Thermodynamics & Fluid Mechanics Interview Questions

Latest Thermal Interview Questions and Answers

Why Entropy decreases with increase in temperature?

ds=dQ/T Entropy is inversely proportional to the temperature so, as temp. Increases, entropy decreases.

Why different types of sound are produced in different bikes, though they run on SI Engines?

Engine specifications are different in different manufactures like as Bore Diameter (CC), Ignition timing. Also the exhaust passage takes more responsible for sound.

How much Watt means 1Hp?

746.2 Watt

Explain Bicycle Rear Wheel Sprocket working?

Rear wheel sprocket works under the principle of ratchet and pawl.

Definition of Octane Number and Cetane Number?

Octane No.- Octane number is defined as the percentage, by volume, of iso octane in the mixture of iso octane and h-heptane. It is the measure of rating of SI engine.

Cetane No.- Cetane number is defined as the percentage, by volume, of n-cetane in the mixture of n-cetane and alpha methyl naphthalene. It is the measure of rating of CI engine.

Which Mechanism is used in Automobile gearing System?

Differential mechanism

When Crude Oil is Heated, Which Hydro Carbon comes first?

Natural gas (Gasoline)… at 20 Celsius

How to calculate Bearing number ti Diameter of the inner and outer?

Divide the shaft diameter size by 5, it will give last two digit of the bearing no. and according to type of load we have to choose the type of bearing and that will give prior number of the bearing.

The Fatigue life of a part can be improved by?

Improving the surface finish by Polishing & providing residual stress by Shot peening.

What happens if gasoline is used in a Diesel Engine, Siesel Engine will work?

No, It will not work, as the Compression ratio of Petrol engine is 6 to 10 & that of Diesel engine is 15 to 22. Thus on such high compression, gasoline gets highly compressed & it may blast.

Poissons Ratio is Higher in, Rubber/Steel/Wood?

When a material is compressed in one direction, it usuallytends to expand in the other two directions perpendicular tothe direction of compression. This phenomenon is called thePoisson effect. Poisson’s ratio is a measure of the Poisson effect.

For rubber = 0.5

For steel = 0.288

For wood < 0.2

Thus Poisson’s ratio is higher in RUBBER.

Basic Fluid Mechanics Interview Questions And Answers.

Why the Centrifugal Pump is called High Discharge pump?

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 formulae:

Centrifugal force F= (M*V2)/R.





How Cavitation can be eliminated by Pump?

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.

Why Cavitation will occur in Centrifugal Pump and not in Displacement Pump?

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 vapour due to increase in temperature in impeller. This air bubbles are transmitted to pump which forms cavitation.

Which Pump is more Efficient Centrifugal Pump or Reciprocating Pump?

Centrifugal pump. Because 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.

Why Centrifugal Pump is not called as a Positive Displacement Type of Pump?

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.

What is the difference between Critical Speed and Whirling Speed?

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.

How a Diesel Engine Works as Generator?

Diesel engine is a prime mover, for a generator, pump,and for vehicles etc. generator is connected to engine by shaft. mostly in thermal power plat ,there is an engine is used to drive generator to generate power.

Explain Second Law of Thermodynamics?

The entropy of the universe increases over time and moves towards a maximum value.

Compare Brayton Cycle and Otto Cycle?

The heat addition and rejection processes in Otto cycle are of constant volume, whereas in Brayton cycle, they are of constant pressure.

-Otto cycle is the ideal cycle for spark ignition engines.
-Brayton cycle is the ideal cycle for gas power turbines.

What is the purpose of Scrapper Ring?

scrap the excess lube oil from the cylinder walls. there by preventing oil from entering combustion zone.

What is DTSI Technology?

DTSI stands for Digital Twin Spark Plug Ignition. The vehicles with DTSI Technology use 2 spark plugs which are controlled by digital circuit. It results in efficient combustion of air fuel mixture.

  • Digital – Since the spark generation will be initiated by a microchip.
  • Twin Since two spark plugs will be used.
  • Spark ignition – Since the ignition will be done via a spark.

How to Find, Ductile-Brittle Transition Temperature in Metals?

The point at which the fracture energy passes below a pre-determined point for a standard Impact tests. DBTT is important since, once a material is cooled below the DBTT, it has a much greater tendency to shatter on impact instead of bending or deforming.

What is the importance of Thermodynamics?

All the mechanical engineering systems are studied with the help of thermodynamics. Hence it is very important for the mechanical engineers.

What is the difference between P11 and P12 Pipes?

P11 the chromium molybdenum composition that is 1% ofchromium and 1/4% of molybdenum

P12 the chromium molybdenum composition that is 1% ofchromium and 2% of molybdenum

State difference between AnitiFriction Bearing and Journal Bearing?

Generally, journal bearings have higher friction force, consume higher energy and release more heat, but they have larger contact surface, so normally used in low speed high load applications. In anti friction bearings friction is less. One object just rolls over each other.

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