# Reversible cycles and entropy generation

• Urmi Roy

#### Urmi Roy

I've got a couple of questions about reversible cycles:

So if we have two gaseous systems and have a reversible cycle working between them, then the entropy generation within each gaseous system is zero, right?

Do turbines execute reversible cycles?

Thanks a lot for your help!

I've got a couple of questions about reversible cycles:

So if we have two gaseous systems and have a reversible cycle working between them, then the entropy generation within each gaseous system is zero, right?

Do turbines execute reversible cycles?

Thanks a lot for your help!

Ideally, a turbine could be modeled as a reversible cycle, which means that the expansion of the working fluid would occur isentropically. In the real world, however, this is not the case, and there is an increase in the entropy of the working fluid as it moves through the turbine.

Ideally, a turbine could be modeled as a reversible cycle, which means that the expansion of the working fluid would occur isentropically. In the real world, however, this is not the case, and there is an increase in the entropy of the working fluid as it moves through the turbine.

My point of confusion arose from the fact that in a discussion we were having at school,the fluid coming out of the turbine was used to fill an initially evacuated vessel.
I had a feeling that since the turbine operates a reversible cycle, the entropy generation in the vessel when the fluid got into it would be 0.

In a reversible cycle such as Carnot cycle, however, the entropy generation in all components, including the heat reservoirs would be zero...

However, as you mentioned, since the turbine operates in an open system, the entropy generation in the fluid as it moves through the turbine is zero. It is not necessary that the Sgen in the vessel is zero, even though it is directly connected to the reversible cycle...is that right?

The word "cycle" means that the system comes back to its initial state over and over again. A cycle doesn't have to be reversible in order for this to happen, but, since entropy is a state function, the change in entropy must be zero for a complete cycle.

A turbine doesn't usually operate in a cycle, but it often does operate close to adiabatically and reversibly. So the change in entropy of the working fluid in passing through a turbine often is close to zero. However, this is usually not because it is operating in a cycle.

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