> Physics says that the heat transfer is better, when the speed of the fluid is higher.
> The Waermeuebergangskoeffizient (Heat-Transfer-Factor, I don't know the exact word in
> english) becames better with higher fluid speed. So the efficiency of a radiator is
> better, when the water flows faster. Too high water speed in the water jacket of the
> engine is another question.
I agree with that analysis. As I mentioned earlier, if coolant flows too slowly through a
radiator the thermal transfer rate goes down, partly due to the temperature differential
between the water and the outside air being reduced as the water flowing through the tubes
cools. If you can keep the temperature differential up along the length of the tubes, the
thermal transfer rate will remain higher. The increased rate of flow also makes up for the
lower amount of time in the heat transfer area, meaning that net heat transfer essentially
does not go down due to increased velocity.
Something else just came to mind (my latest theory...someone correct me if I'm wrong). I
believe that when a fluid flows very slowly along a surface (such as the inside surface of
the radiator tubes), it tends to be a more "laminar" flow than would take place with a
faster flowing fluid. A smooth laminar flow would have a relatively undisturbed "boundary
layer" next to the surface of the tube. This boundary layer could act as an insulator, not
allowing the hotter fluid toward the center of the tube to interact with the surface of
the tube, thus not transferring heat as rapidly. Faster flow would tend to break up the
boundary layer and allow the hotter fluid to interact with the surface, thus producing
faster heat transfer.
As to speed of coolant flow through an engine block, things get complicated. The geometry
of a radiator is pretty straightforward compared with the complex maze of passages,
angles, openings, intersections, etc. inside of an engine's water jacket. It's easy to see
that as coolant velocity changes, the currents, eddies, areas of higher/lower pressure,
etc. will also change, providing more coolant to some areas and reducing flow to others.
Thus there would be a good chance of improper cooling from running the coolant either too
slowly OR too quickly through the system. I would hope that the engine designers would
have experimented with varying flow rates and water jacket design to come up with a
combination that would deliver fairly even cooling. Apparently, removing the thermostat on
some engines can cause hot spots, while other engines are not as affected. If you get a
hot spot, it will cause local boiling, and thus become even hotter. And the bubbles will
affect heat transfer downstream from the hot spot, causing even more areas of the engine
to overheat and produce more bubbles, etc.
But as to overall speed of coolant flow through a "theoretically perfect" system, faster
is better as far as heat transfer rate is concerned.
Again, my advice is to ALWAYS use a thermostat. Keep the engine in the correct operating
temperature range.
Rod Diery made an excellent point that thermal expansion and contraction change the
operating clearances in an engine. You want to keep those clearances within their design
range. And there's the oil sludge problem as well. If you open up an engine that has been
run for a long time without a thermostat, you'll often see large areas where oil sludge
has built up on the sides of the block, due to those areas running too cool and allowing
condensation of a mix of moisture and oil vapors. This sludge can come loose and cause
problems. It can also act as an unwanted insulator, with varying consequences.
Alan Bowes
(Salt Lake City, Utah)
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