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 ------

From: "Sboese sboese@uwyo.edu" <flyrotary@lancaironline.net>

To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>

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 

On Feb 28, 2021, at 12:47 PM, Finn Lassen finn.lassen@verizon.net <flyrotary@lancaironline.net> wrote:



◆ 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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