Low speed axial compressor design and evaluation : High speed representation and endwall flow control studies

Abstract

This Thesis reports the design, build and test of two sets of blading for the Cranfield University low speed research compressor. The first of these was a datum low speed design based on the fourth stage of the DERA high speed research compressor C 147. The emphasis of this datum design was on the high-to-low speed transformation process and the evaluation of such a process through comparing detailed flow measurements from both compressors. Area traverse measurements in both the stationary and rotating frame of reference were taken at Cranfield along with overall performance, blade surface static pressure and flow visualisation measurements. These compare favourably with traverse and performance measurements taken on C147 before commencement of the PhD work. They show that despite the compromises made during the transformation process, due to both geometric and aerodynamic considerations, both the primary and secondary flow features can be successfully reproduced in the low speed environment. The aim of the second design was to improve on the performance of the datum blading through the use of advanced '3D' design concepts such as lean and sweep. The blading used nominally the same blade sections as the datum, and parametric studies were conducted into various lean/sweep configurations to try to optimise the blade performance. The final blade geometry also incorporated leading edge recambering towards the fixed endwalls of both the rotor and stator. The '3D' blading demonstrated a 1.5% increase in efficiency (over the datum blading) at design flow rising to around 3% at near stall along with an improvement in stall margin and pressure rise characteristic. The design work was completed using the TRANSCode flow solver for both the blade-to-blade solutions (used in the SI-S2 datum design calculation) and the fully 3D solutions (for the advanced design and post datum design appraisal). The 3D solutions gave a reasonable representation of the mid-span and main 3D flow features but failed to model the corner and tip clearance flow accurately. An interesting feature of the low speed flowfield was the circumferential variation in total pressure observed at exit from all rotors for both designs. This was not present at high speed and represents one of the main differences between the high and low speed flow. Unsteady modelling of mid- height sections from the first stage indicate that part of this variation is due to the potential interaction of the rotor with the downstream stator while the remainder is due to the wake structure from the upstream stator convecting through the rotor passage. Finally, the implications for a high speed design based on the success of the 3D low speed design are considered.