Xavier Carbonneau

Two-equation modeling of turbulent rotating flows

The possibility to take into account the effects of the Coriolis acceleration on turbulence is examined in the framework of two-equation eddy-viscosity models. General results on the physical consistency of such turbulence models are derived from a dynamical-system approach to situations of time-evolving homogeneous turbulence in a rotating frame. Application of this analysis to a (k,ϵ) model fitted with an existing Coriolis correction [J. H. G. Howard, S. V. Patankar, and R. M. Bordynuik, “Flow prediction in rotating ducts using Coriolis-modified turbulence models,” ASME Trans. J. Fluids Eng. 102, 456 (1980)] is performed. Full analytical solutions are given for the flow predicted with this model in the situation of homogeneously sheared turbulence subject to rotation. The existence of an unphysical phenomenon of blowup at finite time is demonstrated in some range of the rotation-to-shear ratio. A direct connection is made between the slope of the mean-velocity profile in the plane-channel flow with span...

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.

Nonlinear Harmonic Simulations of the Unsteady Aerodynamics of a Fan Stage Section at Windmill

In the present study, the unsteady flow through the fan stage of a high bypass ratio turbofan at windmill is studied numerically. The Nonlinear Harmonic (NLH) method is applied to a section (at 70 % of the relative span) of the fan stage. First, steady mixing plane simulations at windmill are used to perform a grid convergence study based on the prediction of the massively separated flow occurring on the lower side of both the rotor and stator due to highly negative angles of attack. The unsteadiness of the flow is then examined for the isolated rotor and stator, showing that, for this 2D case, negligible natural unsteady flow effects arise. This supports the use of the NLH method to account only for deterministic unsteady rotor/stator interactions. NLH simulations are then performed, and the influence of the num- ber of harmonics is assessed, based on the analysis of wakes. Contrasting the results with the nominal operating point simulations shows that less harmonics are needed for the windmilling case: this is due to the much larger wake behind the rotor associated to massive separation at windmill, which is more conveniently represented by Fourier series than the sharp narrow wake of the nominal point. Finally, the unsteady flow pattern is examined: the velocity defect of the rotor wakes, which periodically increases the flow angle on the stator, is shown to trigger a periodic movement of the reattachment point at the trailing edge of the stator, associated with vortex shedding from the lower side of the vane.

Body-force modeling for aerodynamic analysis of air intake – fan interactions

Purpose The purpose of this paper is to explore a methodology that allows to represent turbomachinery rotating parts by replacing the blades with a body force field. The objective is to capture interactions between a fan and an air intake at reduced cost, as compared to full annulus unsteady computations. Design/methodology/approach The blade effects on the flow are taken into account by adding source terms to the Navier-Stokes equations. These source terms give the proper amount of flow turning, entropy, and blockage to the flow. Two different approaches are compared: the source terms can be computed using an analytic model, or they can directly be extracted from RANS computations with the blade’s geometry. Findings The methodology is first applied to an isolated rotor test case, which allows to show that blockage effects have a strong impact on the performance of the rotor. It is also found that the analytic body force model underestimates the mass flow in the blade row for choked conditions. Finally, the body force approach is used to capture the coupling between a fan and an air intake at high angle of attacks. A comparison with full annulus unsteady computations shows that the model adequately captures the potential effects of the fan on the air intake. Originality/value To the authors’ knowledge, it is the first time that the analytic model used in this paper is combined with the blockage source terms. Furthermore, the capability of the model to deal with flows in choked conditions was never assessed.

Sensitivity Analysis and Experimental Validation of Transient Performance Predictions for a Short-Range Turbofan

Transient effects are important features of engine performance calculations. The aim of this paper is to analyze a new, fully transient model implemented using the PRopulsion Object Oriented Simulation Software (PROOSIS) for a civil, short range turbofan engine. A transient turbofan model, including the mechanical inertia effect has been developed in PROOSIS. Specific physical effects such as heat soakage, mass storage, blade tip clearance and combustion delay have been implemented in the relevant components of PROOSIS to obtain a fully transient model. Since a large number of components are concerned by all the transient effects, an influence study is presented to determine which are the most critical effects, and in which components. Inertia represents the relevant phenomenon, followed by thermal effects, combustion delay and finally mass storage. The comparison with experimental data will provide a first validation of the model. Finally a sensitivity study is reported to assess the impact of uncertain knowledge of key input parameters in the response time prediction accuracy.