Adoption of a tandem bladed rotor configuration brings special flow features at the exit compared to the conventional rotor. For tandem bladed rotor, there is the presence of strong dual-tip leakage flow, atypical exit flow angle distributions, corner blade separations leading to thicker dual wakes at the exit of the rotor to name a few. This makes the aerodynamic design of downstream stator more challenging in terms of overall performance as well as operational stability. The modern compressor requisite of being lighter and cost-efficient needs to be taken care of both aerodynamic and mechanical requirements. To overcome all these challenges, the cantilever type stator (without hub rotation) has been chosen and been analyzed for the present study. The effects of different hub gap sizes of the cantilever stator in combination with the tandem bladed axial compressor stage are investigated in order to explore passive flow control mechanism near the hub. The goal of the work is to get further insights into the aerodynamic aspects of flow using a detailed flow field analysis.
The numerical study was performed using ANSYS TurboGrid® for mesh generation and the commercial package ANSYS CFX® 18.0 was used as solver for steady-state simulation. Stationary hub boundary conditions have been employed for the stator in all 3 cases [baseline, 1% and 2% (of span) part clearance].
For no clearance case, the regions of momentum deficit were observed in the vicinity of the hub endwall and suction surface of the stator. The region keeps growing along both streamwise and spanwise direction as a low momentum bubble is formed near trailing edge. This low momentum bubble seems to be transported along the span and moved more towards the suction surface. The solution strategy explored to mitigate the effect of hub corner separation by adapting hub clearance. The role played by secondary flow in feeding the low momentum flow along the span is seen to be moderated by the high momentum leakage flow from the pressure side. The hub leakage flow from the blade pressure side reenergized the low momentum fluid on the suction side refraining it to travel along the span and mitigate its effect by suppressing the separation tendency near end wall region. The formation of large size bubble gets reduced in overall size both in the circumferential and span-wise direction. This phenomenon compels the low momentum flow to pass along the low span region.
Numerically obtained results provide an insightful mechanism of the interaction of secondary flow structures and the influence of hub clearance flow. Hub corner stall, which is the consequence of low momentum fluid sweeping across the blade passage near the end wall got wiped out in the presence of hub clearance. This phenomenon diminishes the extent and overall effect of the hub corner stall. The interaction of hub leakage vortex and passage vortex leads to mitigation of overall secondary flow adverse effects. As a result, performance improvement at design flow conditions have been elucidated by implementation of cantilever stator. The peak pressure operation is dominated by mid-span flow complexities and as a result cantilevered stator doesn’t show much improvements. Nevertheless, the improvements in design point operating conditions do justify the study for gaining physical insights.