Some people believe religiously in smooth laminar flow
of fluid past an object to cool that object. But turbulence
actually acts to scrub away the boundary layer of air (or
other fluid) that surrounds the object to make closer contact
between the object and the passing fluid. It is the blind
belief of many with non-technical educations that flow should
be laminar (i.e. "smooth") because fluid flows fastest and
unimpeded by the drag that turbulent flow suffers. But it is
the very nature of the contact between the flowing fluid and
an object that causes both the drag and the transfer of heat.
In other words, drag goes hand in hand with heat transfer,
and drag against a surface to be cooled is a measure of
the success one will have in cooling that surface.
Below are random FALLACIOUS arguments for laminar flow:
1) A layer composed of air does not oppose other air.
Laminar flow past a boundary layer will not leave that
boundary layer undisturbed even if #2 isn't true.
Laminar flow is by definition flow over and past a
boundary layer. If a fluid were to flow normal (i.e. in a
"perpendicular" direction) to the surface, it wouldn't
be laminar flow
2) The surface being cooled is radiating heat even without
ANY airflow, your boundary layer is no layer at all, it is
already moving air with no addt'l chassis airflow added.
Well, it's hard to argue with nonsense grammar,
but in a PC case, almost ALL the heat removed is
by forced convection, e.g. air moved by a fan, not
by radiation.
Even ON the surface being cooled, while creation of
turbulence is of benefit, it can't be done without consider-
ation of the reduction in airflow rate.
Reduction in flow rate as fluid flows across a surface
is one measure of the contact between the fluid and the
part. If the fluid does not slow down, it is because it didn't
actually contact the part.
Here are some interesting discussions and comments:
http://www.thermaflo.com/crosscut.shtml
"Turbulent air breaks the stagnant air boundary layers
around the pins and, as a result, enhances the heat sink's
thermal performance."
http://www.frostytech.com/articleview.cfm?articleID=2001
"To induce turbulence within the fins and improve thermal
transmission between the air and metal, Thermalright have
modified the aluminum fins by adding 'proprietary bent winglets'."
http://sound.westhost.com/heatsinks.htm
"Simple convection is not as effective (even for the same rate
of flow of air), because of the "laminar" flow of air (where the
air at the surface of the heatsink moves slower than that further
away). This effect can be easily seen on a windy day. If you stay
close to a wall or other large area (lying on the ground works too),
it will be noticed that it is less windy than out in the open. Exactly
the same thing happens with heatsinks (but on a somewhat
reduced scale). Creating turbulence is an excellent way to defeat
this process, but this requires fans, and fans are noisy."
http://www.fischerelektronik.de/fischer2002/fischer/Fachb-Act_Rep/kuehlkonzepte/KKoneng_e.htm
"The heat transfer towards the flowing air that can be achieved
with plain fins is relatively restricted. The laminar air flow that
emerges is not sufficient to carry off the heat. Therefore, attempts
are being made to improve heat transfer (fins to air) by producing
more turbulent flow using an appropriate fin geometry."
http://www.hilltech.com/products/uv_components/UV_irradiators.html
"Optimizing cooling efficiency in an LIA is achieved by using
a heatsink-based aluminum reflector, where the material has
a high thermal conductivity and the design maximizes the effects
of surface area and turbulence. Within reason, the more surface
area the better the lamp cooling. Also important is turbulence,
because of the skin effect in cooling. A thin layer of air surrounding
a cooling surface acts as a thermal insulator impeding the effect
of forced air-cooling. This layer needs to be disrupted by turbulent
airflow, which can be created by providing irregular fins and fin
geometries."
http://www.freepatentsonline.com/6729383.html
"at least some said protrusions affect said streaming of said
fluid so as to enhance the turbulence of said streaming of said
fluid, thereby enhancing convective heat transfer from said
object to said fluid."
two words: thermal gradient.
You have to get the heated air away from the part,
efficiently if you don't want a high noise:airflow ratio.
"Thermal gradient" is a measure of the change in
temperature per unit of length. The closer that a
passing fluid gets to an object of a differing temperature,
i.e. the thinner the boundary layer, the greater the gradient
in the direction normal (i.e. "perpendicular") to the surface.
That means that the more turbulence, and the resulting
thinning of the boundary layer, results in a higher temperature
gradient and a faster transfer of heat. That's what a high
temperature gradient does.
... and subsequently, not so useful to cool anything else in
the system,....
The goal of cooling an object is to transfer its heat to
something else. If the cooling fluid gets hot, it's because
it drew away heat - just what you want.
In short, the goal of PC component cooling is not to just get the
largest volume of air into and out of the case per unit time, it's
to get the largest volume of air IN CONTACT WITH THE HEATED
PARTS per unit time, the turbulent flow in the vicinity of those
parts, regardless of how that turbulence is generated, is most
important.
*TimDaniels*...
