ESS227
Prof. Jin-Yi Yu
Apply the
Energy Conservation to a Control Volume
•The first law of thermodynamics is usually derived by
considering a system in
thermodynamic equilibrium, that is, a system that is initially at rest and after exchanging heat with its
surroundings and doing work
on the surroundings is again at rest.
•A Lagrangian control volume consisting of a specified
mass of fluid may be
regarded as a thermodynamic system. However, unless the fluid is at rest, it will not be in thermodynamic
equilibrium. Nevertheless,
the first law of thermodynamics still applies.
•The thermodynamic
energy of the control volume is
considered to consist of
the sum of the internal energy (due to the kinetic energy of the individual molecules) and the kinetic energy due to the macroscopic
motion of the fluid. The rate of change of this total thermodynamic
energy is equal to the rate of diabatic heating plus the rate at which work is done on the fluid
parcel by external forces.