Open system

In an open system, there are transfer of three quantities:

1. Mass of fluid
2. Energy (heat and wok)
3. Entropy

Fig. 1.17 An open system

For mass flow, continuity equation is

1.52

where = rate of mass accumulation in the system

The energy equation gives

1.53

where = rate of energy accumulation in the system

Now, entropy principle gives

1.54

where

= rate of entropy transfer into the system (control volume) due to mass m_i

= rate of entropy transfer out of the system (control volume) due to mass m_e

= net rate of entropy transfer

= rate of change of entropy of control volume

Difference of entropy change of control volume and net rate of entropy transfer is the entropy generated within the control volume due to irreversibility.

1.55

By second law of thermodynamics

Sgen ≥ 0

1.56