Quiz: Mechanical Engineering 25th May

Quiz: Mechanical Engineering
Exam: DFCCIL
Topic: Thermodynamics

Each question carries 2 mark
Negative marking: 1/3 mark
Time: 10 Minutes

Q1. High pressure steam is expanded adiabatically and reversible through a well-insulated turbine which produces some shaft work. If the enthalpy change and entropy change across the turbine are represented by ∆H and ∆S, respectively, for this process:
(a) ∆H=0 and ∆S=0
(b) ∆H≠0 and ∆S=0
(c) ∆H≠0 and ∆S≠0
(d) ∆H=0 and ∆S≠0

Q2. Consider the following statements:
1. Availability is generally conserved
2. Availability can either be negative or positive
3. Availability is the maximum theoretical work obtainable
4. Availability can be destroyed in irreversibility
Which of these statements are correct?
(a) 3 and 4
(b) 1 and 2
(c) 1 and 3
(d) 2 and 4

Q3. A reversible power cycle operates with temperature limits of 800 K and 300 K. If it takes 600 kJ of heat, then what would be the unavailable work?
(a) 180 kJ
(b) 290 kJ
(c) 200 kJ
(d) 225 kJ

Q4. Which of the following is not a conserved quantity?
(a) Mass
(b) Energy
(c) Momentum
(d) Exergy

Q5. Which of the following thermodynamic relation is Keenan function?
(a) U+PV
(b) H-T_0 S
(c) E-T_0 S
(d) U-T_0 S

Q6. The main cause for the irreversibility is
(a) mechanical and fluid friction
(b) unrestricted expansion
(c) heat transfer with a finite temperature difference
(d) all of the above

Q7. Neglecting change in kinetic energy and potential energy per unit mass, the availability in a non-flow process becomes a=∅-∅_0, where ∅ is the availability function of the
(a) open system
(b) closed system
(c) isolated system
(d) steady flow process

Q8. In which one of the following processes is there an increase in entropy with no degradation of energy
(a) Polytropic expansion
(b) Isothermal expansion
(c) Isochoric heat addition
(d) Isobaric heat addition

Q9. The second law efficiency (η_II) of a process is defined as (where A_minis minimum exergy intake to perform the task, A_max is maximum exergy intake to perform the task, A is exergy intake to perform the task)
(a) η_II=A_min/A
(b) η_II=A_max/A
(c) η_II=A/A_min
(d) η_II=A/A_max

Q10. In order to determine the quality of wet steam, by a separating and throttling calorimeter, the steam should be throttled such that the final state is
(a) saturated vapor only
(b) superheated vapor only
(c) at a pressure lower than the original pressure
(d) a mixture of saturated liquid and saturated vapor

Solutions

S1. Ans (b)
Sol. For reversible adiabatic process, ∆S=0
After expansion in the turbine there would be an enthalpy change i.e. ∆H≠0

S2. Ans (a)
Sol. The availability of a given system is defined as the maximum useful work that can be obtained in a process in which the system comes to equilibrium with the surroundings or attains the dead state and availability can be destroyed due to irreversibility.

S3. Ans (d)
Sol. Efficiency of reversible cycle (η)=1-T_2/T_1 =W/Q_1
(η)=1-300/800=W/600
W=375 kJ
Unavailable work or heat rejected to sink (Q_2 )=600-375=225 kJ

S4. Ans (d)
Sol. According to second law of thermodynamics exergy or available energy is never conserved.

S5. Ans (b)
Sol. K= H-T_0 S , this is known as Keenan function.
F=U-TS, this is known as Helmholtz function.
G=H-TS, this is known as Gibbs function.

S6. Ans (d)
Sol. Main causes of irreversibility,
Dissipative effects such as Mechanical friction, Fluid viscosity, Magnetic hysteresis, Inelasticity, Electrical resistance etc.
Heat transfer with finite temperature difference

S7. Ans (b)
Sol. Availability function for a closed system ∅=u-T_0 s
Availability function for a open systemψ=h-T_0 s

S8. Ans (b)
Sol. From First law of thermodynamics δQ=δW (Isothermal expansion process)
From second law of thermodynamics dS=R.ln⁡(V_2/V_1 ) (Isothermal expansion process)
We can say that there is no degradation of energy due to no temperature gradient (i.e. no external irreversibility).

S9. Ans (a)
Sol. Second law of efficiency for a work consuming device
(η_II)=(minimum work required to complete the task )/(actual work required to complete the task)=A_min/A

S10. Ans (b)
Sol. Throttling and Separating Calorimeters are used to measure the dryness fraction of steam. If the steam is sufficiently dry to enable it to reach a supersaturated state by throttling process, then a solitary throttling calorimeter can serve the purpose. If on the other hand, the steam is very wet, then a separating calorimeter is used. When the dryness fraction of the steam is somewhere in between, then a combined setup of Throttling and Separating Calorimeter is used

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