Methods of heat transfer and thermal properties of soil
1. Methods of heat transfer and
thermal properties of soil
DATHAN C S
2. Heat
Heat is a form of energy. The physical cause due to
which hotness or coldness is felt is termed as heat. Heat
energy is the kinetic energy (Energy of motion) due to
vibration of molecules.
Its defined as the kinetic energy of random
motion of particles with which the material bodies
are composed of.
Unit of heat: Calorie- The quantity of heat required to
raise 1gm of pure water through 10
C.
3. Temperature
It is the measure of hotness of body.
Defined as thermal condition of a body which
determines whether it will receive or communicte
heat to another body, if two bodies having different
temperatures are put together for thermal
communication
4. Heat and temperature
HEAT TEMPERATURE
1 Form of energy Indicates thermal condition of body
2 Amount of heat may vary at same
temps
Temp of two bodies can be the same but
Quantity of heat can vary
3 Heat flow unaffected by quantity of
heat
Heat Flow b/w bodies depend only on
temp difference
4 Heat flow from High to low temps
irrespective of quantity of heat
When temps become same no further
transfer of heat
5 Unit- Calorie Unit-degree centigrade or degree
Fahrenheit
5. Latent heat (Hidden heat)
Heat for which there is external manifestation such as
rise or fall.
Defined as quantity of heat required to change
the state of matter without changing its temperature.
Two types
1. Latent heat of fusion-
2. Latent heat of vapourisation
6. 1.Latent heat of fusion
Latent heat of fusion of a solid is the quantity of
heat required to change unit mass of the substance
from solid to liquid at its melting point withiut changing
the temp.
Example: Latent heat of fusion of ice 80 calories. This means
that 80 calories of heat req. to change 1gm of ice at 00
c into 1gm
of water at 00
C
7. 2.Latent heat of Vaporization
Latent heat of vaporozation of a liquid is the
quantity of heat required to change unit mass of the
substance form liquid to vapor state at its boiling point
without changing the temp.
Example: Latent heat of vaporization of water at 1000
C is 537
calories. This means that 537 calories of heat req. to change 1gm
of water at 1000
c into steam at same temp.
8. Modes of heat transmission
1.Conduction- Process by which heat
transf. between hotter to cooler substances
or parts of the same substance by internal
molecular motion without any transference
of material particles.
-Caused by transf. of kinetic energy of
numerous inter molecular collisions
- Agency of a natural medium necessary
-- In solids
9. 2.Convection
- Process by which heat transf. from one point to another by
actual movnt. of heated particles from a place of higher temp to
lower temp.
-In liquids and gases.
-Molecules of the hotter
Body transfer heat continuously
Towards the colder parts of the body
Modes of heat transmission
10. Modes of heat transmission
3. Radiation- Heat transmitted from hotter to colder
body without intervention of of any material medium
-Heat reaches to earth from sun through vaccum
-Radiation from sun occurs as short wane radiation(0.3 to 5
microns). Soil and atms. Re-radiates this as long wave
radiation(6.8 to 100 microns)
-Radiation measured as heat energy/unit area/unit time.
(cal/cm2min)
11. Modes of transmission
Conduction Convection Radiation
Mode of
transmission
Through mat.
particles
Through mat.
particles
Through vaccum
Movt. of heat Hotter part to
cooler part of body
Particles move Wave motion
Meduim solids Liquids and gases Empty space
12.
13. Partitioning of solar radiation
Absorbed by Atms
Scattered and diffused by
small particles in air
45%
11%
16%
28%
14. Albedo
Fraction of incidental radiation that is
reflected back is Albedo.
Albedo = Reflected radiant energy
Incidental radial energy
Energy balance of earth
Value ranges from 0 to 1 [value zero=100% absorption
(black body), Value 1= 100% reflection(absolute white surface)]
Larger the albedo , cooler the soil
Albedo high for light col. Soils.
Albedo low for dark colored soils, wet soils
16. Factors affecting albedo
• Seasonal and diurnal radiation changes
• Vegetation cover- forest vegetation, albedo less
• Moisture content-High moisture content, low albedo
• Surface color- high OM content, low albedo
• Surface roughness- Fine textured dry soils, high albedo
17. Soil temp. and diurnal variation
Intensity of heat in soil is soil temperature.
•Surface of soil heated by radiation and heat moves
downward by conduction
•At any time, temp varies from layer to layer
•Heat taken in or given out=Mass x Sp. Heatx Diff. in temp
19. Soil Temp. regimes
• Soil temp. regimes depend on heat flow into soil and heat
transfer processes in the soil and processes between soil and
air
• Study of STR helps in
• Thermal characteristics of soil and their influe. On soil phy,
chem, and biol processes in soil
• Defining soil classes in soil taxonomy
• Soil temp regimes recorded at depth of 50cm or at lithic or
paralithic contact(Boundary between soil and coherent
underlying material)
20. Soil Temp. regimes
• Pergelic: Mean annl. ST less than 00
C
• Cryic: More than 00
C but less than 80
C
• Frigid: Mean annl. ST less than 80
C and the diff between
mean winter and mean summer ST is more than 50
C
• Mesic: Mean annl. ST between 80
C and 150
C and the diff
between mean summer and mean winter ST is more than 50
C
• Thermic: 150
C or more but less than 220
C and the diff
between mean summer and mean winter ST is more than 50
C
• Hyperthermaic: Mean annl. ST more than 220
C
21. Factors affecting soil temp.
Temp. of soils in the field is affected by three
factors.
