Joe Curry wrote:
>
> Randall,
> While I don't dispute the referenced passage, I think there is a bit (if
> not a lot) of truth to this story you have been hearing. Here is my
> explanation of the dynamics of what happens.
>
> In order for the heated water to be cooled, it has to transfer its
> energy to the heat sinks (cooling fins) in the radiator coils. If it is
> flowing too rapidly through the radiator, the amount of energy that is
> absorbed by the fins is minimized, and reduces the effectiveness of the
> radiator.
>
> On the other hand, restricting the flow to a certain extent will cause
> the water to stay in the engine too long and pick up more heat than is
> dissipated by the radiator. The trick is to find that magic point where
> the water flows at just the right speed to dissipate just enough heat to
> keep the engine at the desired temperature. That is why the thermostats
> are temperature rated to open at a desired temperature.
I waded in on this subject about a year or so ago, and a number of
people said, "restricting flow is a myth." I abjured, but I still have
these nagging questions with which to contend:
1) Most engineering guides (see Marks' as an example) suggest, or state
plainly, that heat transfer increases as the delta T increases (the
difference in temperature between the radiating medium--the radiator
system (influenced by airflow through the radiator)--and the circulating
coolant, in this case).
2) Heat transfer is a _rate_ equation. That means it is
time-dependent--so much heat transferred in so much time. This is
because heat transfer from surfaces and through materials occurs as a
function of time.
Most of the naysaying in this matter is based on two presumptions--flow
restriction is unnecessary because, without restriction, the rate of
coolant flowing through the radiator is increased and, therefore, heat
dissipation is increased, and, second, higher rates of flow mean lower
times of coolant residence in hot spots in the engine, and, therefore
(because of a presumed lowering of the effects of nucleate boiling),
increased heat transfer from engine to coolant to radiator.
However, there are some practical and theoretical considerations to be
puzzled over in the above. First, there is the question of coolant
pressure supplied from the water pump. The site mentioned suggests that
early upright radiator systems suffered from explosive discharge from
the cap because of water pump pressure. Hmmm.... A practical example is
required. My GT6 has an upright radiator. It was normally fitted with a
6 lb. cap, I believe.
Currently, it is fitted with a 13 lb. cap, a significantly higher-rated
cap than is used on current crossflow designs. Moreover, older design
radiators discharged coolant when the heat-related expansion forced the
cap off its seat. Newer designs, regardless of cap placement in relation
to flow, allow for fluid escape to the overflow bottle when internal
pressure exceeds the cap rating--that fluid is drawn back into the
system when the system cools.
The only time this system has vented significant amounts of coolant was
at a time when the water pump was _failing_. A seal failure allowed the
pump to draw in significant amounts of outside air and introduce that
air into the cooling system, progressively increasing the system
pressure, until, with pressure and temperature combined, the cap vented
to atmosphere.
Disregarding this information, one must ask why the so-called myth about
restriction came about. Professional engine builders, well into a time
beyond that of ancient radiators, still recommend restrictors in the
absence of a thermostat. Practical experience suggests that this is a
good thing. Why, if it is wrong?
A person a couple of years ago said to me, "I take my thermostat out in
hot weather in my TR3, and it runs great." The person did not say, "my
radiator is new and completely free of debris and scale, as is my block
jacketing." The restrictions in the system may be so great that they are
unnoticeable in the cold months of fall and winter, but may inhibit flow
and heat transfer in summer to such an extent that thermostat
restriction reduces flow so much that normal system cooling is impaired.
(!) That's not a suggestion that restriction is a bad thing in a normal
system. Saying that too high a flow rate is bad is not the same thing as
saying that too low a flow rate is good. System balance is required.
Hmmm. What I'd like to do is get a laptop with some data acquisition
software, install a few thermistors and then compare heat rejection
rates through the same radiator and engine combination under the same
conditions, with and without restriction, charted over time. Empirical
data counts.
Cheers.
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