Fluid Mechanics PUMPS Fluid Machines Is a device

Fluid Mechanics PUMPS

Fluid Machines • Is a device which adds energy (pump or compressor) into or removes energy (turbine) from a fluid by dynamic interactions between the device and the fluid • Four categories of fluid machines (Pumps): – – Positive Displacement (PD) Pump Centrifugal fluid pump Axial fluid pump Jet fluid pump • Energy equation or “Pressure rise” “Head”

P. D. Fluid Machines • Works by allowing a fluid to flow into some enclosed cavity from a low pressure source, trap the fluid and then force it out into a high pressure receiver by decreasing the volume of the cavity

Centrifugal Fluid Machines • Raises the pressure of a fluid by moving it outward in a centrifugal force field, passing it radially across the impeller and giving it a high kinetic energy and then converting that kinetic energy into fluid pressure energy.

Axial-flow Fluid Machines • Fluid passes through several rows of blades arranged in an annulus and is successively accelerated by moving rows of blades, and then slowed by a stator blade. Fluid enters and leaves in the direction of the axis of the rotor

Jet Fluid Machines • Used to convert kinetic energy into pressure energy, usually with the objective of creating an overpressure at one point and a vacuum at another.

What do they look like? P. D. Pump Axial flow Pump Centrifugal Pump Jet fluid Pump

Pump Vs. Compressor • Both pumps and compressors are fluid machines, which add energy into the fluid • The key difference – Pumps deal with incompressible fluid, e. g. water – Compressors deal with compressible fluid, e. g. air • Our focus is on PUMPS – – Pump performance curves System curve Pump selection Dimensionless numbers, which are important to pumps

How P. D. Pumps Work?

Performance Curve of P. D. Pumps “Constant-volumetric flow rate device” At fixed motor speed “Must always have some kind of safety valve to relieve pressure if the discharge line is suddenly blocked”

How Centrifugal Pumps Work? • Fluid enters the eye of the impeller in an axial direction and passes radially across the impeller acquiring a large tangential velocity • The kinetic energy is converted to pressure • Radial-flow fluid machine

Kinetic Energy to Pressure (or Head) or b is the width of the impeller

Centrifugal Pump Performance Curve

Centrifugal Pump Performance Curve

System Curve

System Curve (Cont’d) Energy equation:

System Curve (Cont’d)

System Curve (Cont’d)

Net Positive Suction Head (NPSH)

Below.

Net Positive Suction Head- Required (NPSHR)

System Curve (Cont’d) (NPSHR)

Pumps in Series or Parallel

Pump Dimensionless Numbers • Dimensionless Analysis on Pump – Key Variables • The actual head rise (or the actual head rise in terms of energy per unit mass) • Shaft power • Pump efficiency

Pump Dimensionless Numbers • For geometrically similar Pump

Dimensionless Performance Curves Centrifugal Pump Axial-flow Pump

Pump Scaling Laws • Similitude Law

Specific Speed Dimensionless number and independent of the system of units In US unit:

Specific Speed

Examples

0. 225 m 3/s