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# Important Questions and Answers: Thermodynamics Second Law and Availability Analysis

Mechanical - Engineering Thermodynamics - The Second Law of Thermodynamics

SECOND LAW AND AVAILABILITY ANALYSIS

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. Define Kelvin Planck Statement.

It is impossible for a heat engine to produce network in a complete cycle if it exchanges heat only with bodies at a single fixed temperature.

3. Define Heat pump.

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

4. Define Heat engine.

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

5. 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.

6. State Carnot theorem.

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

7. 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.

8. What is meant by irreversible process?

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

9. 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.

10. 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.

11. Explain the term source and sink.

Source is a thermal reservoir, which supplies heat to the system and sink is a thermal reservoir, which takes the heat from the system.

12. 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.

13.        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.

14.        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.

15.        Define an isentropic process.

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

16.        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.

17. 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.

18. 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.

19. Define the term COP?

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

COP = input. Heat extracted or rejected / Work input

20. Write the expression for COP of a heat pump and a refrigerator?

COP of heat pump:

COP HP = Heat Supplied / Work input = T2 / T2 - T1

COP of Refrigerator:

COP Ref = Heat extracted / Work input = T1 / T2 - T1

21.            Why Carnot cycle cannot be realized in practical?

(i)  In a Carnot cycle all the four processes are reversible but in actual practice there is no

process is reversible.

(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.

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