[Lookid]

Hello,

I compute a low specific speed pump with splitter blades. Inlet + Full impeller + Volute. No casing.

Periodically, I have an hydraulic efficiency > 1, it happens when the main blade (sometimes the splitter also) passes in front of the cutwater. The average efficiency is somehow realistic (~0.77)

I changed the mesh - the turbulence model - the boundary conditions, but I always have the same results (exact same amplitude). So I believe there is 3 ways out of this :

1 - My CFD is still wrong
2 - CFD can't model this case (I'm really not experienced enough to pretend this, just putting it here)
3 - It's physical, and if I try to explain it, here I go :

Citing Gulich, chap 10.1 :

"When the blade passes under the diffuser vane or volute cutwater, there is a pressure minimum due to the effect that the fluid is kind of “squeezed” through gap B"

In my case, this whole pressure minimum is present on the whole channel (see image, in this image, the efficiency is > 1). So locally (at this time), this low pressure on the pressure side causes a low torque > a low shaft power which is locally lower than water power > an efficiency higher than 1.

► Note on the impeller: It's a low specific speed impeller (nq = 9, high diameter, lot of blades), but the outlet width is way higher than the "recommended" one. b2 recommended is 4mm, I have 10mm. So basically, even at the designed point, my impeller behaves like a "part-load" classic impeller.

Any comments is appreciated because I am completely stuck right now