To study the characteristics of internal flow in the gap of interstage seal in supercritical boi-ler feed pumps, a few interstage seal models with various gap sizes were established and the fluid flows in these seals were simulated numerically. The pressure and velocity distributions in the axial and circumferential directions, and the pressure variation along the axial direction were calculated under a pulse inlet pressure. The results show that the fluid hydrostatic pressure and velocity have three variation stages through the seal in the axial direction, namely sharp drop, ascendant and linearly descen-dent stages. The greater the gap′s size is, the smaller the static pressure is, and the faster the velocity is. The static pressure distribution is not uniform in the circular cross-sections and presents a certain periodicity in the circumferential direction. In the annular cross-sections the high-velocity region appears near the rotating inner wall, but the low-velocity region occurs near the stationary outer wall. In addition, under a pulse inlet pressure, the pressure pulsating frequency in the middle and at the outlet of the seal is the same as the frequency at the inlet. The tail pressure is mainly determined by the outlet pressure, thus it fluctuates only within a certain amplitude.
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