Never inject cold fluid into a hot exhaust pipe with hot gas traveling at 500-600mph or 0.7 Mach or higher. It will induce back pressure, acoustic compression and thermal shock in the flow. Normally a jet factory will void the warranty instantly the moment you put smoke fluid in the tube. Try to do that on full scale and you will have a biggest fireball in the sky.

The best way is to preheat the fluid and inject it 1/2 in away from the exhaust pipe's exit.

Mike