Define stations and sign conventions before extracting turbomachinery performance. Use mass-flow-consistent total properties, calculate torque about the correct axis, convert angular speed consistently, and use an efficiency definition that matches the machine and the reference map or experiment.
Performance starts with consistent stations
Before calculating pressure ratio or efficiency, define where inlet and outlet quantities are sampled. The station should be far enough from highly local blade or interface disturbances to represent the intended machine-level quantity, but not so far that unrelated duct losses are included.
For multi-row machines, distinguish rotor inlet/outlet, stage inlet/outlet and full-machine stations. Comparing a rotor-only CFD pressure ratio with a stage-level test map can create an apparent discrepancy that is purely definitional.
Mass flow is the first conservation check
Check inlet and outlet mass flow before interpreting efficiency. In a statistically steady solution, the time-averaged mass imbalance should be small relative to throughflow. For transient sliding-mesh cases, instantaneous imbalance can fluctuate while the cycle average closes.
- Use surface-normal mass flux through complete stations.
- Check leakage, cooling, purge or bleed paths separately.
- Account for rotating-frame interfaces and periodic sectors consistently.
- Do not infer throughflow from velocity magnitude alone.
Pressure ratio usually requires total pressure
Compressors, fans and turbines are commonly evaluated using total-pressure and total-temperature quantities rather than static values. The exact definition depends on the application and reference data.
In rotating domains, verify how the solver reports absolute and relative total properties. A total pressure defined in the rotating frame is not automatically interchangeable with an inertial-frame total pressure used in a performance map.
Torque and shaft power require the correct axis and angular speed
Torque is the moment of pressure and viscous forces about the rotation axis. The reported value depends on origin, axis direction, selected surfaces and sign convention.
For a compressor or pump, shaft work is supplied to the fluid. For a turbine, the fluid extracts work onto the shaft. Software sign conventions can differ from the engineering convention, so document the sign explicitly.
Cross-check torque, RPM and shaft power
Use an independent rotating-machinery calculation to catch unit and sign errors before interpreting efficiency.
Efficiency must match the machine and thermodynamic definition
There is no single universal turbomachinery efficiency expression. Compressor, turbine and fan efficiencies can be based on isentropic enthalpy change, total-to-total or total-to-static definitions, and different station selections.
Total-to-total
Uses total states at both inlet and outlet and is common when downstream kinetic energy remains useful.
Total-to-static
Uses inlet total state and outlet static state, often relevant when exit kinetic energy is treated as a loss.
Mechanical power basis
Uses τω and mass flow together with an enthalpy-based ideal or actual work definition.
Map convention
Corrected mass flow, corrected speed and reference thermodynamic conditions may be required for comparison with a performance map.
Do not compare two efficiencies until you know that both use the same stations and total/static convention.
Area averaging and mass-flow averaging are not interchangeable
For non-uniform turbomachinery stations, simple area averages can overrepresent low-mass-flux regions. Mass-flow-weighted averages are often more appropriate for total enthalpy and total pressure when evaluating throughflow performance, but the exact convention should match the reference dataset.
Transient rotor-stator interaction also requires time averaging. Average over enough blade-passing cycles or periodic repetitions for torque, pressure ratio and efficiency to become statistically stable.
A practical turbomachinery performance workflow
- Define machine mode and performance objective.
Compressor, fan, pump or turbine; design point, map point or local diagnostic.
- Define inlet and outlet stations.
State exactly what each station includes.
- Verify mass conservation.
Include all leakage, cooling and bleed flows.
- Extract consistent total properties.
Check inertial versus rotating-frame definitions.
- Calculate torque about the correct axis.
Use all wetted rotating surfaces that transfer momentum.
- Convert RPM to angular speed and calculate power.
Verify units and sign.
- Apply the correct efficiency definition.
Match the reference map or experiment.
- Average and verify sensitivity.
Check mesh, timestep and cycle convergence for the performance quantities.