# Simple compressible system and flow system

1. Apr 30, 2017

### mech-eng

I would like to ask about the difference between the simple compressible system and the flow systems.

"In the absence of such effects as magnetic, electric, and surface tension, a system is called a simple compressible system. The total energy of a simple compressible system consists of three parts: internal, kinetic, and potential energies. On a unit-mass basis, it is expressed as $e=u+ke+pe$. The fluid entering or leaving a control volume possesses an additional form of energy—the flow energy $\frac{P}{\rho}$. Then the total energy of a flowing fluid on a unit-mass basis becomes $e_{flowing}=\frac{P}{\rho}+e=h+ke+pe=h+\frac{V^2}{2}+gz$(2–8)
where $h=\frac{P}{\rho}+u$ is the enthalpy, V is the velocity, and z is the elevation of the system relative to some external reference point.

Source: Fluid Mechanics Fundamentals by Çengel/Cimbala.

Are flowing systems different than simple compressible systems? Simple compressible systems have kinetic, potential and internal energies but flowing system have one more energy form, the flow energy. So something seems different to me.

Thank you.

Last edited: Apr 30, 2017
2. Apr 30, 2017

### Staff: Mentor

You need to look up the derivation of the open system (control volume) version of the first law of thermodynamics in a book that presents it better than this book. Try Moran et al, Fundaments of Engineering Thermodynamics or Smith and Van Ness, Introduction to Chemical Engineering Thermodynamics.