Course Content
Thermal conductivity
This is a summary of the unit.
0/6
Introduction
03:34
What is thermal conductivity?
03:10
Applications of thermal properties
06:21
Transient methods for measuring thermal properties
05:39
Methods and standards for thermal properties measurements
05:27
Additional reading
Thermal Conductivity
 

What is thermal conductivity?

 

Thermal properties of materials govern heat flow, storage, and propagation, and they are particularly important in applications ranging from insulation and electronics to soil science and food processing. This article outlines the primary thermal properties that are typically measured in porous materials such as soil, whose thermal behavior can vary significantly with environmental conditions.

 

Fundamental Thermal Properties

Four key thermal properties are commonly measured to characterize heat behavior in materials:

  1. Thermal conductivity (k) quantifies the rate of heat flow through a material per unit area and per unit temperature gradient. Materials with high thermal conductivity transfer heat more efficiently.
  2. Thermal resistivity (ρ) is the reciprocal of thermal conductivity. It is particularly useful for describing insulating materials and assessing heat flow in systems such as the soil around buried power cables.
  3. Volumetric specific heat (C) refers to the amount of heat required to raise the temperature of a unit volume by 1 K (or C): J/(m3K). It determines how much energy a material can absorb to change its temperature.
  4. Thermal diffusivity (D) measures the rate at which a thermal disturbance propagates through a material. It combines thermal conductivity, specific heat, and density into a single parameter that reflects how quickly a material responds to changes in temperature.

 

Thermal behavior in porous materials

Unlike non-porous materials such as aluminum or glass, which have relatively stable thermal properties, porous materials like soil exhibit significant variability. Their thermal properties can change by an order of magnitude depending on factors such as:

  • Water content
  • Bulk density
  • Temperature

 

Thermal conductivity and water content

In porous materials like soils, thermal conductivity depends on water content, temperature, and density.
Figure 1. In porous materials like soils, thermal conductivity depends on water content, temperature, and density.

In soils, thermal conductivity generally increases with water content.

Explore the water content course

At low temperatures, this relationship is nearly linear. However, at higher temperatures, the increase in thermal conductivity becomes more pronounced due to latent heat transfer mechanisms. This phenomenon is explained by the heat pipe effect in moist soils:

  • Water evaporates on the warmer side of soil pores.
  • Vapor diffuses across the pore and condenses on the cooler side.
  • The condensation releases latent heat, which contributes to heat transfer.
  • Liquid water then returns to the warm side, completing the cycle.

At elevated temperatures (e.g., 90°C), vapor-phase heat transfer can surpass that of liquid water, resulting in a decrease in thermal conductivity with water content at high water contents.

Moist soil acts as a heat pipe. The water evaporates on the high temperature side of the pores, defusing across the pore in the vapor phase and condenses on the cold side of the pores.
Figure 2. Moist soil acts as a heat pipe. The water evaporates on the high temperature side of the pores, defusing across the pore in the vapor phase and condenses on the cold side of the pores.

 

Specific heat and water content

The volumetric specific heat of a porous material, such as soil, depends on the water content, texture, and density. The change in the volumetric specific heat with change in water content is always linear.
Figure 3. The volumetric specific heat of a porous material, such as soil, depends on the water content, texture, and density. The change in the volumetric specific heat with change in water content is always linear.

Volumetric specific heat in porous materials also varies with water content and density. Unlike thermal conductivity, the relationship between specific heat and water content is consistently linear across different materials. This predictable behavior makes specific heat relatively easy to model.

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