Turbine air start -- too cold?

Colibri wrote:
Sean Trost wrote:

I stand corrected. Thanks Jim. Learned somthing new everyday.
Sean

Jim Carriere wrote:

Sean Trost wrote:

Juan
I believe that N1 is related to the velocity of the airstream
introduced. Which is good. But not related to the ignition problem
Now You have said that you have an excellent spark and good fuel.
the only other thing you need for start is the right airflow. or
volumn. Dont know about your engine specifically but most everything
else runs right around 14 to 1 on the ratio wise.



Actually, jets are a different beast from piston engines. Between
about 25:1 and 50:1 is normal, even for lightoff fuel flow.
Stoichiometrically correct combustion is too hot.


50:1 would be the total airflow:fuel ratio through the engine core. Of
those 50 parts air, 15 are primary air that mixes with the fuel for
stoichiometrically correct combustion.

The other 35 parts are secondary air (not to be confused with a
turbofan's bypass air, which doesn't pass through the core). Some
secondary air passes around the exterior of the combustion chamber liner
to cool it, and some passes through holes into the chamber to shape the
flame and hold it away from the liner wall to prevent damage.

This is a better explanation than I gave, but I'll elaborate further
to this that while technically only 15 of those parts of air are
"burned" (use to support combustion), and the secondary air begins
mixing with the flame very close to where the fuel nozzle(s) are
located. Depending on the design of the engine, some or even all the
secondary air flowing around the combustor liner mixes with the
primary air/flame before entering the turbine.

Complex engines will route some of the secondary air through tiny
cooling passages in the turbine blades themselves (I'm pretty sure
you, Colibri, understand this, I'm speaking to the group and not
trying to insult you), but I strongly doubt this is the case with
Juan's engine.

None of this is impingement starting air, which hits the turbine blades
and then goes right out the exhaust duct.

This is an important distinction, so is the difference between
secondary and bypass.