Senior Design Small Jet Engine Project

[thecolbster]

Howdy y'all,

My name is Colby, I'm from Texas studying my last year in Mechanical Engineering at Texas Tech University where for the past semester I've been part of a year long senior design group tasked with designing, manufacturing and testing a small jet engine.  We've had some struggles on the way, we thought we were gonna use kerosene but this past week we couldn't get nozzles to work right so we decided to just use a pure propane system. Our design consists of a bought turbo compressor wheel from a Garrett GTX 4508R turbocharger with an inducer of 80mm and an exducer of 108mm. We plan to machine everything else besides the bearings. we're using JetSpecs as our primary calculator on our combustor design and we think we got good numbers, especially comparing our data to here. We just sketched up the layout of our engine a couple days ago since we got our compressor wheel in the mail, and we're CADing it right now.

I've uploaded the rough sketch of the design below, I apologize for the crap handwriting.

I'd like to ask about fuel injection, since we're using propane, our plan is to machine a ring with "spurs" sticking out of it where the propane will flow out of. This ring will be at the very front of the combustion chamber with the "spurs" in line with the length of the chamber.

I've read on here that propane injection in line with the air flow is bad. Should our injection design be changed?

Thank y'all




Dso: diameter of the outer part of the driveshaft
Dsc: diameter of the center part of the driveshaft in the combustion chamber
Dol: diameter of the outer liner of the combustion chamber
Dil: diameter of the inner liner of the combustion chamber
Dh: diameter of the housing
Dt: diameter of the turbine
Dn: diameter of the nozzle
L: length of the combustion chamber

[racket]

Hi Colby

May I suggest you guys also join the GTBA www.gtba.co.uk/ , it has the best "library" of information for anyone wanting to do what you intend , its been around since the very beginning of RC micro turbines , it was the Group whose Members did all the development work that latter was commercialised into what we have today .

Theres some very experienced engineers on the Site , smart guys :-)

Cheers
John

[thecolbster]

Perfect thank you so much Racket!

[thecolbster]

Nov 6, 2022 15:22:17 GMT -5 racket said:

Hi Colby

May I suggest you guys also join the GTBA www.gtba.co.uk/ , it has the best "library" of information for anyone wanting to do what you intend , its been around since the very beginning of RC micro turbines , it was the Group whose Members did all the development work that latter was commercialised into what we have today .

Theres some very experienced engineers on the Site , smart guys :-)

Cheers
John

We have a question sir, according to JetSpecs our combustion chamber length should be near 19in. There is debate that we should shorten the length of our chamber, do you see any issues with shortening the combustion chamber length?

Thank you,
Colby R.

[racket]

Hi Colby

Jetspecs is designed for "sore thumb" type combustors on a turbo based engine, we need a certain "length/width" ratio of ~3:1 , an annular combustor as you are intending still requires the same ratio , but because the annular ring is much smaller across the width , its length also becomes smaller .

The length of my flametube is smaller than the diameter of the compressor inducer diameter imgbb.com/HzBXn2H , but the flametubes cross sectional area is still 3 inducer areas , same as Jetspecs , they can become shorter but they still need the cross sectional area in the Primary Zone to allow combustion , with a too small a cross sectional area the airspeeds get too high and combustion becomes unstable .

Cheers
John

[thecolbster]

Nov 8, 2022 23:07:16 GMT -5 racket said:

Hi Colby

Jetspecs is designed for "sore thumb" type combustors on a turbo based engine, we need a certain "length/width" ratio of ~3:1 , an annular combustor as you are intending still requires the same ratio , but because the annular ring is much smaller across the width , its length also becomes smaller .

The length of my flametube is smaller than the diameter of the compressor inducer diameter imgbb.com/HzBXn2H , but the flametubes cross sectional area is still 3 inducer areas , same as Jetspecs , they can become shorter but they still need the cross sectional area in the Primary Zone to allow combustion , with a too small a cross sectional area the airspeeds get too high and combustion becomes unstable .

Cheers
John

Ah ok, I think I understand now but I wanna make sure. So our inducer diameter is 3.14in. the area of that comes out to 7.74371 in^2.

To get the flametube cross sectional area we do 3(7.74371)=23.231in^2.

Now to solve to the outer flametube diameter from that area we got approximately 5.5in.

We assume that the inner flametube diameter is half of the outer diameter which gives us 2.75in. So the annular diameter will become 2.75in too.

With a length/width ratio of 3:1 if i understand, then the length of our combustion chamber needs to be 3(2.75) which equals 8.25in
Does this seem like a correct calculation? Thank you for your time also.

[racket]

Yep , your cross sectional area is OK but your FT outer diameter needs to be larger.

with a 2.75" inner wall dia =5.94 sq ins , add on your 3 inducers of 23.23 sq ins = 29.17 = 6.094 " ID

6.094 minus 2.75 =3.344 div by 2 = 1.672" wide annulus , 3 times 1.672 = 5.016 inch long .

When you draw up your engine , the diameter of your shaft tunnel will dictate the FT inner wall diameter , the 2.75" might be too small as you need to allow space between shaft tunnel and FT for the airflow to the inner wall holes , generally the pinch point is at the front inner corner where the air needs to turn from radially ineards to axial , you need at least 1.5 times the area of the holes downstream .

Cheers
John

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