2. Thermal Diffusivity?
• Abstract :The two main properties related to heat
conduction are thermal conductivity (K) and thermal
diffusivity (D). However, while the meaning of K is
very well known, the role played by D is usually
undervalued and misunderstood. In this paper we will
try to clarify the meaning of thermal diffusivity, first in
homogeneous materials and then in composite
materials. In this latter case, we will find that
sometimes the association of two good thermal
diffusers gives a bad diffuser. Moreover, by properly
choosing the constituents of the composites we can
manufacture materials with thermal properties that
are not found in nature.
3. Definition of Thermal Diffusivity
• Thermal diffusivity is the thermal
conductivity divided by density
and specific heat capacity at constant
pressure. It measures the ability of a material
to conduct thermal energy relative to its ability
to store thermal energy. High diffusivity
means heat transfers rapidly.
4. Unit & Equation of Thermal Diffusivity
• It has the SI derived unit of m2/s. Thermal diffusivity is
usually denoted α but a.
• Thermal diffusivity is a m easure of the transient
thermal response of a material to a change in
temperature and the term thermal diffusivity (α) is
defined as α= k/(ρ x cp)
Where, α is the thermal diffusivity (m2/sec)
k is the thermal conductivity (W/m-K)
ρ is the density (kg/m3)
cp is the heat capacity (J/kg-K)
5. Thermal Diffusivity of Materials
Examples of varies materials:-
Materials Thermal Diffusivity
(cm2/sec) @300 K
Silver 1.74
Copper 1.15
Gold 1.27
Aluminum 0.97
Stainless Steel 0.042
Wood 0.082
Many Plastics 0.08
Air (300 K) 0.34
Glass, window 0.024
Rubber 0.089 - 0.13
6. Thermal Diffusivity
• It should be noted that each of these quantities can vary with
temperature. Thermal diffusivity is a convenient collection of
physical properties for transient solutions of the heat equation.
For a homogenous material with constant properties, the heat
equation (1) with three physical properties is expressed as (2)
with just one coefficient.
7. Thermal Diffusivity
A material with a high thermal diffusivity (such as silver) is a
good diffuser of thermal energy while a material with a low
thermal diffusivity (such as plastic) is much slower at diffusing
thermal energy. If the thermal environment around a material
changes, heat must flow in or out of the material until thermal
equilibrium is achieved, assuming the environment is constant
after the change. Materials with a high thermal diffusivity will
achieve thermal equilibrium faster than materials with low
thermal diffusivity.
8. Thermal diffusivity
• Figure 1 compares the thermal conductivity to thermal
diffusivity for a wide range of materials. Notice that the
data points noted as condensed matter can be described
as being near a straight line. This is because the range of
heat capacity per unit volume for condensed matter
(liquids and solids) is small (the range of heat capacity per
volume for condensed matter is from 1×10-6 J/m3 -K to
4×10-6 J/m3-K). With respect to thermal diffusivity, gases
and condensed matter are different. For example, air has
a low thermal conductivity but a relatively high thermal
diffusivity � this means that while air can only absorb a
relatively small amount of thermal energy, it is effective at
diffusing the energy.
9. Why is thermal diffusivity important?
• Thermal diffusivity is an important term as it states, how fast heat
diffuses into the material. The lower value of thermal diffusivity
indicates that the material has a higher rate of thermal diffusion
and hence such materials heat or cool slowly. In such cases,
the specific heat and density of the material are higher or the
thermal conductivity of the material is less.
• While the material with a higher value of thermal diffusivity has
lower heat diffusion and it can heat or cool quickly. Such material
has a lower value of specific heat or the density of the material
has a higher value of thermal conductivity.
• Therefore in such an application where cooling or heating is
required at a faster rate, in that case, the material with higher
thermal diffusivity is preferred.
• While in some applications, the cooling or heating of material has
been done at a slower rate, hence in such application the material
with lower thermal diffusivity are preferred.
10. Thermal diffusivity vs conductivity:
• The thermal diffusivity states the relation between heat
transferred and heat stored while thermal conductivity is the
ability of the material to conduct heat.
• The thermal diffusivity states how fast heat diffuses into the
material while the thermal conductivity indicates the rate of
heat transfer per unit area and per unit temperature gradient.
• The thermal diffusivity is considered only in unsteady-state
cases, while the thermal conductivity is considered in both
steady-state and unsteady-state cases.
11. What is thermal diffusivity used for?
• In physical terms, thermal diffusivity gives a
measure of how quickly the temperature will
change when it is heated or cooled. Materials with
a high thermal diffusivity will heat or cool quickly;
conversely, substances with a low thermal diffusivity
will heat or cool slowly.
12. Conclusions
• In this work the role played by thermal diffusivity, as the quantity
that governs the speed of heat propagation in transient
problems has been analysed.
• It has been demonstrated that in homogeneous materials the
relationship of better/worse thermal conductor–better/worse
thermal diffuser is not always correct.
• In the case of layered composites it has been shown that while
the associations of two good/bad thermal conductors always
give a good/bad conductor,
• the association of two good diffusers gives a bad diffuser when
their effusivities differ greatly.
• All these results reinforce the role played by thermal diffusivity
and effusivity in all time varying thermal phenomena.
