## Cyclic Process

A process that eventually returns a system to its initial state is called a **cyclic process**. At the conclusion of a cycle, all the properties have the same value they had at the beginning. For such a process, the **final state** is the** same as the initial state**, and so the **total internal energy** change must be zero. Steam (water) that circulates through a closed cooling loop undergoes a cycle. The first law of thermodynamics is then:

**dE _{int} = 0, dQ = dW**

Thus, the net work done during the process must exactly equal the net amount of energy transferred as heat. It must be noted, according to the** second law of thermodynamics**, not all heat provided to a cycle can be transformed into an equal amount of work, some **heat rejection** must take place.

## Example of Cyclic Process – Brayton Cycle

Let assume the **ideal Brayton cycle** that describes the workings of a **constant pressure** **heat engine**. **Modern gas turbine** engines and **airbreathing jet engines** also follow the Brayton cycle. This cycle consist of four thermodynamic processes:

Ideal Brayton cycle consist of four thermodynamic processes. Two isentropic processes and two isobaric processes.

**isentropic compression**– ambient air is drawn into the compressor, where it is pressurized (1 → 2). The work required for the compressor is given by**W**_{C}= H_{2}– H_{1}.**isobaric heat addition**– the compressed air then runs through a combustion chamber, where fuel is burned and air or another medium is heated (2 → 3). It is a constant-pressure process, since the chamber is open to flow in and out. The net heat added is given by**Q**_{add}= H_{3 }– H_{2}**isentropic expansion**– the heated, pressurized air then expands on turbine, gives up its energy. The work done by turbine is given by**W**_{T}= H_{4}– H_{3}**isobaric heat rejection**– the residual heat must be rejected in order to close the cycle. The net heat rejected is given by**Q**_{re}= H_{4 }– H_{1}

As can be seen, we can describe and calculate (e.g. thermal efficiency) such cycles (similarly for **Rankine cycle**) using enthalpies.

See also: Thermal Efficiency of Brayton Cycle