Quiz:Mechanical Engineering 12 June 2020

Quiz: Mechanical Engineering
Exam: ISRO (TECHNICAL ASST.)
Topic: Thermal

Each question carries 4 mark.
Negative marking: 1 mark
Time: 15 Minute

Q1. If maximum surface temperature of sea is 30 Deg C and temperature in depth is 4 Deg C, how much can be the maximum thermal efficiency of Ocean Thermal Conversion (OTEC) System?
(a) 8.58%
(b) 13.3%
(c) 86.7%
(d) None of these

Q2. Critical pressure of a liquid is the pressure
(a) above which liquid will remain liquid
(b) above which liquid will become gas
(c) above which liquid becomes vapour
(d) above which liquid becomes solid

Q3. In binary vapour cycle
(a) Mercury is used in the bottoming cycle
(b) Steam is used in topping cycle
(c) Mercury is used in topping cycle
(d) Either mercury or steam is used in topping cycle

Q4. In fire tube boilers, pressure is limited to
(a) 16 bar
(b) 32 bar
(c) 48 bar
(d) 64 bar

Q5. The efficiency of Diesel cycle approaches to Otto cycle efficiency when
(a) cut-off is increased
(b) cut-off is decreased
(c) cut-off is zero
(d) cut-off is constant

Q6. In a low speed S.I. engine, the inlet valve closes approximately:
(a) 40° after BDC
(b) 30° before BDC
(c) 10° after BDC
(d) 10° before BDC

Q7. The main characteristic feature of Air Refrigeration System is
(a) High COP
(b) throughout the cycle the refrigerant remains in gaseous state
(c) Weight of air circulate is less
(d) High refrigerating cost

Q8. Saturation temperature of the water vapour in the moist air is known as:
(a) Dry bulb temperature
(b) Dew point temperature
(c) Wet bulb temperature
(d) Critical temperature

Q9. A thermocouple responds well when
(a) its coefficient of heat transfer is high
(b) its probe diameter is less
(c) its density is more
(d) its specific heat is more

Q10. If Buoyancy force is 30000 N on a body and viscous force is 3 N, then considering natural convection, what is the value of Grashoff number?
(a) 10000
(b) 1000
(c) 0.0001
(d) 100

Solutions

S1. Ans (a)
Sol. We know that
η=(T1-T2)/T1
=(30-4)/303=8.58%

S2. Ans (c)
Sol. Critical pressure is that pressure above which boundary difference between liquid and vapor vanishes.

S3. Ans (c)
Sol. In the binary vapour cycle, mercury is used in topping cycle because of the physical and thermodynamic properties of the mercury working fluid, require a heat source that operates in the temperature range of about 535°C to 700°C.

S4. Ans (a)
Sol. Fire tube boiler are the low-pressure boilers, they operate below a pressure of 20 bar. E.g. Cochran, Lancashire, Locomotive boilers etc.

S5. Ans (c)
Sol. Cut-off ratio is the ratio of volume of charge at which heat addition stops and swept volume. When cut-off ratio (ρ) is zero then the efficiency of diesel cycle approaches to that of Otto cycle.

S6. Ans (c)
Sol. In a low speed S.I. engine, the inlet valve closes approximately 10° after BDC because inertia of the charge is low which reduces volumetric efficiency. To maintain volumetric efficiency, we make the inlet valve to close 10° after BDC.

S7. Ans (b)
Sol. Air refrigeration cycle works on the reverse Brayton cycle. The main characteristic feature of Air Refrigeration System is throughout the cycle the refrigerant remains in gaseous state.

S8. Ans (b)
Sol. The temperature of water vapour at which air becomes saturated is called dew point temperature.

S9. Ans (a)
Sol. Time constant =ρvc/(hoA)
As, time constant decreases then thermocouple responds quickly. For decreasing this value, we have to increase heat transfer coefficient (ho).

S10. Ans (a)
Sol. We know that
Grashoff number=(Buoyancy force)/(Viscous force)
Gr=30000/3=10000

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