← MES Jr. Grade
Past Questions — 2015
Gas turbine usually operates on
A)
constant volume cycle
B)
constant pressure cycle
C)
dual combustion cycle
D)
constant temperature cycle
A gas turbine works on the Brayton cycle, in which heat addition and heat rejection ideally occur at constant pressure. Hence, the usual operating cycle is the constant pressure cycle. The other options refer to different ideal cycles: constant volume is Otto cycle, dual combustion is used in diesel engine analysis, and constant temperature is not the standard gas turbine cycle.
Total heat of a substance is also known as
A)
internal energy
B)
entropy
C)
latent heat
D)
enthalpy
Total heat of a substance is called enthalpy. Enthalpy is defined as H = U + pV, where U is internal energy. It represents the total heat content of a system at constant pressure. Internal energy is only the energy contained within the system, entropy is a measure of disorder, and latent heat is the heat absorbed or released during a change of state.
By taking the ratio of pressure force and inertia force, the following number may be obtained
A)
Weber number
B)
Euler number
C)
Froude number
D)
Mach number
The Euler number is defined as the ratio of pressure force to inertia force in fluid flow. It is commonly written as Eu = p / (ρV^2) in nondimensional form.
Why others are incorrect:
- Weber number = inertia force / surface tension force.
- Froude number = inertia force / gravitational force.
- Mach number = flow velocity / speed of sound.
Therefore, the correct answer is Euler number.
The maximum pressure ratio in an actual single stage centrifugal compressor will be about
A)
1 : 15
B)
1 : 10
C)
1 : 4
D)
1 : 1.5
A single-stage centrifugal compressor can achieve only a limited pressure ratio in practice because of diffuser losses, temperature rise, and flow limitations. The maximum pressure ratio for an actual single-stage centrifugal compressor is about 4:1. Hence, among the given options, 1 : 4 is correct. The other ratios are either too high for a single stage (1:10, 1:15) or too low (1:1.5).
Thermal power plant works on
A)
Carnot cycle
B)
Joule cycle
C)
Rankine Cycle
D)
Otto cycle
A thermal power plant (steam power plant) generates electricity by producing steam in a boiler, expanding it through a turbine, condensing it, and pumping it back. This sequence is the Rankine cycle. Carnot cycle is an ideal theoretical cycle, Joule cycle is used for gas turbines, and Otto cycle is used in petrol engines.
Which of the following thermodynamic properties is not an example of point function?
A)
volume
B)
temperature
C)
pressure
D)
work
A point function depends only on the state of the system and not on the path taken. Volume, temperature, and pressure are state functions, so they are point functions. Work depends on the path followed during a process, so it is a path function, not a point function.
The refrigerant used in vapour absorption refrigerator is
A)
freon-12
B)
ammonia
C)
aqua-ammonia
D)
carbon di-oxide
In a vapour absorption refrigerator, the working pair is commonly ammonia as the refrigerant and water as the absorbent, together called aqua-ammonia. Hence, aqua-ammonia is the correct answer. Freon-12 is mainly used in vapour compression systems, and carbon dioxide is not the standard refrigerant for vapour absorption refrigerators in such exam context.
Surge tank is provided to protect the
A)
generator
B)
hydraulic turbine
C)
scroll case
D)
penstock
A surge tank is installed in a hydropower system to absorb sudden pressure changes caused by rapid load variations. It protects the penstock from water hammer and excessive pressure rise or drop. Hence, the correct answer is penstock. The generator, turbine, and scroll case are not the primary components protected by a surge tank.
Heat is rejected by a refrigerant, during a refrigeration cycle in a
A)
condenser
B)
evaporator
C)
compressor
D)
throttle valve
In a refrigeration cycle, the refrigerant absorbs heat in the evaporator and rejects heat to the surroundings in the condenser. The compressor only raises the pressure and temperature of the refrigerant, while the throttle valve reduces pressure. Therefore, heat is rejected in the condenser.
