Heat Flow to ground
Heat Flow to ground
(OP)
In calculating heat transfer of an object sat on the ground, what assumptions can be made as to the heat flow into the ground? I would assume that at an infinite distance then there would be no heat flow, however I'd also expect the temperature to tend to a constant value. What temperature and at what distance from ground level is it constant?
corus





RE: Heat Flow to ground
Neither the ground surface nor the space above it will have unchanging temperatures because of wind, moisture changes, radiant flux and etc.
The problem is wayyyyyy too unspecific.
RE: Heat Flow to ground
corus
RE: Heat Flow to ground
Are you asking "At what depth does the ground reach an equalibrium temperature?"
I know here in the Chicago USA area, it's about 3.5 feet down and the temperature is around 50oF.
Patricia Lougheed
Please see FAQ731-376 for tips on how to make the best use of the Eng-Tips Forums.
RE: Heat Flow to ground
For some cases you can assume a constant ground temperature. For long term operation of buried pipes, to model the ground's resistance there are certain "shape factors" that are based on flow lines that curve to the surface. There probably is something similar for your case. There may be cases where you model deep into the ground, then you may need to include the temperature gradient (you know, the deeper the warmer).
RE: Heat Flow to ground
http://www.buildingphysics.com/slab.htm
RE: Heat Flow to ground
When you are establishing heat BCs through the ground. You have to have some info regarding the existing thermal gradient in that region. Every region has different thermal gradient (which represents the steady state condition at that region). So if you have no measured data within the earth, you can know the thermal gradient and the surface temperature you can get the ground temperature at any desired depth. I beleive that The temperature below the ground surface will never reach a constant value with depth by going deeper within the earth. You have to assume an ending depth to your model
RE: Heat Flow to ground
TTFN
RE: Heat Flow to ground
The software looks good, however I don't know what assumptions are being made in the calculation. The program assumes semi-infinte ground and hence I would presume that the assumption is being made that there is no heat loss at infinity (or some large distance). For a fixed temperature at the surface, such as a hot slab, then with no heat loss at infinity, then the ground would be at the same temperature as the slab for an infinite depth.
I would presume that heat flow in the ground would be characterised by a heat sink to some fixed temperature (as Patricia says) so that at infinity there is no heat flow, but the temperature tends to some fixed value. Are there referenced guidelines as to the heat sink, whether it's in Chicago or otherwise?
IRstuff,
I'd never heard of Fick's law, however without the transient term then the equation is just Laplace's equation for heat transfer.
corus
RE: Heat Flow to ground
Fick's law is d^2N/dx^2 ~ dN/dt, the concentration drives the diffusion
Since the heat is spreading in two dimensions, assuming negligible gradients lengthwise to the pipe, the expanding front eventually drops the "concentration" of heat to the point where heat transfer is neligigle and the ground is close to its normal temperature. For example, if at a certain radius of expansion, the further increase in temperature of the ground is less than, say, 0.1ºC, you could consider that as having reached the background temperature, since the change is smaller than the local variations in temperature, probably smaller than the measurement accuracy, and smaller than the temperature variations of the pipe itself.
TTFN
RE: Heat Flow to ground
Thanks
corus
RE: Heat Flow to ground
BigInch
-born in the trenches.
http://virtualpipeline.spaces.msn.com
RE: Heat Flow to ground
ht
will show the soil temperature variation as a function of depth. According to the model, the soil temperature is essentially constant beyond depths of 10 feet at a value equal to the average of the annual maximum and minimum air temperatures. The temperature at depths of 10 feet or more typically vary less than 1 degree from the annual average temperature.
Heat transfer to the soil is dependent on the soil temperature which changes depending on the time of the year (for depths less than 10 ft), the soil moisture content, soil type, whether the soil is underneath an object (such as a floor) or adjacent to the side of the object (such as a basement wall), object shape, (flat slab, corner of a building, pipe), soil surface orientation (north vs south), ground cover type. So, the calculation of heat transfer can carry a significant uncertainty.
A report from the US Department of Energy (http://www.nrel.gov/docs/fy03osti/33954.pdf) discusses the modeling of heat transfer from buildings to the ground. Although the report relies on a sophisticated numerical model, key conclusions are 1) “soil thermal conductivity is the most important parameter in determining” ground heat transfer; 2) the soil thermal conductivity can change by a factor of 10 based on soil moisture content; 3) the soil moisture content varies significantly only near the surface of the soil.
A simplified method for heat transfer calculations from the University of Colorado can be found here: htt
The key parameter in this simplified model is the soil thermal conductivity.
A paper from University of California, Berkeley (http://ww
Hope this helps.
RE: Heat Flow to ground
corus