# General mechanical interview questions • #### What is Mechanical Engineering?

Mechanical Engineering is the branch of engineering which deals with the designing, constructing , maintaining and manufacturing various mechanical systems.

• #### What is the difference between machine and engine?

Engine converts heat energy to Mechanical Energy but machine converts all forms of energy to mechanical energy except heat energy.

• #### What is viscosity? What is newton’s law of viscosity?

Viscosity is the property of  fluid which measures the resistance of the fluid to the gradual deformation due the shear stress.

Newtons Law of Viscosity states that : the shear stress on the fluid is proportional to the velocity gradient.

• #### What are newtonian and non-newtonian fluids ? Give examples.

Newtonian fluids follow the newtons law of viscosity and non-newtonian fluids do not follow the law.

Newtonian fluid : Water, air etc. Non-Newtonian Fluid : blood, tar , paste etc.

• #### What is Pump? What is turbine? / What is the difference between turbine and pump?

Pump is a device which transfers mechanical energy to fluid. Turbine does the opposite. It transfers flow energy of the fluid to the mechanical energy.

• #### What are turbo machines?

Turbo machines are devices which transfer energy to or from the fluid by the dynamic action of rotating blades.

• #### How the characteristics of a flow is determined?

By Reynolds Number . If Re <2300 then the flow is laminar . if 2300 <Re<4000 then the flow is transient and if Re>4000 then the flow is turbulent .

• #### What is Reynolds number ?

Reynolds Number is the ratio of inertia force to viscous force . Re = ρvL/μ

• #### What is Bernoulli’s equation? Why it is used?

pressure head + velocity head + datum head = constant

P1/γ + (v)1/2g + z= P2/γ +( v2)2 /2g + z1 = constant

• #### What is the difference between centrifugal pump and +ve displacement pump?

In Centrifugal pump the flow rate changes with the head but in positive displacementpump the flow rate remains the same.

• #### What is cavitation in centrifugal pump? Why priming is done in centrifugal pump?

If the suction side develops a pressure less than the vapor pressure of the water then water will start to boil and create water bubbles and it will spoil the impeller material. It is called cavitation.

When impeller rotates it adds a rotating motion to the adjacent fluid and as a result K.E and Pressure in the fluid increases and a negative pressure is developed in the suction side of the impeller . So water/fluid flows continuously through the impeller. But when the pipe is filled with air , the impeller rotates but the pressure difference is negligible as a result no water flows through the pump. Thats why priming is done.

• #### What is the difference between heat engine & heat pump?

Heat Engine produces work and transfers energy from high temp. to low temp source medium. In Heat pump external work is needed to transfer energy from low to high temp. medium.

• ### What is the difference between Impulse Turbine and Reaction Turbine?

In Impulse Turbine all the pressure energy is converted to kinetic energy but in reaction turbine a part of the pressure energy is converted to kinetic energy and rest remains as pressure energy.

• #### What is NPSH?

NPSH = Net Positive Suction Head. It is the measure of minimum pressure which is required at the suction pipe to keep the pump from cavitation.

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