Which of the following can be used as a coolant in nuclear plant?
A)
light or heavy water
B)
molten lead
C)
carbon dioxide
D)
freon
In nuclear power plants, water is commonly used as a coolant. Both light water (ordinary water) and heavy water can remove heat from the reactor core. Carbon dioxide is also used in some reactor types, but among the given options, the standard and most widely recognized coolant is light or heavy water. Molten lead and freon are not typical primary coolants for standard nuclear plants.
For evaporators and condensers, for the given condition, the logarithmic mean temperature difference (LMTD) of parallel flow compared to counter flow is
A)
equal
B)
more
C)
less
D)
none of these
For the same inlet and outlet temperature conditions, the logarithmic mean temperature difference (LMTD) is always higher for counter flow than for parallel flow. Hence, for evaporators and condensers under the given condition, the LMTD of parallel flow is less compared to counter flow. Therefore, option (c) is correct.
If the clearance volume of an air compressor is increased from 5 percent to 10 percent, the net work input required to compress a given mass of air will
A)
increase slightly
B)
decrease slightly
C)
remain unchanged
D)
become double
In a reciprocating air compressor, increasing clearance volume increases the amount of compressed air left in the cylinder at the end of delivery. During the suction stroke, this trapped air re-expands, so the piston must travel more before fresh air enters. This reduces volumetric efficiency and slightly increases the net work input for compressing a given mass of air. The increase from 5% to 10% clearance does not double the work; it only causes a small rise. Therefore, the correct answer is 'increase slightly'.
The maximum velocity which can be attained at the throat of a steam nozzle is
A)
half the sonic velocity
B)
equal to sonic velocity
C)
double the sonic velocity
D)
unpredictable
In a steam nozzle, the throat is the critical section where the flow becomes choked. At this point, the Mach number is 1, meaning the flow velocity equals the local speed of sound (sonic velocity). Therefore, the maximum velocity attainable at the throat is equal to sonic velocity. The other options are incorrect because the velocity at the throat cannot exceed the sonic condition in normal nozzle flow, nor is it half or double the sonic velocity.
For a forced convection, Nusselt number is a function of
A)
Prandtl and Grashof numbers
B)
Grashof number only
C)
Reynold and Grashof numbers
D)
Reynold and Prandtl numbers
In forced convection, the Nusselt number (Nu) depends primarily on the Reynolds number (Re) and the Prandtl number (Pr).
- Reynolds number represents the effect of fluid flow/inertia.
- Prandtl number represents the relative thickness of momentum and thermal boundary layers.
Grashof number is associated with natural convection, not forced convection. Therefore, options involving Grashof number are incorrect.
Which of the following items is a path function?
A)
pressure
B)
temperature
C)
work
D)
density
A path function depends on the process or path taken between two states, not just the initial and final states. Work is a path function because its value changes with the way a process is carried out. In contrast, pressure, temperature, and density are state functions, since they depend only on the thermodynamic state of the system.
In any natural process, the difference between the final entropy and initial entropy is
A)
positive
B)
negative
C)
nil
D)
indeterminate
For any natural (spontaneous) process, the entropy of the universe increases according to the second law of thermodynamics. Therefore, the final entropy is greater than the initial entropy, so the change in entropy is positive. It is not negative or nil for a natural process. 'Indeterminate' is incorrect because the direction is fixed by the second law.
The resultant hydrostatic force acts through a point known as
A)
centre of gravity
B)
centre of pressure
C)
centre of buoyancy
D)
centre of force
In hydrostatics, the pressure on a submerged surface varies with depth, so the total resultant force does not act at the centroid. It acts at a specific point called the centre of pressure.
- Centre of gravity: point where weight acts.
- Centre of buoyancy: point through which buoyant force acts.
- Centre of force: not the standard term for resultant hydrostatic force here.
