Return Channel Vanes Research Articles

Detecting Multidimensional Pareto Fronts During the Optimization of a Return Channel Within a Centrifugal Compressor

Abstract We explain the advantages of using big-data methods for evaluating numerical optimization. The investigation focuses on the performance potential of three-dimensional return channel vanes under realistic manufacturing constraints. Based on an analysis of an optimization database, this paper presents a systematic approach for analysis and design guidelines for three-dimensional return channel vanes. To this end, a validated numerical setup was developed on the basis of experimental investigations, followed by a numerical optimization using a genetic algorithm and an artificial neural network. The optimization database was analyzed with a dimension reduction method called t-stochastic neighbor embedding. This method enabled linking geometric design features with physical correlations and, finally, with the objective functions of the optimization. With the help of the detected correlations within the database, it has been possible to work out a method for deciding on the selection of a design on the Pareto front and to draw new relevant conclusions. The systematic use of big-data methods proposed enables a more penetrating insight to be gained into numerical optimization, which is more general and relevant than those gained by simply comparing a single optimized design with a reference design. An analysis of the Pareto front reveals that 0.6% efficiency can be exchanged for 20% more homogeneous outflow.

Numerical simulation and optimization of centrifugal compressor return channels: methods and results

The authors actively use their engineering Universal Modelling Method for gas-dynamic design of centrifugal compressors for industrial partners. Currently, there are two approaches to improving the Method: improving the preliminary design and increasing the accuracy of gas-dynamic characteristics calculation. Computational Fluid Dynamics (CFD) methods give good results for the flow path stator part. Recently, massive CFD calculations of the vaneless diffusers (VLD) characteristics were generalized by a system of algebraic equations, which replaced the previous more complex mathematical model in the Method. Then, based on CFD optimization of a large series of return channels (RCh), corrections were made to the preliminary design of this element of the flow path.This paper presents the results of the joint CFD optimization of a vaneless diffuser and a return channel. Stator elements have many geometric parameters. For a stage with the flow rate coefficient of 0.0597 and the loading factor of 0.60, only the VLD relative radial length D 4/D 2, the number and the inlet angle of the return channel vanes were optimized. Engineering calculations and design experience predicted an optimal value of D 4/D2 within the interval 1.9 - 2.0. CFD optimization demonstrated almost linear reduction of the total head loss towards the end of the investigated range D 4/D2 = 2.3. After careful optimization of the U-bend, the optimization of D 4/D2 was repeated. The influence of on the loss coefficient has decreased, but the value of D 4/D 2 = 2.3 is still far from optimum. It significantly exceeds technically acceptable radial size. The result obtained influenced the design plan of future calculation experiments with a large series of stator elements of the stages in a practically significant range of design parameters.

Flow Field Analysis and Feasibility Study of a Multistage Centrifugal Pump Designed for Low-Viscous Fluids

A multistage centrifugal pump is designed for pumping low-viscosity, highly volatile and flammable chemicals, including hydrocarbons, for high head requirements. The five-stage centrifugal pump consists of a double-suction impeller at the first stage followed by a twin volute. The impellers for stages two through five are single-suction impellers followed by diffuser vanes and return channel vanes. The analytical performance is calculated initially in the design stage by applying similarity laws to an existing scaled-down pump model designed for low flow rate applications. The proposed pump design is investigated using computational fluid dynamics tools to study its performance in design and off-design conditions for water as the base fluid. The design feasibility of the centrifugal pump is tested for other fluids, such as water at a high temperature and pressure, diesel and debutanized diesel. The pump design is found to be suitable for a variety of fluids and operating ranges. The losses in the pump are analyzed in each stage at the best efficiency point. The losses in efficiency and head are observed to be higher in the second stage than in other stages. The detailed flow behavior at the second stage is studied to identify the root cause of the losses. Design modifications are recommended to diminish the losses and improve the overall performance of the pump.

