Thermodynamics most frequently asked question at the viva or Interview:

1. What is the importance of the
Thermodynamics in the field of Mechanical
 All the mechanical engineering systems are
studied with the help of thermodynamics.
Hence it is very important for the mechanical
2. How many Laws of Thermodynamics are
 There are three laws of the thermodynamics.
 First Law: 
Energy can be neither created nor
destroyed. It can only change forms.In any
process in an isolated system, the total energy
remains the same. 
Second Law: 
When two isolated systems in
separate but nearby regions of space, each in
thermodynamic equilibrium in itself, but not
in equilibrium with each other at first, are at
some time allowed to interact, breaking the isolation that separates the two systems, and
they exchange matter or energy, they will
eventually reach a mutual thermodynamic
equilibrium. The sum of the entropies of the
initial, isolated systems is less than or equal to
the entropy of the final exchanging systems. In
the process of reaching a new thermodynamic
equilibrium, entropy has increased, or at least
has not decreased. 
Third Law:
 As temperature approaches absolute
zero, the entropy of a system approaches a
3. State Laws of conservation of energy? 
According to the laws of conservation of
energy, “energy can neither be created nor be
destroyed. It can only be transformed from one
form to another.” 
4. Is the boiler a closed system? 
Yes definitely the boiler is a closed system. 
5. What is Carnot engine?
It was being designed by Carnot and let me tell
you that Carnot engine is an imaginary engine
which follows the Carnot cycle and provides
100% efficiency.
6. Difference between Gas and Vapour
  1. Vapour can turn back and forth into liquid and solid states but a gas cannot.
  2. Gases cannot be see while vapours are visible.
  3. Vapours settle down on ground while gases do not.
7. Difference between Boiling Point and Melting Point
  1. The melting point is a defined for solids when it transfers from solid state to liquid state.
  2. The boiling point is defined for liquids for a state change from liquid to gas.
8.What is the effect of reheat on rankine  cycle? 
1.efficiency increases output increases 
3. both 
4. none of these.
1.Efficiency increases.
this prevents the vapor from condensing during its expansion which can seriously damage the turbine blades, and improves the efficiency of the cycle, as more of the heat flow into the cycle occurs at higher temperature.
9. What is flashpoint?
Flash point: the lowest temperature at which the vapor of a combustible liquid can be ignited in air.
10. What is refrigerant? 
Any substance that transfers heat from one
place to another,
creating a cooling effect. water is the
refrigerant in absorption machines. 
11. Explain Otto cycle. 
Otto cycle can be explained by a
pressure volume relationship diagram.
It shows the functioning cycle of a four
stroke engine. The cycle starts with an
intake stroke, closing the intake and
moving to the compression stroke,
starting of combustion, power stroke,
heat exchange stroke where heat is
rejected and the exhaust stroke. It was
designed by Nicolas Otto, a German
12. How do you measure temperature in a
Wet Bulb Thermometer? 
Wet bulb temperature is measured in
a wet bulb thermometer by covering
the bulb with a wick and wetting it with
water. It corresponds to the dew point
temperature and relative humidity.
13.) In the Thermal Power Plant why Deaerator (D/A) placed on height?
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.
14.) What is the differences between gas
turbine and a steam turbine? 
Gas turbine works on Bryton cycle where
as steam turbine works Rankine cycle.
Construction, operation of a gas turbine are
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.
15.) What is a stage in a steam turbine?
In an impulse turbine, the stage is a set of moving blades behind the nozzle. In a reaction turbine, each row of blades is called a “stage.” A single Curtis stage may consist of two or more rows of moving blades.
16) In which part of the steam turbine does stress corrosion cracking (SCC) occur?
In the wet stages of the low-pressure turbine.
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