers
John

[slittlewing]

Nice work. I particularly like the “snail” feed to the combustor at least from a fabrication standpoint as looks challenging to produce!!


For the oil feed and return make sure your tank is below engine level and the return is close to vertical, and yeah stainless pipes or long banjos would be better to get tubing further from the turbine heat.

Using AN lines is better for heat resistance as they are stainless overbraided

Cheers

Scott

[dieselworks]

Hi John, Scott,


Thanks for the information on flame tube design. I had some real trouble getting flames to be steady without "flameholders" inside the burner tube. Those aren't idea as I can tell they have limited lifespan and would be catastrophic if broken off somehow.


I did go to some lengths to make a smooth transition into the turbo. 



I made a form out of some steel:



Formed some sheet on the lathe with this homemade ball bearing thing:



I made this scroll thing on the milling machine:



Then welded it onto the outer wall:



It's rather ironic that I went to a lot of extra effort to make my combustion chamber spin the air more. I assumed this would be better, not worse! At least it looks nice I guess...

I really appreciate the advice given on this thread. Given that it would take a pretty serious overall of this engine to reach its relatively modest maximum potential, I think I will let it live out its retirement years in peace. I have been contemplating building something bigger and better using lessons learned. I will start a new thread with that subject. 

Best,
Brandon

[racket]

Hi Brandon

Thanks for the pics , that is a nice "scroll" you made for the air delivery :-)

Imagine your air delivery scroll as a turbine scroll , as the air spins around and attempts to travel to a smaller radius within the flametube , the velocity needs to increase and the pressure to drop, this then creates a vortex within the flametube that can result in fuel travelling down the centre of the flametube and not encountering the air correctly .

If you want swirl within the flametube its better to add a swirler surrounding the fuel nozzle imgbb.com/0KpBfWf ...... ibb.co/zNcnSHh and feed your delivery air to a simple "funnel" at the Tertiary end of the combustor ibb.co/kKVD4rk where it can spread out and move axially along the flametube to the wall holes , there will be less losses and a better outcome , simply copy what the large aero engines do :-)

Cheers
John

[slittlewing]

Hi Brandon

Top marks for effort haha.

I made a "simple" swirler after suggestion (copying) from Jetric, in the post below jetandturbineowners.proboards.com/post/31139/thread

Its easier to produce than the vane type, and also cools the fuel nozzle itself. Could be adapted onto your FT quite easily if you needed/fancied.

Cheers

Scott

[cebaero]

Hello,

I am only asking because I am very close to attaching the compressor tubing to my combustor so I want to make sure I do it correctly, but I was under the impression that the compressor intake should enter around the primary holes? It looks like in this build it is entering near the tertiary. Also, everyone is saying that his snail feed is causing too much vortex, but I was also under the impression that was a good thing. Is it just that he did it too much? Or should the intake always hit the combustor straight on?

Beautiful build!

[richardm]

Here are some pictures of my swirl vane and the "head" of the flame tube 

[dieselworks]

Hi everyone,

That makes perfect sense. I can see how extra velocity would add losses. I will probably incorporate a swirler in one of the styles shown here in my next engine and use a different method to supply air to the combustion chamber in a less lossy way.

Working through that initial design currently.

Best,
Brandon

[racket]

Hi Cebaero

The delivery tube must NEVER be attached to the Primary Zone area, it invariably causes problems as the incoming air blows the flame to the opposite side.

I've been trying to counter this design perception for decades but without success , guys look on You Tube and see dozens of engines with it attached at the Primary end and think thats how it should be, unfortunately it ain't :-(

The engine will probably work, but its performance and life span will be less than ideal .

Cheers
John