PT-6 Annular Combustor

[Ozgrade3]

Most annular combustors have airflow the flows in a linear path through the combustor. A combination of swirlers and air dilution holes forming an insluting jacket of cooling air around the inner surface of the combustor to prevent it melting.

So how does the PT-6 combustor with its side entry of air, form the flame vortex that keeps the hottest gases in the centre of the combustor and the cooling jacket of air attached to the inner surface of the combustor. I have been searching for months and can't come up with a definitive answer.

The photo below is not a PT6 but is the best I could find for illustration.

https://cimg2.ibsrv.net/gimg/pprune.org-vbulletin/971x615/image_0f65f76819d069d3a0faa09f9dc8351ef2a2bd44.png

[magyarflyer]

PT6A has no direct connection with the front part. Air comes from the back snd the hot gas is expelled forward . you can actually start the engine and hold the propeller (if you are brave enough)
Garrett’s are directly connected to the prop

[rigpiggy]

https://foxtrotalpha.jalopnik.com/this-animated-cutaway-of-a-pratt-and-whitney-pt6a-turbo-1751484622 good video

can https://www.picclickimg.com/d/l400/pict/323866631936_/Pratt-Whitney-PT6-Combustion-Liner.jpg
https://cimg1.ibsrv.net/gimg/pprune.org-vbulletin/400x300/pratt_whitney_pt6_combustion_liner_925d416d5e850cbb198076907 c5022b58410e13f.jpg

I used to hold the bosses prop on the PC-6 from underneath........ he would then get out to remove the none existent prop ties, hilarity ensued.

[pattern_is_full]

Here's a theory - the PT6 combustor is externally air-cooled by some bleeding of compressor air.

Stealing some "less-hot" air from the compressor output and blowing it around the outside of the combustor produces a roughly-equivalent internal temperature gradient inside the combustor as would a guided central flame vortex, thus eliminating the need for one.

Suggestive support for that theory:

1) This discussion of the PT6 mentions that "This free turbine compressor is designed to provide the necessary compressed air to the Hot Section of a PT6A engine for use in cooling and fuel–air mixture combustion."

Which raises the question - cooling what exactly? How about the combustor?

PT6A Operating Parameters - (http://blog.covingtonaircraft.com/2015/07/14/pt6a-operating-parameters/)

2) Now let us go to a cutaway photograph of an actual PT6 (not a schematic diagram, which may be subject to over-simplication of the exact relationships of parts):

https://upload.wikimedia.org/wikipedia/commons/0/0e/EBACE_2018%2C_Le_Grand-Saconnex_%28BL7C0410%29_%28cropped%29.jpg

....where we can notice two things. (the image can be clicked to zoom in to a higher magnification)

First, the PT6 combustor is the most external, and largest-diameter, part of the hot section (except for the overall engine casing). It is a fat ring surrounding the turbines, not trapped deeper inside as some schematics may show it.

Second, the combustor appears to sit in a large vented or ported plenum chamber (blue) formed by the casing - which (this is ambiguous from the photo) may be simultaneous the combustion air feed - and a cooling airbath, both supplied from the compressor.

With luck, you will get further input from a real PT6 maven, who can answer your question more definitively (and in the process, confirm or deny my theory).

[DuncanDoenitz]

It works exactly like your "linear" example of gas turbine theory except that, by putting some processes in parallel and reversing the direction of flow, it is shorter.

Imagine, if you will, the digestive tract of a 3-metre python. (I know, but its a McGuffin, stay with me). Lay the python out in a straight line and feed it a rat; it eats, digests, poops. Now let the python adopt a coiled, snakey, posture and feed it another rat. It eats, digests and poops, but its now a one-metre python and will fit more conveniently inside the cowling of a King Air.

Getting back to the PT6, all the components and processes of the linear gas turbine are present. Having passed through a centrifugal compressor the annular gas flow has a conveniently large diameter, large enough to accommodate a turbine within it. Yes, the compressor discharge air flow reverses, but retains its pressure and velocity as it enters the combustion liner through the myriad of cooling holes you can see in Post #3. Combustion gas then reverses flow again, but the annular flow now has a smaller diameter so that it, and the turbines, fit within the combustion section. Same processes, same cooling theory.

Edit; I've re-read the OP and looked again at the diagram; its a gross simplification. The light-blue compressor discharge air does not just flow into the side of the combustion liner as illustrated. It discharges into the complete void surrounding the liner and enters from all directions; inwards, outwards, backwards.

[Ozgrade3]

No one has answered the question so far. watch this video. In typical combustors the fuel and air enter the can flowing in the same direction. The swirlers suround the fuel nozel. In the PT-6 the air enters the air at 90 deg to the fuel nozzel. I can't work out how the air is made to swirl around the fuel.

https://www.youtube.com/watch?v=QPK_eiNng7Q

[Vessbot]

https://cimg3.ibsrv.net/gimg/pprune.org-vbulletin/1506x837/20210522_072238_b19389535f0d0f077a011e0fe65495062f5f8ed1.jpg
The cold air completely surrounds the combustor and flows outside-in, like any other.

[pattern_is_full]

So how does the PT-6 combustor with its side entry of air, form the flame vortex that keeps the hottest gases in the centre of the combustor and the cooling jacket of air attached to the inner surface of the combustor.

This what you actually asked - how does the PT6 keep the combustor-liner surface relatively cool compared to the hottest gasses in the center.

A flame vortex is one way of doing that. It is not the only way.

The PT6 surrounds the combustion chamber liner with colder air (like putting a drink into a ice bucket). Doesn't need a vortex for cooling that surface.

Now, some swirling of the air is important for mixing the fuel and air and getting the most efficent burn. The PT6 combustor-chamber liner does this by putting shallow deflection "cooling rings" just inside the side air intake holes, so that air does not actually enter in a straight line, but is deflected into swirls after it passes through the holes. This is one of the things simplfied schematics will not show correctly in detail.

See diagram on page 16 of this PT6 Training Manual: http://www.mautone.eng.br/apostilas/propulsao1/PT6%20Training%20Manual.pdf

Once combustion has been achieved, the curved duct that reverses the flow forwards to the turbines (and encounters the hottest gas) is ceramic-lined for heat resistance.