Numerical and Experimental Investigation of Return Channel Vane Aerodynamics With Two-Dimensional and Three-Dimensional Vanes

The present paper deals with the numerical and experimental investigation of the effect of return channel (RCH) dimensions of a centrifugal compressor stage on the aerodynamic performance. Three different return channel stages were investigated, two stages comprising three-dimensional (3D) return channel blades and one stage comprising two-dimensional (2D) RCH vanes. The analysis was performed regarding both the investigation of overall performance (stage efficiency, RCH total pressure loss coefficient) and detailed flow-field performance. For detailed experimental flow-field investigation at the stage exit, six circumferentially traversed three-hole probes were positioned downstream the return channel exit in order to get two-dimensional flow-field information. Additionally, static pressure wall measurements were taken at the hub and shroud pressure and suction side (SS) of the 2D and 3D return channel blades. The return channel system overall performance was calculated by measurements of the circumferentially averaged 1D flow field downstream the diffuser exit and downstream the stage exit. Dependent on the type of return channel blade, the numerical and experimental results show a significant effect on the flow field overall and detail performance. In general, satisfactory agreement between computational fluid dynamics (CFD)-prediction and test-rig measurements was achieved regarding overall and flow-field performance. In comparison with the measurements, the CFD-calculated stage performance (efficiency and pressure rise coefficient) of all the 3D-RCH stages was slightly overpredicted. Very good agreement between CFD and measurement results was found for the static pressure distribution on the RCH wall surfaces while small CFD-deviations occur in the measured flow angle at the stage exit, dependent on the turbulence model selected.

Flow analysis in a return channel of a multi-stage centrifugal pump

Flow recirculation is an unpleasant and even hazardous phenomenon that can cause mechanical damage in turbomachinery and has to be prevented during their operations. We numerically studied the effects of return channel blade curvature on reducing the recirculation of flow inside the return channel vanes of a multi-stage centrifugal pump. Computational fluid dynamics (CFD) analyses were performed for a wide range of volumetric flow rates. The standard k-e turbulence model was adopted as the turbulence model, and the impeller rotation simulated employing the Multiple reference frames (MRF) method. First, a baseline model together with five different modified geometries for return channel was studied and compared using a two-stage pump framework. The results reveal that decreasing the curvature of the return channel blade makes smooth streamlines and eliminates the flow recirculation inside the return channels. As the second part, two return channels with the highest pump performance were selected to be used in the simulation of a multistage pump. The simulations of the multi-stage pump show that the flow inside the baseline return channel includes considerable areas of flow recirculation, while the modified return channels again have attached flow stream. It is concluded that the return channels with the smooth curvatures and outlet blade angles above 90° remove the flow recirculation inside the return channels, resulting in higher pump efficiencies.

Experimental and Numerical Investigation of the Flow Inside the Return Channel of a Centrifugal Process Compressor

This paper presents the first detailed experimental performance data for a new centrifugal process compressor test rig. Additional numerical simulations supported by extensive pressure measurements at various positions allow an analysis of the operational and loss behavior of the entire stage and its components. The stage investigated is a high flow rate stage of a single-shaft, multistage compressor for industrial applications and consists of a shrouded impeller, a vaneless diffuser, a U-bend, and an adjoining vaned return channel. Large channel heights due to high flow rates induce the formation of highly three-dimensional flow phenomena and thus enlarge the losses due to secondary flows. An accurate prediction of this loss behavior by means of numerical investigations is challenging. The published experimental data offer the opportunity to validate the used numerical methods at discrete measurement planes, which strengthens confidence in the numerical predictions. CFD simulations of the stage are initially validated with global performance data and extensive static pressure measurements in the vaneless diffuser. The comparison of the pressure rise and an estimation of the loss behavior inside the vaneless diffuser provide the basis for a numerical investigation of the flow phenomena in the U-bend and the vaned return channel. The flow acceleration in the U-bend is further assessed via the measured two-dimensional pressure field on the hub wall. The upstream potential field of the return channel vanes allows an evaluation of the resulting flow angle. Measurements within the return channel provide information about the deceleration and turning of the flow. In combination with the numerical simulations, loss mechanisms can be identified and are presented in detail in this paper.

