The smoke generators I've seen work similarly to the ones used in the film industry. A very light oil (often mineral oil) is passed through a stainless tube which is connected to a current source. The tube acts as a resistance heater and causes the oil to evaporate. Usually the oil reservoir is pressurized as well to ensure a steady stream of smoke.
A company named corona makes smoke generators specifically for wind tunnels.
The labs at my school drip standard 2-cycle engine oil down heated Nichrome wire. This method gives large sheets of smoke instead of lines. Also, since the smoke is not injected under pressure, the particles don't have their own velocity relative to the flow.
