Christian Aalburg

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

Extension of the Stator Vane Upstream Across the 180° Bend for a Multistage Radial Compressor Stage

The design, analysis and optimization of a new stator concept for multistage centrifugal compressors using numerical methods is presented. The first objective was to further improve the performance of a well-optimized stage with a short vaneless diffuser, see Aalburg et al [1]. The second objective was to achieve a significant increase in the flow turning in the stator part. In order to achieve these goals an extension of the return channel vane upstream, over the U-turn bend, was considered. This design poses challenges that are quite different from those encountered for a conventional design. For example, a conventional vane angle distribution leads to lean angles across the bend that are not feasible from a manufacturing and aerodynamic perspective. In addition, conventional design tools for geometry generation were found to have limited applicability for this concept. To address these issues a geometry generator was developed that facilitated the design of three-dimensional across-the-bend type vanes with unconventional vane angle distributions. The geometry generator was based on an analytical design procedure similar to that outlined by Veress and Braembussche [2]. This procedure allows a desired loading distribution to be specified. In this paper the vane concept will be introduced, the development of the geometry generator will be outlined and the effect of varying design parameters will be considered. An optimized design will then be presented that outperformed the reference conventional design in terms of efficiency by up to one point across the operating range. This improvement was achieved despite a significantly higher vane loading.Copyright

INTRODUCTION OF CIRCUMFERENTIALLY NON-UNIFORM VARIABLE GUIDE VANES IN THE INLET PLENUM OF A CENTRIFUGAL COMPRESSOR FOR MINIMUM LOSSES AND FLOW DISTORTION

In the Oil & Gas industry, large variations in flow rates are often encountered which require compression trains with a wide operating range. If the stable operating range at constant speed is insufficient, variable speed drivers can be used to meet the requirements. Alternatively, variable guide vanes (IGVs) can be introduced into the inlet plenum to provide pre- or counter-swirl to the first stage impeller, possibly eliminating the need for variable speed. This paper presents the development and validation of circumferentially non-uniform IGVs that were specifically designed to provide maximum angle variation at minimum losses and flow distortion for the downstream impeller. This includes the comparison of three concepts: a baseline design based on circumferentially uniform and symmetric profiles and two circumferentially non-uniform concepts based on uniquely cambered airfoils at each circumferential position and a multi airfoil configuration consisting of a uniquely cambered fixed part and a movable part. The idea behind the circumferentially non-uniform designs was to take into account non-symmetric flow features inside the plenum and a bias towards large preswirl angles rather than counter-swirl during practical operation. The designs were carried out by CFD and first tested in a steady, full-annulus cascade in order to quantify pressure losses and flow quality at the inlet to the impeller at different IGV setting angles (ranging from −20° to +60°) and flow rates. Subsequently, the designs were mounted in front of a typical Oil & Gas impeller on a high speed rotating rig in order to determine the impact of flow distortion on the impeller performance. The results show that pressure losses in the inlet plenum could be reduced by up to 40% with the circumferentially non-uniform designs over the symmetric baseline configuration. Furthermore, a significant reduction in circumferential distortion could be achieved with the circumferentially non-uniform designs. The resulting improvement in impeller performance contributed approx. 40% to the overall efficiency gains for inlet plenum and impeller combined.© 2015 ASME

Annular Cascade for Radial Compressor Development

A full-annulus cascade for radial compressor stator development has been designed and commissioned. The cascade has been developed for the rapid screening of novel stator concepts to facilitate risk mitigation in the early design phase and the validation/calibration of numerical predictions. The rig consists of two main parts. The first part is comprised of an exchangeable set of stationary preswirl vanes that have been designed to mimic discrete points on the operating characteristic of the impeller. The second part consists of a diffuser, bend and return channel with return channel vanes that can also be quickly exchanged. All exchangeable parts are manufactured by rapid prototyping, allowing rapid turnaround times from aerodynamic design to full validation. This is achieved at a significantly lower cost than that of a full rotating test. This investigation summarizes the experimental results and numerical predictions of two test rigs that were designed to study the effect of diffusion ratio, i.e. the ratio of the maximum outer diameter at the top of the bend to the exit of the impeller, on stator performance. To further investigate the sensitivity of the aerodynamic performance to different flow conditions, metal gauzes were positioned immediately downstream of the trailing edges of the preswirl vanes. This allowed the modification of angle and pressure distributions in the diffuser and bend as well as the setting of different turbulence conditions (intensity and length scale) in the downstream sections.© 2011 ASME