/Just revisiting this subject, The theory of operation of a solar HWS
vacuum tube for solar HWS application, using a vacuum to prevent
convectional heat loss, from the black aluminium heatsink.
These tubes have a vacuum between two glass layers and work similar to a
thermos flask allowing it to retain up to 95% of the solar energy they
capture.
Now if the inner tube black heat sink could radiate heat as well as it
absorbs radiated heat, as suggested by some , then these solar HWS
evacuated tubes would not be as effective as they are,
as radiated heat will pass through a vacuum, but convectional heat
transfer is impossible via a vacuum.
Adrian ... vk4tux
/
Re: [Amps] Fan for SB220
from [/Adrian/
<http://lists.contesting.com/archives//cgi-bin/namazu.cgi?query=%2Bfrom%3Avk4tux%40bigpond.com&idxname=Amps&sort=date%3Alate>]
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*To*: amps@contesting.com <mailto:amps%40contesting.com>
*Subject*: Re: [Amps] Fan for SB220
*From*: Adrian <vk4tux@bigpond.com <mailto:vk4tux%40bigpond.com>>
*Date*: Mon, 20 Dec 2010 11:16:55 +1000
*List-post*: <amps@contesting.com
<mailto:%3Ca%20href=>">mailto:amps@contesting.com
<mailto:amps%40contesting.com>>
On 12/20/2010 08:12 AM, Fuqua, Bill L wrote:
/ Adrian,/
/ I give up. I think most everyone else gets it. It is like a
conversation/
/ I had with some old hams, not much older than I am aboiut some large coax./
/ They were convinced that since the center conductor was hollow you could run/
/ wires up thru it to carry current to lights and rotators without affecting
the/
/ impedance of the coax. Their argument was that current only flows on the/
/ outside of the conductors. But that is not always true./
/ Current flows on the surfaces and in the case of a hollow conductor it is/
/ true it will flow on the outside as long there is nothing to electric/
/ create fields on the inside. There are electric/magnetic fields between the/
/ outside of the inner conductor and the inside of the outer conductor. But/
/ once you put a conductor on the inside of the hollow inner conductor you
have/
/ created a new bit of transmission line, there are fields now between the new/
/ conductors and the inside on the inner conductor. That changes everything./
/ 73/
/ Bill wa4lav/
Bill, after seeing this fromhttp://en.wikipedia.org/wiki/Radiator;
Radiation and convection
One might expect the term "radiator" to apply to devices that transfer
heat primarily bythermal radiation
<http://en.wikipedia.org/wiki/Thermal_radiation>(see:infrared heating
<http://en.wikipedia.org/wiki/Infrared_heating>), while a device which
relied primarily on natural or forced convection
<http://en.wikipedia.org/wiki/Convection>would be called a "convector".
In practice, the term "radiator" refers to any of a number of devices in
which a liquid circulates through exposed pipes (often with fins or
other means of increasing surface area), notwithstanding that such
devices tend to transfer heat mainly by convection and might logically
be called convectors. The term "convector" refers to a class of devices
in which the source of heat is not directly exposed.
Theory of operation
From an engineering perspective, a radiator varies from an idealblack
body <http://en.wikipedia.org/wiki/Black_body>by a factor,?, called
theemissivity <http://en.wikipedia.org/wiki/Emissivity>, which is a
spectrum-dependent property of any material. Commonly, a fluidthermal
mass <http://en.wikipedia.org/wiki/Thermal_mass>, containing the heat to
be rejected, is pumped from the heat source to the radiator, where it
conducts to the surface and radiates into the surrounding cooler medium.
The rate ofheat flow <http://en.wikipedia.org/wiki/Heat_flow>depends on
the fluid properties, flow rate, conductance to the surface, and the
surface area of the radiator.Watts
<http://en.wikipedia.org/wiki/Watt>persquare metre
<http://en.wikipedia.org/wiki/Square_metre>are the SI units used
forradiant emittance <http://en.wikipedia.org/wiki/Radiant_emittance>.
If the system is not limited by theheat capacity
<http://en.wikipedia.org/wiki/Heat_capacity>of the fluid, or thethermal
conductivity <http://en.wikipedia.org/wiki/Thermal_conductivity>to the
surface, then emittance, M is found by a fourth-power relation to
theabsolute temperature
<http://en.wikipedia.org/wiki/Absolute_temperature>at the surface.
TheStefan-Boltzmann
<http://en.wikipedia.org/wiki/Stefan-Boltzmann>constant is used to
calculate it, as/M/= ??/T/^4 . Since heat may be absorbed as well as
emitted, a radiator's ability to reject heat will depend on the
difference in temperature between the surface and the surrounding
environment. For particularoperating temperatures
<http://en.wikipedia.org/wiki/Operating_temperature>, a system's overall
heat flow may be given inthermal watts
<http://en.wikipedia.org/wiki/Watt#Electrical_and_thermal_watts>,
abbreviated W_t .
I see that the word radiate is used to describe heat transfer to "the
surrounding cooler medium ".
In this case I would see this as convection heat transfer to air, but
not radiation in the same sense as infra red radiation absorbed by black
material. The terminology seems a bit loose.
However my point was that black material heated by an 3-500z infra red
radiation source due to its absorption properties, releases that heat
mainly by conduction and convection heat transfer within the amplifier
framework and air flow within.
If you say that the process of convection is the black radiating heat
to the surrounding air, then we are on the same page.
Adrian ... vk4tux
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