Entropy production in Newtonian fluids
In fluid dynamics, the general equation of heat transfer is a nonlinear partial differential equation describing specific entropy production in a Newtonian fluid subject to thermal conduction and viscous forces:[1][2]
where
is the specific entropy,
is the fluid's density,
is the fluid's temperature,
is the material derivative,
is the thermal conductivity,
is the dynamic viscosity,
is the second Lamé parameter,
is the flow velocity,
is the del operator used to characterize the gradient and divergence, and
is the Kronecker delta.
If the flow velocity is negligible, the general equation of heat transfer reduces to the standard heat equation. It may also be extended to rotating, stratified flows, such as those encountered in geophysical fluid dynamics.[3]
Derivation
Extension of the ideal fluid energy equation
For a viscous, Newtonian fluid, the governing equations for mass conservation and momentum conservation are the continuity equation and the Navier-Stokes equations:
where
is the pressure and
is the viscous stress tensor, with the components of the viscous stress tensor given by:
The energy of a unit volume of the fluid is the sum of the kinetic energy
and the internal energy
, where
is the specific internal energy. In an ideal fluid, as described by the Euler equations, the conservation of energy is defined by the equation:
where
is the specific enthalpy. However, for conservation of energy to hold in a viscous fluid subject to thermal conduction, the energy flux due to advection
must be supplemented by a heat flux given by Fourier's law
and a flux due to internal friction
. Then the general equation for conservation of energy is:
Equation for entropy production
Note that the thermodynamic relations for the internal energy and enthalpy are given by:
We may also obtain an equation for the kinetic energy by taking the dot product of the Navier-Stokes equation with the flow velocity
to yield:
The second term on the righthand side may be expanded to read:
With the aid of the thermodynamic relation for enthalpy and the last result, we may then put the kinetic energy equation into the form:
Now expanding the time derivative of the total energy, we have:
Then by expanding each of these terms, we find that:
And collecting terms, we are left with:
Now adding the divergence of the heat flux due to thermal conduction to each side, we have that:
However, we know that by the conservation of energy on the lefthand side is equal to zero, leaving us with:
The product of the viscous stress tensor and the velocity gradient can be expanded as:
Thus leading to the final form of the equation for specific entropy production:
In the case where thermal conduction and viscous forces are absent, the equation for entropy production collapses to
- showing that ideal fluid flow is isentropic.
Application
This equation is derived in Section 49, at the opening of the chapter on "Thermal Conduction in Fluids" in the sixth volume of L.D. Landau and E.M. Lifshitz's Course of Theoretical Physics.[1] It might be used to measure the heat transfer and air flow in a domestic refrigerator,[4] to do a harmonic analysis of regenerators,[5] or to understand the physics of glaciers.[6]
See also
References
- ^ a b Landau, L.D.; Lifshitz, E.M. (1987). Fluid Mechanics (PDF). Course of Theoretical Physics. Vol. 6 (2nd ed.). Butterworth-Heinemann. pp. 192–194. ISBN 978-0-7506-2767-2. OCLC 936858705.
- ^ Kundu, P.K.; Cohen, I.M.; Dowling, D.R. (2012). Fluid Mechanics (5th ed.). Academic Press. pp. 123–125. ISBN 978-0-12-382100-3.
- ^ Pedlosky, J. (2003). Waves in the Ocean and Atmosphere: Introduction to Wave Dynamics. Springer. p. 19. ISBN 978-3540003403.
- ^ Laguerre, Onrawee (2010-05-21), Farid, Mohammed M. (ed.), "Heat Transfer and Air Flow in a Domestic Refrigerator", Mathematical Modeling of Food Processing (1 ed.), CRC Press, pp. 453–482, doi:10.1201/9781420053548-20, ISBN 978-0-429-14217-8, retrieved 2023-05-07
- ^ Swift, G. W.; Wardt, W. C. (October–December 1996). "Simple Harmonic Analysis of Regenerators". Journal of Thermophysics and Heat Transfer. 10 (4): 652–662. doi:10.2514/3.842.
- ^ Cuffey, K. M. (2010). The physics of glaciers. W. S. B. Paterson (4th ed.). Burlington, MA. ISBN 978-0-12-369461-4. OCLC 488732494.
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Further reading
- Vallis, G.K. (2006). Atmospheric and Oceanic Fluid Dynamics: Fundamentals and Large-Scale Circulation. Cambridge University Press. ISBN 978-0-521-84969-2.
- Yilmaz, T.; Cihan, E. (September 1993). "General equation for heat transfer for laminar flow in ducts of arbitrary cross-sections". International Journal of Heat and Mass Transfer. 36 (13): 3265–3270. doi:10.1016/0017-9310(93)90009-U. ISSN 0017-9310.