How does a turbomachine transfer energy to or from the fluid?

By dynamic action of rotating blades changing fluid momentum

By trapping fluid in a closed volume changed by a piston

By sliding seals and gear meshing

By heating the fluid at constant volume

Which statement about flow is correct?

Turbomachine flow is continuous and steady; positive displacement flow is intermittent or pulsating

Turbomachine flow is pulsating; positive displacement flow is steady

Both give perfectly steady flow

Which list contains only positive displacement machines?

Centrifugal pump, steam turbine

Centrifugal compressor, reciprocating compressor

Practice · 5 marks

Turbomachines versus positive displacement machines

Short answer

Turbomachines use dynamic blade action for continuous, steady flow, high speed, and large clearances; examples: centrifugal pump, gas turbine. Positive displacement machines trap fluid in a closed volume, produce pulsating flow, run at low speed with tight seals; examples: gear pump, reciprocating compressor.

Differentiate between turbomachines and positive displacement machines. Give two examples of each.

Both types transfer energy between a mechanical element and a fluid. They differ in the way the energy transfer takes place.

Point	Turbomachine	Positive displacement machine
Principle	Dynamic action of rotating blades changes the momentum of the fluid (Euler equation)	Fluid is trapped in a closed volume that is changed by a piston, vane, gear or screw
Flow	Continuous and steady	Intermittent or pulsating
Working element motion	Pure rotation	Reciprocating, or rotary with sealed chambers
Clearances	Large running clearance, no sealing contact	Close clearances or sliding seals needed
Speed	High (thousands of rpm)	Low to moderate
Capacity and pressure	Large flow, moderate pressure ratio per stage	Small flow, high pressure possible
Size and weight per kW	Small, light	Large and heavy
Vibration, balancing	Easy to balance, smooth running	Unbalanced forces, more vibration
Efficiency curve	Peaks at the design point, falls off-design	Almost constant over the range
Viscous fluids	Poor	Good

Examples

Turbomachines: centrifugal pump, axial-flow gas turbine, steam turbine, centrifugal compressor.
Positive displacement machines: reciprocating compressor, gear pump, reciprocating IC engine, Roots blower.

✓ Answer checked by Claude Opus 5.5 (high effort)

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