# Entropy equation

### From Thermal-FluidsPedia

To obtain the differential form of the second law of thermodynamics, the surface integrals in eq. (2.35) can be rewritten as volume integrals:

Substituting eqs. (1) and (2) into eq. (2.35) and considering eq. (2.48) yields

In order for eq. (3) to be true for any arbitrary control volume, the integrand in eq. (3) should always be positive, i.e.,

Equation (4) can be rewritten as

Considering the continuity equation, eq. (2.51), and definition of the substantial derivative, eq. (2.53), eq. (5) can be reduced to

where the three terms on the left-hand side represent rate of change of entropy per unit volume, rate of change of entropy per unit volume by heat transfer and internal heat generation, respectively. Equation (6) means that the entropy generation per unit volume must not be negative at any time or location.

For a multicomponent system without internal heat generation (*q*''' = 0), Curtiss and Bird (1999; 2001) obtained the entropy flux vector and the entropy generation as

where *d*_{i} is the diffusional driving force [see eq. (2) – (6)], is partial molar Gibbs free energy, is total heat flux, obtained by eq. (2.95), including conduction, interdiffusional convection, and Dufour effect.