Unsteady Flow Mechanism of Non-Synchronized Disturbance During Gas Turbine Compressor Start-Up

In this paper, unstable unsteady flow phenomenon that occurs during startup in the compressor of a gas turbine for power generation are investigated in detail. After the rotating stall disappears during startup, a large-scale recirculating flow occurs very rarely at the tip of the 1st rotor over the entire circumferential direction. This stage mismatch causes a large-scale recirculating flow on the casing wall and thus, increases the gas temperature of the tip. This phenomenon ends as the rotational speed increases, and the condition stabilizes at the rated rotational speed, so basically it is not considered to be problematic. However, under certain IGV-VV schedule conditions, there is a rare risk of Non-Synchronized Disturbance (NSD) flow even at the rated speed. Based on these cases, the differences in aerodynamic unsteady flow mechanisms between large-scale recirculating flow and NSD flow were investigated using full annulus unsteady numerical simulations. Although it has been investigated in low-speed rig tests of axial and centrifugal compressors by researchers around the world, there are almost no reported cases in actual gas turbine compressors for power generation, and valuable knowledge about these phenomena was obtained in this paper. In the case of the large-scale recirculating flow with a deeply closed IGV at 70% part-speed, the large recirculating flow occurs in the range over 75% span from hub (%Ht) to the tip from the leading edge to about 80% axial chord upstream. On the other hand, if the IGV is half closed than the design opening angle at 100% speed, the reverse flow region occurs locally on the tip side near the leading edge of 95%Ht or higher in the 1st rotor tip. It was found that the rotation of the local reverse flow region in the circumferential direction causes NSD with a peak around 40%–60% of the 1st BPF. The NSD is caused by Kelvin-Helmholtz instability, which occurs when the shear layer between the leading-edge separation and the main flow oscillates. The mechanism of the unsteady and unstable flow phenomenon which occurred at the start of the gas turbine compressor was clarified by the comparison analysis between the full annulus unsteady numerical simulation and the actual measurement. As a result, it is possible to clarify the direction of improvement measures and contribute to the gas turbine and turbomachinery communities.