Steve, et al.
Are you just trying to stir up trouble, or is this really just an innocent
question??? :>) :>). Throwing in some happy faces so at least some of you
will know I'm just kidding. The argument you pose is the one most often
offered to explain the reason the restriction works. However, I believe
this is actually a specious argument. Now before you get bent out of shape
over my use of the word "specious", let me quote from my Webster's Noew
World Dictionary: "seeming to be, sound, correct, logical, etc. without
really being so". I do not intended by the use of this term to defame or
otherwise impugn you or members of your family. With that disclaimer, let
me explain why I think the "dwell time" argument is faulty and, in fact,
more flow is better. One of the things we know for sure is that the rate of
heat transfer is proportional to temperature difference. This applies both
on the engine side and at the radiator. Heat is carried away from the
radiator by forced convection. Keeping all other conditions the same, the
rate at which heat is transfered from the radiator to the air is
proportional to the temperature difference between radiator and air. This
seems pretty obvious. I notice, for example, that my water temperature runs
about 90 degrees F over the local air temperature. This will vary somewhat
with humidity and elevation (air density), and whether you have a tail wind
or head wind, etc.; but I'm sure you get the idea. What I'm leading up to
here is that the radiator works best when its hottest, and hot all the way
across from inlet to outlet. To the extent that the temperature decreases
as it flows through the radiator, you lose efficiency. Now I know what you
are probably thinking; if the water doesn't cool off while it goes through
the radiator, how can it cool the engine? This is where the "specious"
thinking comes in. Let's break the problem up into four components; engine,
coolant, radiator, and air. The engine produces heat. The coolant
transfers the heat to the radiator. The radiator transfers the heat to the
air. Seems simple enough. Now consider what happens as we vary the
coolant flow rate from zero to infinity. At very slow flow, the coolant
temperature in the engine is the engine temperature and the coolant
temperature in the radiator is at air temperature. In this example,
essentially no heat is removed from the engine. Obviously, this case
doesn't work. As we increase coolant flow rate, its temperature difference
decreases continuously, appoaching a uniform temperature at a sufficiently
high flow rate. Under this circumstance the coolant is at an intermediate
temperature between the engine and the radiator and has ceased to be a
limitation on the transfer of heat between the engine and the radiator.
Whether the coolant temperature is exactly half-way between the engine
temperature and radiator temperature would depend on the relative heat
transfer efficiencies at the engine and at the radiator. At least the
radiator, I think we can assume almost no temperature difference between the
coolant and the radiator. There may be a more significant temperture
difference inside the engine; especially in dead-end sections, etc. But,
I'm probably getting into a little too much detail here. In general, with
sufficient flow, the coolant will come to the optimum temperature for
transfering heat between the engine and radiator and other factors will then
become limitations on the removal of heat from the engine; e.g., the flow of
air past the radiator.
Based on this simple analysis alone, the use of restrictors in the cooling
loop would seem counterproductive. However, too many varifiable examples
exist that show a positive effect of restrictors. Does this mean that the
above reasoning is also specious? I don't think so, otherwise I would not
have offered it. What I do believe is the above reasoning is basically
valid and that a reduction in flow rate per se is never benficial. However,
there must be one or more factors other than flow that benefit from the
restriction. I have offered some ideas as to what these may be, but these
are just speculations on my part. It's at least fun to try and divine
what's going on.
A friend of mine passed on anothe anecdotal story about a particulary
variety of Ferrari V12 heads that had a cooling problem. The heads were
sectioned and flaws in the casting were revealed that left lots of pockets
where water could stagnate. The heads were redesigned and the problem went
away. I don't really see how this story relates to our particular problem
though. The problem with the Ferrari heads would lead to hot spots, not
overall cooling problems as with Tigers and I guess Panteras, etc. One of
the possible ways that the use of a restrictor helps is by building more
pressure inside the engine, especially the heads, to reduce local boiling.
