Do you still deny that convection is the primary energy
transfer mechanism in a car radiator?
Do you still deny that convection can only exist if it's
natural convection? If the answer to the above is yes, please
A - Convection ovens, which use a FAN . I have two
here so I know they exist.
B- Why numerous references, some of which have
been cited, talk about natural and forced convecton.
I have yet to see your reference that says convection
can only be natural, ie without a fan or pump.
C- Why engineers treat air to water heat exchangers using CONVECTION,
not mass transfer
And citing a book cover on Amazon is not a scientific
On Thursday, March 14, 2013 10:13:27 PM UTC-4, email@example.com wrote:
Do chemical engineers consider mass flow and mass transfer to be very diffe
rent things? If mechanical engineers do, then that's either a recent chang
e or my memory has completely faded. I haven't actually done this calculat
ion in a long long time, I became a suit.
But anyway. The very FIRST thing a mechanical engineer does in analyzing a
n air to fluid heat exchanger, after drawing the system boundaries of cours
e, is a Mass Balance. The second thing is an Energy Balance. The third th
ing is an Entropy Balance. Normally all three are required for a solution.
Sometimes the Entropy Balance is called the Availability Balance.
The mechanism by which heat is carried away from a car radiator is the flow
of mass called convection. Convection can be forced or free in the mechan
ical engineer's world, that may not be true for physicists, who knows?
I looked it up, my memory was correct. A car needs a good 3 tons of AC, mu
ch like an average sized house.
I've been looking into adding a unit and have found that most of the
underdash units are 15000 BTU. Some of the bus type units are 24000
BTU. I also found some info that suggests that most OEM factory units
are around 22000 BTU's. I also found info on the ratings of
compressors at various rpm. The smaller ones may only deliver 10,000
BTU's at 1000 rpm and max out at 20,000 at their peak. Some of the
larger ones will peak at around 25000 or a bit more. So it looks to
me like the most you can expect for a car is a bit more then 2 tons of
capacity. Still, that's a lot considering it would cool a 1200 sf
Why do you answer a question with a question and try to
take this even further off point? I posed a few very simple,
yes or no questions that go directly to the point of the discussion:
There are 3 modes of heat transfer, conduction, convection
and radiation, correct?
We're talking about how the heat leaves the car
radiator. I say the vast majority, probably 90%+ is by
convection, that is the air moving through the radiator.
A small amount is by conduction, that is heat transfering
from the radiator to the surrounding metal that it's touching, etc.
And a small amount is leaving via radiation.
Do you agree that convection is the predominant heat
tranfer mode? Or do you agree with JB that convection is
The above is the core of the issue.
These are misnomers. The terms have come into use through attempts by
salespeople to differentiate between normal and fan ovens.
The correct term would be "forced air circulation ovens"
Commercial ovens are identified as such.
Without a fan, the correct term is "passive/natural air circulation".
Convection is the concerted, collective movement of ensembles of
molecules within fluids (e.g., liquids, gases) and rheids.
The term convection may have slightly different but related usages in
different scientific or engineering contexts or applications. The
broader sense is in fluid mechanics, where convection refers to the
motion of fluid regardless of cause. However in thermodynamics
"convection" often refers specifically to heat transfer by convection.
Additionally, convection includes fluid movement both by bulk motion
(advection) and by the motion of individual particles (diffusion).
However in some cases, convection is taken to mean only advective
phenomena. For instance, in the transport equation, which describes a
number of different transport phenomena, terms are separated into
"convective" and "diffusive" effects, with "convective" meaning purely
advective in context.
In forced convection, also called heat advection, fluid movement
results from external surface forces such as a fan or pump. Forced
convection is typically used to increase the rate of heat exchange.
On 3/15/2013 6:28 AM, firstname.lastname@example.org wrote:
Let me take a shot at in a different way. If you have a gas/liquid the
density of the g/l is determined by the size of the molecules. It seems
to me that adding energy to a molecule would would increase its size and
make it less dense than the surround molecules in your g/l. When there
is gravity, the lighter molecules would tend to migrate to the top of a
container and if collected and cooled to a temperature below
that of your existing g/l, those cooled molecules would have less energy
thus less density than the g/l and would tend to migrate toward the
bottom of your container. If you are a PhD and this is wrong thinking
don't howl at me because it's a SWAG made from observation and
could be totally wrong and too simplistic. ^_^
On Mar 15, 9:42 am, The Daring Dufas <the-daring-du...@stinky-
Hide quoted text -
Yes, that's the essence of the idea of how fluids move
via natural convection. Except that it's not the size of the
that changes, it's the spacing between them. The more energy
they have, the more they are bouncing around, the more
space between them.
On 3/15/2013 8:58 AM, email@example.com wrote:
That's what I was trying to remember. When energy is added to a
molecule I believe it gains electrons which would account for the
increase in spacing so the f/l itself gets less dense not the molecule.
Hell, I wrote the original post after I had just awakened hoping my
pain meds would kick in. I had a rough day yesterday since I figured
out what was giving me chest pains, it was Naproxen Sodium. Now I must
switch to another anti-inflammatory so I won't have so much trouble
On 3/15/2013 11:32 AM, firstname.lastname@example.org wrote:
It could be more stuff I forgot after all these years. I can't remember
the specifics but I suppose I was thinking the change in energy level
was caused by added electrons but now I seem to recall the added energy
is simply winding up the molecule increasing the speed of particles and
changing their orbit around the nucleus. Since stable identical
molecules repel each other perhaps this increase in energy causes the
increase in spacing. Heck it's been so many years since I was immersed
in physics and I been busy making a living so I've used those brain
cells for other things. It's a good thing I have Internet access, now I
can go back and refresh my memory. I remember living in libraries but I
can't sit still that long anymore. ^_^
Thanks, my physician friend who helps me is a cardiologist and we discus
what works best since I despise drugs and having to take the
darn things. I'll probably wind up on another anti-inflammatory at
a lower dose that won't melt my brain. o_O
Well, it's close, I forgot it's not the molecule itself that gets less
dense but as Trader4 pointed out it's the spacing between the molecules
that increases which makes the gas or liquid less dense. I was a physics
major in college 40 years ago and I can't remember how to write a fancy
calculation even if you put a gun to my head. ^_^
I had that discussion a few years ago with people in
sci.physics about my house new outside a/c unit.
The conclusion was that had I painted the outside
unit black, the infrared radiation would have been infinitesimal
compared to mass heat transfer due to air flow (as JB mentioned).
The paint keeps the aluminum from developing an oxide layer. Aluminum oxide is a
insulator. Usually the units get formed, brazed then tested. Then into an acid
clean off any oxide and prep the metal then into a dip tank to get a THIN coat
The plain aluminum ones usually have either clear or clear anodizing to keep
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