Hub Corner Separation Research Articles

Optimization of endwall contouring in axial compressor S-shaped ducts

This paper presents a numerical investigation of the potential aerodynamic benefits of using endwall contouring in a fairly aggressive duct with six struts based on the platform for endwall design optimization. The platform is constructed by integrating adaptive genetic algorithm (AGA), design of experiments (DOE), response surface methodology (RSM) based on the artificial neural network (ANN), and a 3D Navier–Stokes solver. The visual analysis method based on DOE is used to define the design space and analyze the impact of the design parameters on the target function (response). Optimization of the axisymmetric and the non-axisymmetric endwall contouring in an S-shaped duct is performed and evaluated to minimize the total pressure loss. The optimal ducts are found to reduce the hub corner separation and suppress the migration of the low momentum fluid. The non-axisymmetric endwall contouring is shown to remove the separation completely and reduce the net duct loss by 32.7%.

Investigation of Unsteady Effects on a Two-stage Counter-rotating Fan

The rotor-rotor interaction on a two-stage counter-rotating fan has been studied by unsteady numerical simulation based on non-linear harmonic method. The results show that the efficiency of the 2nd blade row above 55% span is reduced. Additionally, the unsteady effects increase the efficiency of the 2nd row below 55% span. Further analysis reveals that the unsteady benefits result from the wake boundary layer interaction, which could enhance the exchange of energy between boundary-layer and mainstream, suppress the hub corner separation

Vane Clocking Effects on Stator Loss for Different Compressor Loading Conditions

Vane clocking is the relative circumferential indexing of adjacent vane rows with similar vane counts, and it affects discrete frequency noise, forced response, and stage performance in axial compressors. The objective of this paper is to investigate vane clocking effects on stator 2 loss for many loading conditions in a three-stage axial compressor. Although previous studies have focused on two or three operating conditions, this is the first to investigate vane clocking effects over many loading conditions. Detailed circumferential traverses of stagnation pressure were acquired for six clocking configurations at six different loading conditions: a low-loading condition, two conditions near design point, two high-loading conditions, and a near-stall condition. Stator 2 wake widths and depths were similar for different clocking configurations at low loadings, but the high-loading cases exhibited larger changes associated with vane clocking, especially near the stator hub. This resulted in different radial total pressure profile shapes for different clocking configurations that, coupled with flow visualization, showed that vane clocking modulates the hub corner separation for the high-loading operating condition. For design-loading and low-loading conditions, the differences in area-averaged stator 2 loss coefficients with vane clocking were small, but changes in loss increased at the high-loading condition and resulted in a 20.6% difference in loss coefficient between the minimum and maximum clocking cases, with local changes near the hub as much as 46.6%. The effect of clocking on vane loss lessened as the stall boundary was approached.

Influence of Unsteadiness on the Control of a Hub-Corner Separation Within a Radial Vaned Diffuser

The present work aims at evaluating the effect of the impeller–diffuser interaction on the control of a hub corner separation, which develops in the radial vaned diffuser of a centrifugal compressor designed and built by Turbomeca, Safran group. Unsteady numerical simulations of the flow in the aspirated centrifugal compressor are then performed. Numerical results are validated by comparison with the available experimental results. The analysis of the numerical flow field shows that the hub-corner separation is not completely removed by the suction, on the contrary to the steady-state results that were obtained in previous work. The boundary layer separation is only translated downstream. Its location is explained by the scrolling of the pressure waves generated by the impeller–diffuser interaction, which strengthen when crossing the diffuser throat. This result highlights the major role played by the impeller–diffuser interaction, which should be taken into account for developing control strategies in radial vaned diffusers, and stresses the shortcoming of the steady-state numerical model when suction is applied.

Low-Speed Model Testing Studies for an Exit Stage of High Pressure Compressor

This paper discusses detailed experimental studies of a low-speed large-scale axial compressor, which is typical of an exit stage of HPC. Numerous measuring techniques were performed, and detailed experimental results were obtained, including inlet boundary layer total pressure distributions, overall compressor and model-stage performance, traverse flow field between blade rows and inside the stator for the model stage, static pressure on the stator blade and casing dynamic pressure of the rotor. The objective of the study is to assess the low-speed model compressor design and verify 3D computational fluid dynamics (CFD) code. Results show that inlet endwall blockage requirement of HPC exit stage is achieved; the low-speed model compressor design is fundamentally successful; the flow rate and pressure rise requirements are met at the design operating point, although the flow loss is relatively larger than design values for the lower half span, which can be attributed to a certain hub-corner separation. Furthermore, the reliability of adopted 3D commercial CFD code is validated. It is proved that the low-speed model testing technique is still a prospective way for the design of high performance HPC.

Interaction of shrouded stator flow and main flow and its influence on performances of a three-stage high pressure compressor

The prediction of the complex flows within a turbomachinery remains a challenging issue for computational fluid dynamics (CFD). The real features of a turbomachinery must be taken into account in the simulation but they increase its complexity. One of these features is the leakage flow due to the axial gap between the fixed and rotating parts of the hub. This article focuses on this flow within the scope of a high-pressure compressor. The results are compared with experimental data obtained in the main flow path. The leakage flow is characterized by several vortices, especially near the hub and between the seal-teeth. It interacts with the main flow: the hub separation is quite wider while the casing one is reduced in comparison with the smooth case. In this article, the smooth case is related to the simulation without any shrouded stator cavity. The leakage flow influences strongly on compressor performances. Taking into account the leakage flow in the CFD improves the agreement between CFD and experimental values. Due to the vortex prediction, the influence of the turbulence modelling is also assessed. The explicit algebraic Reynolds stress model improves the flow prediction while the presently investigated differential Reynolds stress model predicts too wide hub corner separation which leads to high underestimation of compressor performances.

