This is going to be a long post. My apologies, but I restrained myself as
long as I could on this subject.
Jim Fuerstenberg wrote:
>
> RE: the cooling problem, you do not want coolant to flow too fast or
> the radiator(s) will not remove enough heat. That is why when we
> remove our thermostats for racing, we install a blanking plate,
> which slows down the coolant a little.
>
> jim
>
> "It has been said that motor racing shares in common with sex the distinction
> of being of the most popular, most maligned and least understood of human
> activities.
>
I know nothing of motor racing, and not enough about sex. But it seems
that the effect of coolant flow on engine cooling ranks right up there
with these more entertaining activities as far as being misunderstood is
concerned.
And Martin Frankford wrote:
>
> I am amused by the lack of knowledge of heat flow out there in
> net-land. Heat flow is exponential with exposure time. That
> means that if x calories was loss when y gallons spent s seconds
> in the radiator, then the square root of x calories would be loss
> by y gallons if it spent s/2 seconds in the same radiator.
>
And then John Lupien quoted M. Frankford:
>
> > I am amused by the lack of knowledge of heat flow out there in
Etc.
And added:
> Saying that heat flow is exponential with exposure time ignores
> the basic fact that the air flow over the radiator is what removes
> heat from a radiator - the transfer of the heat from the radiator
> to the air depends on the relative temperatures of the air and the
> outside of the radiator, and NOT on the flow rate of the coolant
> (except indirectly). The reason that a higher flow rate of coolant
> may improve the cooling is that more total heat is entering the
> radiator, so more of the radiator may come up to the temperature
> of the coolant. Normally, radiators are hot at the top, and cooler at
> the bottom. If the flow of coolant goes up, you'll have a radiator that
> is hot at the top AND the bottom, and it will transfer more heat to the
> air as a result. This is the only way that heat can actually leave the
> system, since the coolant is closed loop (residual heat in the coolant
> leaving the radiator will eventually come back in at the top).
I want to add my $.02. I basically agree with John. Martin's approach
is too simple, and moreover it neglects the other end of the circuit.
First, it probably oversimplifies to say that the square root of x
calories, rather than x, would be lost if the flow rate doubled. The
exponential heat flow with time is, I believe, for heat flow between two
objects in a case where the temperature of one decreases and the other
either is constant or increases during the transfer. But in a radiator,
the water is constantly being replaced, as is the air outside. Thus, I
don't think the exponential relation holds in any simple way. I would
have to read Carslaw and Jaeger's "Conduction of Heat in Solids" to be
sure, and that would be a fate worse than overheating.
But it may not matter much. If we assumed that heat flow between coolant
and air were exponential as Martin describes, it would also be true that
the square root of x, rather than x, would be absorbed by the water as it
passes through the engine faster. In other words, if the flow rate
increases, each bit of water presumably does lose less heat while it is in
the radiator, but it gains less heat during its faster passage through the
engine. I agree with John that the radiator should be hotter at the
downstream side with faster flow, and this would help cooling. Of course,
the water re-entering the engine will now be hotter, because the
downstream side of the radiator is hotter, which would reduce heat
transfer in the engine. :-} (wry smile)
I haven't the time, energy, or expertise to do a detailed analysis, but I
think John has pretty much got it right. IMHO, increasing flow either
will improve or not change heat transfer. I reason as follows. The point
of the coolant is to transfer heat from the engine block to the air around
the radiator. If the coolant were stationary, there would be no heat
transfer between the engine and the air, so the engine would overheat. If
the coolant moved infinitely fast, then the water temperature in the
radiator and in the engine would be identical, and the net effect would be
as if the engine transferred heat directly to the air (I am probably
neglecting some surface area effects here, but no matter). I argue that
this would be optimal.
But, but, BUT, a caveat is that in the real automotive world one may be
wrong in his predictions about what will increase flow. Something like
the blanking plate Jim spoke of should increase the total resistance to
flow in the system (if the engine is one that requires a collar to direct
flow to part of the engine, all bets are off). Thus, if the pressure
driving coolant were constant, the blanking plate could only reduce flow,
which would, I think, decrease cooling. However, it is just possible that
the removal of the thermostat in some engines might cause turbulant flow
(cavitation) around the water pump, as someone already suggested,
especially at high RPM, and maybe this would decrease the pressure and
flow. So, if racers find that a blanking plate helps, great. I would bet,
however, that it doesn't help because it decreases flow.
This all reminds me of my intellectual and financial turmoil when a new
$3000 hot water furnace and circulating pump hooked to my old radiators
could not keep my house at 65 on cold nights, whereas the old furnace had
worked fine. The guy who had installed the new furnace said that the new
furnace got water as hot as the old one, but the new improved pump moved
it faster. So, he argued that I needed more radiators, because the faster
moving water could not transfer heat as well as the slower moving water
from the old pump. Sound familiar?
Of course, if the plumber were right, slowing the flow would be beaucoup
cheaper than adding new radiators. There was a gate valve in the system,
so one cold night I experimented by closing it in stages. As I restricted
the system and slowed the flow, the house got colder and colder. After I
kicked enough a**, the plumbers discovered that the thermometer on the
furnace was way off, and the water was nowhere near as hot as in the old
furnace. When the water temperature was made the same as before, the
system with faster flow worked better than the old one, because the faster
flow caused hotter and more uniform temperature throughout the radiator
circuit.
Ray Gibbons, in Vermont,
where cooling stuff is rarely a problem.
|