Numerical Investigation On Rotating Stall In A Low-Speed Axial Compressor

To improve the performance and reliability of aero-engine and other gas turbines, it is of great importance that the mechanism of compressor rotating stall is well understood, and furthermore, successful control of this aerodynamic instability realized. The work described in this dissertation is aimed towards providing an improved understanding of the internal aerodynamics mechanism of blade passage flow during rotating stall and its inception, making use of the numerical simulation method rather than experimental approach.In addition to the high demand of computing capacity and the need of appropriate and relatively accurate CFD method and program, the main difficulties of the study are the choice of simulation algorithms, the setting of various restricting conditions, and the effective technique to analyze the calculated results, so that the complex flow physics of the phenomena can be simulated. Within the frame of unsteady Navier-Stokes equations, a throttle model capable of simulating the transient process of unstable flow is adopted, and the unsteady compressor system characteristics and various computational domains are extensively studied, the latter including the circumferential (number of blades) and axial (single stage or isolated rotor and their upstream and downstream regions) domains. Thus, the rotating stall in a single stage and an isolated rotor of a three-stage low-speed axial compressor is simulated by two- and three-dimensional numerical analyses respectively. In analyzing the computational results, the numerical probes analogous to those used in experiments are employed, which not only can reproduce the development of stall inception process, but also is suitable for acquiring the dynamic features of blade loading during the stalling process.As for the simulation of stall inception characteristics, without imposing external disturbances, the type of stall inception from the first stage two-dimensional computation is modal wave, whereas in the isolated rotor or the stage with enlarged rotor-stator axial gap, the rotating stall is initiated by spike disturbances. The obtained difference between these two types of stall inception shows that in the stage environment the short length scale disturbances in a rotor will be suppressed by its neighboring stator. It is more favorable for spike disturbances to grow by removing the stator or increasing the axial gap. Therefore the three-dimensional computation for the isolated rotor in the compressor first stage gives spike stall inception as well. The three-dimensional calculations with tip clearance also indicate that the development of clearance leakage vortex plays a dominant role in the onset of spike inception.In analyzing the internal flow patterns during the stalling process, for the case ofmodal wave stall inception simulated by the single stage two-dimensional computation, along with the throttling of the compressor, the separated regions first emerge in suction surfaces of all rotor blades simultaneously, then circumferential asymmetries gradually build up and continuously evolve into one stall cell with its circumferential width of four to five blade passages. However, in solving the three-dimensional flow in the isolated rotor, from nearly the same flow pattern in each passage, the clearance leakage vortex moves upstream and to the adjacent blade pressure surface, and within few rotor revolution periods, the growth of the leakage vortices quickly concentrate into two to three passages, which shows the accelerating process of spike inception to the establishment of stall cell. In this process, the stall cell has experienced three-dimensional flow structure changes of radial migration, axial expansion and circumferential propagation.In the two- and three-dimensional computation mentioned above, to comprehend the internal aerodynamics mechanism of rotating stall in blade passages, the transient flow details of recirculation regions, blocked regions, separation vortex and tip leakage vortex are described through analyzing pressure cont...