A brief view on different modes of heat conduction in soil system

1. MODES OF ENERGY TRANSFER
IN SOIL &ENERGY BALANCE and thermal conductiv
diffusivity
Presented By
SHAHEEN PRAVEEN
Dept. of SS&AC
COA, IGKV, Raipur

2. MODE OF ENERGY TRANSFER
1.
Conduction
2.
Convection
3.
Radiation

3. The transfer of heat from one particle of
matter to another by direct particle to
particle contact or vibration of particle.
– Conduction occurs primarily in solids because
the particles are tightly packed together.
– The particles themselves DO NOT change
positions.

4. Convection is the transfer of thermal
energy (heat) through the bulk
movement of matter.
– Convection occurs in FLUIDS (liquids and
gases).
– Convection produces CURRENTS in both
gases and liquids.
– Thermal Energy heat is carried by the particles as
they move from one location to another.

5. Continue…
Convection currents occur in the
environment as well.
They produce:
– Global winds that contribute to Earth’s
weather.
– Ocean and lake currents

6. Radiation is the transfer of (thermal) energy by
electromagnetic waves.
– Radiation does not require matter to transfer
thermal energy.
• All the sun’s energy that reaches Earth travels
through millions of kilometers of empty space (a
vacuum).
– All matter can radiate energy at a
temperature above 0 k.
– Measured in cal/cm²-min.
– Dimension= M/T³.

7. Distinction Between Conduction,
Convection and Radiation
Particulate Conduction Convection Radiation
Medium
requirement
Yes Yes No
Movement of
particles
Only vibration of
particles for heat
transfer
Actual movement
of particles
-
Medium solid Liquid , Gas vacuum
Speed of process Slow Slow Very quick
Direction of heat
transfer
Any direction Any direction Straight line

9. Net Radiation At The Soil Surface
 Net radiation = (the sum of all incoming) –
(outgoing radiant energy fluxes)

10. Net Radiation At The Soil Surface
• Jn = net radiation (W m-2, J s-1 m-2)
• Js = direct beam incoming short-wave
• Ja = diffuse incoming short-wave
•  = albedo = the fraction of incoming short-wave radiation
reflected by the surface
• Jli = incoming long-wave
• Jlo = outgoing long-wave
   loliasn JJJJJ  1

11. Surface
Energy Balance
For the soil surface layer (infinitely thin):-
energy in = energy out
LEASJn 
• Jn = net radiation at the surface
• S = heat flux into the soil
• A = sensible heat flux to the atmosphere
• L = latent heat of vaporization (J kg-1)
– temperature dependent, 2.4 x 106 J kg-1 @ 25C
• E = rate of evaporation (mm d-1, kg m-2 d-1)

12. Schematic representation of (a) the daytime energy balance and (b) the
night-time energy balance. Net radiation = (solar radiation + sky
radiation) – (reflected radiation + back radiation).

13. Solar Radiation
 Solar radiations come as short wavelength rays but
reradiated as long wavelength radiation after
absorption by Earth surface.
 Incoming short waves are of wavelength 0.3-5.0
microns of which 0.3- 2.2 microns are most efficient for
heat radiation and have high penetrating power.
 Reradiated long waves have wavelength 6.8- 100
microns and have less penetrating power.
 Radiation of 2.2- 6.8 microns are inefficient for heat
radiation.
 Percentage of reradiated long wave radiation is 34% of
insolation.

14. Continue…
Solar constant:- the maximum amount of heat
from sun that reach to the earth’s surface.
It’s value is 2.0 gm cal/ cm2- min or, 2.0 Langley/
min.
99%of the solar radiation reaching earth surface
is short- wave radiation. Of the total sun’s
radiation about 9% in UV region, about 46% is in
IR region and 45% is received in visible region.

15. Partitioning Of Solar Radiation
(Shortwave Radiation)
28%
19%
15%
11%
29%
Reflection by cloud(28%)
Direct beam penetration to
earth surface(19%)
Absorption by
Atmosphere(15%)
scattering radiation returned
to outer space by small
particles(11%)
Diffuse sky radiation(29%)

16. Thermal Conductivity
Ability to conduct heat or ability to convey
heat.
Represented by= k
Unit of measurement is= watt per Kelvin per
meter ( WK¯¹m¯¹).
Reciprocal of thermal conductivity is thermal
–resistivity.

17. It is also called Fourier’s law ,
developed by Joseph Fourier(1822).
According to this law:-
“Flow of heat will always be from hot to cold.”

18. Fourier’s law
Fourier’s law of heat conduction is
mathematically analogous to
a. Darcy’s law of water conduction in porous
media.
b. Ohm’s law of electricity conduction.
c. Fick’s law of diffusion of gases.

