> What is the heat conductivity of dirt, rock, and nickle-iron? Does
> anyone one know?
Dirt and rock are similar, in the range 0.2 - 2 W / m K.
Iron is about 84 W / m K
Nickel is 92 W / m K
air at 300K is 0.026 W / m K
Thank you. Am I right in thinking that for air, this is the heat
conductivity for still air, and not the heat transfer capabilities of
moving air?
What I have been saying is that if I were to perform the
experiment by=20 obtaining a 2-meter or so probe with a sharp end,
mount a thermocouple at=20 that end, and drive it into the ground,
then place an identical=20 thermocouple at on the surface, the one
on the surface would record=20 substantial variations in
temperature both diurnally and annually, while=20 the one a couple
of meters below the surface would report a nearly constant=
Yes, this is a good description of my experience going into cellars
and caves: the surface temperature may be high or low, but the
temperature in the cellars and caves does not change much.
This suggests either that not much heat is taken from the human body
by a cold wind or that dirt is a pretty good insulator. I can tell
you by personal experience that a cold wind can chill a human body
fast. Hence the practical effect in any environment in which air can
move, such as the interior of a spinning space habitat, is that air
has a much higher possible heat transfer capacity than might be
indicated by its still air heat conductivity value.
Put another way, if the choice is between waiting out a blizzard while
exposed to the wind, or waiting out the blizzard in a cave, choose the
cave. You are less likely to freeze to death.
The heat capacity of the endcaps is much more than the air, so it
is better to say that the air is affected by the temperature of
the endcaps.
Yes, the heat capacity of the endcaps is much more than the air. But
is that the critical factor? Or should the question ask how much heat
is transferred to a volume of air near an endcap by conductivity from
the endcap, versus how much heat is transferred (by mass exchange) by
air moving from a spot on the surface at the rim to that volume?
Since dirt and rock are better thermal conductors than air, ...
My experience is that they insulate better than the wind.
Are you saying that there will be no air movements in the spinning
space habitat? If so please explain, because I think there will be,
on account of different surface materials on the rim, or different
amounts of light falling on different regions. But maybe I am wrong.
there will be some heat transfer from the endcaps to the air.
So you are saying that heat from the surface a the `rim' will be
transferred through the rock and the regolith to the air near the spin
axis of the habitat, rather than by air movements. I don't think so.
Certainly, air at the top of a mountain I went to Friday did not gain
temperature from heat conducted through the rock that day from the
warm valley floor. That air at the top of the mountain was cool.
The rocks at the top of the mountain were cool, too, they could not
transfer much heat to the air.
> I would expect ... that with sufficiently good insulation, the
> surface of the end caps would come to the same temperature as
> the air fairly quickly and would neither contribute nor take
> much heat from the air.
Same temperature as which air? The air on the axis is colder than the
air at the rim.
I meant the rock and the air at the same distances from the axis.
Yes, the air on the axis is colder than the air at the rim.
If the endcap had the same temperature vs. radius profile as the
air, then there would be heat flowing from the rim to the center
of the endcaps (heat flows from high to low T), and the center
would heat up until in equilibrium the endcaps are at the same
temperature as the rim.
Yes, but what is the rate of heat flow from the interior of the rock
through its surface into the air? How would it compare to heat
transfered by moving masses of air? What wind speed would it produce?
Suppose that the spinning space habitat is oriented so its spin axis
goes through the North star. This way, the `sides' of its tuna can
shape would spin in and out of sunlight, receiving on average about
the same as a place on the earth does at a latitude of 45 degrees.
(Is that right?) The habitat's end caps would receive slanting sun
light (just like the North and South poles on earth, so here the
metaphor of naming them that way is accurate.)
So the end caps would not receive as much solar energy as the sides.
Would the heat from the sides flow to the end caps and then be
radiated into space? Would the air by the end caps get chilled by the
heat being radiated into space? Or is the insulating quality of rock
and dirt good enough to make that fairly irrelevant?
(I am assuming here a hull of nickle-iron overlaid on the inside by
perhaps 5 meters of rock or regolith. Is that right? Perhaps on the
outside the end caps are painted white, or are of reflective
nickle-iron, so they don't radiate as much heat as sides. I don't
know how the sides should be colored. Perhaps they should be equally
reflective.)
--
Robert J. Chassell Rattlesnake Enterprises
http://www.rattlesnake.com GnuPG Key ID: 004B4AC8
http://www.teak.cc [EMAIL PROTECTED]
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