Connection between inlet velocity field and diffuser flow instability

Simulations of the flow in a straight diffuser were carried out for different inlet velocity profiles equivalent to the real turbine operating points. Frequencies and magnitudes of pressure pulsations of the rotating vortex rope were recorded. Two types of vortices were detected. The first one is a thin vortex with higher frequency and relatively low pressure amplitudes corresponding to higher part load turbine operation. For lower loads, further from best efficiency point, a huge vortex rotating on higher diameter appears, which is accompanied by massive pressure pulsations, but with lower frequency. Connections of these two regimes with inlet circumferential velocity component are discussed in the paper