Classic Mustangs List Archive

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Mail From: (email redacted) (email redacted)

Well it seems as though this thread is not answering the original
question about higher coolant flow yielding higher temperatures. The
missing link in the discussion on heat transfer, and most of the
statements have been true, is that this is really a thermodynamic
problem in which heat transfer takes places. The governing equation
includes a velocity factor, and since this is by no means a
steady-state problem, the time rate of change makes a difference. A
earlier post mentioned that heat transfer increases with water
temperature, but what really changes is the specific heat, or the
ability of water to absorb heat. The key to the wide open thermostat
problem is that the high flow rate does not give the coolant enough
time to transfer the heat to the radiator and the air flowing through
it.
Incidently, antifreeze/coolant does not appreciably change the
specific heat, it changes the boiling and freezing temperature of the
coolant mixture so that a change of state from liquid to gas or solid
occurs at a lower/higher temperature. The radiator cap does the same
thing by increasing the operating pressure, and therefore, temperature
of the coolant. The water wetter refered to in another post reduces
the surface tension of the coolant, which reduces the thickness of the
boundary layer along the engine and radiator passages. The thinner
boundary layer yields and increased heated heat transfer coefficient
and better cooling.

Lane Decker
65 Coupe, 94 GT Conv, 89 Yamaha FJ1200

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Mail From: Mmeswarb (email redacted)

If it was true that you remove less heat with a higher flow rate, then
the temperature gauge would not indicate a higher temperature, but a
lower one. Afterall, if you look at the temperature sender, it
protrudes down into the water flow and measures the temperature of the
water not the block.

(email redacted) wrote:
>
> Well it seems as though this thread is not answering the original
> question about higher coolant flow yielding higher temperatures. The
> missing link in the discussion on heat transfer, and most of the
> statements have been true, is that this is really a thermodynamic
> problem in which heat transfer takes places. The governing equation
> includes a velocity factor, and since this is by no means a
> steady-state problem, the time rate of change makes a difference. A
> earlier post mentioned that heat transfer increases with water
> temperature, but what really changes is the specific heat, or the
> ability of water to absorb heat. The key to the wide open thermostat
> problem is that the high flow rate does not give the coolant enough
> time to transfer the heat to the radiator and the air flowing through
> it.
> Incidently, antifreeze/coolant does not appreciably change the
> specific heat, it changes the boiling and freezing temperature of the
> coolant mixture so that a change of state from liquid to gas or solid
> occurs at a lower/higher temperature. The radiator cap does the same
> thing by increasing the operating pressure, and therefore, temperature
> of the coolant. The water wetter refered to in another post reduces
> the surface tension of the coolant, which reduces the thickness of the
> boundary layer along the engine and radiator passages. The thinner
> boundary layer yields and increased heated heat transfer coefficient
> and better cooling.
>
> Lane Decker
> 65 Coupe, 94 GT Conv, 89 Yamaha FJ1200

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The temperature of the coolant would increase because the higher flow
rate through the radiator, with a given temperature difference, say
180øF(coolant) - 85øF(air), does not have enough time to dissipate all
of the heat it absorbed while in the engine, with a temperature
difference of several hundredøF(block) - 180øF(coolant). The
difference in temperature is what drives heat transfer to bring a
system into equilibrium. When the coolant flow is regulated, by the
thermostat, so that the temperature remains relatively constant at the
thermostat opening temperature, the equilibrium is balanced by the
coolant volume flow, temperature differences, and the interface
surface areas. If any these, or other variables, are changed,
equilibrium is lost and usually that means the temperature goes up.

Lane Decker
65 Coupe, 94 GT Conv, 89 FJ1200, 97 Ducati M900(maybe)


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If it was true that you remove less heat with a higher flow rate, then the
temperature gauge would not indicate a higher temperature, but a lower one.
Afterall, if you look at the temperature sender, it protrudes down into the
water flow and measures the temperature of the water not the block.

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Mail From: (email redacted) (email redacted)

It has been a long time since I've had to use heat transfer, but I've
been thinking about this damn problem for a week now and think I'm
finally starting to understand what is going on.

