Numerical simulation of flow in rotating devices

The purpose of this demonstration is
  • demonstrate contemporary techniques and tools available for flow simulation in rotating devices
  • how it is different from standard contact wall flows
  • special considerations from boundary conditions points of view
  • formulations of the governing differential equation
  • post-processing considerations especially for vector quantities
  • how to check if the results are physically consistent (may or may not be accurate)!
Few features of Rotodynamic pumps (a nomenclature to collectively refer to Centrifugal, Axial Flow and Radial Flow Pumps - the rotating device which use rotation to convert mechanical energy into pressure energy) are:
  • When the working fluid approach the blades at zero angle of incidence, it is called "shockless entry" and cavitation as well as noise is minimum at this operating conditions.
  • At flow rates higher than BEP (Best Efficiency Point), is the cavitation zone.
  • At low flow rates, inlet flow recirculation affects the performance.
  • The specification at the two extreme ends of the operations are: (a) "Closed-valve head" or "shut-off head" when there is no flow through the device (b) "Run-away flow" or "free-flow rate" when there is no back pressure.
  • All such devices follow "affinity laws" or "fan laws" described here.
  • An interface is a MUST between rotating and stationary regions (domains or zones). Sometimes, such surface zones can be set as 'interior' instead of an interface.
  • However, such 'interfaces' must be a surface of revolution with axis of revolution coinciding with the axis of revolution of the rotating zones and walls.
  • The recommended practice (which is most obvious 1st guess sometimes) to chose the location of such interface(s) is the mid-way between the tip of rotating walls (or the blade tip) and the nearest stationary housing walls. Sometimes, the location of interfaces are also governed by meshing considerations (such as number of boundary layers on the walls of the blades) and free mesh size beyond this region.
  • For rotating domains embedded in relative large domains (such as a fan relative very small as compared to room), the recommendation is to have the interface at a location where flow is likely to be uniform.
  • Note that there is a significant difference between "Rotating Frame" and "Rotating Mesh".

Cyclone Separators

  • Cyclone separators are being used in industries for more than a century. This device falls under the category of what is called "Industrial Duct Collectors".
  • Dust separation process utilize different methods ranging from fabrics (such as Air Cleaners in Automotive Intake Systems) to Electrostatic Precipitators in Coal-fired power plants
  • Cyclone separators fall under the category of inertial separators which uses combination of the 3 most prevalent mechanical forces namely gravitational, centrifugal and inertial.
  • The pressure loss and collection efficiency are two key performance parameters of this device.
    Cyclone Separators Design of Cyclone Separators
  • Despite such a long history of application in industry, the design principles so far as mostly based on empirical data. Recently, CFD techniques is being used to optimize the designs.
  • 4 geometrical parameter which is tightly linked with the performance of cyclone separators are
    • Vortex finder diameter
    • Inlet width
    • Inlet Height
    • Total Height of the Cyclone
  • The cone-tip diameter of the separator does not have noticeable effect on its performance.

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