Therefore, the correct answer is centre of pressure.
The clearance volume in reciprocating air compressors is provided
A)
to reduce the work done per kg of air delivered
B)
to increase the volumetric efficiency of the compressor
C)
to accommodate valves in the head of the compressor
D)
to create turbulence in the air to be delivered
In a reciprocating compressor, clearance volume is the small space left between the piston and cylinder head at the end of the compression stroke. It is necessary so that the suction and delivery valves can be housed in the cylinder head and the piston does not strike the head. It is not provided to reduce work done, increase volumetric efficiency, or create turbulence. In fact, clearance volume generally reduces volumetric efficiency.
The gray body shape factor for radiant heat exchange between two long parallel plates of equal area and each having an emissivity of 0.4 is
A)
1
B)
4
C)
¼
D)
0.4
For two long parallel plates of equal area, the geometric view factor between the plates is 1. In gray-body radiation exchange, the effective gray-body shape factor reduces to the surface emissivity when the two plates are identical and each has emissivity ε = 0.4. Hence the gray body shape factor is 0.4. The other options 1, 4, and 1/4 do not match the given emissivity-based result.
The refrigeration effect from a flooded evaporator as compared to a dry evaporator fitted in a similar plant is
A)
more
B)
less
C)
equal
D)
unpredictable
A flooded evaporator generally gives a higher refrigeration effect than a dry evaporator in a similar plant. In a flooded system, the evaporator surface remains fully wetted with liquid refrigerant, so heat transfer is better and the refrigerant can absorb more heat before leaving the evaporator. In a dry evaporator, part of the surface may not be effectively utilized, so the refrigeration effect is comparatively lower. Hence, the correct answer is 'more'.
The drag produced by end effects due to the finite length of an aerofoil is known as
A)
profile drag
B)
form drag
C)
induced drag
D)
friction drag
Drag caused by end effects due to the finite span/length of an aerofoil is called induced drag. It arises because pressure differences at the tips create wing-tip vortices, which increase resistance. Profile drag is the sum of skin friction and form drag, form drag is due to shape, and friction drag is due to viscous shear on the surface.
If the head of a turbine is more than 300 m, the type of turbine used should be
A)
Pelton
B)
Kaplan
C)
Francis
D)
Propeller
For very high heads, typically above 300 m, an impulse turbine is preferred. The Pelton turbine is designed for high head and low discharge conditions, so it is the correct choice. Kaplan and propeller turbines are used for low heads, while Francis turbines are suitable for medium heads.
Willan’s line is a straight line graph between the rate of steam consumption and
A)
pressure of steam
B)
temperature of steam
C)
indicated horse power
D)
none of these
Willan’s line is the straight-line relationship between steam consumption and indicated power (indicated horse power, IHP) of an engine at constant speed and cut-off. As load increases, steam consumption varies approximately linearly with IHP. It is not a graph with pressure or temperature of steam, so options (a) and (b) are incorrect. Hence, the correct answer is indicated horse power.
To produce high head by multi-stage centrifugal pumps, the impellers are connected in
A)
series
B)
parallel
C)
series and parallel
D)
none of these
In multi-stage centrifugal pumps, impellers are connected in series to increase the pressure head. The discharge from one impeller enters the next, so the heads developed by each stage add up. Parallel connection is used to increase discharge, not head. Hence, the correct answer is series.
An enclosure is formed by two concentric spheres. The view factor of a point of inner sphere with respect to outer sphere is
A)
0.0
B)
0.5
C)
1.0
D)
none of these
For two concentric spheres forming an enclosure, the entire radiation leaving any point on the inner sphere can only strike the outer sphere. Hence, the view factor of a point on the inner sphere with respect to the outer sphere is 1.0. Option (a) is incorrect because the outer sphere is not hidden. Option (b) is incorrect because only half the radiation does not go to the outer sphere in this geometry. Therefore, the correct answer is (c) 1.0.