I'm glad you ask the question because it points out the source of a lot
of misunderstandings arising out of "intuitive" thinking. The human mind
tends to look at things in a linear fashion which works fine as an
approximation for most every day problems. However, in nature physical
phenomenae tend to be non-linear. Such is the case with heat transfer.
As has been pointed out in a couple of posts by Mike and/or Candy as
coolant passes across a hot surface its temperature increases as heat is
removed from the source. Mathematically heat transfer for a fluid
passing over a heat source (in its simplest expression) is proportional
to the product of a heat transfer coefficient (a function of the material
properties of the interface), the transfer unit area, the temperature
difference between the heat source and the coolant, and the fluid flow
rate. That is, Q is proportional to: Cp x A x Mdot x (Ts-Tc). This
formula clearly shows that heat transfer increases as flow velocity
increases. To help understand this envision your copper tube as being
heated and having a constant temperature along its length. Now envision
a flow of cool water into the tube. At the entry end of the tube the
coolant has a low temperature and the tube is hot. Heat transfer per
unit mass of coolant is at its greatest at this point since the
temperature difference is at its widest margin. However, as the coolant
absorbs heat its temperature increases, thus as it flows down the tube
the rate of heat transfer progressively decreases. The total heat
transfer will be the product of the change in temperature of the flowing
fluid, its heat capacity and the mass of the fluid. If we slow down the
flow rate, more heat per unit mass is absorbed at the beginning but the
rate of heat transfer down the length of the tube decreases, because the
temperature difference is smaller, and the quantity of mass flowing is
less because the velocity is lower. Therefore we may be maximizing the
amount of heat absorbed by the fluid but we are decreasing the amount of
heat transferred from the source. Now conversely imagine the velocity
of the fluid increasing. Two factors are working to increase the heat
transfer rate; First the temperature difference throughout the length of
the tube is increased, and secondly the quantity of mass absorbing heat
per unit time is increased. The product of the two factors dramatically
increases the total heat transfer rate.
That being said let me point out that you (Greg) are correct in noting
that other factors affect heat transfer and you specifically pointed out
pump cavitation. In fact there are many important factors and the
mathematics for describing real world heat transfer problems can become
quite complicated. Factors such as coolant physical properties, system
pressure, geometry, surface roughness, turbulence, fouling etc. all
effect the heat transfer. Nevertheless, the fundamental property of heat
transfer increasing with flow rate is pretty much a hard and fast rule in
mother nature. Pump cavitation (or cavitation anywhere in the system)
while it can dramatically decrease heat transfer is not a violation of
this. Rather cavitation is introducing a change in the thermophysical
property of the coolant as it is no longer a liquid but a vapor.
Understanding the nature of heat transfer is important as it can help us
avoid potential pitfalls. For example, someone suggested using an
underdrive pulley as a possible solution to an overheating problem. In
point of fact this will almost certainly reduce the heat transfer rate
and worsen the cooling problem. Underdrive pulleys are frequently used
to reduce the horsepower consumed by the coolant pump in performance
cars, but, they can only be successfully used in cars where the cooling
system has enough excess capacity to still cool the car with its cooling
efficiency reduced or in situations where the vehicle will only be
running for a short time, as in drag racing.
I hope this helps. If anyone still has questions or is interested in
other aspects heat transfer I will be glad to respond.
Andrew
----------
From: Greg Monfort[SMTP:wingracer@email.msn.com]
Sent: Tuesday, August 24, 1999 3:46 PM
To: Bricklin
Subject: Fw: COOLING SYSTEM
This is a pretty big misunderstanding!
AFAIK, heat transfer is primarily a function of temperature differential
and
a material's ability to dissipate or absorb it.
Using t'stat open Vs closed as proof that "faster is more efficient"
appears
to me to be apples 'n oranges. When closed, coolant is recirculated,
getting hotter with each cycle due to ever increasing resistance to heat
transfer as their differential is reduced. Once opened, it now can be
somewhat dissipated through the radiator. The increased rate of coolant
flow WRT closed isn't relevant per se, but the rate of flow through the
radiator WRT air flow through it is.
Using your data, John's temp should have increased when he added a
t'stat, since it reduces flow speed. At best, it would have remained the
same, if the t'stat caused no back pressure as some purport, but he
claims it dropped ~25deg. Please explain the mechanism that solves this
conundrum.
As an experiment, heat up some water to near boiling and pour it through
a
piece of copper pipe at different flow rates. Compare heat transfer with
your hand while blowing air across it.
I'll wager your hand gets hotter (more heat transfer) the lower the flow
rate WRT a ~constant airflow (car idling or creeping in traffic). Proving
in
this case what women already know; that slower for longer periods of
contact are better for achieving the desired results. :^))
Please let us know the results. Inquiring minds want to know.............
GM
----- Original Message -----
>
> Please let me clarify an apparently common misunderstanding regarding
> heat transfer. The statement being made that the rate of heat transfer
> is higher if the fluid flow is slower is incorrect and excactly the
> opposite of what actually occurs. Heat transfer is increased with
higher
> fluid flow rates! This is true both for the radiator fluid to radiator
> surface and engine interfaces and for the radiator surface to air
> interface. Thus increased capacity water pumps and fans will increase
> cooling capability even though the radiator has not been changed.
> Likewise, when engine temperature increases the thermostat opens ...
thus
> increasing the fluid flow rate and velocity to increase the cooling
rate
> and lower the temperature! That being said - maintaining correct
engine
> operating temperature with a thermostat as has been stated - is
> important! Not only to keep the engine from overheating but rather to
> keep it from running to cool which has adverse affects on engine
> lubrication and combustion efficiency.
>
> Andrew Somerville
>
>
> Mechanical Engineer and Bricklin owner
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