# Fluid Mechanics and Thermal Sciences (Test 2)

## Gate Exam : Me Mechanical Engineering

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Q.1
A hinged gate of length 5 m, inclined at 30° with the horizontal and with water mass on its left, is shown in figure below. Density of water is 1000 kg / m2. The minimum mass of the gate in kg per unit width (perpendicular to the plane of paper), required to keep it closed is A. 5000
B. 6600
C. 7546
D. 9623
Explaination / Solution:
No Explaination.

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Q.2
A pump handling a liquid raises its pressure from 1 bar to 30 bar. Take the density of the liquid as 990 kg /m3. The isentropic specific work done by the pump in kJ/kg is
A. 0.10
B. 0.30
C. 2.50
D. 2.93
Explaination / Solution:

Work done by the pump = Workspace
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Q.3
The temperature and pressure of air in a large reservoir are 400K and 3 bar respectively. A converging–diverging nozzle of exit area 0.005m2 is fitted to the wall of the reservoir as shown in the figure. The static pressure of air at the exit section for isentropic flow through the nozzle is 50kPa. The characteristic gas constant and the ratio of specific heats of air are 0.287kJ/kgK and 1.4 respectively The density of air in kg/ m3 at the nozzle exit is
A. 0.560
B. 0.600
C. 0.727
D. 0.800
Explaination / Solution:

Given data:
T1 = 400k, P1 = 300 kPa, P2 = 50kPa, R = 0.289kJ / kgK
v = 1.4, A2 = 0.005m2
The happened process from entrance to exit is isentropic process, therefore Workspace
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Q.4
If a mass of moist air contained in a closed metallic vessel is heated, then its
A. relative humidity decreases
B. relative humidity increases
C. specific humidity increases
D. specific humidity decreases
Explaination / Solution:

Given that mass of moist air contained in a closed metallic vessel is heated, means its specific humidity (𝜔) is constant. So, from the psychometric chart, We can say that at constant specific humidity, as temperature increases relative humidity decreases i.e.,(Φ2<Φ1)

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Q.5
The inlet and the outlet conditions of steam for an adiabatic steam turbine are as indicated in the notations are as usually followed If mass flow rate of steam through the turbine is 20kg/s, the power output of the turbine (in MW ) is
A. 12.157
B. 12.941
C. 168.001
D. 168.785
Explaination / Solution:
No Explaination.

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Q.6
A plastic sleeve of outer radius r0 = 1 mm covers a wire (radius r = 0.5 mm) carrying electric current. Thermal conductivity of the plastic is 0.15 W/m-k. The heat transfer coefficient on the outer surface of the sleeve exposed to air is 25 W/m2-K. Due to the addition of the plastic cover, the heat transfer from the wire to the ambient will
A. increase
B. remain the same
C. decrease
D. be zero
Explaination / Solution: r = 0.5 mm, r0 = 1 mm. Given, the wire is like a cylinder. Hence, critical radius of insulation is Now if we add insulation (plastic cover), the heat transfer will increase. Workspace
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Q.7
A moist air sample has dry bulb temperature of 30ºC and specific humidity of 11.5g water vapour per kg dry air. Assume molecular weight of air as 28.93. If the saturation vapour pressure of water at 30ºC is 4.24kPa and the total pressure is 90kPa, then the relative humidity (in %) of air sample is
A. 50.5
B. 38.5
C. 56.5
D. 68.5
Explaination / Solution:
No Explaination.

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Q.8
Which of the following relationships is valid only for reversible processes undergone by a closed system of simple compressible substance (neglect changes in kinetic and potential energy?)
A. δQ = dU + δW
B. Tds = dU + pdv
C. Tds = dU + δW
D. δQ = dU + pdv
Explaination / Solution:
No Explaination.

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Q.9
The inlet angle of runner blades of a Francis turbine is 90o. The blades are so shaped that the tangential component of velocity at blade outlet is zero. The flow velocity remains constant throughout the blade passage and is equal to half of the blade velocity at runner inlet. The blade efficiency of the runner is
A. 25%
B. 50%
C. 80%
D. 89%
Explaination / Solution:
No Explaination.

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Q.10
Which combination of the following statements is correct ?
The incorporation of reheater in a steam power plant :
P : always increases the thermal efficiency of the plant.
Q : always increases the dryness fraction of steam at condenser inlet
R : always increases the mean temperature of heat addition.
S : always increases the specific work output.
A. P and S
B. Q and S
C. P, R and S
D. P, Q, R and S