Viscosity and thermal diffusivity in liquids and gases

In summary, viscosity and thermal diffusivity are both measures of flux, with viscosity measuring momentum flux and thermal diffusivity measuring heat flux. In gases, both values are typically similar due to collisions between molecules. However, in liquids, there can be a significant difference, with very viscous fluids having a much larger viscosity-to-density ratio compared to thermal diffusivity. This is due to the fact that both phenomena occur due to collisions, but there is also a statistical physics component involved in understanding these quantities.
  • #1
dRic2
Gold Member
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Hi PF, I was wondering about this for some time and I can get my head around it.

Viscosity ##\mu## is a "measure" of the momentum flux
Thermal diffusivity ##\alpha## is a "measure" of the heat flux (kinetic energy of molecules)

In gases both viscous stresses and heat flux take place due to collisions between molecules thus the value of viscosity and thermal diffusivity is similar. That doesn't apply to liquids. In particular for very viscous fluids
##\frac \mu \rho >> \alpha## (##\rho## is the density). Since both phenomena happen for the same reason (collisions) I can't explain myself why there is this huge difference.

Ps: for liquid metals ##\frac \mu \rho << \alpha##
 
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  • #2
The molecular derivation of these quantities and their comparisons (the Prandtl number) are explained in detail in Transport Phenomena.
 
  • #3
Yes, but there is a lot of statistical physics (kinetic theory of gases) which I know nothing about. I was looking for an easier explanation if possible... Otherwise I'll have to wait until I know something about statistical mechanics
 

What is viscosity?

Viscosity is a measure of a fluid's resistance to flow. It is determined by the internal friction between molecules in a fluid and is affected by factors such as temperature, pressure, and molecular size.

How is viscosity measured?

Viscosity is typically measured using a viscometer, which measures the time it takes for a fluid to flow through a small tube or the force required to make a fluid flow at a certain rate. The unit of measurement for viscosity is the poise, with water having a viscosity of 1 poise at 20°C.

What is thermal diffusivity?

Thermal diffusivity is a measure of how quickly heat can be conducted through a material. It is determined by the material's thermal conductivity, density, and specific heat capacity.

How does temperature affect viscosity and thermal diffusivity?

In general, as temperature increases, viscosity decreases while thermal diffusivity increases. This is because higher temperatures allow for molecules to move more freely, reducing the fluid's resistance to flow and increasing its ability to conduct heat.

What are some real-world applications of understanding viscosity and thermal diffusivity?

Viscosity and thermal diffusivity have many practical applications, such as in the design of lubricants for machinery, the development of cooling systems for electronics, and the study of weather patterns and ocean currents. They are also important considerations in industries such as food and beverage production, cosmetics, and pharmaceuticals.

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