Question

Friction in a pipe flow causes a slight pressure decrease and a slight temperature increase. How does that affect entropy?  

Answer 1

The friction converts flow work (P drops) into internal energy (T up if single phase). This is an irreversible process and s increases.

If liquid: Eq.  ds = C/TdT so s follows T

If ideal gas Eq. ds = Cp dT/T − R dP/P (both terms increase)