"Michael D. Porter" wrote:
> 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.
Yes, it does... However, it be absolutely positive of all your results,
you have to check a number of cars' systems including those which are
clean and those which have crud stopping up the radiators and blocks;
some with the low pressure caps, some with high; some with top tanks,
some with side tanks... I hardly think that kind of effort is warranted
for such a nebulous subject anyway. 8^)
Joe
--
"If you can't excel with talent, triumph with effort."
-- Dave Weinbaum in National Enquirer
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