spridgets
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Re: O v e r h e a t i n g

To: Ron Soave <soavero@yahoo.com>
Subject: Re: O v e r h e a t i n g
Date: Sat, 5 Mar 2005 03:35:02 -0600
Cc: spridgets@autox.team.net
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References: <20050305005650.19160.qmail@web30507.mail.mud.yahoo.com>
Ron reminds us that:
> I missed all of this thread (been busy), but in a
> nutshell, boiling point varies with vapor pressure
> (search "steam tables" in yahoo or google to get the
> numbers).  Steam (or water vapor) doesn't transfer
> heat as well as liquid water so although water and
> steam (if not superheated) are at the same temp for a
> given pressure, heat transfer sucks with steam and the
> overheating runs away (not to mention the fact that
> your water pump doesn't appreciate trying to pump a
> gas). 

Ah!!  Someone else with some physics!  Ron is on the
money here.  Thermal conductivity of liquid water is
almost 25 times greater than that of air. Worse, the
thermal conductivity of air is about 50% greater than
that of water vapor (read: steam).  NOW apply what
Ron says about vapor pressure and you can see that
there are huge differences in the cooling ability between
water and steam.

I won't pick nits over the fact that thermal conductivity 
is a function of the cumulative specific heats of its
constituents (think about all the *stuff* in your water
here) and its density. Ya see - its kinda complicated...... 

> With the fan argument, a "puller" is generally
> more effective at cooling a heat exchanger (radiator)
> because the flow thru the HX is more laminar (the fan
> blades "turbulate" the flow), and the pressure drop
> will be much lower (if it's turbulent, meaning
> Reynold's # higher than 3000 or so, the pressure drop
> goes as the square of the flow thru, versus a linear
> relationship for laminar flow).  You'l also use more
> of the hx face, because with a fan in front the
> pressure will be at the blade tips, and very little
> flow goes throught the center axis of the fan. I can
> not overemphasize the importance of a good shroud
> around the fan in either configuration.

Now this is really useful, Ron. And something I 
did not know.  I had not thought of applying 
non-dimensional analysis techniques to airflow
through the radiator but I see now that it is very
important. I was thinking of putting an electric
fan in front of the radiator, but now I'll look more
closely at the idea of removing the existing mechanical
fan and placing an electric fan with a good shroud 
on the inside so that it draws increased laminar
flow through the core.  

Cool stuff, Ron!!

Cheers!!
Jim - who drove the Midget to work in Dodge City 
again tonight...
-- 
1964 R60/2
1968 MG Midget
1976 R90/6
1990 K100LT
**Don't underestimate the power of stupid people in large groups.**





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