The paper describes the flow mechanism of the rotating stall inception in a multi-stage axial flow compressor for an actual gas turbine. Large-scale numerical simulations have been conducted. The compressor investigated is a test rig compressor which was used for development of the industrial gas turbine, Kawasaki L30A. While the compressor consists of 14 stages, the front half stages of the compressor were analyzed in the present study. According to the test data, it is considered that the 5th or 6th stage is the one most suspected of leading to the stall.
In order to capture precise flow physics that could happen at stall inception, a computational mesh was made dense, giving at least several million cells to each passage. It amounted to about two billion cells for the first 7 stages (three hundred million cells in each stage). Since the mesh was still not enough for the large-eddy simulation (LES), the detached-eddy simulation (DES) was employed. In the DES, a flow field is calculated by LES except near-wall and near-wall turbulent eddies are modeled by RANS. The computational resource required for such large-scale simulation was still quite large, so the computations were conducted on the K computer (RIKEN AICS in Japan). Unsteady flow phenomena at the stall inception were analyzed by using data mining techniques such as vortex identification and limiting streamline drawing with the LIC (line integral convolution) method.
The present compressor has stall started from the separation on the hub side instead of the commonly observed leading-edge separation near the tip. The flow phenomenon first observed in the stalling process is the hub corner separation, which appears in some passage of the 6th stator when approaching the stall point. This hub corner separation expands with time, and eventually leads to the leading-edge separation on the hub side for the stator. Once the leading-edge separation happens, it rapidly develops into the rotating stall, causing another leading-edge separation for the neighboring blade in sequence. Finally, the rotating stall spreads to the upstream and downstream bladerows due to its large blockage effect.