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Old   April 5, 2011, 04:05
Default Centrifugal pump analysis
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Hi All,
I am currently performing a simulation of a centrifugal pump with water as the working fluid. At design point itself I am getting negative pressures(absolute) at the leading edge of the blades. The head rise prediction is within 3% of the experimental values. My question is I have read that cavitation occurs if local pressure falls below 3194 Pa ( vapor pressure of water at 25deg celcius) So is cavitation occuring in this case and if so; even without switching on the cavitation model why is my result this close to experimental value.

Thanks in Advance,
Dinesh
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Old   April 5, 2011, 18:52
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If the region which cavitates is small it does not affect the overall flow much.
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Old   April 6, 2011, 02:01
Default Negative Absolute Pressures
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Thanks Glenn,
I have one more doubt. In the some low pressure regions (near the L.E. of the vane) I am getting Absolute Pressure values as negative ( arnd -1000Pa). I find this highly unrealistic. My reference pressure is 1atm( but that wouldnt matter while viewing Absolute Pressure values). Any suggesstions/comments?

-Dinesh
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Old   April 6, 2011, 07:41
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Do you know what the pressure in this region should be?

Either your simulation is inaccurate (see http://www.cfd-online.com/Wiki/Ansys..._inaccurate.3F) or you have found a small cavitation region which does not really affect much.
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Old   April 7, 2011, 00:17
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Hi,
I have checked my simulation over and over again and I couldnt find any problem. Incidentally I ran the sam mesh and boundary conditions with different advection schemes 1) High resolution and 2nd) Upwind till convergence and then High resolution. I assumed convergence when the total pressure rise across my domain became constant with iterations and max residuals below 10e-4 and RMS below 10E-5.
The contrast was that despite having almost the same pressure rise across the domain I found these physically impossible pressure values in the 2nd case while they were absent in the 1st case. I am clueless to what might the reason be..any sugesstions ?

-Dinesh
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Old   April 7, 2011, 07:27
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Why is it unrealistic to have a small low pressure region? Happens all the time in my experience.
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Old   April 8, 2011, 01:22
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Dear Glenn,
I am getting NEGATIVE ABSOLUTE PRESSURE ( not wrt atmospheric ) what does it even mean to have a negative absolute pressure ??? ( a black hole ???? ). I am clueless.
I found it to be in my every simulation for this pump for 4 different meshes for all kinds of boundary conditions ( the earlier thing I said about not getting negative absolutes in a certain simulation was incorrect, pls ignore it )
The simulation is very very simple. Pump with total pressure inlet mass flow outlet. Impeller with the volute front and back wall in a rotating domain. Impeller wall - no slip wall. Volute wall with counter-rotating no slip wall. Even my colleague tried it on his mesh and he is getting the same result. Pls help !!!!

-Dinesh
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Old   April 8, 2011, 01:37
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Could mean a few things.

Most likely you have a small region which cavitates. The cavitation is small so does not affect the flow much. But if you model without cavitation you get the correct answer but small regions of negative abs pressure.

Also possible is the negative absolute pressure is real. It takes time for materials to change phase, so if you reduce pressure fast enough you can get transient negative absolute pressures.
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Old   April 8, 2011, 12:07
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Yes, I do get negative pressures on the suction side of the leading edge, where the pressure is at its lowest across the entire pump. But this happens at all flow rates along the pump curve ( also it is a small domestic pump, I am not sure if they are that heavily loaded).

But when you mean, absolute negatives are possible...do you mean numerically or physically..cause if you mean physically, (correct me if I am wrong)..isnt negative absolute pressure impossible to achieve ( or rather undefined ) just like absolute zero of temperature...

If this is due to cavitation how do I ensure that my simulation is correct ( since I have only head rise across the pump, which is already within 3% of experimental values )

I apologize for the barrage of questions, but I am out of ideas and there seems to be no way out of this.

- Thanks & Regards,
Dinesh
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Old   April 9, 2011, 06:32
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You can get very low pressures regardless of pump loading.

Yes, negative absolute pressures are possible. Just like water does not boil at 100C, it needs a superheat to start it off. Cavitation needs the equivalent of a sub-cool to start it off. And if the cavitation pressure is near absolute zero pressure then it is possible for fluids to have absolute pressures less than zero. For most applications this discussion is academic, but I work in the MEMS field and I can assure you in MEMS water boils at about 300C (because of the rapid time scales it is non-equilibrium boiling) and if you wack it hard enough you can get sub-zero absolute pressures.

But in your case (as you are not dealing in MEMS but more "normal" scale stuff) the low pressure is almost certainly associated with a small cavitation bubble which does not affect the flow much.
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