Application of Flow Control in a Novel Sector Test Rig

An experimental and numerical study has been performed to evaluate the effectiveness of steady injection flow control for the reduction of losses in the return channel of a radial compressor. This investigation formed part of an overall attempt to develop a strategy for reducing the diffusion ratio of radial compressors. It is envisaged that this flow control would be activated at off-design conditions, where separation levels on the return channel vanes are considerable. A novel radial compressor sector test rig, supported by a blow-down facility and equipped with a range of instrumentation, was used for the experimental portion of the study. This allowed multiple flow control configurations to be studied in a simplified environment. A set of exchangeable, inlet guide vanes provide the test vanes with the correct inlet three-dimensional flow-field, while airfoil static pressure taps allowed the blade loading to be assessed. The numerical portion of the study was conducted using 3D-computational fluid dynamics (CFD) and involved simulations of both the sector test rig and a “substitute system”. In this paper, the rationale for the inclusion of flow control in a radial compressor return channel is discussed. The sector test rig is then described, including the implementation of flow control. The results of the matrix of flow control experiments are then discussed with comparison to the numerical results.

Unsteady Acoustic Forcing on an Impeller Due to Coupled Blade Row Interactions

This article contains an investigation of the unsteady acoustic forcing on a centrifugal impeller due to coupled blade row interactions. Selected results from an aeromechanical test campaign on a GE Oil and Gas centrifugal compressor stage with a vaneless diffuser are presented. The most commonly encountered sources of impeller excitation due to upstream wake interaction were identified and observed in the testing campaign. A 30/rev excitation corresponding to the sum of upstream and downstream vane counts caused significant trailing edge vibratory stress amplitudes. Due to the large spacing between the impeller and the return channel vanes, this 30/rev excitation was suspected to be caused by an aero-acoustic excitation rather than a potential disturbance. The origin of this aero-acoustic excitation was deduced from an acoustic analysis of the unsteady compressor flow derived from CFD. The analysis revealed a complex excitation mechanism caused by impeller interaction with the upstream vane row wakes and subsequent acoustic wave reflection from the downstream return channel vanes. The findings show it is important to account for aero-acoustic forcing in the aeromechanical design of low pressure ratio centrifugal compressor stages.

English

Aerodynamic studies in the static components of a centrifugal compressor stage were conducted using the computational fluid dynamics solver FLUENT. For the simulation study, a typical centrifugal compressor stage geometry with a flow coefficient of 0.053 was chosen, The study is confined to the static components of the centrifugal compressor stage, i.e., the crossover bend (180° U-bend), a radial cascade of return channel vanes, and the exit ducting (90° L-turn). The aerodynamic performance is reported in terms of total pressure loss coefficient, static pressure recovery coefficient, return channel vane surface static pressure distribution, and stage exit swirl angle distribution. The simulated flow through the static components covered five different operating conditions of the actual centrifugal compressor stage: the design point with 100% flow rate, and the off-design operating conditions with 70%, 80%, 110%, and 120% flow rates. The standard k-e model was used with standard wall functions to predict the turbulence. A minimum total pressure loss coefficient was observed near 80% flow rate when the average flow angle at the U-bend inlet was 24°. Better static pressure recovery was observed with 70%, 80%, and 100% flow rates. The swirl angle distribution at the stage exit was recognized as satisfactory.

Flow Investigations in the Crossover System of a Centrifugal Compressor Stage

The performance of the crossover system of a centrifugal compressor stage consisting of static components of <TEX>$180^{\circ}$</TEX> U-bend, return channel vanes and exit ducting with a <TEX>$90^{\circ}$</TEX> bend is investigated. This study is confined to the assessment of performance of the crossover system by varying the shape of the return channel vanes. For this purpose two different types of Return Channel Vanes (RCV1 and RCV2) were experimentally investigated. The performance of the crossover system is discussed in terms of total pressure loss coefficient, static pressure recovery coefficient and vane surface pressure distribution. The experimentation was carried out on a test setup in which static swirl vanes were used to simulate the flow at the exit of an actual centrifugal compressor impeller with a design flow coefficient of 0.053. The swirl vanes are connected to a mechanism with which the flow angle at the inlet of U-bend could be altered. The measurements were taken at five different operating conditions varying from 70% to 120% of design flow rate. On an overall assessment RCV1 is found to give better performance in comparison to RCV2 for different U-bend inlet flow angles. The performance of RCV2 was verified using numerical studies with the help of a CFD Code. Three dimensional sector models were used for simulating the flow through the crossover system. The turbulence was predicted with standard k-<TEX>$\varepsilon$</TEX>, 2-equation model. The iso-Mach contour plots on different planes and development of secondary flows were visualized through this study.