> I swore to myself I wasn't gonna throw in my two cents worth about
> this whole fast/slow coolant flow thing,
Fine with me -- it's always a pleasure to hear from a REAL
rocket scientist... :-)
> One thing this arguement doesn't include is the effects from turbulent
> flow. Laminar flow through the coolant passages would not transfer
> heat as effectively as would turbulant flow.
Yet another point where what seems counter-intuitive makes sense on
closer examination. I'd originally ignored the issue of turbulence
increasing with the cube of velocity, but the picture of the open
wing mounts in Carroll Smith's book kept haunting me. (For those who
haven't read Tune to Win, the picture I'm talking about is in the
chapter on aerodynamics, and outlines the flow disturbances caused
when you leave the mounting holes open on the top of a wing; basically,
you lose 20% of your wing surface due to turbulence screwing up the
nice attached flow, at least for the configuration Smith chose to
illustrate.)
In vehicle aerodynamics, turbulence is BAD and laminar flow is GOOD.
Laminar flow does things like push the car onto the track or slip
evenly with little drag over the body (and it's almost always OR,
not AND). Turbulence does things like build up air in front of the
car acting as a brake, or trying to pick the rear end of the car up
and make it come around (or sometimes over), or at the very least
reduce your car's grip, stability, and fuel economy (which is even
an issue in racing, as it's difficult to win if you run out of gas
before the finish line).
So in my mental model of the fluid dynamics of the situation, I had
assumed that the same held true. Of course, it doesn't, if you think
about it; laminar flow in a cooling system would lead to cooler
portions of the fluid running near the metal of the radiator, and
warmer portions of the fluid running in the center of the tubes, in
nice even layers. You *want* turbulence in the core so that the
temperature of the coolant-radiator unit is as homogeneous (and
therefore as high, entropy being what it is) as possible.
> I would expect in this
> case that turbulence introduced at the thermostat would have a
> negligable effect upon heat transfer.
I'd expect that too, if only because so little of the heat transfer
happens in the thermostat. However, increasing the flow rate through
a fixed orifice is going to have SOME kind of effect on turbulence,
laminar flow, and other fluid dynamic (as opposed to thermodynamic)
effects. I've convinced myself that a well-mixed, homogeneous quantity
of fluid passing through the radiator will contribute to the highest
mean surface temperature at the radiator core, if only due to the second
law of TD, and because water is typically a better insulator than metal.
I am open to reasoned discussion of opposing viewpoints.
It's not yet clear to me, though, how much effect an unrestricted
thermostat housing would have on increasing the flow to such a degree
that these kinds of effects really occurred. In fact, I would suspect
that the actual flow-rate increase caused by removing the thermostat is
more likely on the order of 5% than 50%. It is my considered professional
opinion that there is far more waste heat involved in this discussion than
in the actual engines we're talking about.
But none of that matters very much, and all of you thermodynamics types
are still way off base, regardless of how many times you took the class
or whether or not you can spell enthalpy. The thermodynamic envelope of
the typical car radiator is so broad, and the flow range with or without
thermostat so narrow, that it just doesn't apply.
Let me repeat: the reason your car runs hotter with the thermostat OUT
than with it IN is most likely because the thermostat is there as a
flow-direction device, moving coolant through passages in the head as
well as through the radiator. For Jim's Mini in particular, I can
speak with some knowledge: the heads on A-series and B-series engines
have a small passage outside the brass sleeve that hangs down from
the thermostat plate. The sleeve is there to direct flow at a known
rate through that passage. See Vizard, Porter, and the BMC Special
Tuning Manual for source information (some of us not only read the
manuals, we also read additional technical material on the actual
construction of the motors in question). Specifically, see page
415 of the blue edition of Vizard's "Tuning the A Series Engine."
The primary purpose of this passage, of course, is to recirculate the
coolant before the thermostat opens. If it didn't go anywhere, the
water pump would stall or the engine would bust a hose. But the
engineers cleverly guide the coolant back through the cylinder head,
because that's the area that most needs cooling as it's where the
explosions take place. Vizard specifically says that if you run an
A Series engine without the thermostat in place, you end up with
uneven cooling of the cylinders because the flow is disrupted, and
that you will probably end up *running hotter than if you left the
thermostat in place.* B Series engines (MGAs, MGBs, some early TVRs
and Austin/Morris Marinas, Itals, and other vehicles) have a similar
passage coming off the thermostat's resting place in the head, a
passage that perhaps I ought to photograph and post as a GIF on Hoosier.
Again, while I have been specifically addressing a pair of very similar
BMC-designed engines, the issue applies to any engine with a thermostat
for the simple reason that if the thermostat is closed, and the coolant
doesn't circulate through some other passage, Bad Things will Happen.
In short, the thermodynamic issues are orthogonal to the real reason
it's not a good idea to run without a thermostat. The REAL reason is:
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
* If you take the thermostat out, the car will run hotter because it *
* isn't getting enough coolant directed through the head, which is where *
* something on the order of 90% of the heat is generated in an *
* internal combustion engine. *
* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
This is why Jim's Mini runs the blanking sleeve (whether he knew that
or not :-) It has nothing to do with entropy, or enthalpy, or thermal
coefficients. It has to do with not getting enough coolant to the
right places in the motor in the first place. How much heat gets out
of the radiator is unimportant if you can't get the heat out of the
engine in the first place.
--Scott "Physics is fun, but don't overlook mechanics" Fisher
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