Investigation of the Stability and Surge Limit in an Industrial Centrifugal Compressor With Variable Inlet Guide Vanes

Abstract The instability phenomena of an industrial centrifugal compressor are experimentally investigated under various pre-swirl settings and rotational speeds, ranging from subsonic to transonic inflow. The compressor under consideration was installed in an open-loop test rig and consists of variable inlet guide vanes (IGV), a non-splittered impeller, a vaneless diffuser and a volute. The IGV allows pre-swirl angles of −15° (counter-rotating pre-swirl) to +60° (co-rotating pre-swirl). The compressor map measurement in the off-design region at different IGV pre-swirl angles shows that co-rotating pre-swirl facilitates rotating stall. A nominal non-integer stall cell number of 2.4 to 4.6 and a rotational speed of 10 to 25% of the impeller rotational speed characterize the rotating stall phenomena observed. In contrast, mild surge is observed on speed lines without pre-swirl and with counter-rotating pre-swirl at a low rotational speed. At some operating points rotating stall occurs in addition to the mild surge, whereby the rotating stall shows a broadband signal rather than a distinct frequency. The surge limit mass flow rate of the speed lines without pre-swirl stagnates at an approximate isentropic relative Mach number at the leading edge of 0.69 and then rapidly increases at a Mach number of 0.78. As such, the operating range of the compressor stage significantly decreases and the stability and surge limit coincide. In summary, the results presented in this paper show that the IGV has a major impact on the type of instability preceding the deep surge of the compressor stage. The surge limit does not follow any obvious pattern and therefore its prediction during the pre-design remains challenging.