Twin Turbocharger-based Turboshaft engine

[marvnero]

Hey guys,

At first I'd like to write a short introduction of myself and how I came across building my own Gasturbine.

I've always been interested in combustion engines of any kind. But as I got more and more familiar with piston engines (build a turbocharged, intercooled EFI scooter) they became kind of an usual thing for me. That was about the time I came in contact with valveless pulsejet engines. After building a "giant" RC-Style valveless pulsejet I tried to start working on a gas turbine. But as I was unable to do any calculations and didn't know enough about thermodynamics o understand exactly what is going on I quit the project.
Now, 5 Years later I'm close to completing my bachelor of engineering. I signed up for a lecture that could maybe be translated to "basics of turbo machines ". That lecture immediatley reignited all my thoughts about building a gas turbine...
with the difference to back in the day that I have a quite good understanding of thermodymanics and at least a basic understanding of gas dynamics and how a compressor and turbine work. Futhermore I learned how to roughly design axial turbines and compressors without the use of numeric simulations. 
And well, it's been 5 years since I wanted to build a gas turbine but I'm more eager than ever to get that turbine running!

Here's my current configuration and what I'm planning to do:
 
The first step is to build a gas generator with only one Turbocharger not modified. This is to develop a compact combustion chamber to be used in a configuration quite similar to Anders configuration.
Later on I'll decide whether I'll pull the turbos apart or use the not as compact design of an external combustion chamber.
After sorting out the inevitable design flaws I'd like to connect the two exhaust streams to propel a power turbine.

Here are some specs of the parts I already got/planning to get:

Already got:
2x Borg Warner KKK K31 Turbochargers (I only found a compressor map of the K29 Charger but I assume that the K31 will only push a little bit more air) 
Automotive fuelpump and PWM regulator
Planning to get:
Automotive gearpump as the oil supply
2x temperature gages to measure the temperature before and after the turbine (to do some calcs around heat transportation and how hot I can run it before turbine wheel meltdown)
pressure gage to measure P2
some kind of rpm measurement system (maybe I'll build my own)

If you have any questions or ideas, let me know!!

Greetings from Germany,
Marvin

PS: Sadly I am unable to upload any pictures of the turbos or compressor map as the upload tool says "unknown error"

[racket]

Hi Marvin

Is your K31 compressor wheel inducer ~71 mm diameter ??

If it is then you'll be designing for a flow of ~1.1 lb/sec - 0.5 Kg/sec................with 2 of them you'll be ending up with roughly the same flow as Anders is getting , so a 4th stage wheel probably required for the freepower .

As they are modern turbos you should be able to get those T I T temperatures up ~900C out of the combustor without any problems.

Cheers
John

[marvnero]

Hi John,

the inducer appears to be 67mm, but I haven't disassembled the compressor housing yet to get an accurate measurement.
As Mark offered me a nicely priced 2nd stage turbine wheel, I think it would be best to go for a single setup first and get some experience before trying to run 2 gas generators in parallel to propel the power turbine.
Thanks for your advice on the T I T temperatures. This will radically increase my turbine's output and efficiency.

[racket]

Hi Marvin

The second stage wheel will be big enough for a "smallish" single turbo based gas producer , I have all the relevant "numbers" for the second stage flow areas etc so should be able to point you in the right direction .

The second stage wheel is 163 mm dia with a flow area ~5.8 sq ins - 3740 sq mms, the NGV area is 5.2 sq ins - 3355 sq mms...............potential power output >50 Kw using your turbo based gas producer .

Cheers
John

[marvnero]

Hi John,

Thanks for supporting my project!
After my current exam period I'll draw some velocity triangles to determine the shape of the stator vanes.
I would be very thankful if you could check whether they make sense at all

As for the combustion chamber I'd like to use an annular one but set a cone at the end. Do you have any concerns why that shouldn't work?
I'd design the combustors Flow Area for a gas stream velocity of Mach 0.15 to 0.2...do you think that'll do?

Cheers,
Marvin

[racket]

Hi Marvin

I'll be happy to go over your "numbers" to check them out :-)

When you say an "annular" combustor , do you mean like Anders and I use for our engines postimg.cc/image/qs6kyispl/ ??

If so , it is possible to use such a flametube fitted to a turbo scroll , it would shorten the length considerably compared to a can type as you can fit multiple fuel injection points , I once designed one up for myself for some reason , its filed away somewhere .

Yep , air speeds need to be low in the primary zone , I general work on a cross sectional area in the flametube of 3 times inducer area for both annular and can type .

