The relationship between the growth of the stall cell and variation in the surge behavior was experimentally investigated. The aim of this study was to reveal the effect of the stall cell on the surge behavior from the viewpoint of the inner flow structure. In the experiment, the unsteady compressor characteristics during the surge and rotating stall were obtained by using a precision pressure transducer and a one-dimensional single hotwire anemometer. Under the coexisting states of surge and rotating stall, various surge behaviors were observed by throttling the mass flow rate. When the flow rate was set such that the surge behavior switched, an irregular surge was observed. During the irregular cycle, two different cycles were selected randomly corresponding to the stall behavior. When the amplitude of the plenum pressure is relatively large among the measurement results, the absolute value of the time-change rate in the flow coefficient and the static pressure-rise coefficient tend to be high. This shows that the flow field during stable operation near the peak point of the unsteady characteristics changes rapidly. In this case, an auto-correlation function of the wall-pressure fluctuation data showed that the stall inception of the compressor was induced earlier in the large cycle compared with the case of the top cycle. When studying the growth of the stall cell during the stalling process of the large cycle, the wall-pressure fluctuation data showed that the stall cell rapidly grew by gathering more than one spike-type disturbance into one stall cell. In this case, the stall cell fully expanded along the circumferential direction and developed into a deep stall. Therefore, the key factors that determine the surge behavior are the sudden change in the flow field near the peak point of the unsteady characteristics and the rapid growth in the stall cell during the stalling process.
In this study, we aim to elucidate the effect of a forward-swept rotor on the stall margin and flow field at the distorted inflow conditions. The rig in this research is a low-speed, single-stage axial compressor, which has two types of rotor blades: the radially stacked blade (Radial) and the forward-swept blade (Sweep). The distortion screen that circumferentially generates distorted inflow is located upstream of the rotor. The stall margin of Sweep was found to be larger than that of Radial. Sweep was considered to improve the flow fields at the distorted inflow conditions. From the results of the study, it was observed that Sweep suppressed the circumferential expansion of the high-load regions and the spiketype disturbances generated at the distorted sector. Therefore, Sweep enlarged the stall margin more than to Radial.
Interaction between surge behavior and internal flow field under coexisting phenomena of surge and rotating stall was experimentally investigated. In the experiment, the tank pressure of the compressor during surge was measured to detect the effect of the back-pressure fluctuation on the change in the internal flow field. Furthermore, the rotating stall in the compressor was investigated to define the influence of an unsteady internal flow field change on the surge behavior. From the tank pressure measurements, the amplitude of the tank pressure fluctuation was found to vary depending on the cycle. A larger maximal value for the tank pressure fluctuation led to a higher flow rate where the stall inception occurred. This difference in the flow rate indicated that the stall was induced by a severe adverse pressure gradient in the compressor. Then, the absolute rate of change in the flow coefficient was increased by both a large decrease in the compressor back pressure and performance degradation from stalling. In a case where the rate of decline in the flow rate was large, the scale of the stall cell developed up to a deep stall according to the movement of the operating point. Thus, a large trajectory for the surge cycle was selected, where the unsteady operating point went through the deep stall region. This development in the scale of the stall cell suggested to be influenced by the instability of the inner flow field caused by the rapid change in the flow rate.
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