Your assertion that slowing the flow of fluid through the radiator will
increase heat transfer to the air flow is incorrect. The principle of
heat transfer I attempted to describe apply to any fluid whether it is a
liquid or gas. It is also independent of the direction of energy flow,
i.e. whether you are heating or cooling the fluid. Increasing fluid
velocity increases the rate at which heat will be transferred from the
system to the environment just as it increases the rate at which the
fluid in the system will absorb heat from the environment. Notice the
equation I provided does not make a distinction as to which direction the
heat flows. I gather from your statements that perhaps you are
envisioning a static case and considering the amount of energy absorbed
by a fixed mass of fluid. It is true that for a fixed mass or discrete
quantity that increasing the dwell time over the hot (or cold) surface
will increase the energy exchange to that mass of fluid. However, that
is not what occurs in the automotive cooling system. There is a
continuous flow of fluid and the heat transfer is proportional to the
product of the temperature difference and the mass flow rate. By
increasing mass flow rate you increase the average temperature difference
over the transferring surface and multiply by the velocity. Increasing
velocity within the limits of applicability for the equations (which I
guarantee is the case of all automotive cooling systems) will always
increase heat transfer regardless of the direction it is being conducted.
Now if I increase the surface area over which the fluid is flowing by
installing a larger radiator or increasing the number of parallel flow
channels in the block, and keep a constant mass flow rate the fluid
velocity will decrease (mass flow rate is the product of flow velocity
and flow area). As the equation shows, you will still an increase in
the heat transfer rate even with the lower velocity. However that is due
to the increased heat transfer surface area. Increasing the flow
velocity and hence the mass flow rate will even further increase the heat
transfer.
All of this is easy to test. You can decrease the velocity of airflow
through the radiator, which will test the same principle as reducing the
fluid flow velocity through the radiator, by simply removing your fan or
placing an obstruction in front of it and comparing your engine
temperature (with thermostat removed of course for obvious reasons) to
the temperature without the alteration. I guarantee you will see an
increase in operating temperature. Truckers occasionally use this very
technique to increase diesel engine operating temperature in cold
weather. You will see truck radiators often covered with a cloth flap or
even louvres in the winter time.
Andrew
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From: Phil Martin[SMTP:pmartin@isgtec.com]
Sent: Wednesday, August 25, 1999 3:37 PM
To: Bricklin
Subject: Flow rate and thermal transfer
I think that one very important factor has been ignored in this
discussion
so far.
You want your cooling system to do _two_ things that are in some respects
at
odds with one another:
1) Extract heat from the engine
2) Give off heat via the rad
It's clearly true that circulating a greater volume of coolant through
the
engine in a given amount of time will increase the cooling capacity _IF_
the
temperature of the coolant as it enters the engine is kept constant.
Note
that this breaks down for very high flow rates (where particle speeds
reach
a few hundred miles per hour) where the kinetic energy of the coolant
particles slamming into the water jacket walls would give up energy to
the
engine block. But realistically, at the temperatures and flow rates
you're
going to see in an engine, this effect can almost certainly be completely
ignored.
However, it's also clearly true that leaving a given volume of water in
the
rad for a longer time will allow it to cool more. So increasing the flow
rate through the rad decreases the amount of heat that can be dissipated
from a given volume of coolant for one cycle through the rad. Of course,
if
the universe is gravitationally closed, then after a few dozen billion
years, the big crunch is going to drive the temperature in that rad way,
way
up. But in that case, you'd have bigger problems. ;)
So, there are two opposing forces here: the faster the coolant flows
through
the engine, the better able it will be to dissipate heat, and the slower
it
flows through the rad, the cooler it will be when it re-enters the
engine.
Depending on the slopes of these two curves, increasing flow through the
system as a whole could sometimes increase cooling effectiveness, and
sometimes decrease effectiveness. There are a lot of inter-related
variables.
You can't generalize and say that a greater flow rate through the system
increases or decreases operating temperature, because the cooling
effectiveness depends on both the flow rate through the engine (more flow
through engine = better cooling), AND the coolant temperature as it
enters
the engine (more flow through rad = higher engine inlet temperature =
worse
cooling).
Which is why the damn thermostat is in there. The thermostat regulates
flow
through the engine to keep the temperature within a certain range so that
you don't have to figure all this stuff out for yourself. If the
engine's
consistently running too hot, change the thermostat and flush the system
to
get rid of air pockets and/or whatever gunk might be blocking the cooling
passages. If it's still running too hot, go with a cooler thermostat.
If
it still can't keep up, you probably need a more effective rad, or the
engine's out of tune somehow.
Of course, as has been pointed out, if you only drive the car very rarely
the risk of the thermostat seizing might outweigh the inefficiency that
results from running without one altogether, so there are no easy
answers.
Hope this is worth something,
--
Phil Martin pmartin@surgnav.com
"Who's my cream-puff?"
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