Chapter 6: Entropy Concepts

Entropy is a fundamental thermodynamic property that measures the amount of disorder or randomness in a system. It can be viewed as a measure of the system's energy dispersal. A higher entropy implies a greater degree of disorder and randomness.

Entropy transfer occurs when the system interacts with its surroundings and exchanges entropy with the environment. This can happen through heat transfer and mass transfer carrying entropy across the boundary.

Internal irreversibilities within the system lead to entropy production. These irreversibilities could be due to friction, heat transfer across a finite temperature difference, or other processes that deviate from ideal reversibility.

The second law of thermodynamics states that the total entropy of an isolated system always increases over time. This means that irreversibilities always lead to an increase in entropy, making it impossible to reduce the entropy of a system without external intervention.

Isentropic processes are theoretical, idealized processes that happen without entropy production. This means they are reversible, frictionless, and occur without any heat transfer across a finite temperature difference. In reality, perfectly isentropic processes don't exist.

The Clausius inequality is a fundamental principle used to determine the direction of spontaneous processes. It states that for any cyclic process, the integral of the heat transfer divided by temperature is always less than or equal to zero. This inequality directly links entropy changes with irreversibilities within the system.

An entropy balance is used to track the changes in entropy within a system. It considers the entropy transfer across the system boundary and the entropy production within the system. Just like energy or mass balances, it helps us understand how entropy evolves in a system.

Isentropic efficiency is used to quantify the performance of various thermodynamic devices like turbines, nozzles, compressors, and pumps. It reflects how closely the actual process resembles the ideal isentropic process. A higher isentropic efficiency implies a greater resemblance to the isentropic ideal, indicating fewer losses and better efficiency.

Entropy change is calculated by integrating the heat transfer divided by temperature along an internally reversible process path.

Entropy is a property of a system, meaning that its change is independent of the process taken to reach a particular state. In other words, the change in entropy between two states is the same for all processes connecting those states.

Entropy is an extensive property, meaning that its value depends on the size or mass of the system. A larger system will have a larger entropy value.

Entropy can increase, decrease, or stay the same depending on the process involved.

Entropy units are kJ/K (kilowatt-hours per kelvin) and Btu/°R (British thermal units per Rankine). Specific entropy (s) is calculated by dividing entropy (S) by mass (m), and its units are kJ/kg·K and Btu/lb·°R.

In thermodynamics, we can use tables to find specific entropy values for different substances and states. This enables us to analyze and solve problems related to entropy changes in systems.

For two-phase liquid-vapor mixtures, we find the specific entropy using the quality (x) and the specific entropy values of the liquid and vapor phases.

The entropy term "s" in the entropy balance equation is zero when there are no irreversibilities and is positive when there are irreversibilities present in the system.

Entropy change (ΔS) for a closed system during an internally reversible process is equivalent to the area under the temperature-entropy (T-S) diagram.

The change in entropy of a closed system that undergoes a process can be calculated as the integral of the heat transfer divided by temperature, from the initial state to the final state.

The entropy change of a closed system during an adiabatic process is zero for reversible systems and positive for irreversible systems.

For an adiabatic expansion, the entropy increase within the system is greater when the final temperature is 200°C compared to 100°C. This indicates more irreversibilities in the process with a higher final temperature.

The process is irreversible because entropy is produced within the system. This is due to internal irreversibilities during the expansion, such as friction.