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Fluid 4.1

Centrifugal pumps utilize backward-curved or radial vanes for stability and efficiency, while Pelton wheel buckets avoid a 180° turning angle to minimize radial thrust. Cavitation in turbomachines leads to reduced efficiency and potential structural damage, necessitating adequate Net Positive Suction Head. Centrifugal pumps vary flow with pressure and are suited for low-viscosity fluids, whereas positive displacement pumps provide constant flow and handle viscous fluids, with impulse turbines converting kinetic energy and reaction turbines utilizing both pressure and kinetic energy.

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11 views2 pages

Fluid 4.1

Centrifugal pumps utilize backward-curved or radial vanes for stability and efficiency, while Pelton wheel buckets avoid a 180° turning angle to minimize radial thrust. Cavitation in turbomachines leads to reduced efficiency and potential structural damage, necessitating adequate Net Positive Suction Head. Centrifugal pumps vary flow with pressure and are suited for low-viscosity fluids, whereas positive displacement pumps provide constant flow and handle viscous fluids, with impulse turbines converting kinetic energy and reaction turbines utilizing both pressure and kinetic energy.

Uploaded by

mosesmusuko
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© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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1. Why are pumps not designed with forward-curved vanes?

Most centrifugal pumps are thus designed with backward-curved or radial vanes, which offer more
stable, efficient, and durable performance.

2. Why is the turning angle of the Pelton wheel bucket not set at β = 180° even though this would
give maximum power?

 Avoiding Water Interference.

 Mechanical Constraints.

 Minimizing Radial Thrust: A turning angle just under 180° ensures that the force direction
remains tangential, minimizing the unwanted radial force on the turbine shaft and bearings.

3. Describe cavitation in turbomachines and analyze its effects on performance and durability

Cavitation is the formation and subsequent collapse of vapor bubbles in a liquid when the local
pressure falls below the vapor pressure of the fluid. This typically occurs on the suction side of
pumps or near turbine blades.

Causes:

 Low pressure at impeller inlets (e.g., high suction lift, high fluid temperature).

 High flow velocities that reduce static pressure.

Effects on Performance:

 Reduced efficiency due to disruption in flow continuity.

 Fluctuating hydraulic performance, including pressure pulsations.

 Reduced flowrate and head developed by the pump or turbine.

Effects on Durability:

 Erosion of metal surfaces due to bubble collapse near solid boundaries.

 Noise and vibration, often described as a “gravel-like” sound.

 Structural damage over time to impellers, casings, and bearings.

Maintaining an adequate Net Positive Suction Head Available (NPSHA) above the NPSH Required
(NPSHR) is essential to prevent cavitation.

4. Compare centrifugal and positive displacement pumps

Feature Centrifugal Pump Positive Displacement Pump

Working Adds energy to the fluid via rotating Traps a fixed volume and displaces it
Principle impeller blades. mechanically.

Nearly constant, regardless of system


Flow Rate Varies with system pressure.
pressure.

Pressure Moderate pressures. High pressures possible.


Feature Centrifugal Pump Positive Displacement Pump

Output

Viscous fluids or high-precision flow


Best Suited For Large volumes of low-viscosity fluid.
requirements.

Self-Priming Often requires priming. Generally self-priming.

Hydraulic systems, oil and chemical


Typical Use Water supply systems, irrigation, cooling.
dosing.

Centrifugal pump for water treatment


Example Gear pump for hydraulic oil circulation.
plant.

5. Describe the difference between an impulse turbine and a reaction turbine

Feature Impulse Turbine Reaction Turbine

Working Converts kinetic energy of high-speed jets Converts both pressure and kinetic
Principle into mechanical energy. energy.

Fluid Fluid flows fully around blades in a


Fluid strikes buckets or blades as free jets.
Interaction pressurized casing.

Occurs both in nozzles and across the


Pressure Drop Occurs only in the nozzles before the rotor.
rotor.

Rotor Rotor is fully immersed in the


Rotor is not fully immersed in fluid.
Immersion working fluid.

Examples Pelton wheel. Francis and Kaplan turbines.

Application High head, low flowrate. Low to medium head, high flowrate.

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