Archived from groups: alt.comp.hardware.homebuilt (
More info?)
"Bob Adkins" makes a blooper:
> Laminar airflow transfers heat better for several reasons.
OK, let's start with a source a homebuider would
appreciate - http://www.overclockers.com/tips90/
A quote from the above webpage:
"Turbulent air cools better.
Say, for sake of argument, you have a simple tube
with a fan in the middle. The fan pulls air from
one side of the tube, and blows into the other.
If you have a hot component on the exhaust side
of the fan, it will be more efficiently cooled
than on the intake side. This is because the air
on the exhaust side of the fan is more turbulent.
For lack of a better explanation, the loops and
whorls of turbulent air moving across the surface
pick up more heat. The effective surface area of
the object is increased. (Actually, it was explained
to me by saying the effective surface area of the
air is increased.) The total volume of airflow
remains the same, but turbulent air just cools better."
If you want to pay more than $100 for a book or
monograph on heat transfer, you can find a multitude
of very academic books on turbulent flow and heat
transfer. Here's a blurb about one in the following
webpage -
http://www.begellhouse.com/books/497d60632054f587,6ddfe1a32b58c789.html
"Turbulent flow is the most common form of motion
of liquids and gases playing the role of the heat-
transfer medium in thermal systems. The complexity
of turbulent flow and the importance of hydrodynamics
and heat transfer in practice inspired continuing
research for methods of efficient heat augmentation
by the Lithuanian Energy Institute. The solution of
this problem was directly linked with the determination
of the reaction of flow in the boundary layer to the
effect of various factors and heat transfer rate under
given conditions. The investigated factors included
elevated degree of turbulence of the external flow as
well as strong acceleration and turbulization of flow
near the wall by surface roughness. The material in
this volume shows that it is possible to control the
efficiency of turbulent transfer when the vortical
structure of the turbulent flow is known."
You think this investigation of augmentation of
turbulent flow is to *reduce* heat transfer? I don't
think so. But read the book to be sure.
And here's a nice little webpage -
http://www.cougarlabs.com/cool2.html . It's about
water cooling, but it applies to air cooling as well.
Here's a quote from it:
"Boundary Layers
When there is fluid flow across a surface, a velocity
boundary layer must develop. If the flow is in the
'laminar' flow regime, then the flow velocity in the
fluid at the surface is zero. A boundary layer is
formed, within which the shear stresses and velocity
gradients are large. At sufficient distance from the
surface, these same shear stresses and velocity gradients
become negligible."
"The problem, then, is this (simplistically): When
atoms/molecules strike the surface, they take on the
velocity of the surface (zero) and, to an extent, the
temperature of the surface. If these atoms/molecules
were to simply "get out of the way" to be replaced by
other (colder) atoms/molecules, then we could imagine
a great deal of heat being carried away."
"In addition to the velocity boundary layer, if there
is heat being carried away, then there must also be a
thermal boundary layer. Whereas the velocity boundary
layer was characterized by shear and velocity gradients,
the thermal boundary layer is characterized by temperature
gradients and heat transfer."
"Laminar, Transition and Turbulent
For convective heat transfer to work well, we need to
get the heat energy out into the flowing coolant.
Turbulence will do this for us."
"At low flow velocities, we can visualize 'streamlines'
along which the particles of the fluid actually move,
and transport is dominated by diffusion. However, as
the flow velocity becomes larger and larger, fluctuations
and irregularities will force the flow to become turbulent.
In between the extremes of laminar flow and turbulent flow,
we have a transition region where diffusion and turbulent
mixing are of about equal importance. Finally, in the
turbulent portion of the flow, transport is dominated by
turbulent mixing."
Need something more explicit? Try downloading this
..pdf document:
http://www.ceere.org/beep/docs/FY2002/Turbulent_Flow_in_Enclosure.pdf
Here is a quote:
"In engineering applications, turbulence often displays apparent
differences from laminar flows. Comparatively speaking, turbulent
flows often lead to higher transport rate of momentum, energy and
mass than laminar flows. These features are widely made use of in
energy systems in industry. For example, turbulence enhancers such
as ribs are added to cooling systems of turbine blades and micro-
electronic devices to create more turbulent motions so that the
overall heat transfer efficiency can be improved."
There's a whole lot more, especially if you want to pay
for the information, but you get the idea -
Turbulent flow is better than laminar flow for cooling warm surfaces.
*TimDaniels*
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