# Mechanical Engineering Interview Questions with Answers - Fluid Mechanics & Fluid Machinery

### Mechanical Engineering Interview Questions with Answers - Fluid Mechanics & Fluid Machinery (Turbine, Boiler, compressor, Pumps, Bearings Etc…): Set 01

Question No. 01
What is the difference between streamline body and bluff body?
Answer:
In streamline body the shape is such that separation in flow occurs past the near most part of the body so that wake formed is small and thus friction drag is much greater than pressure drag. In bluff body the flow gets separated much ahead of its rear resulting in large wake and thus pressure drag is much greater than the friction drag.

Question No. 02
Define hydraulically efficient channel cross section.
Answer:
The shape of such section is that which produces minimum wetted perimeter for a given area of flow and carries maximum flow.

Question No. 03
What is the difference between Center of Mass and Centroid?
Answer:
If the material composing a body is uniform or homogeneous, the density or specific weight will be constant throughout the body, and then the Centroid is the same as the center of gravity or center of mass.
Centroid: Centroid is the point, where the whole area of plane is going to be act. It is valid only for 2D problems like plane figures, square plate etc. The center of mass is a point that acts as if all the mass was centered there (the mass on one side of the point is equal to the mass on the opposite side). If supported at the center of mass, an object will be balanced under the influence of gravity.

Question No. 04
On what factors does the pressure at a point as a static mass of liquid depends upon?
Answer:
Specific weight of liquid and the depth below the free liquid surface.

Question No. 05
When maximum discharge is obtained in nozzle?
Answer:
At the critical pressure ratio.

Question No. 06
Under what condition the work done in reciprocating compressor will be least?
Answer:
It is least when compression process approaches isothermal. For this purpose, attempts are made to cool the air during compression.

Question No. 07
What is the difference between stalling and surging in rotary compressions?
Answer:
Stalling is a local phenomenon and it occurs when How breaks away from the blades. Surging causes complete breakdown of flow and as such it affects the whole machine.

Question No. 08
State Archimedes principle.
Answer:
Any weight, floating or immersed in a liquid, is acted upon by a buoyant force equal to the weight of the liquid displaced. This force acts through the center of buoyancy, i.e. the e.g. of the displaced liquid.

Question No. 09
What do you understand by center of buoyancy?
Answer:
Center of buoyancy is the center of gravity of the displaced liquid and buoyant force acts through it.

Question No. 10
Why the Centrifugal Pump is called High Discharge pump?
Answer:
Centrifugal pump is a kinetic device. The centrifugal pump uses the centrifugal force to push out the fluid. So the liquid entering the pump receives kinetic energy from the rotating impeller. The centrifugal action of the impeller accelerates the liquid to a high velocity, transferring mechanical (rotational) energy to the liquid. So it discharges the liquid in high rate. It is given in the following formula:
Centrifugal force F= (MV²)/R.
Where, M-Mass, V-Velocity, R-Radius

Question No. 11
Why the electric motor of a fan with backward curved blades is never got overloaded under any condition?
Answer:
The maximum power is consumed at about 70% of maximum flow in case of fan with backward blades. For higher flow, power consumption gets lower.

Question No. 12
Why the work per kg of air flow in axial flow compressor is less compared to centrifugal compressor for same pressure ratio?
Answer:
Isentropic efficiency of axial flow compressor is higher.

Question No. 13
What is pitting? How it is caused?
Answer:
Non uniform corrosion over the entire metal surface, but occurring only in small pits is called pitting. It is caused by lack of uniformity in metal.

Question No. 14
How Cavitation can be eliminated by Pump?
Answer:
Cavitation means bubbles are forming in the liquid. To avoid Cavitation, we have to increase the Pump size to one or two inch to increase the pressure of the Suction Head, or decrease the Pump Speed.

Question No. 15
Which impurities form hard scale and which impurities soft scale?
Answer:
Sulphates and chlorides of lime and magnesium form hard scale, and carbonates of lime and magnesium form soft scale.

Question No. 16
What is the difference between hard water and soft water?
Answer:
Hard water contains excess of scale forming impurities and soft water contains very little or no scale forming substances.

Question No. 17
Which two elements in feed water can cause corrosion of tubes and plates in boiler?
Answer:
Acid and oxygen in feed water lead to corrosion.

Question No. 18
Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?
Answer:
The formation of cavities (or bubbles) is induced by flow separation, or non-uniform flow velocities, inside a pump casing. In centrifugal pumps the eye of the pump impeller is smaller than the flow area of pipe. This decrease in flow area of pump results in increase in flow rate. So pressure drop happened between pump suction and the vanes of the impeller. Here air bubbles or cavities are formed because of liquid vapor due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms Cavitation.

Question No. 19
Why large boilers are water tube type?
Answer:
Water tube boilers raise steam fast because of large heat transfer area and positive water circulation. Thus they respond faster to fluctuations in demand. Further single tube failure does not lead to catastrophic.

