Chapter 7 Flashcards
Clausius equality
The equality in the Clausius inequality holds for totally or just internally reversible cycles and the inequality for the irreversible ones
____ is an extensive property of a system
Entropy
The entropy change between two specified states…
Is the same whether the process is reversible or irreversible
Internally reversible isothermal heat transfer process
This equation is particularly useful for determining the entropy changes of thermal energy reservoirs
The net change in volume (a property) during a cycle is always ____
Zero
The increase of entropy principle
The equality holds for an internally reversible process and the inequality for an irreversible process
Some entropy is generated or created during an irreversible process, and this generation is due entirely to the presence of irreversibilities
The entropy generation Sgen is always a positive quantity or zero
The entropy change of an isolated system…
The sum of the entropy changes of its components, and is never less than zero
A system and its surroundings form an ______
Isolated system
Sgen > 0
Irreversible process
Sgen = 0
Reversible process
Sgen < 0
Impossible process
The entropy change of a system can be _____
Negative
Can entropy generation (Sgen) be negative?
No
Entropy
- Processes can occur in a direction only, not in any direction.
- Entropy is a nonconserved property and there is no such thing as the conservation of entropy principle
- The performance of engineering systems is degraded by the presence of irreversibilities and entropy is a measure of the magnitudes of the irreversibilities during that process. It is also used to establish criteria for the performance of engineering devices
Processes can occur in a direction only, not in any direction
A process must proceed in the direction that complies with the increase of entropy principle (Sgen >= 0)
A process that violates this principle is impossible
Entropy is a non-conserved property
Entropy is conserved during the idealized reversible processes only and increases during all actual processes
Entropy change of pure substances
Entropy is a property, and thus the value of entropy of a system is fixed once the state of the system is fixed
Determined from the tables
A process during which the entropy remains constant
Isentropic process
Isentropic process
During an internally reversible, adiabatic process
Appears as a vertical line segment on a T-s diagram
On a T-s diagram, the area under the process curve represents…
The heat transfer for internally reversible processes
Mollier diagram
The h-s diagram
For adiabatic steady-flow devices, h-s diagrams
The vertical distance (change in h) is a measure of work
The horizontal distance (change in s) is a measure of irreversibilities
The level of molecular disorder (entropy) of a substance _______ as it melts or evaporates
Increases
A pure crystalline substance at absolute zero temperature is in perfect order and its entropy is zero
The third law of thermodynamics
Disorganized energy
Does not create must useful effect, no matter how large it is
In the absence of friction, raising a weight by a rotating shaft…
Does not create any disorder (entropy) and energy is not degraded during this process
The paddle-wheel work done on a gas…
Increases the level of disorder (entropy) of the gas and energy is degraded during this process
During a heat transfer process, the net entropy ____
Increases
Hot body - decreases
Cold body - increases
T ds Relations
Valid for both reversible and irreversible processes and for both closed and open systems
Liquids and solids can be approximated as _____ since their specific volumes remain nearly constant during a process
Incompressible substances
For solids and liquids, an isentropic process is also ____
Isothermal
A reversible turbine delivers more work than an irreversible one if…
Both operate between the same end states
The adiabatic compression requires the ____ work
Maximum
The isothermal compression requires the ____ work
Minimum
The gas is compression in stages and cooled between each stage by passing it through a heat exchanger called an ____
Intercooling
To minimize compression work during two-stage compression, the pressure ratio across each stage of the compressor…
Must be the same
Isentropic process
Involves no irreversibilities
Serves as the ideal process for adiabatic devices
Can you use isentropic efficiency for a non-adiabatic compressor?
No
Can you use isothermal efficiency for an adiabatic compressor?
Yes
Compressors are sometimes intentionally cooled to…
Minimize the work input
Mechanisms of Entropy Transfer
- Heat transfer
2. Mass flow
Entropy transfer by work
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