1.Absorption of net amount of heat energy
2.Amount of heat energy required to bring
about a change in soil temp
3.Amount of heat energy req. for processes
like evpn
22. 1.Absorption of net amount of heat
energy
• 99% of incoming solar radiation as short wave- between 300 um and 4000
um
• Soil re-radiates this as long wave radiation (WL b/w 4000 and 10 000 um)
• Amount of heat that reaches earth is 2 gm cal/cm2 or 2 Langley/min.
• Sunlight- 30-40% -on cloudy humid regions. 70-80% on bright days in arid
regions . Global average 50%.
• Absorption of heat influenced by
• Exposure and slope of land
• Soil water content
• Surface cover
• Vegetation
• Plant residue
• Om content
• Soil color
• Surface roughness and stoniness
23. 2.Amount of heat energy required to
bring about change in soil temp.
It depends on
•Thermal properties of soil
•Radiation exchange over the soil surfaceTC
•Nature of soil surface
Black colored soils good absorbersC
Dry soils good absorbers (sp.heat of water higher( 1 cal/gm)
than dry soil particles(0.2 cal/gm)
Compacted soils & naturally aggregated soilshave higher
thermal conductivity
Soils having blocky and platy structure, high TC
Soils having high mineral fraction, more TC than Org. soils
24. 3. Chemical or physical processes
• Evaporation of water from soil requires large amount of heat
energy(580 kcal/kg of water).
• Evaprn has cooling effect on soils
25. Specific heat
Ratio of quantity of heat required to raise a little
mass of a substance to a range of temp, to the qnty. of
heat required to raise the same mass of water to same
range of temp.
Sp. Heat= Heat req. to raise m g of sub. From 15-160
0C
Heat req. to raise m g of water From 15-160
0C
= Heat req. to raise 1 g of sub. Through 10
C
1 cal
Unit= cal gm-10
C-1
or J gm-10
C-1
26. Volumetric heat capacity
Heat capacity of a body is the quantity of heat required to raise
the temp. of a unit volume of body by 10
C
Defined as change in heat content of a unit volume of
soil per unit change in temp
-Denoted by Cv.
-Unit = cal cm-30
C-1
-It is equal to product of Density and sp. heat
27. Sp. Heat and Vol. heat capacity of some
soils and minerals
28. Conduction of heat in soil under
steady state condition
1. Thermal conductivity
It’s the ability of a substance to transfer heat from a hotter
molecule to a cooler molecule.
The thermal conductivity is a physical property of the solid.
It the a measure of the materials ability to conduct heat.
29. Thermal properties of different soil constituents and ice
at 200C and 1 aatm. pressure
30. • Typical values of conductivity for common
rock types are:
• sandstone k = 1.5 - 4.2 J s-1m -1K -1
• gneiss k = 2.1 - 4.2 J s-1m -1K -1
• granite k = 2.4 - 3.8 J s-1m -1K -1
• salt k = 5.4 - 7.2 J s-1m -1K -1
• iron k = 73 J s-1m -1K
31. Fouriers first law of conduction
•In 1822 Fourier postulated that the rate of heat transfer is
proportional to the temperature gradient present in a solid,
assuming that the heat flow is unidirectional.
•He found out that the total quantity of heat(Qh) flowing
through a soil column is directly proportional to the
•Cross section area of soil(A)
•Diff in temp(T1-T2), to the time ‘t’ secs
and inversely proportional to
the distance or thickness between two points assuming that
heat flow is unidirectional and there is no heat loss.
•The laws governing conduction of heat are very similar to
Ohm’s Law, which governs electrical conduction.
34. 2. Thermal diffusivity
• When soil receives heat temp rises. The rate of change is dir.
propotional to (Kh) thermal conductivity and inversely propnl. to
heat capacity on volume basis ie. Volumetric heat capacity. The
ratio of Kh/Cv is constant which determines the rate of rise of temp
in a given soil. This constant is Thermal diffusivity(Dh).
Dh=kh/Cv
• Unit- cm2/sec.
• A measure of a material’s ability to respond to changes in its
thermal environment.
• It measures the ability of a material to conduct thermal energy
relative to its ability to store thermal energy
• It expresses the rate at which a body warms up under a given heat
gradient
35. Factors affecting thermal conductiity
and diffusivity of soil
1. Composition of soil
2. Soil texture
3. Water content
4. Om content
5. compaction
