Steve, testing on the system showed a flow-rate of about 0.2 gallons per minute = 5 g consumed in 25 minutes. I used nylaflow 1/4" tubing - pinch-melted the tube end and then drilled 2 tiny holes on each side of the tube such that they spray UP at a 45 degree angle.  The spray point is about 12" ahead of the radiator inside the diffuser, so at 80+ mph the water is splashed against the plenum walls and mixes with the incoming air for atomization.  The pump came from Amazon, is designed to work in mobile home water systems, is self-priming and runs off +12V dc; I believe the hose-barb connections are 3/8".  Similar to this : https://www.amazon.ca/Keenso-Pressure-Diaphragm-Priming-Sprayer/dp/B07FL49495/ref=asc_df_B07FL49495/?

Thanks very much Jeff, most helpful.
Do you have a volume per minute figure of water you sprayed to get that cooling?
Was your spray pattern very fine as far as atomisation?

Steve