This is a pretty big misunderstanding!
AFAIK, heat transfer is primarily a function of temperature differential and
a material's ability to dissipate or absorb it.
Using t'stat open Vs closed as proof that "faster is more efficient" appears
to me to be apples 'n oranges. When closed, coolant is recirculated,
getting hotter with each cycle due to ever increasing resistance to heat
transfer as their differential is reduced. Once opened, it now can be
somewhat dissipated through the radiator. The increased rate of coolant
flow WRT closed isn't relevant per se, but the rate of flow through the
radiator WRT air flow through it is.
Using your data, John's temp should have increased when he added a
t'stat, since it reduces flow speed. At best, it would have remained the
same, if the t'stat caused no back pressure as some purport, but he
claims it dropped ~25deg. Please explain the mechanism that solves this
conundrum.
As an experiment, heat up some water to near boiling and pour it through a
piece of copper pipe at different flow rates. Compare heat transfer with
your hand while blowing air across it.
I'll wager your hand gets hotter (more heat transfer) the lower the flow
rate WRT a ~constant airflow (car idling or creeping in traffic). Proving in
this case what women already know; that slower for longer periods of
contact are better for achieving the desired results. :^))
Please let us know the results. Inquiring minds want to know.............
GM
----- Original Message -----
>
> Please let me clarify an apparently common misunderstanding regarding
> heat transfer. The statement being made that the rate of heat transfer
> is higher if the fluid flow is slower is incorrect and excactly the
> opposite of what actually occurs. Heat transfer is increased with higher
> fluid flow rates! This is true both for the radiator fluid to radiator
> surface and engine interfaces and for the radiator surface to air
> interface. Thus increased capacity water pumps and fans will increase
> cooling capability even though the radiator has not been changed.
> Likewise, when engine temperature increases the thermostat opens ... thus
> increasing the fluid flow rate and velocity to increase the cooling rate
> and lower the temperature! That being said - maintaining correct engine
> operating temperature with a thermostat as has been stated - is
> important! Not only to keep the engine from overheating but rather to
> keep it from running to cool which has adverse affects on engine
> lubrication and combustion efficiency.
>
> Andrew Somerville
>
>
> Mechanical Engineer and Bricklin owner
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