Convection
•Convection: transfer of heat by a
flowing liquid or gas
•2 forms:
–Convection of sensible heat (heat in the
form of a change in temperature)
–Convection of latent heat (heat in the form
of a change of phase)
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Convection - Soil Physics - Lecture Slides, Slides of Soil Physics
Soil physics is the study of soil physical properties and processes. It is applied to management and prediction under natural and managed ecosystems. (Wikipedia). Keywords in this lecture are: Convection, Heat, Liquid, Sensible Heat, Latent Heat, Heat Conduction, Fourier's Law, Volulmetric Heat Capacity, Soil Thermal Properties, Diffusivity
Typology: Slides
2012/2013
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Convection
- Convection: transfer of heat by a flowing liquid or gas
- 2 forms:
- Convection of sensible heat (heat in the form of a change in temperature)
- Convection of latent heat (heat in the form of a change of phase)
Convection of sensible heat
- Liquid water has a high volumetric heat capacity
C (^) L = 4.17 X 10 6 J m-3^ K -1^ (at 20 °C)
- Convection of sensible heat can significantly alter the soil thermal regime when liquid water fluxes are relatively large
Convection of latent heat
Heat conduction
- Fourier’s Law: the heat flux is proportional to the temperature gradient
q (^) h = heat flux by conduction (W m -2) κ = thermal conductivity (W m -1^ K-1) T = temperature (K or °C) z = position (m)
d z
d T q (^) h = − κ
Transient heat conduction
- Combine Fourier’s law:
with the heat conservation equation:
to describe transient heat conduction:
∂
∂ = − ∂
∂ z
q t
T C h
d z
d T q (^) h = − κ
∂
∂ − ∂
∂ = − ∂
∂ = − ∂
∂ z
T z z
q t
T C h κ
Soil thermal properties
- Volumetric heat capacity
- Soil thermal conductivity
- Soil thermal diffusivity
Soil thermal conductivity
Volumetric Water Content
Volumetric heat capacity, C
- C , the volumetric heat capacity, is the energy required to heat a unit volume of soil by one degree. Units: J m -3^ K^ -
- C varies with
- Water content
- Bulk density
- Mineralogy
- Organic matter content
- temperature
Soil thermal diffusivity, D T
- Change in temperature produced in a unit volume by the quantity of heat flowing through the volume in unit time under a unit temperature gradient.
- Units: m^2 s -1^ (same as molecular diffusion)
- More useful definition: the ratio of the thermal conductivity to the volumetric heat capacity:
D T = κ/ C
- Makes the heat flow
equation more convenient: (^2)
2
z
T D t
T T ∂
∂ = − ∂
∂
Soil thermal diffusivity
- Used to characterize the rate of transmission of heat through the soil
- Influenced by
anything that changes κ or C.
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