Question No. 20
What type of boiler does not need a steam drum?
Answer:
Supercritical pressure boiler.

Question No. 21
Why manholes in vessels are usually elliptical in shape?
Answer:
Elliptical shape has minimum area of opening and thus plate is weakened the least. Further it is very convenient to insert and take out the cover plate from elliptical opening.

Question No. 22
Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?
Answer:
Centrifugal pump. Because of flow rate is higher compared to reciprocating pump. Flow is smooth and it requires less space to install. Lower initial cost and lower maintenance cost.

Question No. 23
Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?
Answer:
The centrifugal has varying flow depending on pressure or head, whereas the Positive Displacement pump has more or less constant flow regardless of pressure.
Likewise viscosity is constant for positive displacement pump where centrifugal pump have up and down value because the higher viscosity liquids fill the clearances of the pump causing a higher volumetric efficiency. When there is a viscosity change in supply there is also greater loss in the system. This means change in pump flow affected by the pressure change.
One more example is, positive displacement pump has more or less constant efficiency, where centrifugal pump has varying efficiency rate.

Question No. 24
Low water in boiler drum is unsafe because it may result in overheating of water tubes in furnace. Why it is unsafe to have high water condition in boiler drum?
Answer:
High drum level does not allow steam separation to be effective and some water can be carried over with steam which is not desirable for steam turbine.

Question No. 25
What is the difference between Critical Speed and Whirling Speed?
Answer:
In Solid mechanics, in the field of rotor dynamics, the critical speed is the theoretical angular velocity which excites the natural frequency of a rotating object, such as a shaft, propeller or gear. As the speed of rotation approaches the objects natural frequency, the object begins to resonate which dramatically increases system vibration. The resulting resonance occurs regardless of orientation. Whirling Speed is due to the unbalanced forces acting on a rotating shaft.

Question No. 26
Maximum combustion temperature in gas turbines is of the order of 1100 to 10°C whereas same is around 0°C in I.C. engine? Why?
Answer:
High temperature in I.C. engine can be tolerated because it lasts for a fraction of second but gas turbines have to face it continuously which metals can't withstand.

Question No. 27
What is basic difference between impulse turbine and reaction turbine?
Answer:
(A) In impulse turbine, jet is used to create impulse on blades which rotates the turbine and in reaction turbine, no jet is used pressure energy is converted into kinetic energy.
(B) In impulse turbine fluid enter& leave with same energy, but in reaction turbine fluid enter with pressure energy& leaves with kinetic energy
(C)In impulse turbine all the pressure drops in nozzle only & in reaction turbine pressure drops both fixed & moving blades. The difference is due to blade profiles.

Question No. 28
What is the Difference between a Generator and Inverter?
Answer:
An inverter is only effective if there is already a source of electrical energy. It cannot generate its own. It can simply convert electrical energy that is already there. On the other hand, a traditional generator cannot make AC current into DC current.

Question No. 29
What are the causes of failure of boiler tubes?
Answer:
Boiler tubes, usually are made from carbon steel and are subject to
(A) High rates of heat transfer
(B). bending stresses due to uneven heating, especially at expanded or welded joints into headers or drums,
(C) External erosion from burners and flue gas,
(D) Possible corrosion on the boiler side, and
(E) Occasional manufacturing defects.

Failure may occur due to following reasons:
(A) High thermal ratings may lead to rapid failure if the internal fluid flow is reduced for any reason. The resultant overheating leads to a failure by creep, characterized by the bulging of the tube with the eventual development of a longitudinal split.
(B) Fatigue cracking due to bending stresses occurs. These are associated with change of section and/or weld undercut, where tubes are expanded or welded into headers.
(C) Failure may arise due to over-stressing of a reduced section of metal.
(D) Sudden failure of the boiler tube due to corrosion arises from embrittlement of the carbon steel due to interaction between atomic hydrogen from the corrosion process and the iron carbide present in the steel.
(E) Defects in tube manufacture, although far from being a regular occurrence, can be a cause of serious trouble. Lamination in boiler tubes or score marks arising from the cold drawing of tubes give rise to premature failure and may promote corrosion at these regions.

Question No. 30
Why is sound faster in warm air?
Answer:
The speed of sound in air Cair = 331.3 + (0.66 x T) m/s, where T is the temperature in °C.
The speed of sound is proportional to gas temperature and inversely proportional to its molar mass.
Sound is transferred by collisions of molecules. Therefore sound waves will travel faster on warm air because collision of molecules of air in warm air is greater.

Question No. 31
What is a Newtonian fluid?
Answer:
A Newtonian fluid possesses a linear stress strain relationship curve and it passes through the origin. The fluid properties of a Newtonian fluid do not change when any force acts upon it.

