Numerical Investigation On Tip Clearance Flow In Compressor

Tip clearance flow has profound effects on the performance and stability of the compressor. To improve the performance and reliability of aero-engine and other gas turbines, it is of great importance that the mechanism of the compressor tip clearance flow is well understood, and furthermore, successful control of it realized. In the past fifty years, numerous studies were mainly focused on the mechanism of steady tip clearance flow and its impacts on the compressor performance. Recently, the unsteady characteristics of tip clearance flow, which appears in some flow conditions, was discovered and its influence to the compressor performance was recognized. However, the detailed transient flow field, the originating mechanism and the potential application of unsteady tip clearance flow are unclear or not enough understood to date. Therefore, this dissertation, making use of the numerical simulation, is aimed towards providing a revelation of the transient flow field, a new understanding of the originating mechanism of unsteady tip clearance flow, and a possible approach to improve compressor performance through utilizing the unsteadiness of tip clearance flow.This dissertation is organized as follows: Part one focuses on the reliability assessment of several numerical methods. Part two concerns the analysis of steady tip clearance flow behavior. Part three concentrates on the unsteady tip clearance flow behavior, and the effects of some geometric and aerodynamic parameters on such behavior are evaluated. Part four discusses the originating mechanism of unsteady tip clearance flow. Part five explores the possibility of the improvement of the compressor performance by taking advantage of the unsteady tip clearance flow.Results of the reliability assessment indicate that adequate spanwise grid resolution in the blade tip and endwall region is the key to achieve accurate clearance flow predictions. Our studies from the steady numerical simulations show that the intensifying of tip leakage vortex and the weakening of the tip passage vortex can be achieved either by the relative motion of the casing wall or by the increase of the tip clearance gap size.Unsteady tip clearance flow in an isolated axial compressor rotor and a two-stage axial compressor is simulated respectively by solving three-dimensional time-accurate Reynolds-averaged Navier-Stokes equations. The result shows that the periodic unsteadiness of the tip clearance flow occurs with approximately 50% of blade passing frequency when the flow coefficients reduced to 0.586 at the design tip clearance size. Decreasing the flow coefficients or increasing the tip clearance size, and/or the thickness of inlet boundary layer and/or the rotor rotation speed wouldresult in the enhanced unsteadiness and decreased frequency of the tip clearance flow, whereas increasing the upstream and downstream stator interactions, which can also intensify the unsteadiness of the tip clearance flow, has no effects on its frequency. For various compressor geometries and flow conditions, the dimensionless frequencies defined by the natural frequency of the tip clearance flow, the inlet relative velocity and the chord length in the rotor tip region are all in the range of 0.60-0.76.An originating flow mechanism is proposed to explain the unsteady flow phenomenon that this periodic unsteadiness of tip clearance flow is a result of dynamic balance, as opposed to static balance, between two counter-acting driving "forces". One such "force" is the aerodynamic loading of the blades, i.e. the pressure difference across the pressure and suction sides of the compressor blades created by the main through flow. Its counter-acting "force" is the unloading of the blades, i.e. the reduction of the pressure difference caused by the tip leakage flow. If these two "forces" are strong enough to push the tip leakage vortex to interact with the pressure side of the neighboring rotor blade, the static balance between the two "forces" will be broken and unsteady oscillation forms.Improved compressor performance can be achieved through the control of tip clearance flow by setting on the blade tip surface an unsteady excitation with alternate air injected and aspirated. Given the equal frequency of the unsteady excitation and the tip clearance flow, increasing the amplitude of the unsteady excitation can lead to improved compressor overall performance. At the same unsteady excitation amplitude, the compressor has an approximately monotonic increase in the overall performance with the increase of the unsteady excitation frequency, and the improved compressor performance can keep approximately constant in a wider range of the unsteady excitation frequency.