TM 5-852-4/AFM 88-19, Chap. 4
(k) The above calculations neglected any
approach utilizing a digital computer. However, some
useful relationships can be obtained via a steady state
consideration of the thermal benefit of finning the pile. A
analysis assuming that the mean ground temperature,
1/4-inch-thick, 6-inch-wide fin of 4-feet length would
g
T , remains unchanged at a distance of ten radii from the
dissipate 6.2 Btu/lin ft hr at 20 F air temperature and 20.7
pile.
Btu/lin ft hr at 0F. The use of four fins would have the
The heat removed by the pile is:
effect of essentially doubling the heat transfer rate from a
cylindrical surface. The use of six fins, a number
commonly used, would reduce the freezeback time from
while the heat input from the soil is:
16.5 and 10 days to 12 and 6 days for ambient air
temperatures of 20 F and 0 F respectively.
(I) The above example assumed that the
air would be quiescent during the freezeback period; the
effect of an average wind of only 2 mph is considered
next:
(m) The freezeback time is reduced from
20 to about 12 days, representing a reduction of 40%
from the natural freezeback time without thermal pile
assistance. Had the temperature averaged 0 F, the
time would be reduced by about 60%o to 7-3/4 days.
These results are tabulated below:
Freezeback Time (Days)
Without Fins
With Fins (6)
20F
Wind
0F
20F
0F
For the sample problem above in which the average wind
speed was 2 mph, it is estimated that the permafrost
0 mph
16 1/2
10
12
6
temperature would be depressed from 28 F to about 19
2 mph
12
7 3/4
--
--
at the pile/soil interface.
5 mph
9 1/2
5
--
--
(2) Single-phase piles.
Natural Freezeback Time 20 days (Permafrost at 28 F)
(a) As previously described, the single
(n) As indicated by these calculations, the
phase system, or convection cell, operates by virtue of a
amount of surface area presented to the cold outside air
density gradient induced by temperature difference
is critical and thus it is essential that snow (which is a
between the above-ground (exposed to air) and the
rather good insulator) not be allowed to impede heat
below-ground portions of the pile. Such systems may
transfer from the pile. Heat transfer by the emission of
use a confined liquid, or gas, or ambient air as the heat
long-wave radiation from the pile will accelerate the heat
transfer medium within the pile. As the fluid extracts
transfer process while absorption of solar radiation tends
heat from the soil surrounding the pile, its density
to retard heat transfer. The use of high albedo paint to
decreases, thereby causing the fluid to rise and be
reflect the incoming solar radiation is a common practice.
replaced by overlying cooler fluid. Heat exchange to the
(o) Computation of heat transfer in
atmosphere is accomplished either through the pile wall
service. An indication of the magnitude of temperature
for liquid systems or by
depression below the mean temperature of the ground
surrounding the pile is useful in appraising the potential
adfreeze strength provided by the thermal pile. This
problem is best solved by means of a finite difference
4-126
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