Learning Forum

[deepak]

I have a simulation for axial flow pump with the following details;
Inlet BC = Pressure (0.4 bar)
Outlet BC = massflow rate ( 6000 m3/hr)
Ref. Pressure = 0 atm
Kw SST Turbulence model
Steady state simulation.
Y+ = ~2

When I plot contours of total pressure, very large negative pressure values are observed ( in the orders of -500 KPa / -5.0 bar ). 
How can this be possible when absolute pressure cannot be < 0 in real world.

We know, In Ansys, The total pressure is = static + dynamic 
and Absolute pressure = Ref. operating pressure + static pressure

 

1. In the Total Pressure contor, -ve pressure values are observed (at the impeller) which is expected as the impeller egde will have a lower pressure naturally.
2. But looking at the second picture (i.e. Total pressure contour at Impeller blades), very large -ve pressures are observed which cannot be physically possible. 
3. What inference can i deduce from the above contours. (for e.g. cavitation? incorrect physics settings? numerical error?)

Any comments / suggestions would be apperciated as it will help me make my case stronger.
Thank you

[Amine Ben Hadj Ali]

The pressure values reported are gauge pressure values: what is the reference pressure?

Solver pressure is acutally not bounded only when it get's used to evaluate material properties.

In real case (most engineering problems) you will then start cavitating by going below the vapor pressure.

[deepak]

DrAmine, Thank you for your prompt response. 
The reference pressure is 0 atm (mentioned above)
1. If the pressure values are gauge pressures, how can it be in orders of -440 KPa. 
2. Could you please elaborate on your second point.
3. Yes i agree to your point, but how can one show negative total pressures? Is there a better way to justify negative pressures?
4. Would you like to see any additional contours/plots?

Regards.

[Amine Ben Hadj Ali]

Negative values aca n arise due to numerical resons but aslo can aslo means that we need to account for cavitation. Again the pressure variable in Ansys Fluent is not bounded and allowed to float. It is only bounded to a minimum value for evaluation of material property (escpecially density).

[deepak]

Hello DrAmine, sorry for the delayed response. Based on your latest reply, is it fair to say that large negative values can be ignored as long as it does not hinder our solution and the very low pressure zones are most likely prone to cavitation.
Regards.

[NickFL]

Large negative values of pressure indicate that cativation could occur here. Overall the model may be very good, but in this small region where the "negative" pressure occurs, the model is not going to be very reliable. Nature would account for this negative pressure by having the liquid transition into a vapor. Since you are not including that in your model (it is possible to do), your model will not be accurate here.

Cativation can be a very big problem with pumps that can lead to failures. For full disclosure, when documenting the model, you should point out that cavitation could be occuring there.

[Amine Ben Hadj Ali]

Yes but it is good to enable the cavitation model to track the regions or at least to create a post-processing variable to "identify" these regions.

[deepak]

Thank you for your valuable inputs, DrAmine.
Thank you too NFLynn.