Reversed flow at the centrifugal pump outlet faces
 

Hi everybody!

I am having a problem and I' ll appreciate any help! I am simulating the water flow through a centrifugal pump. The BC are velocity in-Pressure out, expecting the results to give me the value of suction pressure which will be very helpful for the rest of this project. Now the first problem comes to be that I have a reversed flow at the out let just after starting the calculation, and the number of reversed flow faces keeps on increasing instead of vanishing with the time. Don't you think this is a problem maybe to the results accuracy?! if yes, How can I cope with this problem ?! The second problem is the presence of Negative Suction pressure at the pump inlet ! This could be a problem when I need to calculate the NPSHa ! How can I proceed to have positive suction pressure ?! Please ..I need your help ! I just started using this software (ansys Fluent) so I think I still have a lot to learn ! Thank you

Can you post the numerical values of your BCs? I understand you set a velocity inlet (which equals to a mass flow through the pump) and a static pressure outlet?

The problem of reversed flow at the outlet is sometimes caused because the position of the outlet is not appropiate. To avoid this problem you have change the position of the outlet or extend your mesh about 5-10 times the diameter of the pipe.

The negative suction pressure at the inlet is usual if you are talking of relative pressures (if there wasn't relative pressure lower than atmospheric pressure, the pump wouldn't be able to suction fluid to the impeller).

Hope this helps.