Methods of heat transfer and thermal properties of soil

DATHAN C S

HeatHeat 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 10C.

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

Heat and temperatureHEAT 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

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

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 00c into 1gm of water at 00C

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 1000C is 537 calories. This means that 537 calories of heat req. to change 1gm of water at 1000c into steam at same temp.

Modes of heat transmission1.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

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 continuouslyTowards the colder parts of the body

Modes of heat transmission

Modes of heat transmission3. 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)

Modes of transmissionConduction 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

Partitioning of solar radiation

Absorbed by Atms

Scattered and diffused by

small particles in air

45%

11%

16%

28%

Albedo

Fraction of incidental radiation that is reflected back is Albedo.

Albedo = Reflected radiant energy Incidental radial energy

Energy balance of earthValue ranges from 0 to 1 [value zero=100% absorption

(black body), Value 1= 100% reflection(absolute white surface)]Larger the albedo , cooler the soilAlbedo high for light col. Soils.Albedo low for dark colored soils, wet soils

Albedo values of natural surfaces

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

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

Day time heat balance

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)

Soil Temp. regimes

• Pergelic: Mean annl. ST less than 00C

• Cryic: More than 00C but less than 80C

• Frigid: Mean annl. ST less than 80C and the diff between mean winter and mean summer ST is more than 50C

• Mesic: Mean annl. ST between 80C and 150C and the diff between mean summer and mean winter ST is more than 50C

• Thermic: 150C or more but less than 220C and the diff between mean summer and mean winter ST is more than 50C

• Hyperthermaic: Mean annl. ST more than 220C

Factors affecting soil temp.

Temp. of soils in the field is affected by three factors.

1.Absorption of net amount of heat energy2.Amount of heat energy required to bring

about a change in soil temp3.Amount of heat energy req. for processes

like evpn

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

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 surfaceBlack colored soils good absorbersCDry 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 conductivitySoils having blocky and platy structure, high TCSoils having high mineral fraction, more TC than Org. soils

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

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 15-1600CHeat req. to raise m g of water From 15-1600C

= Heat req. to raise 1 g of sub. Through 10C1 cal

Unit= cal gm-1 0C-1 or J gm-1 0C-1

Volumetric heat capacityHeat capacity of a body is the quantity of heat required to raise

the temp. of a unit volume of body by 10C

Defined as change in heat content of a unit volume of soil per unit change in temp

-Denoted by Cv. -Unit = cal cm-3 0C-1

-It is equal to product of Density and sp. heat

Sp. Heat and Vol. heat capacity of some soils and minerals

Conduction of heat in soil under steady state condition

1. Thermal conductivityIt’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.

Thermal properties of different soil constituents and ice at 200C and 1 aatm. pressure

• 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

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.

Fouriers law of heat conduction

Fouriers law of conduction

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

Factors affecting thermal conductiity and diffusivity of soil

1. Composition of soil2. Soil texture3. Water content4. Om content5. compaction