Your analysis is correct, but I believe that the range of k (fluid
conductivity) is
relatively small between the different fluids (e glycol/H2O and H2O, primarily),
so while k is significant, it is of secondary importance since (I think this
is correct,
but perhaps not) it varies over a limited range and you can cause the Nusselt,
Reynolds and Prandl numbers to vary much more by varying flow rates, tube
diameters and viscosity.  The issue is one of "is it a variable with a
significant
range that I can control".  I think the answer for k  is "less so than many
other
variables". Of your variables, I might take some issue with the order, but the
 key point is that I can make the flow rate vary at least one order of
magnitude,
I can vary the tube diameter at least one order of magnitude, while the
viscosity
and Cp and k vary over perhaps 2-4X.  Also note that the exponents of the
key physical properties are less than unity.  i.e. not ultra critical.  I warn
young engineers to first look at the exponents of any new equation of interest.
Find any second order or third order terms and watch them like a hawk.
They'll either kill you or make you a hero, depending on which side they
come down on.  Less that unity exponent terms are unlikely "killers".

So, certainly you want to choose a coolant with all the things going in your
favor as much as you can, but I submit that we STILL use water at least 75%
because it is cheap and readily available.  That the Cp is high and viscosity
OK, and k reasonable are just frosting on the cake.  They also mean that we
won't be going very far looking for better coolant, but we WILL design our
cooling systems to use water ("fixed" for it's two big disadvantages - it
freezes
and corrodes the hell out of stuff) cause it is cheap, available and good
enough.
The fact  that it is actually VERY good is neat, too.  If it were a worse
coolant,
we would probably still use it, and just have bigger radiators and water pumps.

So I agree, the system needs to be balanced, and we absolutely need a good
enough heat transfer to the air - the final step.  I think that my point was
that
nucleate boiling is mandatory to get a high enough heat transfer from the very
hot parts of the engine to the fluid, since phase change heat transfer is about
3 orders of magnitude higher than conduction.  Regardless of the size of the
radiator, if there were no nucleate boiling, the engine would overheat locally.
Higher k helps on radiator size, and cannot be ignored, but is not a show
stopper if it varies moderately.

I think we are really agreeing about 97% here, with a slight disagreement on
semantics and "why", both of which are not worth much more effort.

Thanks for the good commentary.  Excellent points.

Bill

Harry Sheldon wrote:

Youv'e got some front Paul,   :-)  .... up @ P&W.Havn't heard a thing. Which is why
it's good to see (your/ you're) back again.
As you know we're antiphase seasons here in Oz.
In Adelaide winter min maybe 0->  -1 nightime; day 11 maybe 9.
Just approaching Summer season. Then your'e up for min maybe 24 max 40+ (all deg C).
Iv'e had a few (Email) words with one helpfull dude up in Yellowknife. Very diferent
climatic conditions!
Seems to me that the point is that straight (& pure) H2O you can't go much better for
removing therms both in 'sensible heat' or in latent heat of vaporisation. The only
other major 2 issues of importance are :-  corrosion (dissimilar metals) and bustin'
your tubes with ice. Soooo corosion inhibition -- essential.  antifreezing protection
---- check your local weather records (& wherever your'e gonna go). I think to
ammalgammate and therefor confuse the 2 (almost) saparate issues of nucleate boiling
and the processes of transfer into 'sensible' heat Xfer ----de dit de da  vis-a-vis
the ACTUAL opperating conditions temp (& humidity) confounds a clear understanding.
I think, look at the minimum anti-corrosion additives and then look at minimum temps
encountered, add a fudge factor saftey margin and then run as close as you can to the
de-mineralised/distilled Adams' Ale.
'Course if your'e experimental and not to sure that you might not only cook the goose
but burn his arse as well, then an increase beyond 212F is a wise policy as well.
Rotary Regards'
    Harry S

 
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