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Fluid Mechanics

Positive Displacement Machines More:

Screw Pump:

  • Can handle debris
  • Used to raise the level of wastewater
  • Abrasive material will damage the seal between screw and the housing
  • Grain augers use the same principle





Rotadynamic Machines (Turbomachines):

  • All rotodynamic machines have a rotating component through which the fluid passes.  In a turbine this is called the rotor which has a number of vanes or blades.  
  • The fluid passes through the blades and drives the rotor round transferring tangential momentum to the rotor
  • Rotodynamic machines are smooth and continuous in action with a consequent pulsation free flow from pumps and smooth rotation from turbines. 
  • In the event of pump discharge flow being suddenly stopped there are no high shock loads. 
  • Positive displacement machines can easily be damaged if a discharge valve is suddenly closed.  
  • Rotodynamic pumps are ideal for high flow low discharge head duties and provide compact reliable solutions.
  • Some of the important rotadynamic machines are as below:
Radial Pumps (Centrifugal Pump):
  • also called centrifugal pumps
  • broad range of applicable flows and heads
  • higher heads can be achieved by increasing the diameter or the rotational speed of the impeller





Axial Flow
  • also known as propeller  pumps
  • low head (less than 12 m)
  • high flows (above 20 L/s)






Turbines and pumps

  • A turbine directly converts fluid energy into rotating shaft energy.
  • If the fluid motion is converted, initially to reciprocating mechanical motion the machine is an engine e.g and internal combustion engine or a steam engine ).
  • A machine for converting mechanical energy into fluid flow is called a pump.

Compressors or Fan

  • If the machine converts mechanical energy to increase the potential energy of a compressible fluid by increasing its pressure the machine is called a compressor.  
  • If the machine is primarily provided to increase the kinetic energy of a compressible fluid e.g. air, the machine is a fan.  
  • With a fan or blower the pressure head developed is usually relatively small and fluid calculations can often be done assuming the fluid is incompressible.

Positive Displacement Machines:

  • A pump can be a positive displacement machine or a rotodynamic machine Ref.
  • Pumps Positive displacement machines are designed such that there is virtually zero fluid slippage in the energy transfer process. 
  • The general principle of these type of pumps is that fluid is drawn into a chamber at a low pressure.
  • The inlet to the chamber is closed and the outlet is opened, and the fluid is then forced out of the chamber by reducing its volume.
  • The type of pump can be used to generate very high pressures in a compact mechanical envelope.
  • The main disadvantage is that the operation is an intermittent one resulting in a high level of pressure fluctuation throughout the operating cycle.


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