Availability and Second Law | Thermal Interview Question and Answers

Availability and Second Law | Thermal Interview ,viva ,oral Question and Answers

1. Define Clausius statement.

It is impossible for a self-acting machine working in a cyclic process, to transfer heat from a body at lower temperature to a body at a higher temperature without the aid of an external agency.

2. What is Perpetual motion machine of the second kind?

A heat engine, which converts whole of the heat energy into mechanical work is known as Perpetual motion machine of the second kind.

3. Define Kelvin Planck Statement.

It is impossible to construct a heat engine to produce network in a complete cycle if it exchanges heat from a single reservoir at single fixed temperature.

4. Define Heat pump.

A heat pump is a device, which is working in a cycle and transfers heat from lower temperature to higher temperature.

5. Define Heat engine.

Heat engine is a machine, which is used to convert the heat energy into mechanical work in a cyclic process.

6. A heat engine with a thermal efficiency of 45 percent rejects 500 kJ/kg of heat. How much heat does it receive?

Thermal efficiency=(Qs-Qr)/Qs
0.45=(Qs-500)/Qs
Qs=909 kJ/kg

7. What is a reversed heat engine?

The reversed heat engine works on the principle of reversed Carnot cycle. The heat engine produces work by absorbing heat from source and liberating some heat to sink. The reversed heat engine transfers the heat from sink to the source with the help of external work.

8. What are the assumptions made on heat engine?

1. The source and sink are maintained at constant temperature.
2. The source and sink has infinite heat capacity.

9. State Carnot theorem.

It states that no heat engine operating in a cycle between two constant temperature heat reservoir can be more efficient than a reversible engine operating between the same reservoir.

10. What is meant by reversible process?

A reversible process is one, which is performed in such a way that at the conclusion of process, both system and surroundings may be restored to their initial state, without producing any changes in rest of the universe.

11. What is meant by irreversible process?

The mixing of two substances and combustion also leads to irreversibility. All spontaneous process is irreversible.

12. Explain entropy?

It is an important thermodynamic property of the substance. It is the measure of molecular disorder. It is denoted by S. The measurement of change in entropy for reversible process is obtained by the quantity of heat received or rejected to absolute temperature.

13. What is absolute entropy?

The entropy measured for all perfect crystalline solids at absolute zero temperature is known as absolute entropy.

14. 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 a adiabatic system is always constant. (S=C) S1=S2

15. Define availability.

The maximum useful work obtained during a process in which the final condition of the system is the same as that of the surrounding is called availability of the system.

16. Define available energy and unavailable energy.

Available energy is the maximum thermal useful work under ideal condition. The remaining part, which cannot be converted into work, is known as unavailable energy.

17. What is a thermal energy reservoir? Explain the term source and sink.

A thermal reservoir, a short-form of thermal energy reservoir, or thermal bath is a thermodynamic system with a heat capacity that is large enough that when it is in thermal contact with another system of interest or its environment, its temperature remains effectively constant.
Source is a thermal reservoir, which supplies heat to the system and sink is a thermal reservoir, which takes the heat from the system.

18. What do you understand by the entropy principle?

The entropy of an isolated system can never decrease. It always increases and remains constant only when the process is reversible. This is known as principle of increase in entropy or entropy principle.

19. What are the important characteristics of entropy?

1. If the heat is supplied to the system then the entropy will increase.
2. If the heat is rejected to the system then the entropy will decrease.
3. The entropy is constant for all adiabatic frictionless process.
4. The entropy increases if temperature of heat is lowered without work being done as in throttling process.
5. If the entropy is maximum, then there is a minimum availability for conversion in to work.
6. If the entropy is minimum then there is a maximum availability for conversion into work.

20. What is reversed Carnot heat engine? What are the limitations of Carnot cycle?

1. No friction is considered for moving parts of the engine.
2. There should not be any heat loss.

21. Define an isentropic process.

Isentropic process is also called as reversible adiabatic process. It is a process which follows the law of pVy= C is known as isentropic process. During this process entropy remains constant and no heat enters or leaves the gas.

22. Explain the throttling process.

When a gas or vapour expands and flows through an aperture of small size, the process is called as throttling process.

23. What are the Corollaries of Carnot theorem?

(i) In the entire reversible engine operating between the two given thermal reservoirs with fixed temperature, have the same efficiency.
(ii) The efficiency of any reversible heat engine operating between two reservoirs is independent of the nature of the working fluid and depends only on the temperature of the reservoirs.

24. Define – PMM of second kind.

Perpetual motion machine of second kind draws heat continuously from single reservoir and converts it into equivalent amount of work. Thus it gives 100% efficiency.

25. What is the difference between a heat pump and a refrigerator?

Heat pump is a device which operating in cyclic process, maintains the temperature of a hot body at a temperature higher than the temperature of surroundings.
A refrigerator is a device which operating in a cyclic process, maintains the temperature of a cold body at a temperature lower than the temperature of the surroundings.

26. Define the term COP?

Co-efficient of performance is defined as the ratio of heat extracted or rejected to work input.

COP = Heat extracted or rejected / Work input

28. Why Carnot cycle cannot be realized in practical?

(i) In a Carnot cycle all the four process are reversible but in actual practice there is no process irreversible.
(ii) There are two processes to be carried out during compression and expansion. For isothermal process the piston moves very slowly and for adiabatic process the piston moves as fast as possible. This speed variation during the same stroke of the piston is not possible.
(iii) It is not possible to avoid friction moving parts completely.

29. Why a heat engine cannot have 100% efficiency?

For all the heat engines there will be a heat loss between system and surroundings. Therefore we can’t convert all the heat input into useful work.

30. What are the processes involved in Carnot cycle.

Carnot cycle consist of
i) Reversible isothermal compression
ii) Isentropic compression
iii) Reversible isothermal expansion
iv) Isentropic expansion

31. What are the causes of irreversibility?

Four of the most common causes of irreversibility are friction, unrestrained expansion of a fluid, heat transfer through a finite temperature difference, and mixing of two different substances.

32. State Clausius statement of II law of thermodynamics.

Clausius statement states “it is impossible for a self-acting machine working in a cyclic process without any external force, to transfer heat from a body at a lower temperature to a body at a higher temperature. It considers transformation of heat between two heat reservoirs.