Passive control of hub-corner separation/stall using axisymmetric-hub contouring

Numerical simulations were performed to investigate the forming mechanism of the compressor/fan hub-corner separation/stall. Taking the small-scaled vortex in the detached regions into account, scale adaptive simulation model coupled with shear stress transport model (SAS–SST turbulence model) does well in capturing the flow structures in the compressor. At near stall condition, low-momentum stall cell originates from the leading edge of the stator near hub, contributes to the greater turning angle at the suction side because of endwall viscosity, rolls, and lifts up in spanwise direction. The same then interacts with the wake in the downstream passage of stator, nearly taking up the whole passage downstream. Three vortex centres are generated at the hub induced by the boundary layer separation flow, one near the stator leading edge, a second one at the blade trailing edge near the suction side, and a third one near the middle passage, and further swirled up to the blade suction surface which can be recognized as thepassage vortex developed by the radial and circumferential pressure differences. Axisymmetric-hub contouring (a type of passive flow control techniques) has resulted in performance improvement of the compressor. Main contribution of performance enhancement is attributed to the suppression of hub-corner separation. Significant influence of axisymmetric contouring has been found on boundary layer separation structures near hub and passage vortex, which affects the formation and development of hub-corner stall.

Topological Studies of Three-dimensional Flows in a High Pressure Compressor Stator Blade Row without and with Boundary Layer Aspiration

This paper presents a numerical study of the flow topologies of three-dimensional (3D) flows in a high pressure compressor stator blade row without and with boundary layer aspiration on the hub wall. The stator blade is representative of the first stage operating under transonic inlet conditions and the blade design encourages development of highly complex 3D flows. The blade has a small tip clearance. The computational fluid dynamics (CFD) studies show progressive increase of hub corner stall with the increase in incidence. Aspiration is implemented on the hub wall via a slot in the corner between the hub wall and the suction surface. The CFD studies show aspiration to be sensitive to the suction flow rate; lower rate leads to very complex flow structures and increased level of losses whereas higher rate renders aspiration effective for control of hub corner separation. The flow topologies are studied by trace of skin friction lines on the walls. The nature of flow can be explained by the topological rules of closed separation. Furthermore, a deeper analysis is done for a particular case with advanced criterion to test the non-degeneracy of critical points in the flow field.

Far-Field Radiation of Tip Aerodynamic Sound Sources in Axial Fans Fitted With Passive Noise Control Features

Abstract This study investigates the causal relationship between flow aerodynamics and radiated noise in low-speed axial fans. Using blades with three distinctive tip configurations, including two that were developed with a view to reducing noise emissions, the pressure fluctuations of the exhaust flow in the near field are correlated with the noise measured in the far field in an anechoic chamber. By varying the far-field microphone’s azimuthal position, the study investigates the source signatures and directivity of noise sources distributed along the blade span. Several distinctive features in the noise directivity pattern are identified and correlated with the noise sources of aerodynamic origin dissected along the blade span. Utilizing the directional far-field autospectra of the three blade configurations in combination with the near-field/far-field cross-spectra, the emission characteristics of the aerodynamic sources are analyzed and their roles with respect to the overall acoustic signature of the fan are discussed. It is apparent that the ability to decompose the output of the aerodynamic noise sources in the near field is a useful tool in designing fans to achieve desirable low-noise targets. The results confirm that the tip-flow appendages influence the noise radiation pattern for the investigated family of fans. These phenomena are linked with the control of aerodynamic noise sources related to the tip-leakage vortex and the hub corner separation.

Simulating the Multistage Environment for Single-Stage Compressor Experiments

The performance of a single-stage low-speed compressor has been measured both before and after the introduction of certain features of the multistage flow environment. The aim is to make the single-stage rig more appropriate for developing design rules for multistage compressors. End-wall blockage was generated by teeth on the hub and casing upstream of the rotor. A grid fitted upstream produced free-stream turbulence at rotor inlet typical of multistage machines and raised stage efficiency by 1.8 percent at the design point. The potential field that would be generated by blade rows downstream of an embedded stage was replicated by introducing a pressure loss screen at stage exit. This reduced the stator hub corner separation and increased the rotor pressure rise at flow rates below design, changing the shape of the pressure-rise characteristic markedly. These results highlight the importance of features of the flow environment that are often omitted from single-stage experiments and offer improved understanding of stage aerodynamics.

Three-Dimensional Flows and Loss Reduction in Axial Compressors

Measurements have been performed in a low-speed high-reaction single-stage axial compressor. Data obtained within and downstream of the rotor, when correlated with the results of other investigations, provide a link between the existence of suction surface–hub corner separations, their associated loss mechanisms, and blade loading. Within the stator, it has been shown that introducing a small clearance between the stator blade and the stationary hub increases the efficiency of the stator compared to the case with no clearance. Oil flow visualizaton indicated that the leakage reduced the extensive suction surface–hub corner separation that would otherwise exist. A tracer gas experiment showed that the large radial shifts of the surface streamlines indicated by the oil flow technique were only present close to the blade. The investigation demonstrates the possible advantages of including hub clearance in axial flow compressor stator blade rows.