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Flow bench

To: shop-talk@autox.team.net
Subject: Flow bench
From: Veeduber@aol.com
Date: Sat, 13 Jan 1996 18:35:56 -0500
Some years ago I assembled engines for light aircraft using Volkswagen
components.  In order to improve the castings I made for the intake manifold
I constructed a primitive flow bench that proved very useful.  I say
primitive because it did not provide specific numbers with regard to cubic
feet per minute, but reflected relative values, above or below a base-line.

I used a squirrel-cage type blower about 3 feet in diameter and about a eight
inches wide.  It was belt-driven by a one horsepower electric motor.  The
blower was mounted outside of the shop for a number of reasons, one being the
noise, another being the possibility of being blown out the door.  The blower
was used simply because it was there.  I believe it had once been part of a
commercial air conditioning system. 

Air flowed through a plenum constructed on the underside of a work bench, to
the blower via large diameter -- ten inches or so -- plastic pipe to another
plenum to which the blower was attached.  I installed a simple 'valve' on the
lower plenum chamber, a sliding panel cut at a sixty degree angle that
allowed me to partially block the inlet to the fan.

In use, plates of aluminum or steel were made to fit the top of the plenum on
the workbench with an opening suitable for the part being tested.  To measure
air-flow I constructed an awkwardly long tube of clear plastic filled with
colored water.  The tube was mounted on  the wall behind the bench, ascending
the wall.  To use the thing, a wand of finer gauge tubing was attached to the
upper end of the tube and inserted into the object being tested.  The drop in
pressure inside the tube as a result of the air flowing past would cause a
change in the level of the colored water.  Initially, the gauge tube was
nearly vertical.  By the time I'd worked out all of the bugs, it was nearly
horizontal and of smaller diameter than my first effort.  With the tube
vertical the changes were often too small to be read.  With the tube laid out
at an angle, the changes could span a number of inches, making it easy to
see.

I used the bench to learn why one cylinder habitually ran lean while the
other ran rich, both being fed by the same manifold pipe.  The problem turned
out to be too abrupt an angle in the casting.  But since the shape of the
casting was partially the result of the space available, a smoother curve of
greater radius was not practical.  The solution was to introduce a degree of
turbulance in the manifold.  This either slowed the fuel/air mixture so both
cylinders received a more equal charge, or perhaps prevented impact
condensation of the fuel charge as it rounded the bend.  In either case, the
flow bench allowed me to solve the problem.

The bench also helped when the diameter of the engine cylinders were
increased from 86mm to 92mm.  The larger opening in the heads left a
considerable lip in the combustion chamber.  Using the flow bench, I was able
to figure out where best to remove that portion of the lip shrouding the
valves, the best shape with regard to flow rate, and so forth.

An important accessory to the flow bench was several pounds of modeling clay,
which served as a gasket when installing something on one of the test plates.
 When working with aluminum cylinder heads I found the optimum shape only by
going beyond it.  When the flow rate began to drop, I would fill in my last
grinds with modeling clay until the maximum flow rate was restored.  When
doing a pair of heads 'for real', the flow bench served to confirm when I'd
achieved the right shape and helped me keep the flow rate equal among the
four combustion chambers, which I felt was necessary for best volumetric
efficiency.

Getting the bubbles out of the gauge tube was quite difficult until I added a
few drops of dishwashing detergent to the water.

Some interesting points learned from using the flow bench:  At the flow rates
I was using there was no difference between a finely polished manifold and
one finished with #600 grit paper.  This may not be true if the gas happens
to be a mixture of gasoline vapor and air but running the engines on the test
stand seems to bear this out.

At certain flow rates through objects having an irregular surface, the air
moving through the object appears to have a resonant frequency which
coencides with maximum flow.  Any increase or decrease in the pressure
differential across the object will produce less flow through it.  This
'sweet spot' can often be found by increasing or decreasing the length or
diameter of the object.  This can be very important in constant-speed
engines, such as used in aircraft.  I'm sure someone has worked these things
out mathmatically but learning them empirically and putting them to use to
increase the power output of an engine was a very satisfying experience.

-Bob

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