Constant entropy, often referred to by the term isentropic, is a state or process characterized by the lack of change in entropy. Based on the provided information, the term isentropic, meaning constant entropy, is specifically used to describe a reversible cycle that is adiabatic.
Understanding Isentropic Processes
An isentropic process is a theoretical thermodynamic process where the entropy of the system remains constant. According to the definition provided:
- Isentropic means Constant Entropy: This is the fundamental definition.
- Describes a Reversible Cycle: The process must be reversible, meaning it can be reversed without leaving any change in the surroundings.
- That is Adiabatic: An adiabatic process is one where there is no heat transfer into or out of the system. This is a key condition for achieving constant entropy in a reversible process.
Essentially, for a process to be isentropic, it must be both reversible and adiabatic.
Key Characteristics
Here are the key characteristics based on the definition:
- Entropy Remains Unchanged: S = constant
- No Heat Transfer: Q = 0 (Adiabatic)
- Process is Reversible: The system can return to its initial state without external work or energy input (in a theoretical sense).
Constant Entropy in Practice: The Gas Turbine Example
The concept of constant entropy is important in the analysis of various thermodynamic cycles. The provided reference highlights its relevance in the context of a gas turbine cycle.
- Gas Turbine Cycles: While real-world processes are never perfectly isentropic due to irreversibilities (like friction), ideal models often assume isentropic compression and expansion steps.
- Compressor Stage: As noted in the reference, air enters the compressor in a gas turbine cycle. The compression process, ideally, can be modeled as isentropic. Air is compressed significantly, increasing its pressure from atmospheric levels to 10–23 times its initial value. An ideal, frictionless compressor where no heat is lost would perform this compression isentropically.
Modeling processes like compression or expansion as isentropic simplifies calculations and provides a benchmark for evaluating the efficiency of real equipment, which will always have some degree of irreversibility and thus an increase in entropy.
