When the air makes a right angle turn towards the combustor ;)
There's still pressure after the turbine so might as well add some air (um, I've never heard of an engine being ran extra lean when afterburner is added, but it would have to be, no?) and fuel, plus some extra exhaust nozzle to make use of the burning, expanding gas and, um there you have it. (I'm too tired to correct that paragraph gramatically.)
Probably something like putting your thumb over the end of the garden hose.
But you *are* moving thousands of pounds of fuel from zero (relative the engine) to several mach, aft-ward. That makes for a nice reaction force. Same goes for *any* other jet engine, or fluid mover (propeller in air or water) for that matter: the net effect is the fluid medium being thrown backwards with respect to what's throwing it. Note that drag (parasite drag, induced drag, drag of a turbine to spin the compressor, etc.) displaces this air foreward, or at least less aftward than the thrust. So thrust has to be that much more to counter it.
Makes sense because for a given source of limited pressure and flow rate, there is an ideal nozzle dimension which produces a maximum velocity output (without compromising flow rate by restricting, nor pressure by being too open). If you had some detailed spec's on the engine's output behavior, I bet it'd be pretty easy to find with some calculus. (What can I say, I'm in a calc. class, everything's starting to look like a derivative, erm, slope now...)
I would bet that the places of main thrust production are those most highly pressurized: the compressor, because it's producing the pressure in the first place; the combustor, because pressure again increases here; the exhaust nozzle because the air is able to do more physical work before exiting the engine. In each place there are surfaces whose normals are pointed in the general direction of thrust, although some mildly. Obviously these won't contribute much thrust, instead having to simply retain their internal pressure. (Take a piece of pipe for instance: blow air through it -- its walls are parallel to the flow direction so despite the pressure inside it, what net force, if any, is acting on the pipe? However, if you curved the pipe around 180° so it points back at the source, the back side will be contributing a net outward force. You might also be able to argue that the inside of the bend is contributing "negative" force (um... double negative kind of "negative") if the conditions are able to reduce its pressure below outside pressure.)
That was too long. I'm going to sleep. lol
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