Date: Thu, 4 Jul 2002 09:20:20 -0700
Reply-To: Keith Hughes <keithahughes@QWEST.NET>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Keith Hughes <keithahughes@QWEST.NET>
Subject: Re: TIICo cooling issues/HUH?
Content-Type: text/plain; charset=us-ascii
Richie,
> << No, your thinking is correct. The coolant has to "hang around" for a bit
> for
> it to absorb the heat from the engine and the same goes for it to get rid of
> the heat in the radiator. >>
>
> No, your thinking is flawed. The rate of heat transfer is proportional to
> the temperature differential and mass flow.
Well, you're sort of correct. The *Heat Transferred* through the
system is proportional to the temp. difference and mass flow, as:
H = Mf * dT * Cp
Where H = Heat Gain
Mf = Mass Flow Rate
dT = delta T (change in temp.)
Cp = Specific heat of the transfer medium
The heat transfer *process* is proportional to the temperature
differential at the transfer interface. It is also constrained by
the heat capacity of the transfer medium and the overall heat
transfer coefficient of the "source" (i.e. motor on one end,
radiator on the other).
> So, the more mass per time
> (coolant flow) through the engine and the radiator, the more heat transfer
> will occur, assuming all else is up to snuff in the system, of course. If
> you cannot bring yourself to trust my word, then you can experience the dual
> of this process firsthand.
Another thing to keep in mind, however, is that the water pump is
subject to cavitation. The thermostat, even open, provides a
significant backpressure (head) on the pump, which minimizes
cavitation. Allowing the coolant to stay in the motor longer, and
heat up a little higher, also generates higher pressure on the
suction side of the pump, which also helps control cavitation.
Under high RPM, high temps., without a t-stat, in some
applications, the pump will cavitate, resulting in a *lower* flow
rate than would have been the case had the t-stat been installed.
As I replied to the original poster, I've seen this happen a number
of times before (not in vanagons). You have to look at the system
in terms of fluid dynamics, not just thermodynamics, before you can
be sure that a higher flow rate will result in greater overall heat
transfer (and, you have to add in the additional frictional heat
gain from using the higher flow rate).
Keith Hughes
'86 Westy Tiico "Marvin"
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