As the gas turbine becomes smaller and is operated at high altitude, the aerodynamic condition frequently lies at the low Reynolds number. In the present study, three-dimensional computations were performed to understand the effects of the low Reynolds number on the loss characteristics in an axial compressor. The numerical results showed that the performance of the axial compressor like the static pressure rise is reduced by the full-span separation on the suction surface and the boundary layer on the hub, caused by the low Reynolds number. Compared with that at the reference Reynolds number, the total pressure loss at the low Reynolds number was found to be greater from the hub to 85 per cent span and smaller above the 85 per cent span. For a detailed analysis, the total pressure loss was scrutinized through three major loss categories available in the subsonic axial compressor: profile loss, tip leakage loss, and endwall loss.
A three-dimensional computation was conducted to understand effects of low Reynolds numbers on loss characteristics in a transonic axial compressor, Rotor 67. As a gas turbine becomes smaller and it operates at high altitude, the engine frequently operates under low Reynolds number conditions. This study found that large viscosity significantly affects the location and intensity of the passage shock, which moves toward the leading edge and has decreased intensity at low Reynolds number. This change greatly affects performance as well as internal flows, such as pressure distribution on the blade surface, tip leakage flow and separation. The total pressure ratio and adiabatic efficiency both decreased by about 3% with decreasing Reynolds number. At detailed analysis, the total pressure loss was subdivided into four loss categories such as profile loss, tip leakage loss, endwall loss and shock loss.
A 3D computation was conducted to investigate the role of hub-corner-separation on the rotating stall in a low-speed axial compressor. It is generally known that tip leakage flow plays an important role in stall inception. However, not much attention has been paid to the role of hub-corner-separation on the rotating stall although it is a common flow feature in an axial compressor operating near the stall point. During our time-accurate unsteady simulation, we suspected that hub-corner-separation might be a trigger for the rotating stall. After an asymmetric disturbance is initiated at hub-corner-separation, this disturbance is transferred to the tip leakage flows and grows to become an attached stall cell, which adheres to the blade passage and rotates at the same speed as the rotor. When the attached stall cell reaches a critical size, it moves along the blade row and becomes the rotating stall. The rotating speed of the stall cell decreases to 79% of the rotor so the stall cell rotates in the opposite direction to the rotor in the rotating frame.
A three-dimensional numerical simulation was conducted to study an effect of the inlet boundary layer thickness on the rotating stall in an axial compressor. The inlet boundary layer thickness had significant effects on the hub-corner-separation in the junction of the hub and the suction surface. The hub-corner-separation grew significantly for the thick inlet boundary layer as the load was increased, while it was diminished to be indistinguishable from the rotor wake for the thin inlet boundary layer and a new corner-separation was originated near the casing. The difference in the internal flow at the near stall condition also had a large effect on characteristics of the rotating stall, especially the first asymmetric disturbance and the size of the stall cell. While a pre-stall disturbance arises firstly in the hub-corner-separation for the thick inlet boundary layer, an asymmetric disturbance was initially generated in the tip region because of the corner-separation for the thin inlet boundary layer. This disturbance was transferred to the tip leakage flow and grew to be an attached stall cell. When this attached stall cell reached a critical size, it moved along the blade row and became a short-length-scale rotating stall. The size of the stall cell for the thick inlet boundary layer was larger than that for the thin inlet boundary layer. The difference of the stall cell’s size affected the performance of the single rotor, causing large performance drop for the former case but a continuous performance change for the latter case.
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