Using a scaled down J79 combustor for my DIY jet engine

[totalsnafu]

Hey everyone, a question on the black art of combustors. I am about 70% done making what is basically a scaled down version of a J79 combustor for use in my turbocharger jet engine. I had seen that older radial flow jet engines use combustor cans rather than annular cans, and since that is simpler, that's what I decided to do. The standard DIY jet engine diagrams show the combustor as some sort of glorified space heater; just a cylinder with the inlet air coming in just about anywhere and swirling around the liner in an uncontrolled manner. No way that is stable or efficient, I think to myself. Well, jet heads....am I right? Will a combustor with an axial flow be more efficient, more stable, make more power than your standard flame can from a Joe Schmoe turbocharger jet engjne? Or have I been wasting my time?

I have the outer and inner liner, swirler vanes, fuel spray nozzle, igniter, convergent inlet and divergent outlet etc. So, will it be worth it?

Thanks!

[racket]

Hi

If you reduce the losses in the combustor you'll have a bit more pressure for making thrust , but don't expect a huge amount of difference , maybe 5-10% at most over a reasonably constructed "simple" combustor

A lot of commercial APUs have "sore thumb" combustors with good power/efficiencies , power output has more to do with the temperature of the gases going into the turbine stage, and those are limited by considerations of the engine surviving.

I used a flametube with swirler surrounding the fuel nozzle in my first turbine bike and it worked OK , getting the best power from our engines is all about reducing losses as both rpm and temperatures are fixed at safe levels for the sake of engine integrity................increase either or both and the engine can/will come apart, stick to below 1450 ft/sec tip speed on the comp and 1450 F going into the turbine stage and the turbo should be safe .............go beyond those at your own risk .

Cheers
John

[finiteparts]

Hi "totalsnafu",

I am not totally sure what you are asking. The J-79 combustor is probably not the best thing to base your design on, since it is a very old design that uses flow blockage as the means for flame stabilization. See the image below:




As you can see, there is no primary swirler, only controlled inlet air that forms a recirculation zone in the primary zone domed headend region...there are a bunch of cooling slots that allow air to cool the liner walls, but the bulk primary air fuel ratio is set by the primary holes that are farther back. The basic idea is shown schematically below...



The is an old technique refered to as the "magic circle" which places the primary holes at the tangent to a circle formed in the space between the centerline of the combustor and the head end dome, like what is sketched. It works well for simple combustors.

I am not sure what you are looking at that makes you think that anyone here is suggesting that air can come in anywhere and just swirl around, but that is not the case...it is pretty well established how the air partitioning should be. And making a statements on the performance of a combustor based on a generalization of axial flow entry would not be the right thing to do. Show us a picture of your combustor and maybe we can give you more informed feedback.

Thanks,

Chris

[finiteparts]

I agree with John...don't expect to get a big difference.

The design goal is usually to take the majority of the pressure drop in the combustor at the liner. You want to have the pressure drop across the liner used to really push the air into the combustor through all the well placed air holes and thus get good mixing of the air and fuel mixtures, even in the center core region that is the farthest from the liner wall. If you loose pressure because of losses upstream of the liner, then you are wasting the energy that you used in the compressor to create that pressure rise.

Pressure loss in the combustor is a direct reduction on you cycle's ability to do work, so we want to minimize this...but, you need a certain pressure drop to make sure that you get adequate mixing and thus decent combustion efficiency. The numbers usually thrown around are between 4-10% are required (not including the hot loss), with a common starting point for calculations around 5% pressure loss. So I would say that your efforts to minimize the losses should help even if it is a small amount.

~ Chris

[racket]

Hi

How did you determine the total hole area going thru the flametube wall ??

How did you determine the crossectional area of the flametube??

The J79 ran a 13:1 Pressure Ratio , you'll be running around a quarter of that so your combustion intensity will be roughly 4 times higher , this will require a bit of design work to get the scaling right .

Cheers
John