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PostPosted: Sun Jul 26, 2020 8:09 am 
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I recently raced at Heartland Park and the temperature was 95 degrees F and the humidity was high (heat index was 108). I noticed the car did not cool off as quickly during a cool down lap as normal and temps went up considerably in impound. Caused me to wonder how the radiator cooling capacity is affected by humidity. Any one know the science??

(Cool shirts are great).

Jim S


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PostPosted: Sun Jul 26, 2020 9:07 am 
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Location: Florida
Don't know about humidity affecting radiators but here in Florida the temps match what you just quoted. In the same scenario on the Sebring long course even the 3 1/2 mile cool down doesn't do much for reducing temps. When in impound temps go from 190 running temp up to 230 on average. Got major coolers and radiators with reduced rpm water pumps but can't do any better than stated in that kind of weather. Gave up a few years ago and put an 8 inch fan behind the radiator which I turn on on the cool down lap and leave on while in impound or back in paddock. One thing you have to do is start the engine a couple of times to move the cooler water from the radiator into the block. Note I put the fan behind the radiator. Fan spins while you are racing due to air flow and if it's in front of the radiator and catches sand and gravel you will chew up your fins and tubes.

Ditto on the cool suit. Not a lot of room or electricity in our Sprites so we use the cool shirt hooked up to a freeze can. One poke of the finger and you get cool quick. Assuming 3 or 4 times on the track ranging from 15 minutes to 30 minutes a can lasts all weekend. They are especially good for Red Flag situations, particularly in an open car.

Bob

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HP 13 '59 AH Sprite HP Hybrid
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PostPosted: Sun Jul 26, 2020 9:26 am 
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jstinehelfer wrote:
I recently raced at Heartland Park and the temperature was 95 degrees F and the humidity was high (heat index was 108). I noticed the car did not cool off as quickly during a cool down lap as normal and temps went up considerably in impound. Caused me to wonder how the radiator cooling capacity is affected by humidity. Any one know the science??

(Cool shirts are great).

Jim S


The radiator cooling (forced convection to air) isn't affected much by the humidity of the air, if anything humid air has a tad higher heat capacity thus may cool a tiny bit better. So the 95 matters not the 108.

Humans of course are cooled mostly by evaporative cooling which IS drastically impacted by the ambient air humidity. Which is why I'm not outside working on our project car this afternoon. :(


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PostPosted: Sun Jul 26, 2020 7:16 pm 
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Joined: Thu Aug 18, 2011 10:46 am
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Thanks for the responses. I already have a dual pass oversized radiator, a new expensive dual pass oversized oil cooler... all with air separated and ducted.

I am adding a fan on the back of the radiator. My concern now is (no alternator) will the battery handle the car, a cool shirt pump and a radiator fan. Add to that, my primary racing is done in Colorado at 5,000 feet ... I may be skipping summer races around here.

Thanks again

Jim S


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PostPosted: Mon Jul 27, 2020 3:11 am 
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Joined: Wed Jul 06, 2011 3:48 am
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Location: Florida
We don't run an alternator on either car. One has a deep cycle battery, the other a standard medium size. Got a charge cart that we hook up to the car as soon as we get to the paddock. Takes about 30 to 45 minutes to get the batteries back up to full charge. Been running that setup for 30 years and never killed a battery. Do use the type battery you can service. We don't run the fan on the track until the cool down lap.

Bob

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HP 13 '59 AH Sprite HP Hybrid
HP 49 '60 AH Sprite HP/Vintage


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PostPosted: Mon Jul 27, 2020 6:27 am 
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A couple of nibbles on the food for thought plate..

In talking with aircraft operators, max allowable weights are reduced in high humidity due to lower lift in humid air. H2O molecules have lower molecular mass than N2 and O2, and in an ideal gas situation they all take up roughly the same amount of space in the atmosphere--- so when the humidity is high, there's less mass to the air flowing across the wings/ props. extrapolate that theory to a heat exchanger (I know, dangerous to do!), and the heat exchanger would have less air mass flowing through it, so less heat transfer.

However, I did a little digging and came along a research paper which seems to indicate otherwise. however, that was for an "air conditioner" style heat exchanger where the exchanger was well below ambient temperature. water droplets were forming onto the coils and and eventually dripping off. Thus I suspect this particular test maybe be inverse of what we see in an application where we want to reject heat away from the heat exchanger. dunno. I'm just an electrical engineer, not a thermodynamicist.

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PostPosted: Mon Jul 27, 2020 6:47 am 
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I'm an ME thus a tad closer to a thermodynamicist. though admittedly my thermo and heat transfer classes were a while back.

Water vapor is less dense that air, but water vapor also has a significantly higher thermal capacity than air (ie ability to hold heat, expressed in energy required to raise the temperature one degree). Any changes in heat transfer with ambient air humidity are very small in any event.

(In an airplane, everything is moving against you - air is hotter and more humid (lower density) decreasing lift, and the water vapor in the air displaces oxygen decreasing engine power adding to the power loss from the decreased density. Lose - lose.)

If water is condensing on or evaporating from the heat exchanger (radiator) fins that is a different story. But that should not be happening on a race car radiator.

I run a thermostat in my VW H Prod car, with a well shrouded and probably slightly oversize radiator and no fan. I've (knock on wood) never seen it deviate significantly from the thermostat controlled temperature as long as it is moving at any pace. It does heat up fairly quickly when stopped as it has no fan. This is only really an issue in the impound / scales line.


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PostPosted: Mon Jul 27, 2020 7:09 am 
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Ya, that's why I threw the caveat in there.. a heat exchanger taking heat out of the air will act differently than one putting it into the air. there's also the whole condensation thing which throws a huge heat of vaporization/ evaporation energy buffer in there. usually not an issue in these cases tho...

some other articles I read through basically said the difference in humidity results negligible differences. water has greater thermal capacity, but is very low percentage of the total air mass AND displaces the other gas molecules around it. sooo win/lose some there. too much theory for a monday morning...

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PostPosted: Mon Jul 27, 2020 8:14 am 
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Jim, I had temp issues on Jesse's dyno before HPR. I took a long look, and my super-fancy radiator was just full of rocks and goo, and many fins were compromised.

I put in a new radiator, and at the same Heartland event, I never got over 197 temp despite the horrific 108 heat index.

I do run a fan; I changed the placement down much lower based on where I have my oil cooler etc.

Maybe take a look at your radiator and see what you see.


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