This isn't an original idea with me, but seems plausible. Another thought,
which I think I originated, is that by adding a restriction before the
pressure relief cap, you increase the overall pressure everywhere in the
system. This could have a really beneficial effect on the inlet side of the
water pump. Cavitation, and consequently loss of flow, would worsen as the
coolant temperature rises and also as the pressure at the inlet of the pump
decreases. The article I mentioned in my previous e-mail talks about
pressure differentials of up to 40 psi produced by the water pump. If we
take this as being true, then let's consider what happens if we take our
simplified cooling system and add a restriction. Assume, for the moment
that neither the engine or radiator are significant restrictions. Suppose
we start throttling down at the point where the thermostat usually is.
Initially, the pump is flow limited and is producing very little pressure
differential. Now as we increase the restriction, the pressure difference
increases. Over some range of restriction, there may be very little change
in flow because the pressure difference at the pump increases to compensate.
Let's assume that this is basically true up to, say 40 psi. Assume also,
that we have a 15 psig pressure cap at the usual location; i.e., after the
restriction. In this example, the pressure in the cooling system following
the restriction, through the radiator, and incuding the inlet side of the
pump is all at 15 psig. At the pump we have a 40 psi pressure difference,
so inside the engine the pressure is 15 + 40 = 55 psig. Contrast this case
with the case where all the pressure drop is across the radiator. In this
case we have 15 psig at the inlet to the radiator (and everywhere else in
between the pump and the radiator, including the engine) and a 40 psi drop
across the radiator so the pressure between the outlet of the radiator and
the inlet to the pump is 15 -40 = -25 psig. I can definitely see the
possibility of problems with this situation. Any given "real world"
situation probably lies somewhere between these extremes.
Does the above model really give the right explanation for the beneficial
effect of a restriction? Maybe so, maybe not. It does seem plausible, but
I would be happy just to be able to have elevated the discussion a little
bit above what has preceeded which has seemed to me to be logically
inconsistent. (I'm trying to be as gentle as possible here.) If we were to
fully instrument a car with pressure and temperature sensors, we could
probably really nail this topic down. I think as a practical matter, must
of us find it easier to do the things that make sense and/or copy what
others have done that seems to work. However, using anecdotal information
without any theoretical foundation can be dangerous. Seldom does one do a
controlled experiment with changing just one variable at a time and it's
easy to make erroneous assumptions when this is the case. If you have at
least the basic ideas right you can avoid at least some of the pitfalls.
One example in my case comes to mind; I wanted a new water pump and was
thinking about getting the Motorsport impeller and having my old water pump
rebuilt with this impeller. So I called up the local rebuild shop and asked
about a price. The guy in the shop immediately wondered why is was using a
high performance impeller. I explained my application and he started off on
the usual crap about flowing the water too fast through the radiator and
what he usually does for high rpm motors is to cut every other vane out of
the cheap stamped metal impeller. I listened as politely as I could, and
then called Sunbeam Specialties and ordered their HiPo pump. Seems to work
fine. Can't say if it is the absolute best, but I have no indication of a
problem from idle up to the 7-8000 rpm range. Somehow it strikes me as a
little ridiculous to take the advice of someone at the local water pump
rebuild shop versus the Ford engineers who are getting the big bucks to
design parts like impellers that work.
Well, I've gone on at much more length than I intended. Now it's someone
elses turn to expound for awhile. Hope this has been at least a little
amusing if not enlightening. By the way, I will get you the information
about my electric fan. I also like the idea of two fans side-by-side which
has been submitted previously.
Bob
>Jim,
>
>RE: Laminar vs turbulent flow, I am not sure that is accurate (see Bob's
>memo). However, I did use a reducing washer on the custom rear
>engine-front radiator car I built, and it did improve cooling. Maybe the
>decrease flow rate increases the "dwell" time of the hot water in the
>heat transfer section of the system (radiator) and allows more heat to
>be removed per pass. This would decrease exit temperature. Think about
>it, there are a lot of washers in use in modified cars, and are
>available in speed shops. They all don't do it if it doesn't work. I do
>believe a valid thermal case can be made for dwell time in a heat
>exchanger. What do you think, Bob?, Jim?
>
>Steve
>--
>Steve Laifman < One first kiss, >
>B9472289 < one first love, and >
> < one first win, is all >
> < you get in this life. >
>
>
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