This graph from Mocal might be helpful. It's for their oil coolers but the trends may be transferable to water exchangers.
The solid line is Pressure Drop. The two dotted lines tell the story of two different oil flow rates/tube.
| m/sec |
5 |
10 |
15 |
20 |
25 |
| kmh |
18 |
36 |
54 |
72 |
90 |
| mph |
11.2 |
22.4 |
33.6 |
44.8 |
55.9 |
Increasing the air flow 5 fold from 11 to 56 mph only increases the heat transfer: 2 fold with an oil flow of 0.02 L/sec/ tube and 2.3 fold by doubling the oil flow rate per tube to 0.04 L/sec/tube While pressure drop increased 13 fold.
So, diminishing returns from increasing airflow or fluid flow.
Steve Izett
Charlie, No, no reference, just what I have read and also talking to Rad manufacturers such as BWR in Brisbane. You can check it out by passing your hand through a naked flame. Quickly and there is no heat transfer. Pass slowly and you will see what the argument is. As I said the truth is there somewhere and as Lyn so aptly puts it “I could well be wrong”.. Neil. Hi Neil, Do you have a reference for that? Slowing a medium down so it has time to absorb the heat seems to conflict with physics as I've been led to understand it. Charlie On 7/20/2021 5:01 PM, 12348ung@gmail.com wrote: Charlie, Much wisdom out there, you just have to find the truth! Max cooling is apparently 30 MPH, so Any faster and it does not pick up heat before going past. Look at big trucks, that grill is not only for looks, they slow the air to get max cooling. If too slow they have a quite large fan that kicks in to drag air through at 30 MPH not 100! As you say, what do I know – I have seen too many that do not work – without any degree. Neil. Subject: Inlet cooling article
I remember the Laboda article about enlarging their cooling inlets, but not many of the details.
This:
The plenum receives air through two circular air intake ducts behind the propeller and squeezes it, Bernoulli-style, so that the air accelerates across the cylinders and between their fins, carrying the heat back, down and out an outflow "gate" at the back and bottom of the engine area, forward of the firewall.
Is contrary to everything I've ever read about cooling efficiently. Faster relative flow will always have higher drag, all else being equal. Accelerating the air even faster than freestream just sounds crazy. My understanding is that there's a balancing act between having the room in an a/c to 'recover' (increase) differential pressure across the heat exchanger (engine fins, in this case), and causing too much drag from the air going through the fins too fast (there's aerodynamic drag in the heat exchanger, just like over the a/c itself). It's surprising to me that James made the plenum the way he did. The rest sounds like putting bandaids on stuff. The next-to-last image, of the final inlet, shows what appears to be a *much* smaller plenum inlet than the cowl ring in front of it, and a rather sharp edged lip where the plenum starts. It looks like the air would accelerate until it hits that sharp lip, and immediately go turbulent, which will kill any pressure recovery and actually slow flow into the cylinder fins.
Most Lyc plenums I've seen (even the ones James made for the 4 cyl engines) have significant volume above the cylinders with smoothly expanding ducts feeding the plenum. That allows the air to slow in an organized fashion, which increases *pressure*, which is what actually makes the air move through the fins.
But what do I know; I have an Economics degree....
Charlie
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