Question No. 32
What are the causes of failure of superheater tubes?
Answer:
Superheater tubes are subjected to the most severe combination of stress, temperature and corrosive environment. In addition to high temperature strength, resistance to corrosion is also important. For example, low alloy ferrite steel such as 1% Cr, 1% Mo would not be used at metal temperatures above 580°C because of inadequate resistance to corrosion and oxidation over a full service life of 100,000/150,000 hr.
Failures in superheater tubes may arise from:
(A) Prior fabrication history
(B) Faulty heat treatment
(C) Consequences of welding
(D) Overheating of the tube metal
(E) Gas-side corrosion
(F) Stress corrosion (austenitic steels).

Question No. 33
Why supercritical boilers use less amount of steel compared to non supercritical boilers?
Answer:
Supercritical boilers do not head heavy drum for separation of steam from mixture of water and steam.

Question No. 34
What does a pump develops? Flow or Pressure?
Answer:
A pump does not create pressure, it only creates flow. Pressure is a measurement of the resistance to flow.

Question No. 35
What is a Hydrostatic system?
Answer:
Hydrostatics is the study of fluid bodies that are
(A) At rest
(B) Moving sufficiently slowly so there is no relative motion between adjacent parts of the body

For hydrostatic situations
(A) There are no shear stresses.
(B) There are only pressure forces that act perpendicular to any surface.
It’s a closed loop hydraulic systems. It comprises of motor and pump. Here pump supplies energy to motor and motor gives return energy to pump supply.

Question No. 36
What is the difference between Blower and Fan?
Answer:
Fan is an air pushing device. Either Axial or Centrifugal type systems are used to move the air in low pressure. It is rotated by a motor separately.
When the fan is a housing of blades and motor, then it called as Blower. It directs the air in a single path with high pressure.

Question No. 37
Out of electric heater and heat pump, which is economical in operation?
Answer:
Heat pump.

Question No. 38
What is Hydrodynamic Cavitation?
Answer:
Hydrodynamic Cavitation describes the process of vaporization in a constrained channel at a specific velocity. Bubble generation and Bubble implosion which occurs in a flowing liquid as a result of a decrease and subsequent increase in pressure.

Question No. 39
Is gate valve used for Throttling?
Answer:
Gate valves are not suitable for throttling because the control of flow is difficult for the valve’s design, and the flow of fluid slapping against a partially open gate can cause extensive damage to the valve.

Question No. 40
Where Multi-stage pump used?
Answer:
Pressure washing of Aircraft, Trains, Boats and Road vehicles as well as Spray washing of industrial parts and Electronic components.

Question No. 41
Which furnace burns low ash fusion coal and retains most of the coal ash in the slag?
Answer:
Cyclone furnace.

Question No. 42
How the thickness of thermal boundary layer and thickness of hydrodynamic boundary layer related?
Answer:
Ratio of their thickness = (Prandtl number) - (1/3).

Question No. 43
What is the effect of friction on flow of steam through a nozzle?
Answer:
To decrease both mass flow rate and wetness of steam.

Question No. 44
What is the function of Scoop in BFP (Boiler Feed Water Pump) in Thermal Power Station?
Answer:
The Function of Scoop tube is regulating the varying amount of oil level in the coupling during operation of infinite variable speed.

Question No. 45
In the Thermal Power Plant why Deaerator (D/A) placed on height?
Answer:
To build a Very high pressure and the temperature for a boiler feed water pump and it discharge high pressure water to the boiler. And to provide the required Net Positive Suction Head (NPSH) for the BFW pump and to serve as a storage tank to ensure a continuous supply of feed water during rapid changes in BFP.

Question No. 46
In orifice why the Pressure and Temperature are decreases?
Answer:
Orifice is a small hole like a nozzle. When a high pressure fluid passes through the orifice,
Pressure gets reduced suddenly and the velocity of the fluid gets increased. Also the heat transfer rate increases. We know that Heat transfer rate is directly proportional to the difference in temperature, Area and the Heat transfer coefficient. Heat transfer Coefficient remains constant for a fluid at a particular temperature.
Q= KA (T1-T2)
If the heat transfer rate increases, it seems the difference in temperature gets increased. There is no way in reduction of inlet temperature of the orifice. As a result, the outlet temperature of the orifice gets reduced. Hence the pressure and temperature gets reduced when it passes through orifice

Question No. 47
Why gas turbine power plant needs efficient compressor?
Answer:
Because a large portion of turbine work is eaten away by compressor and its inefficiency will affect net power output and cost of generation.

Question No. 48
Why rockets using liquid hydrogen have higher specific impulse compared to liquid hydrocarbon?
Answer:
Liquid hydrogen has higher burning velocity.

Question No. 49
Why axial flow compressor is preferred for gas turbines for aeroplanes?
Answer:
Because it has low frontal area.

Question No. 50
What is the difference between gas turbine and a steam turbine?
Answer:
Gas turbine works on Brayton cycle where as steam turbine works Rankine cycle. Construction, operation of a gas turbine is entirely different to steam turbine. Gas turbine has a compressor to compress the combustion air, a combustion chamber to burn the fuel and a turbine section to extract the work for burning fuel. Steam turbine is just has a turbine section to extract the work from steam.

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