If in a multistage air preheater von Karman vortices is calculated to say 131Hz is it possible that it will also exist at half of it at 64Hz as that is the frequency that is found on the device? In some places also multiples are found.
It is very flow sensitive, lowering fan speed by 44 RPM from 1444RPM make thing go away and it also reduce flow from 105 000 mx3/h to 100 000. Opening the bypass also makes it go away... When it vibrates as heard in the whole plant the Delta Pressure goes up above nominal accepted, when it stop vibrating DP is normal. Any suggestions? By the way the fan have 8 blades so I can´t find any correlation to that either. Also to be noted is that the inlet and outlet design is less than what I would like, rarely seen so many 90 deg turns on the ducts for a substantial airflow. Olov

This message has been edited. Last edited by: OLI,

Hey Oli, what are you using for the Strouhal number? It typically varies from 0.25 to 0.5 which may explain the difference you are seeing between calculation and measurement.

Oli, Von Karman vortex occur in all fin across air flow, but when attack angle is increased a lot it help to increase noise. Remember that Von Karman vortex is recirculating air flow at low pressure so,when 8 blades fan has high pressure difference it could produce resonance on each one of 8 blades fan. I would like you show spectrum indicating run speed.