What ensures clockwise direction of gas flow in this setup?
Just the way a little cup is installed in the duct.
Photo Credit: nifs (e-bay)
You'll notice in the picture above that there is a hole formed into the shroud, welded onto which is a pipe leading away from the shroud. Above the hole is a cup.
In the left exhaust shroud (pictured), this cup directs some of the exhaust gases into the hole, then through the pipe.
In the right shroud, this cup could be reversed - that is, the open side would face away from you. The right shroud has, if you will, two sources of "exhaust" - one from the engine and one from the heated air intake.