Plenum? - Does it work both ways?

[2seater]

Hi, I'm new here, and I wish I had found this earlier. My intention is to build a pitot style bench and my question is this: I can see the reason for a plenum below the actual flow test opening in the bench but what effect does this have in the reverse, or blow, direction? My intended design will have an approx. 30" vertical drop of 4" pvc pipe directly below the test opening where it will turn and go through a wall where it will turn again and run vertically about 72" to the actual vacuum/pressure box mounted high on the wall. The sensors will be located in the 6 ft. length of pipe. I guess this brings up a second question; is a plenum needed if the test opening in the bench is always smaller than the pipe below? Any thoughts appreciated.

[bruce]

The larger dia for the plenum allows you to take a more stable static pressure reading from under your test peice. Try and keep the static pickup as close to the top of your plenum so its not effected by any velocity/impact pressure.

I have a 4" hole and this opens up into a 6" plenum that is approx 18" long (without me running downstairs to actually measure it) When I had a 4" dia plenum I could see pulsations on my static manometer, now that I went with a 6" they have all but gone away.

I do not reverse flow in my bench, I draw my flow out of the exhaust port and into my bench.

What are you planning on using for your flow sensors?

[2seater]

I plan to use the annubars I just ordered for the flow readings. The reason I want to flow both directions is for intake manifold testing and I will need full capacity in both directions. If pulsing is a problem, would static pressure taken from multiple points around the perimeter possibly help?
Hal

[ozrace]

2 seater
The plenum is there to provide volume to allow the air to settle and dampen pulsations. If you run straight out of a pipe, you can get quite a strong swirl pattern happening which will affect readings at the sensor. This is nearly always a problem with round passages, even on round ports, and even if you allow sufficient pipe length before the sensor.
If the air is swirling past the sensor, you may find that you can flow a head, and then turn the head 90 or 180 degrees on the bench and get a different reading.
If you use flow straighteners before the sensor, you won't have the problem. Then all you need to worry about is having enough volume to reduce pulsing.

[2seater]

The flow sensors shouldn't be a problem as they will be located several feet from the actual test port. My concern was actually introducing turbulence with large change in size in the pipe when flowing backwards out through the test hole, but I guess if I would have thought about it, even the Superflow type benches have a plenum below the test port and they will flow both ways. Thanks for the input, a plenum will be part of the build.