Fluid Mechanics and Thermal Sciences (Test 3)

Gate Exam : Me Mechanical Engineering

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Fluid Mechanics and Thermal Sciences
| Fluid Mechanics and Thermal Sciences |
Q.1
The pressure, temperature and velocity of air flowing in a pipe are 5 bar, 500 K and 50 m/s, respectively. The specific heats of air at constant pressure and at constant volume are 1.005 kJ/kgK and 0.718 kJ/kgK, respectively. Neglect potential energy. If the pressure and temperature of the surroundings are 1 bar and 300 K, respectively, the available energy in kJ/kg of the air stream is
A. 170
B. 187
C. 191
D. 213
Explaination / Solution:
No Explaination.

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Q.2
A spherical steel ball of 12mm diameter is initially at 1000K. It is slowly cooled in a surrounding of 300K. The heat transfer coefficient between the steel ball and the surrounding is 5W /m2 K . The thermal conductivity of steel is 20W /mK . The temperature difference between the centre and the surface of the steel ball is
A. Large because conduction resistance is far higher than the convective resistance
B. Large because conduction resistance is far less than the convective resistance
C. Small because conduction resistance is far higher than the convective resistance
D. Small because conduction resistance is far less than the convective resistance
Explaination / Solution:

Bi = hL/K = 5 ×0.002/20 = 0.0005
For the given condition the Biot number tends to zero, that means conduction resistance is far less than convection resistance. Therefore temperature between the centre and surface is very small.

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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 mass flow rate of air through the nozzle in kg/s is
A. 1.30
B. 1.77
C. 1.85
D. 2.06
Explaination / Solution:
No Explaination.

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Q.4
Water at 25oC is flowing through a 1.0 km long G.I pipe of 200mm diameter at the rate of 0.07 m3/s. If value of Darcy friction factor for this pipe is 0.02 and density of water is 1000 kg/m3 , the pumping power (in kW) required to maintain the flow is
A. 1.8
B. 17.4
C. 20.5
D. 41.0
Explaination / Solution:
No Explaination.

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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
Assume the above turbine to be part of a simple Rankine cycle. The density of water at the inlet to the pump is 1000kg/m3. Ignoring kinetic and potential energy effects, the specific work (in kJ/kg) supplied to the pump is
A. 0.293
B. 0.351
C. 2.930
D. 3.510
Explaination / Solution:
No Explaination.

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Q.6
The INCORRECT statement about regeneration in vapor power cycle is that
A. it increases the irreversibility by adding the liquid with higher energy content to the steam generator
B. heat is exchanged between the expanding fluid in the turbine and the compressed fluid before heat addition
C. the principle is similar to the principle of Stirling gas cycle
D. it is practically implemented by providing feed water heaters
Explaination / Solution:
No Explaination.

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Q.7
Consider one-dimensional steady state heat conduction along x-axis  through a plane wall with the boundary surfaces (x = 0 and x = L) maintained at temperatures 0ºC and 100ºC. Heat is generated uniformly throughout the wall. Choose the CORRECT statement.

A. The direction of heat transfer will be from the surface at 100ºC to surface at
0ºC.
B. The maximum temperature inside the wall must be greater than 100ºC
C. The temperature distribution is linear within the wall
D. The temperature distribution is symmetric about the mid-plane of the wall
Explaination / Solution:
No Explaination.

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Q.8
Considering massless rigid rod and small oscillations, the natural frequency (in rad/s) of vibration of the system shown in the figure is
A. √400/1
B. √400/2
C. √400/3
D. √400/4
Explaination / Solution:
No Explaination.

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Q.9
Steam enters an adiabatic turbine operating at steady state with an enthalpy of 3251.0 kJ/kg and leaves as a saturated mixture at 15 kPa with quality (dryness fraction) 0.9. The enthalpies of the saturated liquid and vapor at 15 kPa are hf = 225.94 kJ/kg and hg = 2598.3 kJ/kg respectively. The mass flow rate of steam is 10 kg/s. Kinetic and potential energy changes are negligible. The power output of the turbine in MW is
A. 6.5
B. 9.1
C. 8.9
D. 27.0
Explaination / Solution:

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Q.10
Which combination of the following statements is correct ?
P : A gas cools upon expansion only when its Joule-Thomson coefficient is positive in the temperature range of expansion.
Q : For a system undergoing a process, its entropy remains constant only when the process is reversible.
R : The work done by closed system in an adiabatic is a point function.
S : A liquid expands upon freezing when the slope of its fusion curve on pressure-Temperature diagram is negative
A. R and S
B. P and Q
C. Q, R and S
D. P, Q and R