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.