Circumferential Propagation of Tip Leakage Flow Unsteadiness for a Low-Speed Axial Compressor

Full-annulus three-dimensional unsteady numerical simulations were conducted for a low-speed isolated axialcompressor rotor, intending to identify the behavior of self-induced unsteady tip leakage flow within multi-bladepassages. There is a critical mass flow rate near stall point, below it, the self-induced unsteadiness of tip leakageflow can propagate circumferentially and thus initiates two circumferential waves. Otherwise, the self-inducedunsteady tip leakage flow oscillates synchronously in each single blade passage. The major findings are: 1) whilethe self-induced unsteadiness of tip leakage flow is a single-passage phenomenon, there exist phase shifts amongblade passages in multi-passage environments then evolving into the first short length wave propagating at abouttwo times of rotor rotation speed after the transient period ends; and 2) the time traces of the pseudo sensors locatedon the rotor blade tips reveal another much longer length-scale wave modulated with the first wave due tophase shift propagating at about half of rotor rotation speed. Features of the short and long length-scale circumferentialwaves are similar to those of rotating instability and modal wave, respectively.

Unsteady flow structures in the tip region for a centrifugal compressor impeller before rotating stall

To get an insight into the occurrence and the mechanism of flow unsteadiness in the tip region of centrifugal compressor impellers, the flow in Krain’s impeller is investigated by using both steady and unsteady RANS solver techniques. It is found that the flow unsteadiness on the pressure side is much stronger than that on the suction side. The periodical frequency of the unsteady flow is around half of the blade passing frequency. The originating mechanism of the flow unsteadiness is illustrated with the time-dependent tip leakage flow and blade loading at the tip region. Due to the blockage caused by the joint effects of broken-downed tip leakage vortex, separated fluids and tip leakage flow at downstream, a low pressure region is formed on the pressure side, consequently the blade loadings is altered. In turn, the changed blade loadings will alter the intensity of tip leakage flow. Such alternative behavior finally results in the periodic process. By comparing the calculated flow field in the cases of single-passage and four-passage models, it is confirmed that the investigated flow unsteadiness is confined in each single passage, as no phase differences are found in the model of four passages. This is different from the situation in axial compressor when the rotating instability is encountered. The flow unsteadiness only occurs at the working conditions with small mass flow rates, and the oscillation intensity will be enhanced with the decrease of mass flow rate. When the mass flow rate is too small, the flow unsteadiness in a single passage may trigger rotating stall, as the disturbance propagates in the circumferential direction.