>From herbie ford-trucks.com Fri Oct 2 06:11:48 1998
Date: Fri, 2 Oct 1998 06:11:48 -0400 (EDT)
From: owner-perf-list-digest ford-trucks.com (perf-list-digest)
To: perf-list-digest ford-trucks.com
Subject: perf-list-digest V1 #106
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perf-list-digest Friday, October 2 1998 Volume 01 : Number 106



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Ford Truck Enthusiasts - Performance
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In this issue:

FTE Perf - Re: Long cooling ramble

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Date: Thu, 01 Oct 1998 23:30:02 -0700
From: Vogt Family
Subject: FTE Perf - Re: Long cooling ramble

I am the one who re-started the beating of this dead horse. The reason
was I have still not seen an answer that satisfies me.

That being said, "Chris Samuel" wrote:
>
> "Technically, as long as the water is in contact with the block heat
> transfer is happening at the same rate regardless of water speed but the
> speed causes more turbulence and I think it's the turbulence that causes the
> problem not speed by itself. Technically, more speed means more
> cooling......IF the radiator can dispose of it."
>
> Said Gary.
>
> This thread is just too much fun and causing me to read too much!
>
> The thermal transfer between any material and water happens at a rate of "X"
> time/Thickness/in.sq.to raise water temperature 1 degree F. So as you can
> see there is a time differential to this equation. To achieve maximum
> cooling the goal is to remove maximum heat from the engine, so the coolant
> should be as hot as it can be when it gets to the radiator. Or stated
> another way the Delta T between the inlet water and the outlet water should
> be as high as possible with in the design limits of the system.
> This is going to happen only if there is enough time to fully BTU load the
> coolant.
> The issue of turbulence is one where turbulence is actually desirable and
> the properties of laminar flow will in-fact reduce cooling.

I can't agree here, I disagree with the time factor: I think Gary got a
little confused. The time is only important in respect to the amount of
heat transferred. Think of it this way: we can equate heat flow with
water flow. You can measure the rate of flow from a spigot in GPM, and
once you know that you can figure gallons based on how long it has been
running. GPM here is equivalent to rate of heat transfer and gallons is
BTUs.

> "Put a big enough radiator on it, take out the thermostat and run enough
> volume past the surfaces in the engine and it will run COLD not hot from a
> physics stand point but as you said the turbulence causes areas where there
> are lots of restrictions and turns to actually cavitate which is what is the
> real culprit IMHO."
>
> Gary said.
>
> But in the real world this is often only what appears to be happening. In
> reality there can be a wonderful amount of flow and yet the engine
> overheats. The key is a mixture of both flow and pressure and time; not just
> one of these.

Water is an incompressible liquid. It does not care what pressure it is
at in an immovable environment such as the engine block (except for
boiling point) its density and therefore its heat accepting capability
remains the same.

> "Heat moves from the block to the coolant at the same speed or rate no
> matter what based on only two things I know of: Temp difference between
> the two mediums and time. If the water moves too slowly the temp difference
> will diminish causing a loss in efficiency but if the coolant is separated
> from the walls for an instant here and there then the time element is
> compromised which is what turbulence does."
>
> Quoting Gary again.
>
> The rate of transfer is not constant as I understand it. Not because the
> rate that the metal gives up heat changes but because colder water accepts
> heat at a slower rate then hotter water. Ok we are splitting hairs but...
> If the water moves too slowly then the water will boil and yes that is a bad
> thing. The turbulence issue is not true up to a point. Picture this if you
> will: The flow of water out of a hose. If viewed as a cross section The
> walls of the hose being the water jacket. If the temperature of the water is
> measured the boundary layer will be higher then the center temperature. The
> observation also correlates that the flow will be slower at the outside then
> the center.

By definition, rate excludes time. In its pure form, rate is an
instantaneous measurement, taken irrespective of time. The rate you get
by taking two measurements and dividing it by the time is an average
unless the rate happened to stay exactly the same for all that time.

I also take issue with your assertion that cold water accepts heat
slower than hot water. Temperature differential affects heat transfer
rate and the greater the differential the higher the rate. This is why
hotter running engines are more efficient; they transfer less heat of
combustion to the cylinder walls which are hotter, thereby leaving more
heat to do work.

> If the flow is increased to some point the center will simply not change
> temperature as it is moving too fast, while the outside may boil. What we
> want is to have turbulence so that we get the center of the flow to deflect
> and scour the boundary layer off the walls, and the hot boundary layer is in
> the center so to speak. Yes too much turbulence is a bad thing but it is
> fairly hard to do with OEM pieces. And again the flow rate is designed to
> have a t/stat restricting it.

I can't see how faster flow in the middle of a path would cause slower
flow in the outside layers but there may be some shear effect I don't
know about.

> Again Gary said:
>
> "I'm sure most OEM cooling systems are perfected through trial and error
> based on these phenomena. Run it too fast and you get good transfer but
> may cavitate some areas of the heads causing hot spots and "after boil" on
> shut down etc.. Run it too slow and you will have even cooling but may not
> be enough transfer to keep the engine cool enough."
>
> This has been a topic that is so misunderstood and difficult to grasp as to
> drive one knutz!
> Slowing the water in the block and heads is deliberate. This is because you
> are not "Cooling the engine" by putting cold water in. You are cooling the
> engine by taking heat out. A small difference in wording but all important.
> The temperature of the water is not important up to a point.
> The temperature of the engine metal is important.
> The amount of heat that is removed by the water is important.
> The amount of heat dissipated by the radiator is vary important.
> The physics of the model are that you want to be able to remove enough heat
> from the engine to hold it at a specified or design temperature; no more no
> less.
> The easiest way to do this is to size the system so that it is capable of
> holding the engine operating at maximum design output for a designed amount
> of time, at a designed temperature, on an "Air Standard Day".
> The inlet and outlet temperatures need to reflect the coolant picking up a
> full BTU load in the engine and a full reduction across the radiator. Here
> you have two time factors, and yes flow rates. The thermostat is the method
> that is used to regulate the temperature when operating at less then maximum
> output and to give quick warm times, or operate on other then standard days.

You are absolutely correct in that at the molecular level there is no
such thing as "cooling" per se. But two bodies of different
temperatures will attempt to equalize their temperatures, the greater
the differential, the quicker they will do it. There is no provision
for the speed of one of these bodies (the water) affecting the heat
transfer rate that I know of. I do not know all about the design
criteria, only that there is a lot of misinformation floating around out
there about basic thermodynamics! There has to be another explanation
out there. I pose my original question again, since I had conjectured
that maybe the increased flow had blown off the radiator cap causing the
appearance of boiling on a down flow ratiator: Has anyone experienced
this phenomenon on a side-flow radiator equipped vehicle (cap on cool
side tank) or noticed a temperature rise without coolant venting?

BTW: In Oregon this summer, I visited a farmer whose grain truck is
powered by a 396 Chebby. He had removed the thermostat because the
heater wouldn't shut off. That thing ran so cold it would not idle,
even after coming off the freeway.

Birken
== FTE: Uns*bscribe and posting info www.ford-trucks.com/faq.html

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End of perf-list-digest V1 #106
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