Quiz:Mechanical Engineering 16 July 2020

Quiz: Mechanical Engineering
Exam: UPPSC AE
Topic: Thermal

Each question carries 3 mark.
Negative marking: 1 mark
Time: 10 Minute

Q1. In a steady flow process, across the control volume mass and energy flow
(a) Varies continuously
(b) Remains constant
(c) Depends on control surface
(d) Depends on type of process

Q2. Which of the following is correct statement?
(a) Entropy of isolated system always decreases
(b) Energy always degrades during the real process
(c) Energy always destroyed during the real process
(d) Heat transfer through a finite temperature difference is reversible process

Q3. Low grade waste heat in bottoming cycle of co-generation is used for:
(a) Feed water heating
(b) Processing
(c) Power generation
(d) None of these

Q4. High load factor indicates that
(a) cost of generation per unit power is increased
(b) total plant capacity is utilized for most of the time
(c) total plant capacity is not properly utilized for most of the time
(d) load on the plant is high

Q5. A 4-stroke reciprocating engine has 4-cylinders with diameter of 3 cm, stroke length of 8 cm and clearance volume 1 cc. The engine capacity in cc is:
(a) 110
(b) 252
(c) 400
(d) 227

Q6. The centrifugal compressors are generally used for refrigerants that require:
(a) Small displacement and low condensing pressures
(b) Large displacements and high condensing pressures
(c) Small displacements and high condensing pressures
(d) Large displacements and low condensing pressures

Q7. Amount of moisture that air can hold increases by:
(a) Decreasing both saturation pressure and temperature
(b) Decreasing saturation pressure and increasing temperature
(c) Increasing saturation pressure and decreasing temperature
(d) Increasing both saturation pressure and temperature

Q8. In transition boiling, heat flux decreases due to which of the following?
1. Low value of film heat transfer coefficient at the surface during 100℃ to 120℃ surface temperature.
2. Major portion of heater surface is covered by vapour film which has smaller thermal conductivity as compared to liquid.
3. Nucleate boiling occurs very fast.
Select the correct answer using the code given below.
(a) 1 only
(b) 2 only
(c) 3 only
(d) 1, 2 and 3

Q9. In which type of flow, the stream function satisfies the Laplace equation?
(a) Rotational flow crest
(b) Irrotational flow
(c) Circular flow
(d) None of these

Q10. Running cost of the hydro-electric power plant is
(a) more than the running cost of steam power plant
(b) less than the running cost of steam power plant
(c) equal to running cost of steam power plant
(d) equal to running cost of nuclear power plant

Solutions

S1. Ans.(b)
Sol. Following assumptions are made in steady flow process:
1. Control volume moves relative to the co-ordinate frame.
2. The state of the mass at each discrete area of flow on the control surface does not vary with time.

S2. Ans.(b)
Sol. Entropy of an isolated system always increases or remains constant, energy always degrades during a real process due to entropy generation, energy losses occur during a real process but energy remains conserved and heat transfer through a finite temperature difference is irreversible process.

S3. Ans.(a)
Sol. Low grade waste heat in the bottoming cycle of a combined cycle (co-generation) power plant is used for feed water heating for process use.

S4. Ans.(b)
Sol. Plant Load Factor (PLF) is the ratio of average power generated by the plant to the maximum power that could have been generated for a given time period. High load factor shows that total plant capacity is utilized for most of the time.

S5. Ans.(d)
Sol. Given,
Diameter (d) = 3 cm
Stroke length (L) = 8 cm
Clearance volume (Vc)= 1 cc
Engine capacity in cc = Swept volume× No. of cylinders
= π/4×d^2×L×4
= π/4×3^2×8×4
= 226.19 cc

S6. Ans.(d)
Sol. Centrifugal compressors are used for the applications where there is a requirement of high discharge and low condensing pressure.

S7. Ans.(b)
Sol. Air contains a certain amount of water vapor. The amount of water vapor any mass of air can contain depends on the temperature of that air i.e. the warmer the air is, the more water it can hold.

S8. Ans.(b)
Sol. As major portion of heater surface is covered by vapour film which has smaller thermal conductivity as compared to liquid, so heat flux decreases in transition boiling.

S9. Ans.(b)
Sol. The stream function is valid for two-dimensional flow. For irrotational flow, the stream function satisfies the Laplace equation.

S10. Ans.(b)
Sol. Hydro-electric power plant consumes no or negligible fossil fuels during its operation and it completely rely on hydro power. So, running cost of the hydro-electric power plant is less than the running cost of steam power plant.

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