TURBOMACHINES

               TURBOMACHINE DEFINATIONAND EXPLANATION



A turbo-machine is a device in which energy transfer takes place between rotating element and stationary fluid. example- fan is the simplest turbo-machine.

The turbo-machine works on basic principle of EULAR'S equation, NEWTON'S second law.

The two kinds turbo-machines are:-

  • Open turbo-machines 
  • Closed turbo-machines

On the basis of flow of fluid turbo-machines are classified as:-
  • Axial flow turbo-machine
  • Radial flow turbo-machines
  • Mixed flow turbo-machines

A axial turbo-machine is one in which the flow of fluid is in direction parallel to the axis of rotation. Such machine which satisfies this type of flow are kaplan turbine. There is no radial flow, means that the flow is not perpendicular to the axis of rotation of blades.

A radial flow turbo machine is one in which the flow of fluid is perpendicular to the axis of rotation of blades. Machines like centrifugal pump corresponds to radial flow machines.

Mixed flow machines have both axial flow and radial flow character. It has certain advantages as it enjoys both characteristics of axial flow and radial flow machines.

Depending on pressure the turbo machines can be classified as
  • Impulse turbine
  • Reaction turbine

IMPULSE TURBINE

Such turbines in which there is no pressure drop or rise across the blade of machine is called impulse turbine. Given below is example of impulse turbine, in such machines the pressure energy of fluid is converted to velocity of fluid before it approaches the blades of turbine. The velocity of fluid rotates the blades of turbine.

 PELTON WHEEL a example of impulse turbine




INVENTOR-LESTER ALLAN PELTON (1870'S)


A Pelton wheel is impulse type water turbine. Such turbine utilizes water energy in form of impulse from nozzle, which is directed to the cup shaped buckets.
As the high impulse water strikes these spoon shaped buckets, it starts rotating. These spoon shaped bucket is fixed on round circumference ring called runner (impeller).


The impulse of water supplies torque to the spoon cups as it strikes to cups. The velocity of water changes after collision and it takes a "U" turn after striking cups.

The impulse provided by the water rotates the bucket cups and impeller and hence turbine. There is transfer of momentum by water to the cups. Basically the spoon cups are provided in pair, this is done for better splitting of water in two steams, hence better balancing of side forces is possible.

VARIOUS COMPONENTS OF PELTON WHEEL

Impeller - These are cups which are mounted on the hub and connected to shaft.
spoon cups - Water strikes these cups and rotates, water momentum is converted into rotary motion.
Nozzle - Increases pressure as the area decreases along the nozzle.
Spear - Used to adjust the velocity of the approaching high pressure fluid towards the cups. The spear is present inside the nozzle which can move to and fro. As it moves back the area of cross section in the nozzle increases and velocity decreases. As the spear move forward the area of cross section decreases and flow velocity increases.
Spear rod - Used for increasing or decreasing the flow of water through nozzle.
Deflector plate - For deflecting water as per requirement.
Efficiencies related to pelton wheel:-
  1. Mechanical efficiency
  2. Hydraulic efficiency
  3. Volumetric efficiency
  4. Overall efficiency
REACTION TURBINE

Such machine have pressure differential across the turbine blades. The thrust is developed by the rotating blades as the fluid strikes the blade. Examples of such machine is Francis turbine.


Objective questions on turbo machines
1) When the flow generated by turbo machine is parallel to the axis of turbine, then such turbine is called_______?

1) Radial flow turbine
2) Axial flow turbine
3) Mixed flow turbine
4) Any of them

2) A nozzle has always _________ shape at the exit?

1) Divergent
2) Convergent
3) Parallel
4) Any of them

3) What kind of turbine is De-laval turbine?

1) Reaction turbine
2) Multi-rotor impulse turbine
3) Single rotor impulse turbine
4) It depends on mode of operation

4) Exit velocity of nozzle is given by?

1) 44.62*sq root(k*hd)
2) 44.72*sq root(k*hd)
3) 44.82*sq root(k*hd)
4) 44.92*k*sq root(hd)

5) Choose the incorrect statement.

1) Turbine blade changes the direction of flowing fluid.
2) When the fluid strikes the blades the turbine, there is large pressure drop.
3) Flow at critical pressure gives velocity of sound equal to velocity of steam at throat.
4) The pressure at which steam leaves the nozzle is called front pressure.

6) What kind of turbine is Parsons turbine?

1) Impulse turbine
2) Reaction turbine
3) Mixed flow turbine
4) None

7) At entry of nozzle the steam or fluid has

1) Low pressure
2) High pressure
3) No pressure
4) Infinite pressure.

8) Depending on fluid flow, how many turbines are there?
1) 4
2) 3
3) 2
4) 6

9) In a nozzle, with decrease in exit discharge pressure, the discharge

1) Increases
2) Decreases
3) Remains same
4) Zero discharge


Be first to answer these questions via comment.
Thank you.

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