Unsteady Flow and Transition Phenomena in an Axial Flow Compressor

The unsteady mid-span aerodynamics of an outlet stator row in a 1.5-stage low-speed axial compressor is investigated experimentally and numerically. Two stator blade rows with characteristically dierent blade proles are studied: one of standard British C4 section and a controlled diusion (CD) blade with a circular arc leading edge prole. A turbulence grid placed at compressor inlet is used to generate turbulence characteristics similar to those occurring in an embedded stage in a multi-stage axial compressor. The stator inlet ow is studied using hot-wire anemometry and compared with previous measurements made in the natural low inlet turbulence conguration of the research compressor. Increased turbulence level enhances the dispersion of inlet guide vane (IGV) wakes. This modies the interaction between IGV and rotor blade wakes, leading to a more circumferentially uniform ow eld at entry to the stator with signicantly lower periodic unsteadiness. Laminar-turbulent transition on a C4 stator blade is studied using an array of surface-mounted hot-lm sensors. Comparisons with measurements made at low inlet turbulence show that the increased inlet turbulence level reduces the extent of periodic transitional ow on the stator blade surface. The blade element behaviour ow behaviour at high inlet turbulence closely resembles the low inlet turbulence case with the stator immersed in IGV wake turbulence. The circular arc leading edge prole of the CD stator produces rapid acceleration and deceleration at the stator leading edge. The in uence of this velocity spike on the stator boundary layer development and transitional ow behaviour is studied using an array of surface mounted hot-lm sensors. A region of favourable pressure gradient on the suction surface following the leading edge spike has a stabilising eect on the boundary layer, with a large region of ow in a laminar or transitional state. Turbulent spots and instability phenomena in this region are examined for convection speed, growth rate and evidence of relaminarisation. In contrast, the ow on the pressure surface becomes turbulent near the leading edge. The study shows that compressor blade leading edge proles have a major in uence on boundary layer development over the whole surface. The eect of upstream rotor wake passing on the stability of stator blade boundary layers is examined. The unsteady quasi-three dimensional ow solver, UNSFLO, is used to interpret surface hot-lm data and unsteady laminar ow behaviour at the leading edge of both C4 and CD stators. Rotor wake chopping is found to stabilise the pressure surface boundary layer and destabilise the suction surface boundary layer. Examination of hot-lm data points to the leading edge as the principal receptivity site for transitional ow phenomena occurring on the suction surface of both the C4 and CD blading.