First off, the "water doesn't have time to cool off" argument doesn't
work. At a higher flow rates, it is true that the water will not have
a chance to cool to as low a temperature, but by the same token the
water will spend less time heating up to a higher temperature in the
engine. As others have pointed out, how fast heat is transfered is a
function of temperature difference. The combustion chamber
temperatures are so much higher than the coolant temperatures that
heat will be transfered to the coolant at a fairly constant rate
regardless of the temperature of the water entering the block. How
much the coolant temperature rises will depend only on how long it is
in the engine.

This isn't true of the radiator - it has a much smaller temperature
range to work with - the difference between ambient air temp, say 80F,
and the boiling point of the coolant - say 200F (actually, the
upper bound is more likely set by what the engine oil can stand, but
that is still somewhere in the 200F range). If you just filled the
radiator with 200F coolant, it would cool very fast at first thanks
to the 100+F temperature difference, but the rate of heat transfer to
the air would drop as the temperature dropped. It might only take
1 minute to cool from 200 to 190F, but would take 2 minutes to cool from
150 to 140F since the temperature difference would be halved.

If the engine can heat coolant from ambient to 200F faster than the
radiator can cool coolant from 200F to ambient, the coolant will have
to leave the radiator at higher than ambient. However, this is OK, since
the radiator is much more efficient in dissipating heat when working at
a higher temp. The best heat transfer out of the system occurs when
the coolant enters the radiator at the max acceptable temp, cools
ever so slightly (but very quickly), and reenters the engine, where
it is reheated ever so slightly before returning to the radiator.

The thermostat valve in a car's cooling system is not a binary
open-or-closed device, but gradually goes from closed to full-open
over a small temperature range. In steady state operation, the
thermostat adjusts the flow rate so that the cooling provided by the
radiator just matches the heating of the engine, keeping the engine at
a near constant temperature. If the thermostat were to stick open,
the steady state coolant temperature would stabilize near the
temperature that the radiator's heat dissipation rate just matched the
rate that heat is produced by the engine. Since the radiator should
be able to keep up with the engine under a modest load - say climbing
a hill with a full load of passengers - at full flow the radiator
should yield an even low temperature just cruising down a level highway.

BUT - it is a fact that *some* cars with stuck-open thermostats will
overheat, so clearly something in the above reasoning is wrong.
Interestingly, such overheating seems to occur while cruising at
highway speeds - when the load on the engine is fairly low, but the
engine revs (and hence the water pump flow rate) would be somewhat
high. For this to happen, something would have to change to make the
radiator less efficient when the coolant flow rate was higher. What I
believe happens is "cavitation" in the radiator. At a high enough
flow rate near a surface, the effective pressure drops enough (due to
the Bernoulli effect) that the boiling point of the coolant can drop
below the temperature of the liquid and bubbles of steam will start to form.
This can actually happen at relatively low temperatures - ship propellers
can form such bubbles in sea water. Steam conducts heat much more poorly
than water, and would account for the efficeincy drop of the radiator
at higher flow rates.

You would be more likely to encounter such cavitation based overheating
if your engine was turning high revs at crusie (due to a "low" - numerically
high - rear gear) or if the radiator was undersized or partially clogged -
causing high coolant velocities in the radiator.

So - does this match anyone's experiences with overheating due to
stuck thermostats? Or should I go back to the drawing board?

Don Schmitz

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Interesting theory, so I thought I would delve into it. Unfortunately,
I am missing a big factor in the equations, the initial state volume
flow. Is this a motor turning 2600 rpm at 65 mph like my 65, 1800 rpm
lik my 94 GT, or 3000 rpm like my friend's 67? Is the radiator a two
row core or a four row Desert Cooler? Is it a new system or partially
clogged? What about the cross sectional area of the tubes? And to
complicate all of this, the water pump is definitely not a positive
displacement pump. And how much do we lose in the bypass hose? At what
pressure is the cooling system operating? It's less than the lower
tolerance of the 14 psig radiator cap, but what is it? Their are a lot
of variables, and therefore flow velocities, in just looking at the
flow rate of the coolant through the radiator. But to actually
calculate a localized pressure drop... Has any had this problem, and
had the radiator core spring a leak due to the cavitation erosion? It
kind of makes you realize what an elegant, and simple, solution the
thermostat is for regulating the flow of coolant at the proper flow
rate to maintain a relatively constant temperature. Anyway, this does
sound like a good project for our Physics major.

Lane Decker

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Interestingly, such overheating seems to occur while cruising at
highway speeds ... What I believe happens is "cavitation" in the
radiator.