|
Al,
“Sorry to say,
the pressure you see has no significant effect on cooling efficiency (heat
transfer). The next time you fly, since you’ve removed most of that air,
you will still see 210F. Just like before”.
If the air was
trapped at the top of the heat exchanger I would expect improved
temperatures. Could be wrong.
Bobby
Hughes
From: Rotary motors
in aircraft [mailto:flyrotary@lancaironline.net]
Sent:
Wednesday, May 06, 2015 11:15 AM
To: Rotary motors in
aircraft
Subject: [FlyRotary] Re: Return to
Flight
Hi Jeff. I’ve
done lot’s of experimenting with cooling sys. Tough to explain all this,
but I’ll give it a try:
Pretty clear you
saw 6 psi only because you took off with 3+ cups of air in the system. If
you’d taken off with 2 cups of air, then pressure would have been 9 psi. 1
cup, 14 psi. No air in system, then you would have seen rated cap pressure
(16 psi in your case).
Sorry to say,
the pressure you see has no significant effect on cooling efficiency (heat
transfer). The next time you fly, since you’ve removed most of that air,
you will still see 210F. Just like before. There’s one huge exception to
that statement, that’s if you have air in sys that can’t move to a high
point out of the flow. In that case air in sys has huge negative effect.
Causes local boiling when it passes hot areas and inflates cooling temp.
You don’t have
to fly to prove these concepts. Ground running at idle is all that’s
needed. Let’s assume all air is removed. Then letting engine warm up to
180 F will result in rapid pressure rise to 16 psi (rated cap pressure).
Fluid will exit system. With 2 cups of air in sys, that same 180F will
yield slow rise in pressure to only 9 psi. No fluid will leave sys.
A good cooling
design pretends air is stuck in block, so you add a path for that air to
rise out of the block coolant flow. This is called a dynamic bleed. Air is
automatically removed from engine coolant flow. Super low risk way to fly
as you no longer care if air is in system. It’s can’t affect cooling.
It’s a bit
higher risk to fly without dynamic air bleed, you just make darn sure you
purge all air from block sys prior to flight. Applying vacuum to rad cap
is great way to remove air.
One of the
ironies about cooling design is that air that resides above engine flow is
a safety asset. For example, your cap is highest point in sys and you have
2 cups of air under cap. Big safety advantage simply because your pressure
gage is then a great predictor of how well your sys is doing. A leak will
be detected long before overheating. A bunch of other assets to this
design.
Conversely, air
in engine flow has overwhelming negative affect. Temps soar and risks boil
over.
Clear as mud
eh?
Fwiw
-al
wick |