Nicolas Binder

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 Variable Geometry Effect in Radial Turbines

The influence of variable geometry stators on the stage behavior is analyzed from both theoretical and experimental points of view. A theoretical analysis of the trajectory of some pressure-ratio lines in a loading-to-flow-coefficient diagram leads to the definition of a specific dimensionless parameter: the reduced section. This parameter is representative of the stator geometric configuration and is thus expected to be a good candidate to describe the variable geometry problem. From a theoretical point of view, this parameter is no less than the formal expression of the link between the geometric configuration of the stator and the behavior of the stage. An experimental approach decomposed in three phases is then led to evaluate this assessment. The results clearly demonstrate the crucial influence of the reduced section in the operating point definition. It leads to the conclusion that from a theoretical point of view, the two solutions mainly used in the industry for variable geometry stages (variation of the height or of the opening position of the stator blades) are equivalent provided that they are sanitized of their respective technological drawbacks. It has also been shown that the geometric configuration of the stator chosen to reach a specific value of the reduced section has some incidence on the efficiency of the stage. This observation gives some opportunities for optimization, for which some axis of reflection is given.

Evaluation of the Thrust Recovery of an Aircraft Flapped Outflow Valve

This paper presents a detailed study of a flapped outflow valve. Well-known as part of the pressurization system of aircraft, this type of valve is also designed for thrust recovery even if the efficiency of the system has never been demonstrated. Aground experimental test rig is first designed to provide global and local measurements to be used as validation data for numerical simulations. Once the validation of the numerical approach is achieved on a ground configuration, additional three-dimensional computations are then conducted for cruise conditions. They lead to a reliable estimation of thrust recovery as well as interesting insight in the aerodynamic behavior of the flow across the valve and its associated three-dimensional effects.

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.

Off-Design Considerations through the Properties of Some Pressure-Ratio Line of Radial Inflow Turbines

Correspondence should be addressed to N. Binder, nicolas.binder@isae.frReceived 18 July 2008; Accepted 17 November 2008Recommended by Gerard BoisRadial turbines are commonly used in applications involving operation through severe off-design conditions. The emergence ofvariable-geometry systems leads to the distinction between two off-design concepts: operational and geometric off-designs. Bothof these operating constraints should be integrated in the design procedure. Recent developments in prediction and optimizationmethods allowed such an integration, but involving complex algorithms is coupled with semiempiric loss models. This paperprovides a basis to obtain simple information from an existing or predesigned machine, for both operational and geometric off-design conditions. An alternative turbine map is defined using loading and flow coefficients. A one-dimensional analysis showsthat the constant pressure-ratio lines are straight lines whose slope is remarkably correlated with the pressure-ratio value andgeometrical characteristics. This theoretical approach is validated against the experimentation of two machines, the linearity isobserved in both cases. The direct influence of the stator configuration on the pressure-ratio lines confirms the applicabilityof this work to variable-geometry stages. A dimensionless cross-section of the stator is thus defined. However, the unexpecteddisplacement of the intercept of the pressure-ratio lines limits the application field of this method. Nevertheless, a simpleperformance prediction analysis is proposed for blocked mass flow operation.Copyright

Theoretical Analysis of the Aerodynamics of Low-Speed Fans in Free and Load-Controlled Windmilling Operation

The present work is a contribution to understanding the windmilling operation of low-speed fans. Such an operating situation is described in the literature, but the context (mainly windmilling of aero-engines) often involves system dependence in the analysis. Most of the time, only regimes very close to the free-windmilling are considered. A wider range is analyzed in the present study, since the context is the examination of the energy recovery potential of fans. It aims at detailing the isolated contribution of the rotor, which is the only element exchanging energy with the flow. Other elements of the system (including the stator) can be considered as loss generators and be treated as such in an integrated approach. The evolution of the flow is described by the use of theoretical and experimental data. A theoretical model is derived to predict the operating trajectories of the rotor in two characteristic diagrams. A scenario is proposed, detailing the local evolution of the flow when a gradual progression toward free and load-controlled windmilling operation is imposed. An experimental campaign exerted on two low-speed fans aims at the analysis of both the local and global aspects of the performance, for validation. From a global point of view, the continuity of the operating trajectory is predicted and observed across the boundary between the quadrants of the diagrams. The flow coefficient value for the free-windmilling operation is fairly well predicted. From a local point of view, the local co-existence of compressor and turbine operating modes along the blade span is observed as previously reported. It is further demonstrated here that this configuration is not exclusive to free-windmilling operation and occurs inside a range that can be theoretically predicted. It is shown that for a given geometry, this local topology strongly depends on the value of the flow coefficient and is very sensitive to the inlet spanwise velocity distribution.

