Description of the unsteady flow pattern from peak efficiency to near surge in subsonic centrifugal compressor stage

This paper aims to describe the flow structure modifications when the operating point moves from peak efficiency to near stall condition in a moderate pressure ratio centrifugal compressor stage consisted of a splittered unshrouded impeller and a vaned diffuser. The investigations are based on three-dimensional U-RANS simulation results. The flow is described in the impeller and in the vaned diffuser through time-averaged flow quantities and unsteady fluctuations. Results show that at low mass flow rate, the effects of secondary flow in the impeller are more pronounced, inducing both, high time-averaged values and temporal fluctuations of the flow angle near the shroud at the diffuser inlet, leading to vane suction side boundary layer separation. Pressure waves due to impeller diffuser interaction spread through the vaned diffuser generating unsteadiness which intensifies at near surge condition

Une Nouvelle Pédagogie pour des Travaux Pratiques en Mécanique des Fluides

National audienceThe Fluid Mechanics Teaching Group of Ecole Centrale de Lyon introduced new teaching methods for the lab works of 2nd year students (eq. Master 1). Instead of a classical linear experiment on a dedicated rig, the students choose a scientific ant technical theme they illustrate on a panel of rigs and numerical simulations. During the first session, the students choose a theme and define objectives and concepts to illustrate, in conjunction with the theoretical classes. They also select appropriate experimental rigs and numerical simulations. During the next two sessions, they have to carry out the experiments and simulations, over the same configurations. Data are analyzed according to the objectives, combining experimental and numerical approaches. Finally, the work is synthesized through a report and a presentation during the last session. This talk is also a way to present the results to the other groups that studied different themes.L'Equipe d'Enseignement de Mécanique des Fluides de l'Ecole Centrale de Lyon pratique une nouvelle pédagogie pour les Travaux Pratiques dédiés aux élèves de 2ème année. Plutôt que de réaliser classiquement une expérience formatée sur une installation dédiée, les étudiants définissent eux-mêmes un thème scientifique et technique qu'ils illustrent grâce à un panel d'installations expérimentales et de simulations numériques. Lors d'une première séance, les étudiants choisissent leur thème d'étude et définissent des objectifs et des concepts à illustrer, en lien avec les enseignements théoriques dispensés. Ils identifient les installations expérimentales et les simulations utiles. Lors des deux séances intermédiaires, ils réalisent leurs expériences et des simulations numériques, menées sur les mêmes configurations. Les données traitées sont analysées en cohérence avec les objectifs prédéfinis, et en confrontant les approches numériques et expérimentales. La dernière séance est consacrée à la restitution. Un rapport écrit et un exposé oral constituent un exercice de synthèse. Par ailleurs, l'exposé oral est un vecteur de transmission des connaissances acquises, à l'adresse des étudiants ayant choisi un autre thème d'étude

Analysis of the Unsteady Flow Field in a Centrifugal Compressor from Peak Efficiency to Near Stall with Full-Annulus Simulations

This study concerns a 2.5 pressure ratio centrifugal compressor stage consisting of a splittered unshrouded impeller and a vaned diffuser. The aim of this paper is to investigate the modifications of the flow structure when the operating point moves from peak efficiency to near stall. The investigations are based on the results of unsteady three-dimensional simulations, in a calculation domain comprising all the blade. A detailed analysis is given in the impeller inducer and in the vaned diffuser entry region through time-averaged and unsteady flow field. In the impeller inducer, this study demonstrates that the mass flow reduction from peak efficiency to near stall leads to intensification of the secondary flow effects. The low momentum fluid accumulated near the shroud interacts with the main flow through a shear layer zone. At near stall condition, the interface between the two flow structures becomes unstable leading to vortices development. In the diffuser entry region, by reducing the mass flow, the high incidence angle from the impeller exit induces a separation on the diffuser vane suction side. At near stall operating point, vorticity from the separation is shed into vortex cores which are periodically formed and convected downstream along the suction side

Analysis of the Interrow Flow Field Within a Transonic Axial Compressor: Part 2—Unsteady Flow Analysis

