[TR] Water pumps.

[Michael Marr]

Without getting into the esoterica of heat exchange, what Randall has described is accounted for by a concept known as the logarithmic mean temperature differential (LMTD) across a heat exchanger.  There is a quite good article on Wikipedia that explains this.  In my earlier note, when I pointed out that Q (heat transferred) is equal to U x deltaT, the delta T for a heat exchanger is in fact the LMTD.

https://en.wikipedia.org/wiki/Logarithmic_mean_temperature_difference

As an aside, we owners of TRs often retrofit the cars with electric radiator fans so that we can increase airflow over the radiator at idle and at low vehicle speeds.  We do this to increase the mass flow of the air across the radiator, so that the radiator's capacity to transfer a fixed maximum quantity of heat is fully utilized.  What we do not do is slow the fan down to give the air more time to linger!

Another way of improving the performance of a radiator at low speeds would be to spray water on the radiator and use the latent heat of evaporation of the water to improve LMTD.  This approach is used in evaporative condensers in the refrigeration industry, and those of you that live in dry climes may have an evaporative cooler on your roof, more commonly known as a swamp cooler.

Mike

From: Triumphs [mailto:triumphs-bounces at autox.team.net] On Behalf Of Randall
Sent: Wednesday, May 10, 2017 11:53 PM
Cc: 'Triumphs'
Subject: Re: [TR] Water pumps.


> The

> flow rate of the fluid through the heat exchanger does not affect how much

> heat is transferred.



I disagree.  As the fluid cools during it's trip through the exchanger, the delta-T across the exchanger drops, causing less heat to be transferred.  So the slower the flow, the more delta-T drops and less heat is transferred. In the extreme case of no flow at all, the fluid temperature drops until delta-T is zero and no further heat is transferred.  If this were not true, the thermostat could not regulate engine temperature.


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