19. Continue..
Fourier’s law :-
Qһ = - Kһ(T₁-T₂)At/x or,
qһ =- Kһ∆T/ x
qһ = Qһ/At
Qһ = Total quantity of heat
Qһ = Thermal flux
Kһ = Thermal conductivity
A = Cross sectional area

20. Thermal conductivity
∆T = T₁ - T₂

21. Thermal conductivity
Kһ depends on the material i.e. soil.
kһ = Qһ (x) / At(∆T)
Thermal conductivity(kһ):- is the rate of
flow of heat through unit area of cross section
of 1m of material when temperature
difference between the surface is 1k.
Unit = Wm¯¹K¹

22. Thermal Conductivity
For a uniform rod the rate of flow of heat
through a conductor(Q/ t) is proportional to :-
# Cross sectional area(A) of conductor
# Temperature gradient(∆q/∆x).
Note:- Heat flows down a temperature gradient
so we put a negative sign to account this.
Q/ At = -k ∆T/ x or,
k = Qx / At.∆T

23. Methods to measure thermal
conductivity
Main methods:-
1. Steady state method
2. Transient state method.

24. Thermal Diffusivity
 Thermal diffusivity is thermo-physical property which
defines-the speed of heat propagation by conduction
during changes of temperature.
 Denoted as Dһ.
Dһ = k∕ C
Unit = cm² per second (cm²/s).
 The higher the thermal diffusivity, the faster the heat
propagation.
.

25. Continue….
The thermal diffusivity is related to the
thermal conductivity, specific heat capacity
and density.
Dһ = thermal conductivity ̸ density х sp. heat capacity
 Reciprocal diffusivity is Thermal retentivity (Rһ).
Rһ= 1/Dһ
Dimension of Rһ= TL¯²

26. Factors affecting Thermal conductivity
and Diffusivity
1. Composition of soil:- 4 major component of
soil i.e. mineral matter, organic matter, soil
water & soil air.
Soil compon-
ents
Density Specific
heat(cal
gm¯¹C¯¹)
Heat
capacity (cal
gm¯¹C¯¹)
Thermal
conductivity(
cal gm¯¹C¯¹)
Thermal
diffusivity(ca
l gm¯¹C¯¹)
Mineral
matter
2.65 0.175 0.46 7 * 10-3 15 * 10-3
Organic
matter
1.30 0.46 0.60 0.6 * 10-3 1.0 * 10-3
water 1.00 1.00 1.00 1.42 * 10-3 1.42 * 10-3
Air 0.00120 0.24 0.00029 0.062 * 10-3 0.22 * 10-3

27. 2. Soil texture:-decreases with decrease in
particle size i.e. sand˃ loam˃ clay.

28. 3. Water content:- positive correlation with content.
K(water) ˃ 23 K(air)
Thermal diffusivity initially shows positive relation but
after reaching maximum start decreasing.
4. Organic matter content:- Shows negative correlation.
D(soil mineral= 7x 10¯³) ˃ D(OM= 0.6x 10¯³).
5. Compaction:- With compaction , Kһ increases.
It is so- (a) Decrease in porosity
(b) Increase in degree of saturation of water.

29. Measurement of soil temperature
Soil temperature are usually measured by
Thermometric methods.
Thermometric methods
Contact type
Example:- 1. liquid in glass type(alcohol or mercury )
2. Bimetallic thermometer
3. Electrical resistance thermometer
4. Thermocouple thermometer
Non-contact type
Example:- 1. Optical pyrometer
2. Total intensity radiometer

30. Liquid in glass type thermometer
Type:- 1. Mercury thermometer(
Wet bulb thermometer)
2. Alcohol thermometer(Dry
bulb thermometer)
 These thermometer work on
the principle of – expansion of
liquids.
 A good thermal contact is
necessary between
thermometer and object i.e.
soil.
 Most commonly used soil
thermometer.

31. Bimetallic Thermometer
 Based on the principle of –
solid expansion.
 It consist of 2 metal rod or,
strip having different thermal
expansion coefficient.
 It can be used for different soil
depth.
 Easy to install, read and are
suitable for outdoor exposure
condition.
 It gives good accuracy.

32. Electrical resistance thermometer
 Measures electrical resistance but that can be calibrated to read in
degree.
 Resistance of common alloy and metal changes with temperature ( 0.008
– 0.33% per °F)
 The calibration is done on a known change in resistance with change in
temperature.
TYPES:-
1.Wire wound resistor –
a. Copper wire type(-250°F to +250°F).
b. Platinum wire type(-324°F to +16.5°F).
2. Thermistor – Most sensitive type and have negative temperature
resistance. Its range is = -150°F to + 550°F.
# For soil temp. measurement commercial thermometers are graduated
in the range of 0°C to 30°C.
3. Positive temp. coefficient semiconductor resistor

34. Thermocouple thermometer
Thermocouples with properly calibrated
galvanometer or, recording potentiometers and a
suitable reference are usually known as
thermocouple thermometer.
It works on the basis of 2 effects –
1. Seebeck effect – a difference of temp. between 2
junctions cause flow of electric current between
them.
2. Peltier effect – a direct current flowing between
2 junctions makes them to assume different
temperature.

35. Continue…
Thermocouple thermometers are specially used
for measurement of temp. differences such as
(a) Differential thermal analysis of soil minerals ,
(b) Temp. difference betweens between dry &
moist soils ,
(c) Rise of temp. due to wetting of soil ,
(d) Decrease of temp. due to evaporation of
water etc.

36. Thermocouple device