Energy Balance for the Stationary Flow Process

The energy balance per unit mass for the stationary flow process is as follows (cf. [3])  

Subscript 1 denotes the initial and subscript 2 the final state, respectively. 

The flow processes treated below are considered not to involve any energy losses. Neither heat nor mechanical work are introduced into or output from the process. 

If the medium is incompressible (this is, as a rule, a good approximation for liquids), the internal energy only depends on its temperature and its specific volume (vi = V2 = 1/p). Therefore the internal energy does not change during the flow process and Eq. (7.4) becomes 

The above relations describe frictionless flow processes. However, the outflow from safety valves and bursting discs is accompanied by friction losses. These are accounted for by the discharge coefiicient K. This coefficient is determined experimentally in the context of the certification process. It represents the ratio between the ideal and real flow velocities. 

---