Dynamic Response in Transient Operation of a Variable Geometry Turbine Stage: Influence of the Aerodynamic Performance

The transient response of a radial turbine stage with a variable geometry system is evaluated. Mainly, the consequences of the variations of the aerodynamic performance of the stage on the response time are checked. A simple quasi-steady model is derived in order to formalize the expected dependences. Then an experimental campaign is conducted: a brutal step in the feeding conditions of the stage is imposed, and the response time in terms of rotational speed is measured. This has been reproduced on different declinations of the same stage, through the variation of the stator geometry, and correlated to the steady-state performance of the initial and final operating points of the transient phase. The matching between theoretical expectation and results is surprisingly good for some configurations, less for others. The most important factor identified is the mass-flow level during the transient phase. It increases the reactivity, even far above the theoretical expectation for some configurations. For those cases, it is demonstrated that the quasi-steady approach may not be sufficient to explain how the transient response is set.

Generic Properties of Flows in Low-Speed Axial Fans Operating at Load-Controlled Windmill

This paper aims at underlining the existence of some generic properties of windmilling flows, partially spread in the literature but never clearly stated. Two kinds of axial machines are investigated from compressor mode to highly loaded windmill: a conventional fan with poor turbine performance and an optimized fan able to reach high efficiencies in both compressor and windmilling operations. Both simulations and experiments are used to perform the analysis. Three particular behaviours were identified as typical of fans operating at windmill: the inverse stacking of the speed lines visible in (P, Qm) turbine maps, the appearance of a slope change on the loading-to-flow coefficient diagram at windmill and a threshold effect occurring at highly loaded windmill.

Assessment of Steady and Unsteady Full Annulus Simulations Predictivity for a Low-Speed Axial Fan at Load-Controlled Windmill

A steady mixing plane approach is compared with the time-averaged solution of an unsteady full annulus calculation for a conventional fan operating at load-controlled windmill. The objective is to assess the added value of a complete unsteady calculation compared with a more classical approach, especially concerning the effect of the spatial and temporal periodicity release in such an unusual operation as windmill. Experiment with global steady measurements and rotor radial characterizations was conducted. Numerical analysis demonstrates that windmilling global performances obtained with the time-averaged solution of the unsteady simulation are not far different from the steady case, especially in the rotor. Some differences arise in the stator, particularly regarding the velocity field. Temporal periodicity release in this row has clearly a significant effect on the flow unsteady response. A detailed analysis highlights that generic patterns of windmilling flows recorded on a steady approach are also reported on the unsteady case.

Numerical analysis of secondary flow topologies of low-speed axial fans from compressor to load-controlled windmill

This paper presents the detailed analysis of the local topology evolution from compressor mode to loaded windmill. A first objective is to identify generic patterns of rotor separation topology at windmill and bring light on the mechanisms responsible for it. This study also aims at numerically providing new elements of understanding on tip leakage flows and curvature effects while shifting from compressor to turbine operation. Two machines were investigated : a conventional fan and an innovative design meant to reach both high compressor and turbine efficiencies. In compressor or turbine operation, the tip leakage flow is located on the blade suction side. However, the location of the latter is inverted from one mode to the other. The inversion of the pressure gradient leads, near the hub boundary layer of a blade passage, to an inversion of the cross-flow direction.