Cheers
John

[marvnero]

Hi John,

The combustor on the image is exactly what i mean. As I'm not quite sure whether I keep using an external combustion chamber or "integrate" it later on like you and Anders did, I think it would make sense to design an annular combustor right away instead of building a tubular one first.
It would help me a lot if you could give me some tips concerning the diameter and hole placement for an annular combustor.
The placement on a tubular design is pretty obvious, but I have no clue where I should start with an annular combustor :/

Cheers,
Marvin

[racket]

Hi Marvin

If you haven't already got Thomas Kamps book , may I suggest you get a copy from Traplet Publications gb.trapletshop.com/model-jet-engines-3rd-edition-by-thomas-kamps

This will provide some guidance and will save me a lot of typing :-)

If you confirm your turbo specs , comp inducer and exducer diameters, turb exducer dia , and the scroll A/R which should be cast unto the scroll near the rectangular inlet , I can then work some numbers for the flametube

Cheers
John

[marvnero]

Hi John,

now I know what that "Kamps" is that everybody is talking about
I'll read that first and ask you if there are some questions left unanswered about building the combustor.
In addition to that book I bought one written by Dr. Voss about model gas turbines. Maybe I can gather some additional knowledge and share it with you guys.

I highly appreciate you for offering me to work out some numbers, but as this project for me is not only about building a gas turbine but also doing the necessary maths behind it,
I'd like to know how you calculate it and the physics behind the calculations. I mean I don't have to start with the universal Navier-Stokes equations, but a little bit background about the physics behind the calculations would be great.

Cheers,
Marvin

[racket]

Hi Marvin

LOL............I'll do my "numbers" in Imperial then convert the results to SI units for you .

Another worthwhile read is Cohen and Rogers early editions www.ebay.co.uk/itm/Gas-Turbine-Theory-by-H-Cohen-and-G-F-C-Rogers-H-Cohen-and-G-/201801696172?hash=item2efc5183ac:g:r0kAAOSwNnRYkHmG
or the later ones in SI units www.ebay.co.uk/itm/Gas-Turbine-Theory-H-I-H-Saravanamuttoo-G-F-C-Rogers-H-Cohen-Book-4th-Ed-/172519835908?hash=item282afbc904:g:jvIAAOSwZQRYcloI

Cheers
John

[marvnero]

Hi John,

Please excuse my late reply. I'm currenty quite busy with some exams.
Thanks for your advises on further literature about gas turbines. I'll keep an eye on those and try to get my hands on them if they appear for a reasonable price.
During the past weeks I did some rough calculations with the flow area you recommended.
A drawing of a cut-through with the most important specs is attached. The calculated area already takes the "dead" area of the flow sticks into account.
Do you think it might work?

As for the holes in the primary zone I chose to use 3 rows. The first row with holes of 3mm diameter (16 on the outside, 12 on the inner tube) directly behind the combustors front cover to keep it cool. The other 2 rows contain holes of 4mm diameter ( 12 on the outside, 8 on the inside).
Added to the flow area of the sticks the primary area should flow ~18-20% of the total air mass flow.

How long should an annular combustor like mine be? I calculated the gas flow velocity to be ~25 m/sec, which will be more likely ~40 m/sec in reality.

Cheers,
Marvin

[racket]

Hi Marvin

I'd go 120 mm long , that'll give you a roughly 3:1 length/width ratio

You need to position your holes so that they discharge between evaporators especially on the outer wall , it gets a bit harder on the inner due to limited space between holes , so with 8 evaps you'll be needing 8 or 16 holes , your 12 holes will place them in less than ideal positions .

Cheers
John

[marvnero]

Hi John,

My Kamps book finally arrived and together with your advice on combustion chambers, I created a chamber which should have roughly the same gas speed entering the c-chamber as the Kamps chamber has.
I placed the holes as you wrote and in a pattern in which the holes of the inner wall don't face those on the outer wall. I created some drawings to get your opinion on my design.

Cheers,
Marvin

[racket]

Hi Marvin

The Kamps book is an invaluable start for any build .

I can't seem to be able to see the attached pics :-(

Cheers
John

[marvnero]

Hi John,

Especially the chapter about rotor dynamics is very interesting to me. At the university I was told, that rotor dynamics are very complicated and it would be nearly impossible to calculate them without numeric simulations...seems to be not that hard to me after reading the Kamps book!
I'm sure these calculations are just approximations but I guess they'll do the job good enough.

Hmm...what's wrong with the pics? They should be jpg standard.
I'll try uploading them again.

Edit: the direct links to the pictures are on the next page!

Cheers,
Marvin