Michael Bernhard Schmitz

Design and Testing of Multistage Centrifugal Compressors With Small Diffusion Ratios

Two stators of a multistage centrifugal compressor with progressively smaller outer radii have been designed, built, and tested. The aim was to achieve a significant reduction in the outer diameter of the compressor stage without compromising performance. The reduction in size was achieved by reducing the diffusion ratio (outer radius/inner radius) of the vaneless diffuser in two steps. In the first step, the outer diameter of the entire stage was reduced by 8% compared with the baseline design. In the second stage, the outer diameter was reduced by 14%. The outer radius of the smallest design was limited by the impeller exit diameter, which was kept constant, as was the axial length of the stage. The large radius baseline design has been tested on a rotating rig in a 1.5 stage setup. This setup aimed at simulating the multistage behavior by applying a pseudostage upstream of the main stage. The pseudostage consisted of a set of nonrotating preswirl vanes in order to mimic an upstream impeller and was followed by a scaled version of the return channel of the main stage. The experimental database was then used to calibrate a 1D analysis code and 3D–computational fluid dynamics methods for the ensuing design and optimization part. By applying an extensive design-of-experiments, the endwalls as well as the vanes of the stator part were optimized for maximum efficiency and operation range. In order to preserve the multistage performance, the optimization was constrained by keeping the circumferentially averaged spanwise flow profiles at the exit of the smaller radius stages within close limits to the original design. The reduced radius designs were then tested in the same 1.5 stage setup as the baseline design. The results indicate that the reduction in size was feasible without compromising the efficiency and operation range of the stage.

Design, Validation and Application of a Radial Cascade for Centrifugal Compressors

A novel sector test rig has been used to evaluate a new airfoil concept for multistage radial compressors. The test rig is supported by a blow-down facility where the operation conditions are adjusted by controlling mass flow, pressure and temperature. At inlet to the sector test rig itself a set of adjustable inlet guide vanes provide the test vanes with the correct inlet three-dimensional flow-field. The rig is equipped with instrumentation to allow a detailed description of the inlet and outlet conditions, as well as the blade pressure loading. This rig, using rapid prototyped vanes, allows design candidates to be screened quickly and is ideal for conducting an experimental investigation of a design space using a Design-of-Experiments approach. In this paper the rationale for the sector approach is described, the design of the test rig with 3D-CFD methods is outlined and a detailed validation of the rig is presented. For the vane in question detailed investigations of different operation points close to stall are reported, blade pressures and inlet and exit flow profiles are given. Where applicable, measurement data from the sector rig was compared to 3D-CFD calculations of the full annulus multistage configuration, to 3D-CFD calculations of the sector rig itself and to the test results from a 1.5-stage rotating test rig. The measurement data are compared to the CFD predictions and served as a calibration basis for the design tools.Copyright