page_title: Variable geometry turbomachine
section_title: Performance characteristics of turbomachines
hierarchical_section_title: Variable geometry turbomachine / Performance characteristics of turbomachines
context_page_description: A variable geometry turbomachine uses movable vanes to optimize its efficiency at different operating conditions. It does not cover the widespread use of movable vanes in gas turbine compressors.
context_section_description: If all fluid velocities at corresponding points within the turbomachine are in the same direction and proportional to the blade speed, then the operating condition of a turbomachine at two different rotational speeds will be dynamically similar.
  If two points, each on dissimilar head-flow characteristics curve, represent similar dynamic operation of the turbo machine, then the non-dimensional variables (ignoring Reynolds number effects) will have same values.
  
  Head coefficient 
  
  Efficiency 
  
  Power coefficient 
  Where,
   is speed of rotation.
   is flow rate.
   is impeller diameter.
  Thus non-dimensional representation is highly advantageous for converging to single performance curve that would otherwise result in multiple curves if plotted dimensionally.
  Figure 1 shows head characteristics of centrifugal pump versus flow coefficient. Within the normal operating range of this pump, 0.03 <Q/(ND³) < 0.06, the head characteristic curves approximately coincide for different values of speed (2500 <N<5000 rev/min) and little scatter appears may be due the effect of Reynolds number. For smaller flow co-efficient, Q/(ND³) < 0.025, the flow became unsteady but dynamically similar conditions still appear i.e. head characteristic curves still coincides for different values of speed. But at high flow rates deviation from the single-curve are noticed for higher values of speed. This effect is due to cavitation, a high speed phenomenon of hydraulic machines caused by the release of vapour bubbles at low pressures.
  Thus during off-design operating conditions, i.e. Q/(ND³) < 0.03 and Q/(ND³) > 0.06, the flow become unsteady and cavitations occurs . So to avoid cavitation increase efficiency at high flow rates we resort to Variable Geometry Turbomachine.
caption: Figure 1. Dimensionless head-volume characteristics of centrifugal pump
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