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Heat transfer and GPM

by ISX
Tags: heat, transfer
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Apr14-10, 08:40 PM
P: 116
I been debating this and figured you guys would know what the truth is. When you take the thermostat out of a vehicle, the flow is so high because of the thermostat not being a restriction, that the temperature gauge will hardly budge from whatever the lowest degree is, and if it does, it takes a long time. Now is this because it cannot extract heat with such high flow since heat needs time to transfer (meaning the engine might actually be overheated since the coolant is not grabbing any heat from it) or is this because the coolant is taking so much heat away from the engine that it keeps the temperature that low?
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Apr14-10, 09:11 PM
P: 22,313
Welcome to PF.

It's because the heat transfer is so good that the coolant is taking heat away so easily it keeps the engine extra cool.

There is a somewhat counterintuitive principle at work here, which is probably why you are asking the question: The relationship between flow and delta-T (or heat removed) is not linear. A doubling of the flow rate does not result in a halving of the delta-T, it results in slightly more than half the delta-T and thus slightly more heat transfer than with the lower flow rate. This is because the approach temperature, the difference betwen the water temperature and the air temperature, is larger when the flow is higher and thus the heat transfer is more effective.
Apr14-10, 09:25 PM
P: 116
Ah, thanks for the help!

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