Question

Why thermal boundary layer thickness in higher for liquid metals

Answer 1

Here, the question needs a simple intuition. Why is the ‘Rate’ of Thermal boundary layer(BL) higher than the ‘Rate’ of Hydrodynamic boundary layer.

As mentioned in the earlier answer,

Ratio of thickness of Hydrodynamic BL to Thermal BL depends on the dimensionless number, Prandtl number(Pr) which happens to be the ratio of Momentum diffusivity to Thermal diffusivity.

Here, the rate of formation of a boundary layer is directly proportional to the governing diffusion variable; i.e.

In Hydrodynamic BL, the diffusion term is the kinematic viscosity and for Thermal BL, it is Thermal diffusivity.

Diffusivity is a measure of the capability of a substance or energy to be diffused(spread) or to allow something to pass by diffusion.

For a material, the rate of formation of either boundary layers will be dependent on their respective diffusion terms.

As mentioned, for liquid metals: Pr < 1 →numerator < denominator → momentum diffusivity < Thermal diffusivity → So, finally:

“Rate of thermal BL formation is higher than the rate of hydrodynamic BL formation in liquid metals.”

Answer 2

Explanation:

Here, the question needs a simple intuition. Why is the ‘Rate’ of Thermal boundary layer(BL) higher than the ‘Rate’ of Hydrodynamic boundary layer.

As mentioned in the earlier answer,

Ratio of thickness of Hydrodynamic BL to Thermal BL depends on the dimensionless number, Prandtl number(Pr) which happens to be the ratio of Momentum diffusivity to Thermal diffusivity.

Here, the rate of formation of a boundary layer is directly proportional to the governing diffusion variable; i.e.

In Hydrodynamic BL, the diffusion term is the kinematic viscosity and for Thermal BL, it is Thermal diffusivity.

Diffusivity is a measure of the capability of a substance or energy to be diffused(spread) or to allow something to pass by diffusion.

For a material, the rate of formation of either boundary layers will be dependent on their respective diffusion terms.

As mentioned, for liquid metals: Pr < 1 →numerator < denominator → momentum diffusivity < Thermal diffusivity → So, finally:

“Rate of thermal BL formation is higher than the rate of hydrodynamic BL formation in liquid metals.”