Thermodynamics - Open Systems

Conservation of Mass and Energy

\( \Sigma \dot{m}_{in} - \Sigma \dot{m}_{out} = \dot{m}_{CV} \)

Where \( \dot{m}_{CV} \) is the rate of change of mass in the control volume.

Since the intake and output flows also have their associated energies, the general energy equation becomes

\( Q - W + \Sigma E_{in} - \Sigma E_{out} = \Delta E_{CV} \)

The work done to move the fluid from the intake to output through the control volume is

\( \dot{W} = \dot{W}_{flow,out} - \dot{W}_{flow,in} + \dot{W}_{CV} ; W_{flow} = PV \)

Where \( \dot{W}_{CV} \) is the work for processes affecting the control volume.


Steady Flow Systems

Assumptions for a steady flow system are as follows:

\( \dot{m}_{in} = \dot{m}_{out} \)

\( \dot{E}_{in} = \dot{E}_{out} \)

\( V_{CV} = Constant \)

The general energy equation

\( Q - W + \Sigma E_{in} - \Sigma E_{out} = \Delta E_{CV} \)

can then be modified for steady flow to

\( \dot{Q} - ( \dot{W}_{CV} - \dot{W}_{flow} ) = \Sigma E_{out} - \Sigma E_{in} \)

Since enthalpy (H) also takes into account flow work,

\( H = PV + U ; \dot{W}_{flow} = PV\)

The general steady flow energy equation can be written as

\( \dot{Q} - \dot{W} = \Delta H + \Delta KE + \Delta PE \)

Where \( \Delta H = m C_p \Delta T ; H = PV+U ; \dot{W}_{flow} = PV \)

and \( KE = \frac{1}{2} m C^2 ; PE = mgz\)


Work for Closed and Open systems

On a PV diagram, work in a closed system is the area under the graph:

\( W = \int P dV \)

For open systems, work done is the area to the left of the graph:

\( W = - \int V dP \)