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 -
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/wikiped...cropped%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).