Radiator Cap--flow rate through radiator
Michael Skolones
michaels at telerobot.com
Wed Oct 15 20:53:47 GMT 1997
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Yes, I agree with you that the temperature differential at the radiator is an
important consideration. As another reader pointed out, a full-fledged analysis
of the cooling system is a major undertaking, but without it we run the risk of
ignoring important details; oversimplification can lead to irrelevant arguments.
However, my point may be better illustrated by considering the following
simplified system. Suppose, for the sake of argument of course, that our model
system consists of an engine block and an infinite supply of coolant at some
temperature T0, which is being pushed through the engine block at some rate V/t.
If we assume a steady-state condition (T of block constant, higher than T0),
then the coolant will remove the thermal energy generated by the combustion at a
steady rate, and the coolant temperature will be some constant value Tf as it
leaves the block.
Now consider increasing the flow rate. A given volume of water passing through
the block will now take in less thermal energy than the same volume of water
passing through at the slower rate, but since more water flows through per unit
time the effect will be the same to first order. A closer look will show that
the steady-state temperature of the block will be slightly lower if the initial
coolant temp is the same, but the flow rate is increased. This is because the
rate of heat flow from the block into the water is proportional to the logarithm
of the difference of their temperatures; since the average temperature of the
fast moving water within the block is lower than the average temperature of the
slow moving water within the block, the block-water temperature difference will
be less for the same amount of heat removed. This makes sense if you consider
the extremes: as flow goes to zero, clearly the block temperature will rise; as
we increase flow from zero, the block temperature will drop.
My point here is that the earlier claim that "fast moving water doesn't have
enough time to soak up the heat" is nonsense.
As for a real block-thermostat-radiator system, that is much more complicated.
If we eliminate the thermostat (I haven't decided if this makes it simpler to
analyze or not), the temperature of the coolant coming out of the radiator is a
function of the coolant temperature when it enters the radiator, and of the
efficiency of the radiator. The radiator efficiency depends on the surface area
and the temperature difference between the coolant and the environment. For a
fixed size radiator, peak efficiency is probably found at some particular
temperature (healthy dose of conjecture here), and so factory cooling systems
are designed to operate properly with a chosen temperature thermostat, flow rate
and radiator size.
I don't think B&B would sell so many high-volume water pumps for hot-rod Chevy
350's if they weren't necessary. Typically when you increase the mechanical
power output of an engine the thermal ouput increases as well, and so bigger
radiators and higher-volume water pumps are called for.
mike skolones
______________________________ Reply Separator _________________________________
Subject: RE: Re[2]: Radiator Cap--flow rate through radiator
Author: diy_efi at coulomb.eng.ohio-state.edu at Internet
Date: 10/15/97 10:40 AM
Don't forget that the amount of heat transfer (both in and out) are
somewhat dependant on differential temperature. The greater the
temperature differential between radiator temperature and ambient, the
larger the temperature drop within the radiator. Water moving too fast
will not sink up as much heat in the heat developing portions of the
engine AND will not cause as high a temperature differential in the
radiator (between radiator and ambient), resulting in higher engine
temperatures, even when the coolant temperature reading is lower.
I once had an MG that ran at it's coolest (per the temperature gauge)
when there was no water in the radiator. Don't get caught up in coolant
temperature readings as the sole source of information.
> ----------
> From: michaels at telerobot.com[SMTP:michaels at telerobot.com]
> Sent: Tuesday, October 14, 1997 10:47 AM
> To: diy_efi at coulomb.eng.ohio-state.edu
> Subject: Re[2]: Radiator Cap--flow rate through radiator
>
> <<File: RFC822 message headers.txt>>
>
>
>
> ______________________________ Reply Separator
> _________________________________
> Subject: Re: Radiator Cap
> Author: diy_efi at coulomb.eng.ohio-state.edu at Internet
> Date: 10/13/97 8:24 PM
>
>
> There's a problem with the argument below. If the water is in contact
> with the
> block long enough to pick up some thermal energy (and no matter how
> small the
> time of contact is, there _will_ be a thermal transfer if there is a
> difference
> in temperature), then it will be in contact with the radiator fins
> long enough
> to get rid of the same amount of thermal energy, provided the radiator
> is
> properly designed. Obviously if the radiator is too small, or
> clogged, or
> bypassed, or what-have-you, then the temperature of the water will
> continue to
> rise as time goes by.
>
>
>
>
>
> Not only does it not absorb enough heat from the engine, it goes
> through
> the rad so fast it doesn't release the heat either. Getting the
> coolant
> up to a high enough temp gets the temp difference at the rad higher,
> allowing the rad to shed more calories or BTUs. This effectively
> removes
> more heat, even if the operating temp is a bit higher. I know it does
> not make a lot of sense, but without a restrictor in an engine running
>
> without a stat, overheating of the valve area, for instance, with
> resulting head cracks and/or detonation can be a real problem.
> --
> _/\_
> --|-----([])-----|--
> S 0/ \0 B
>
> Remove the R from E-Mail Address to reply. Stop the spammers!!!
> It's hard to soar like an eagle when your stuck with a bunch of
> turkeys-- or stuffed with too much Turkey!! Happy Thanksgiving
> to all our Canadian Friends!!!!
>
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<diy_efi at coulomb.eng.ohio-state.edu>
Subject: RE: Re[2]: Radiator Cap--flow rate through radiator
Date: Wed, 15 Oct 1997 10:40:25 -0500
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