ABSTRACT An analysis of the experimental data, obtained by laser two-focus anemometry in the IGV-rotor inter-row region of a transonic axial compressor, is presented with the aim of improving the understanding of the unsteady flow phenomena. A study of the IGV wakes and of the shock waves emanating from the leading edge of the rotor blades is proposed. Their interaction reveals the increase in magnitude of the wake passing through the moving shock. This result is highlighted by the streamwise evolution of the wake vorticity. Moreover, the results are analyzed in terms of a time averaging procedure and the purely time-dependent velocity fluctuations which occur are quantified. It may be concluded that they are of the same order of magnitude as the spatial terms for the inlet rotor flow field. That shows that the temporal fluctuations should be considered for the 3D rotor time-averaged simulations. NOMENCLATURE Symbols M R = relative Mach number N = number of blades P = measurement point location T = temporal period r U , U = rotor velocity vector, modulus V r , V = velocity vector, modulus X = some flow parameter or other r f = external forces n = co-ordinate normal to the shock p = pressure r = radial co-ordinate t = time z = axial co-ordinate r Ω , Ω = vorticity vector, modulus Θ = angular blade pitch α = absolute velocity angle ϕ = phase shift ω = rotation speed (radian/s) θ = azimuthal co-ordinate ρ = fluid density τ = turbulent and viscous stress tensor τ = co-ordinate tangent to the shock Superscripts -= time averaged valuẽ = spatial averaged value ' = time fluctuating value in the absolute frame * = spatial fluctuating value " = purely time-dependent fluctuating value Subscripts R = relative to the rotor n = component normal to the shock r = radial component r = rotor red = reduced value ref = reference value rel = relative to the rotor (for a time averaged process) s = stator θ = azimuthal co-ordinate τ = component tangent to the shock 1 = rotor inlet conditions INTRODUCTION A critical issue for the turbomachinery industry remains the timeaveraged simulation of multi-stage turbomachinery. Classical 3D multi-blade row simulation methods consist in connecting results of stationary calculations applied to isolated adjacent blade rows, the information between the rows usually being exchanged throug

Active Flow Control in a Radial Vaned Diffuser for Surge Margin Improvement: A Multislot Suction Strategy

This work is the final step of a research project that aims at evaluating the possibility of delaying the surge of a centrifugal compressor stage using a boundary-layer suction technique. It is based on Reynolds-Averaged Navier-Stokes numerical simulations. Boundary-layer suction is applied within the radial vaned diffuser. Previous work has shown the necessity to take into account the unsteady behavior of the flow when designing the active flow control technique. In this paper, a multislot strategy is designed according to the characteristics of the unsteady pressure field. Its implementation results in a significant increase of the stable operating range predicted by the unsteady RANS numerical model. A hub-corner separation still exists further downstream in the diffuser passage but does not compromise the stability of the compressor stage

Numerical Investigation of Kelvin–Helmholtz Instability in a Centrifugal Compressor Operating Near Stall

The present work details the occurrence of the Kelvin–Helmholtz instability in a centrifugal compressor operating near stall. The analysis is based on unsteady three-dimensional simulations performed on a calculation domain covering the full annulus for the impeller and the vaned diffuser. A detailed investigation of the flow structure is presented, together with its evolution consequent to the mass flow reduction. It is demonstrated that this reduction leads to an enlargement of the low-momentum flow region initially induced by the combination of the secondary and leakage flows. When the compressor operates near stall, the shear layer at the interface between the main flow and this low-momentum flow becomes unstable and induces a periodic vortex shedding. The frequency of such an unsteady phenomenon is not correlated with the blade-passing frequency. Its signature is thus easily isolated from the deterministic rotor/stator interaction. Its detection requires full-annulus simulations with an accurate resolution in time and space, which explains why it has never been previously observed in centrifugal compressor

Numerical Simulation of Stall Inception Mechanisms in a Centrifugal Compressor With Vaned Diffuser

The present paper numerically investigates the stall inception mechanisms in a centrifugal compressor stage composed of a splittered unshrouded impeller and a vaned diffuser. Unsteady numerical simulations have been conducted on a calculation domain compris- ing all the blade passages over 360 deg for the impeller and the diffuser. Three stable operating points are simulated along a speed line, and the full path to instability is inves- tigated. The paper focusses first on the effects of the mass flow reduction on the flow topology at the inlet of both components. Then, a detailed analysis of stall inception mechanisms is proposed. It is shown that at the inlet of both components, the mass flow reduction induces boundary layer separation on the blade suction side, which results in a vortex tube having its upper end at the casing and its lower end at the blade wall. Some similarities with flows in axial compressor operating at stall condition are outlined. The stall inception process starts with the growth of the amplitude of a modal wave rotating in the vaneless space. As the flow in the compressor is subsonic, the wave propagates upstream and interacts with the impeller flow structure. This interaction leads to the drop in the impeller pressure ratio

Comparison of steady and unsteady flows in a transonic radial vaned diffuser

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Detailed analysis of the flow in the inducer of a transonic centrifugal compressor

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