You may want to lend some consideration to using one of those cans of airbrush propellant to create the effect. When the can is held upside down, the gas leaving the can creates a vapor that looks similar to smoke or steam. The practical effects guys out of Hollywood have used the technique for decades in filming miniatures for films and television to simulate rocket and jet exhaust, like the "Eagle" from Space:1999. Pretty sure there's a YouTube clip out there which shows the rig one of those builders used to hold four cans of propellant and the surgical tubing they used to route the vapor to the model's exhaust system.