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Although the energy change field in the centrifugal pump has been investigated under quasi-steady conditions (QSC), equivalent information is not yet known during the rapid starting period (RSP). A centrifugal pump loop system is constructed to investigate the energy change field in the centrifugal pump during RSP. The RSP is selected as a linear rotational speed from 0 rev/min to 2900 rev/min (design rotational speed) and a constant valve opening of 0.569. Results show that the flow rate lags behind the pump head value with the linear increase in rotational speed. The large values of partial derivations of mechanical energy in normal and tangential components are mainly concentrated at the impeller outlet, whose position is insensitive to rotational speed. The region of dominant energy loss is negatively correlated with rotational speed, and an opposite phenomenon is observed in the region of dominant energy increase. With the rotational speed increasing, the mean energy gradient function in the pump impeller and pump volute gradually increases, and the slope of the former is less than that of the latter. After reaching the design rotational speed, the energy change field gradually approaches that under quasi-steady conditions. [ABSTRACT FROM AUTHOR]