I figure that optimal coolant flow
(speed) is more of a parabolic or bell curve, the top of which is
determined by a collection of other parameters:
rad efficiency, path through engine, airflow through rad. In my
case I'm positive my rad in-and outlets were too restrictive. Once
opened up I can then proceed to optimize airflow or collect enough
data to show that I need to increase rad size.
Again, optimizing for indefinite full power climb on a 100 deg F
day would most likely result in unwanted drag at cruise speeds
(sans a cowl flap). So mission also come into play. Perhaps I err
on the side of too small rads but I have spray bars in the back of
my mind. This RV-4 is slated towards being a cross-country cruise
plane.
Finn
On 3/1/2021 10:40 AM,
eanderson@carolina.rr.com wrote:
Cooling is a fascinating and complex challenge not helped by
urban legens and myths.
I recall many years ago of having a discussion with old man
Lou Ross (maker of the first Rotary PSRU widely used) about
coolant temperature.
His view was (as shown by his and others' experiments) that
the slower coolant flowed through the radiator then greater the
delta T of the coolant - therefore more heat was removed from
slower flowing coolant. While I agreed with that data point,
I tried to point out the objective was to get heat removed from
the engine and that greater coolant flow (up to a point) would
result in more heat removal even though the delta T of the
coolant through the radiator would decrease. We "discussed"
this matter for some time. I finally made the comment that if
slow coolant flow cooled better, then theoretically stopped
coolant flow should cool best. Lou paused - then hung up on me
and we never talked again.
Complicating the situation even more was when some earlier
racers indeed tried to improve cooling by increasing coolant
flow rate by running faster turning water pumps. When in a
number of cases this resulted in poorer cooling, it seemed to
confirm the theory that slower flow was best. Of course, in
many cases the faster turning water pump produced flow
cavitation resulting is less rather than more coolant flow and
an over-heated engine lending credence to the slower flow was
better theory.
So one must keep in mind we are talking about a cooling
system with a number of components each which needs to be
optimized as part of the total system with the objective of
removing the right amount of heat from the engine given the
operational environment. The cooling environment of Southern
Texas may be different than in Northern Alaska.
Ed
------ Original Message ------
Sent: 2/28/2021 3:43:54 PM
Subject: [FlyRotary] Re: N214FL RV-4 First Flight
Finn,
Just ignore the following message if the contents are
obvious:
A higher flow rate of coolant through the system will
result in a smaller delta T of the coolant.
The air doesn’t know or care about the coolant flow
rate. It is just removing heat from the exchanger
regardless of how that heat got there. The air flow was
sufficient to keep temps under control for the
conditions of that flight. With the same air flow rate
and power setting, an increase in OAT will result in an
increase in the exchanger (coolant) temp since the
amount of heat removed is dependent on the air flow rate
and the air delta T. Increasing the power in addition to
the OAT with the same air flow rate will increase the
coolant temps even more.
This all assumes that the overall efficiency of the
heat exchanger is affected minimally by the coolant flow
rate. The efficiency of the exchanger should increase
somewhat with an increase in coolant flow rate but it is
difficult to predict by how much.
There is something to be said for changing only one
thing at a time, though.
Steve Boese
◆
This message was sent from a non-UWYO address.
Please exercise caution when clicking links or
opening attachments from external sources.
Here's pictures of the
small in- and outlets (looking into 5/8" and 3/4
OD tubes).
Finn
On 2/27/2021 11:29 PM, Finn Lassen
finn.lassen@verizon.net wrote:
Yep, inlet/outlets on
rads are very, very restrictive. 7/64x1/2" (0.05
sqin or 1/4" diameter hole equivalent)) at best.
I was aware of it when I JB-welded 5/8" and 3/4"
over existing tubes and "just wanted to see how
it would work" and then forgot about it.
Drilling holes in tank end plates I also found a
small tube inside one tank and baffles in both
tanks. Now removed.
As for airflow, when I get new elbows welded
onto the rads I'll collect new data. I think the
large delta is due to the very slow coolant flow
-- stays in rads way too long (and thus not
circulating through the engine fast enough).
Finn
On 2/27/2021 9:32 PM, Steven W. Boese
SBoese@uwyo.edu wrote:
Finn,
If the
"WTeng" sensor location is somewhere in the
engine block and not at the coolant outlet
and the "WTout" sensor is in the coolant
exiting the engine, then the coolant flow
rate through the system may need to be
increased by enlarging the radiator
connections.� Typically, it seems that at
normal operating temps, the delta-T across
the engine and across the radiators is about
15 deg F.
It also
would appear that the mass air flow through
the radiators may need to be increased since
the air delta T is quite large and the temp
of the air exiting the radiators is close to
temp of the coolant exiting the
radiators.� If your OAT were increased to
100 deg F, you could see "WTout" of ~250 deg
F with the setup as it is now.
The
attached plots show the effect of changing
only the air inlet and outlet areas of my
system which has ~0.9" id plumbing to the
single radiator.��
FWIW
Steve
Boese
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