Influence of Vaned Diffuser on Dangerous Blade Vibration Due to Blade Flow Interactions in a Centrifugal Compressor

Influence of diffuser vane position on dangerous blade vibration due to blade flow interactions at part load operating condition was investigated in a centrifugal compressor. By means of some measurement and analysis techniques a mechanism of interaction between blade vibration and circumferential flow condition in the impeller with vaned diffuser was determined, which is a cause of dangerous blade vibration in resonance. Experiments for the compressor with different radial diffuser positions have been performed to reduce the effect of the blade flow interaction and the dangerous blade vibration. A quantitative estimate of the influence of vaneless space length between diffuser and impeller on the blade flow interaction was made. An optimal diffuser position with lower blade vibration and higher compressor characteristics was determined, INTRODUCTION The experimental results in the past (Haupt et at, 1989) proved dangerous blade vibration in the resonance zone between 4th engine-oMer and I. blade natural frequency on a centrifugal compressor with a vaned diffuser in the part load operation region. In the mechanism of this blade vibration the flow in the impeller plays a major role, which is characterized by considerable reverse flow near shroud from the impeller exit to the impeller inlet These flow characteristics are controlled by the instantaneous blade inlet angle changing with the strong blade vibration. Since the blades vibrate 4 times every rotation and with the same phase at a fixed location on the circumference, a circumferential pressure profile characterized by 4 periodic waves could be detemaned in the impeller exit and inlet. The blade vibration and circumferential flow condition form an accumulation effect. This is the cause of the intensification of the blade excitation. The measured maximum stress of the blade vibration in resonance represents a case beyond allowable stress limit. However, the dangerous blade vibration in resonance for the compressor with vaneless diffuser cannot be observed. It can be suggested that the diffuser vanes play a considerable role to intensify the effect that the flow condition in impeller passage is controlled by the periodical variation of the blade inlet angle. In this work the Influence of diffuser vane position on the dangerous blade vibration due to blade flow interactions was investigated in detail to select an optimal diffuser vane position with lower blade excitation without losing -energy transfer in the compressor. COMPRESSOR TEST FACILITY Blade vibration and flow measurements were performed on a single stage compressor test rig operating with an open loop configuration as show in figure 1 and 2. The compressor is driven by a 1350 kW dc-motor. The maximum values of the compressor pressure ratio and the mass flow rate are it = 5 and InTed = 10 Ices respectively. The flow enters the compressor in axial direction through a short suction pipe from a setting chamber and passes through the impeller and a long radial diffuser. The flow is collected downstream in an annular collecting chamber and exits the Fig. 1 Centrifugal compressor test facility for the investigation of blade vibration with the installation of circumferential static pressure measurements Presented at the International Gas Turbine and Aeroengine Congress and Exposition Houston, Texas June 54 1993 Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 03/22/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use compressor via the outlet tube, which is tangential to the machine. The mass flow rate of the compressor is regulated by a throttle valve located behind the compressor. The unshrouded impeller of the test compressor used for these investigations is shown in figure 3. The impeller has characteristically thin blade with approximately I mm thickness at the impeller inlet and 4 mm at the outlet. The impeller has an outer diameter of 400 mm and 28 radial ended blades with every second blade cut back at the inlet. Because of the working error by milling the impeller, the blade natural frequencies are not same. The L natural frequencies of the blades at static state are shown in table I. The I. natural frequencies of the blade No. land No. 13 are minimum of 782 Hz and maximum of 823 Hz respectively. The difference of the natural frequency for two blades is 41 Hz. The diffuser configuration may be changed to operate as a vaneless diffuser with constant radial flow area or as a varied diffuser with 19 vanes (see figure 2). In these experiments the diffuser vanes were located along the flow line in 4 different radial positions (X. 1.125, 1.15, 1.2 and 1.3) with constant vane inlet angle of 17°. Here is the diameter ratio between the diameters of diffuser inlet leading edge and impeller outlet MEASURING SYSTEM -Steady static pressure measurements Circumferential nonimiformity of the flow in the compressor is an excitation son of blade, which rotates in this variable pressure field. As a result the blade is excited by different NOMENCLATURE Table 1 : first blade natural frequency rotor speed n irpml static state 1230