Wschertz@aol.com wrote:
In a message dated 2/23/99 0:48:01, you wrote:
Paul Yaw wrote:
The stock Mazda oil coolers have a series of wavy wires running through
the tubes. It has been explained to me that they create some turbulence
so that a greater amount of the oil makes direct contact with the
cooling surface. I cannot vouch for this myself, because I simply do
not know enough about cooling, but it does sound reasonable. Does
anyone out there have any input?
PY
-------------------
In a heat exchanger, there are three main resistances to the flow of the heat
from the oil to the air,
1. The film resistance from the oil to the metal wall of the heat exchanger,
2. The resistance to heat flow by the metal wall itself, and
3. the film resistance from the metal wall to the air.
Oil is a relatively poor heat transfer fluid compared to water, in that the
specific gravity, heat capacity, and viscosity all work to give a relatively
poor film coefficient for the transfer from the oil to the wall. Assuming the
Mazda engineers were working off of test data combined with engineering
calculations, the presence of the wires in the tubes was done to enhance
(increase) the heat transfer from the oil to the wall. This in turn means
that the oil-wall heat transfer resistance was the rate-limiting step in the
application as designed. (Doesn't help to increase that rate if the real
limiting step was something else - like the air resistance.) Therefore, in
the car, adequate air was being forced through the oil cooler, and the film
coefficient on the air side was smaller than that on the oil side.
On a separate topic, regarding oil/water heat exchangers, the most efficient
would be a counter-flow exchanger, where the hot oil enters at one end, and
the cooler water enters at the other end. That way you have a maximum delta-T
across the tube at all points. Such an exchanger can be built from two
concentric tubes, with the oil in the center tube, and the coolant flow in the
annulus. This would be external to the oilpan of the engine, and would suffer
the two extra fittings in the high pressure side of the oil that Paul has
mentioned.
The problems with the tube-in-the oil-pan method that Tracy used, is that the
oil is essentially quiescent, moving very slowly past the heat-exchange
surface, therefore the resistance to heat transfer will be greater. The fact
that it works so well indicates that you don't have to get a lot of heat out
of the oil to do some good.
I disagree on this point Bill. There are about 40 gallons of oil moving
through
the pan from top to bottom per minute. The mixing might not be the
greatest
but never the less there is some flow through there. PL
Another concern that I would caution about is the mixed metals in the coolant
loop. As I understand it, Tracy has aluminum radiators, and a copper tube in
the bottom of the oil pan. Coolant is flowing in a closed loop through the
engine, copper tube, aluminum radiator, engine---- . Something to be monitored
-- keep the antifreeze solution with its corrosion protection fresh, or you
could see pin-hole corrosion in the aluminum radiators. The mechanism is that
if a small particle of the copper tube is transported to the aluminum surface,
(i.e. a filing from the construction, or some very small corrosion level of
the copper), the copper will plate out on the aluminum, and then an
electrochemical corrosion cell is created on the aluminum surface. The copper
particle just "eats" its way down through the aluminum, and voila -- a pin-
hole leak. If someone decided to run their system with just plain water, this
is almost guaranteed to happen -- use the antifreeze with corrosion
inhibitors.
These problems surfaced in a lot of the solar energy collectors that were
designed with Aluminum collector plates and copper plumbing in the rest of the
system.
Bill Schertz
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