Injecting prior to turbo comp' impellers

[hotrod]

I agree with most of the preceeding statements. My intention was to have the drip step well back from the turbo inlet. I would be very reluctant to place it right up against the compressor impeller as it could create all sorts of strange effects on the compressor map, moving the surge line around and so on.

This is the mental image I formed. The injection nozzle is placed .5 -.8 meters back from the compressor inlet. Some reasonable distance down stream from that, you flare out the inlet pipe some. This slows airflow speed and encourages any large dropplets to drop out of the flow. After a short distance you began to taper back in, creating a venturi. Liquid water would tend to collect at the point the intake tract begans to converge again. The liquid would be forced to move "uphill" on the converging section. If air speed is low enough at that point it would be impossible for the liquid dropplets to run on the surface. If you place a small lip near the end of this converging section, you would stop any dropplets that try to climb the "ramp" and as they get pushed up over the lip they should get ripped into a fine mist again by the high speed airflow at the throat of the converging section.

A secondary possibility would be to place a small drain at the bottom of the wide point to allow any liquid water to drain away. This may require some form of active extraction, or simply a gravity drain with a vacuum check valve would suffice. For active extraction you could just plumb a simple windshield washer pump with its suction to this low point and its outlet back to the water reservoir or suction side of the WI pump.

For reference, Oakos Automotive on their web site ( http://www.oakos.com/wrx/installs/AEMCAI/index.htm ) did a measured air pressure drop test on a WRX. The stock intake created a 26" of water pressure drop at max flow, the high flow AEM intake reduced that to about 12" of water pressure drop. Any drain system would need to have a check valve so it did not create a vacuum leak post MAF due to the intake pressure drop but allowed the collected liquid to drain away.

My current setup (old version) had the spray injected from the outside of a horizontal 90 degree elbow, with the spray at right angles to the air flow. When I pulled the turbo out that I photographed earlier, I could see clear evidence of liquid water flow on the inside of the intake tract. There were slight but unmistakable markings left by the evaporating water as it ran on the surface. The water mark basically went downward from the jet turning down stream and then followed the bottom center of the intake tract.

In considering the physics of what was happening I remembered some old info I had picked up years ago about water flow in a stream as it enters a turn. The fast moving flow moves to the outside bank and then due to drag with the outside river bank the current turns and moves down toward the river bottom then comes back upward on the inside of the turn toward the surface. In the case of air flow in a duct you would have this same hydraulic flow in the full 3 dimensions of the tube, with the air scrubbing hard against the outside center of the duct at the point of the turn then splitting and 1/2 going up and 1/2 going down (on a horizontal bend) and curling back toward the inside of the bend a few diameters down stream. This sort of flow would carry most of the water dropplets down the bottom center of the duct. In the faint water marks on the inside of my duct I could see exactly this same type of movement. I suspect a large fraction of the liquid drop out occured during the turn on and turn off period of the spray event when atomization is not good, not to mention some after drip that I imagine every nozzle would have.

The most simple solution if you have the room would be to create a P trap intake like on a sink drain where the injection occurs on the downward section of the P and then any liquid water would collect at the bottom as it could not climb the vertical leg of the trap. Air flow would likely quickly evaporate any water collected but it would be a bad thing if a large quantity of liquid were to collect there allowing a water slug to be pulled up the intake during a hard "yump" or other violent change in direction.

I also agree that Pre-compressor injection is best reserved for use only during high airflow situations where the benefits it provides would be best used. My 10 psi turn on was too low and as mentioned above I will move that turn on point up toward peak boost where I need to maximise the compressor effeciency.

FWIW

Larry