1. What is the principle of increase of entropy?
For any infinitesimal process undergone by a system, the change in entropy is given by
dS > dQ/T
For reversible, dQ = 0, hence, dS = 0
For irreversible, dS > 0
So, the entropy of an isolated system would never decrease. It will always increase and remain constant if the process is reversible called principle increase of entropy.
2. When a system is adiabatic, what can be said about the entropy change of the substance in the system?
Entropy change of the substance in the system is zero when a system is adiabatic.
3. Define exergy.
The maximum useful work in a process which brings the system to equilibrium with a heat reservoir is called exergy. It is measured in terms of second law efficiency.
4. Define the term absolute entropy.
The change in entropy of the system with respect to ambient conditions or any other standard reference condition is known as absolute entropy.
5. List the causes of entropy increase.
(i) More energy into a system excites the molecules and the amount of random activity which increase the entropy.
(ii) The sudden change in volume of the working fluid in the systems leads to increase in entropy.
(iii) As a gas expands in a system, the entropy is increased due to more space for bouncing of atoms.
(iv) When a solid becomes a liquid, its entropy is increased.
(v) Similarly, when a liquid becomes a gas, its entropy is increased due to rapid movement of atoms.
(vi) During any chemical reaction, the number of gas molecules increases which lead to increase its entropy.
(vii) Entropy increases during heat exchange if two streams with different temperatures are mixed.
6. How does entropy of isolated system change? Why?
For isolated system, there is no change in entropy, it will be constant. It is because of the fact is that when the isolated system undergoes the transition from one state to other, it exchanges no heat or work with the surroundings.
7. What are 'available energy' and 'unavailable energy'?
Available energy:
The portion of the energy supplied as heat which can be converted into useful work by a reversible engine. It is called available energy.
Unavailable energy:
The portion of the energy supplied as heat which cannot be converted into work due to friction is called unavailable energy.
8. What is loss of availability? How is it related to entropy of Universe?
The portion of the energy supplied as heat which cannot be converted into work due to friction is called unavailable energy.
Unavailable energy, U.A.E
= Total heat energy - Available energy
= Q - A.E = Q - [Q - To ΔS]
= To ΔS [⸪ ΔS Entropy of Universe]
9. A turbine gets a supply of 5 kg/s of steam at 7 bar, 250°C and discharges it at 1 bar. Calculate the availability.
Given data:
m = 5 kg/s
p1 = 7 bar
T1 = 250°C = 250 + 273 = 523 K
p2 = 1 bar
Solution:
Availability, B = m[(h1 - h2) - To (S1 - S2)] for any type of open system
In a turbine, the flow is isentropic.
So, ΔS = S1 - S2 = 0
Therefore, the availability equation reduces to
B = m (h1 - h2)
Corresponding to 7 bar and 250°C, both enthalpy and entropy are read Mollier diagram.
h1 = 2954 kJ/kg
Draw a vertical line up to 1 bar and the enthalpy is read as 2581 kJ/kg.
So, h2 = 2581 kJ/kg
⸫ Availability, B = m (h1 - h2) = 5 (2954 - 2581) = 1865 kW Ans.
10. Define irreversibility.
Irreversibility is defined as the amount of work to be done to restore the system to the original state.
11. What is Helmholtz free energy function?
Helmholtz free energy is a thermodynamic potential that measures the useful work obtainable from a closed thermodynamic system at a constant temperature and volume. The negative of the change in the Helmholtz energy during a process is equal to the maximum amount of work that the system can perform in a thermodynamic process in which volume is held constant.
12. Define Second law efficiency.
Second law efficiency is defined as the ratio between the change in available energy of the system and change in available energy of the source. Otherwise, it may also be defined as the ratio between the availability of output to the availability of input.
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