A number of people seem to include CO2 snuffing for vents but I've not run across any design standards. Perhaps others have.
The end result you're after is that you do not want to support a flame on the end of the vent.
Physically and mechanically you need to avoid details which will help to hold a flame, like recirculation zones and bluff bodies. A clean deburred discharge would be preferred.
You can choose to extinguish an unwanted flame in a number of different ways with CO2.
- chemically, you need to add enough inert to prevent flame propagation. As a rough estimate of how much inert you need for this, calculate the gas air ratio of the lean limit mixture and replace the air with CO2.
- flame destabilization, flames become less stable as they become thermally leaner, so using the dilution techniqe above sometimes works even if you don't have enough gas to be technically enough, simply because a flame doesn't sustain as easily.
- starvation, adding the inert as a "plug" of CO2 in the vent flow so that it cuts off the flame's fuel source when the plug gets to the tip. This needs a high pressure, rapid discharge flow of inert and is the technique used by some commercial vendors, like fire suppression system vendors. you could look at www.fenwalfire.com/products/index.htm
- mechanically, adding the inert as turbulent flow from a ring around the discharge is also used because it partially cuts off the flow of support air and also the inert velocity enhances the discharge velocity and helps in flame destabilization.
In practice, I suspect that satisfactory home made solutions tend to be a serendipitous amalgam of all of the above techniques.