On my last vacation, I took lots of photos from landing aircraft, and here are two showing the engine of two B737:
What is that pipe on the cone of the exhaust for?
I also saw them on other, larger aircraft. Often, the exhaust looks like a large, black hole where you don't see anything inside. Maybe, those engines have such pipes, too, but you can't see them.
My guess is it's for cooling of the shaft or similar, as the exhaust gases should create a vacuum in the pipe.