THE AMERICAN SOCIETY OF MECHANICAL ENGINEERS

In a centrifugal compressor adjustable inlet guide vanes (IGV) in front of the impeller are used to regulate the pressure ratio and the mass flow. The stationary measurement of the velocity profile in front of the impeller with different angles of the IGV displays shock losses at the inlet edge of blade of the impeller. In the partial-load region (e.g. partial-load efficiency) the radial distribution of the flow influences considerably the performance of the impeller. The tested compressor consists of an adjustable IGV with straight vanes, a shrouded impeller and a vaneless, parallel diffuser. In the first measurement location, behind the IGV, total pressure, static pressure and flow angle were measured with a 5hole cylinder probe. In the second measurement location, in front of the impeller, the measurement of the total pressure was carried out with a Kiel probe and the flow angle with a Cobra probe accordingly the static wall pressure was measured. Taking into consideration the fundamental thermodynamical equations it was possible to determine the velocity profiles because of the measured distributions of the flow angle in these two measurement locations. For different angles of the IGV and with various mass flows the distributions of the deflection defect behind the IGV are described. Starting with the measured distributions of the flow in front of the impeller the flow angles at the impeller inlet are calculated and the distributions of the incidence angle at the impeller inlet are figured out. Nomenclature Symbol Units Meanings b [m] width c [m/s] absolute velocity coast [-] constant D [m] diameter E [-] measurement location in front of the impeller I [m] length L [-] measurement location behind the Inlet Guide Vane Symbol Units Meanines p [Pa] pressure, static r [m] radius s [m] length of profile t [-] blade pitch u [m/s] circumferential velocity a [0] absolute blade angle a [0] absolute flow angle [°] relative blade angle [°] relative flow angle S [0] incidence angle Si [-] inlet diameter ratio Act [0] deflection defect 11 [%] efficiency v [-] hub-to-tip ratio a [-] pressure ratio (P [0] angle in a segment (in peripheral direction) 0 [-] mass flow (flow coefficient) Subscripts Symbol Meanines 0 suction pipe, in front of the holder of the blast pipe 1 impeller inlet 2 impeller outlet 7 pressure pipe a outer ax axial bez reference point for max. efficiency with 100% mass flow IGV Inlet Guide Vane L measurement location behind the impeller s impeller blade s isentropic s static conditions t, tot stagnation conditions u circumferential direction z z-direction Presented at the International Gas Turbine & Aeroengine Congress & Exhibition Stockholm, Sweden — June 2-June 5, 1998 Downloaded From: http://proceedings.asmedigitalcollection. asme.org/ on 07/22/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use ml Fuser 5-hole cylinder probe scroll Introduction Matching of compressor performance to different operating conditions is possible by means of e.g. adjustable inlet guide vanes (IGV), Balje (1981). Past investigations of centrifugal compressors with an IGV and a diffuser clearly demonstrate the way in which the latter alter the compressor performance map as reported by Wallace et al. (1975), Whitfield et al. (1976), Simon et al. (1986), Rodgers (1990), Harada (1996) and Rodgers (1996). In order to assess the flow in an axial suction pipe of a centrifugal compressor with an IGV it is necessary to investigate the flow conditions at this location. Usable measured flow data are hardly available in the literature for the inlet region of centrifugal compressors with an IGV. This paper deals with the experimental determination of the flow behind an IGV. The flow field inside the IGV was not measured because the flow distribution behind the IGV was the topic of these measurements. By means of a second plane of measurement in front of the impeller it was also possible to determine the flow evolution and energy conversion processes between the IGV and the impeller. Different mass flows between the surge line and the choke margin in the centrifugal compressor performance map were also considered in order to determine the effects of the flow velocity in relation to the guide vanes. Variable angular settings of the IGV (max. angle of the IGV is 60°) with large whirl changes generate complex flow processes. The complexity of the flow results as an example from the vorticities after the 60° angle of the IGV. CDF computations for this region with big IGV-angles are quite difficult if there are no supporting values from stationary flow measurements. The application of similarity mechanics involving heuristics, on the other hand, offers a more promising design concept for centrifugal compressors. Stationary flow measurements are necessary for both purposes. As the impeller inlet flow is also a decisive factor governing the efficiency of the overall compressor stage, conversion of the flow distribution at the plane of measurement E in front of the impeller is carried out in order to obtain the flow distribution in plane I at the impeller inlet (see Fig. 1). This conversion is necessary due to a change in the flow characteristics on moving towards the impeller inlet, Kammer (1984). In order to compute the flow characteristics at the impeller inlet, use is made of relationships based on stream filament theory. Once the flow angle at the impeller inlet has been determined it is then possible to compute the incidence angle. The incidence angle provides information on the nature of the flow entering the impeller i.e. whether the impeller inlet flow has positive or negative incidence and how the part-load and overload mass flows affect the impeller inlet flow. Test compressor The experimental investigations were carried out at the Institute of Turbomachinery on a centrifugal compressor test rig supplied by the German Babcock-Borsig Company. Fig. I shows a meridional section of the test compressor drawn to scale. The IGV has 11 vanes and the impeller inlet flow is axial. The plane vanes are slightly profiled, likely as a NACA profile. The ratio of the maximum vane thickness to the corresponding profile length is slightly above 0.1 and the pitch ratio t/s = 0.64. The IGV deployed in the tests is shown in Fig. 2. The outer profile of the intake channel (ro = 0.99 m) narrows beyond the centerline of the IGV. Both the outer and inner walls of the intake channel are specially highlighted in the diagrams depicting flow measureFig.1 Test compressor with measurement location Fig.2 Inlet Guide Vane ments. The inner wall marks the boundary of the vaneless circular area in the middle of the IGV, which is blocked by a blast pipe secured by a holder in front of the IGV. The total and static pressures as well as the flow angles were recorded in the measurement plane L behind the IGV by means of a 5-hole cylinder probe. The housing disc accommodating the probe traversing device as well as the outer profile of the intake channel can be rotated in the peripheral direction. Owing to the restricted space available in the proximity of the impeller a 2-hole Cobra probe and a total pressure Kiel probe were used alternately in the plane of measurement E in front of the impeller. The air flow at the impeller inlet is almost axial. The impeller is designed with a front shroud and has 18 backswept blades. The hub-toshroud ratio v is 0.293, the inlet diameter ratio is 4; = 0.583, the length ratio is 1,21D2 = 0.234 and the width ratio is b2/D2 = 0.065. The impeller with the vaneless diffuser is shown in Fig. 3. The impeller tip diameter is 0.252 m. The air is deflected radially in the impeller and finally flows through a vaneless diffuser into the scroll. Downloaded From: http://proceedings.asmedigitalcollection. asme.org/ on 07/22/2018 Terms of Use: http://www.asme.org/about-asme/terms-of-use