page_title: Axial compressor
section_title: Off-design operation
hierarchical_section_title: Axial compressor / Steady-state performance / Off-design operation
caption_attribution_description: English: Off design characteristics curve of an axial compressor
context_page_description: An axial compressor is a gas compressor that can continuously pressurize gases. It is a rotating, airfoil-based compressor in which the gas or working fluid principally flows parallel to the axis of rotation, or axially. This differs from other rotating compressors such as centrifugal compressor, axi-centrifugal compressors and mixed-flow compressors where the fluid flow will include a "radial component" through the compressor. The energy level of the fluid increases as it flows through the compressor due to the action of the rotor blades which exert a torque on the fluid. The stationary blades slow the fluid, converting the circumferential component of flow into pressure. Compressors are typically driven by an electric motor or a steam or a gas turbine.
  Axial flow compressors produce a continuous flow of compressed gas, and have the benefits of high efficiency and large mass flow rate, particularly in relation to their size and cross-section. They do, however, require several rows of airfoils to achieve a large pressure rise, making them complex and expensive relative to other designs.
context_section_description: The performance of a compressor is defined according to its design. But in actual practice, the operating point of the compressor deviates from the design- point which is known as off-design operation.
  from equation (1) and (2)
  The value of  doesn't change for a wide  range of operating points till stalling. Also  because of minor change in air angle at rotor and stator, where  is diffuser blade angle.
   is constant
  Representing design values with (')
  for off-design operations (from eq. 3):
  for positive values of J, slope of the curve is negative and vice versa.
caption: Off design characteristics curve of an axial compressor.  Stage loading coefficient (
    
      
        
          ψ
          
        
      
      {\displaystyle \psi \,}
    
  ) as function of flow coefficient (
    
      
        
          ϕ
          
        
      
      {\displaystyle \phi \,}
    
  )
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