Centrifugal pump Simulation in Cfx ; Head capacity curve not matching
 

Hi,
I am trying to simulate a centrifugal pump to generate head capacity curve and compare it with experimental results. My simulation results of head values much differ from the test data.
Pump specs:
Duty point discharge - 2 litres per second
Head- 21 m
Speed-2800 rpm
Impeller dia - 141 mm

Cfd gives much higher values of head in higher flows.
This pump is supposed to give 0 m head at around 3.5 lps flow, but instead gives about 19 metres of head.

I used SST & K-E models, used wall roughness, but not much advantage. Hope anyone may help me resolve this issue.