Building a Flow bench

[Greg Hermann]

At 1:45 PM 2/1/02, steve ravet wrote:
>Greg Hermann wrote:
>> If you want to do "off-standard" testing, fine. I was just outlining what
>> would be needed to do _repeatable_ work, on standard.
>
>One question from someone who obviously isn't an ME:
>
>Measuring flow at a lower pressure ratio may not be comparable to
>published numbers but is it useful for relative comparisons?  When
>porting or whatever could you do something that would improve your flow
>on the bench but be ineffective or actually worse on the car?

It's OK, up to a point. I don't KNOW for sure, but I suspect that the two
standard numbers (25 and 28 inches of H2O) evolved because somewhere,
sometime somebody figured out that that is about how much pressure drop
there is across a typical intake port under maximum HP flow conditions.

Why would it be important?? Because comparable pressure drop to operating
conditions would mean testing at comparable velocities. Scaling back the
pressure drop (and therefore the velocity) for testing would be sort of OK,
to a point--BUT--when things like flow separation start to raise their
head, low velocity testing wouldn't correlate very well at all to high
velocity testing.

And--flow separation downstream of the short side radius in an intake port
IS a _BIG_ deal in the real world!

As for testing exhaust ports on a flow bench--it can help marginally, but
unless you're in a position to test the exhaust ports with choked (sonic)
flow at or just downstream of the valve curtain area, at low valve lifts,
you're really NOT going to be looking at the most significant flow regime
which occurs in an exhaust port (during blowdown) ! (HI Dave!!)

Would REALLY be fascinating to see what would happen if somebody were to
figure out a way to work a convergent--divergent nozzle into an exhaust
port--so that the first pulse of flow down a header tube would be
trans-sonic !!! _THAT_ would make for some serious scavenging pulses !

Greg
>
>--steve
>
>--
>Steve Ravet
>steve.ravet at arm.com
>ARM,Inc.
>www.arm.com
>
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