In this notes sheet:
The basic form of the first law, ΔQ – ΔW = ΔE, only applies to closed systems - no mass can transfer across the system boundary, only energy in the form of heat and work.
In reality, perfectly closed systems are quite rare (take a turbine, for example: air flows in as well as heat, shaft work and hot air flow out), and as such a different model is required:
The energy transfers across the control volume surface are:
Energy in the working fluid (kinetic, potential, and internal energies)
To simplify the process, we only look at the energy inputs and outputs: what goes on inside the control volume is irrelevant.
We call where the working fluid enters and exits ports.
The control volume above has two ports: one inflow and one outflow port.
The Steady Flow Energy Equation (SFEE)
In order to solve problems involving steady flow through a control volume, we use the Steady Flow Energy Equation (SFEE) instead of the simple first law equation:
In a less mathematically intimidating form:
Left hand side: rate of energy transfer – rate of shaft work
Right hand side: sum of output mass energy flow rates – sum of input mass energy flow rates
Note: The dot above the letters means it is a rate of change with respect to time: