Abstract: When a running axial flow pump deviates from its design condition, especially at very low flows, strong vortices in its flow channel will cause severe pressure pulsation, deteriorating the safe and stable operation of the pump unit. In this paper, by examining a high specific speed axial flow pump as a case study, we simulate numerically three-dimensional flows in its whole flow channel for five operating points by using a code equipped with a RNG k-ε turbulence model, and discuss its unsteady flow characteristics. Its external characteristic curves at five steady operating points are verified, and we find they agree well with the design performances. We calculate the unsteady cases at two flows of 0.2Qd and 1.0 Qd. The results show obvious differences occur between the two conditions in the amplitude and frequency of pressure pulsation mixing at the same measuring points. At the flow of 0.2Qd, the maximum peak-to-peak value of pressure pulsation is four times higher than that at 1.0 Qd, indicating highly uneven flow patterns in the channel. In unsteady cases, the dynamic and static interference, usually causing low-frequency pulsation, will greatly enhance the 3fn and 5fn pulsation amplitudes of its principal frequencies. At the measuring points closer to the impeller and guide vanes, these increases are more significant, and the flows are more sensitive to the interference, with the pulsation amplitudes of 3-5 times that of 1.0 Qd; the influence in the runner and guide vane section is stronger. Thus, pressure fluctuation at the lower flow is more severe, and the impeller and guide vane sections are more affected by the static and dynamic interference, so that severe flow separation and more vortices occur in the guide vane channels.

Key words: axial flow pump, small flow conditions, pressure pulsation, internal flow characteristics, numerical simulation