Francis Leboeuf

Free surface flows simulations in Pelton turbines using an hybrid SPH-ALE method

An Arbitrary Lagrange Euler (ALE) description of fluid flows is used together with the meshless numerical method Smoothed Particle Hydrodynamics (SPH) to simulate free surface flows. The ALE description leads to an hybrid method that can be closely connected to the finite volume approach. It is then possible to adapt some common techniques like upwind schemes and preconditioning to remedy some of the well known drawbacks of SPH like stability and accuracy. An efficient boundary treatment based on a proper upwinding of fluid information at the boundary surface is settled. The resulting SPH-ALE numerical method is applied to simulate free surface flows encountered in Pelton turbines.

Unsteady Simulation of an Axial Compressor Stage With Casing and Blade Passive Treatments

This paper deals with the numerical simulation of technologies to increase the compressor performances. The objective is to extend the stable operating range of an axial compressor stage using passive control devices located in the tip region. First, the behavior of the tip leakage flow is investigated in the compressor without control. The simulation shows an increase in the interaction between the tip leakage flow and the main flow when the mass flow is reduced, a phenomenon responsible for the development of a large flow blockage region at the rotor leading edge. A separation of the rotor suction side boundary layer is also observed at near stall conditions. Then, two approaches are tested in order to control these flows in the tip region. The first one is a casing treatment with nonaxisymmetric slots. The method showed a good ability to control the tip leakage flow but failed to reduce the boundary layer separation on the suction side. However, an increase in the operability was observed but with a penalty for the efficiency. The second approach is a blade treatment that consists of a longitudinal groove built in the tip of each rotor blade. The simulation pointed out that the device is able to control partially all the critical flows with no penalty for the efficiency. Finally, some recommendations for the design of passive treatments are presented.

Experimental Study of Corner Stall in a Linear Compressor Cascade

In order to gain a better knowledge of the mechanisms and to calibrate computational fluid dynamics (CFD) tools including both Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES), a detailed and accurate experimental study of corner stall in a linear compressor cascade has been carried out. Data are taken at a Reynolds number of 382 000 based on blade chord and inlet velocity. At first, inlet flow boundary layer is surveyed using hot-wire anemometry. Then in order to investigate the effects of incidence, measurements are acquired at five incidences, including static pressures on both blade and endwall surfaces measured by pressure taps and the total pressure losses of outlet flow measured by a five-hole pressure probe. The maximum losses as well as the extent of losses of the corner stall are presented as a function of the investigated incidences.

Sensitivity analysis of a concentration fluctuation model to dissipation rate estimates

Lagrangian dispersion models require estimates of the local dissipation rate ( e ) of turbulent kinetic energy ( k ). In this study, we evaluate the sensitivity of a Lagrangian model to different estimates of e in simulating passive scalar dispersion in a turbulent boundary layer over a rough surface. Two different estimates of e are used to simulate pollutant dispersion emitted by a linear elevated source with a Lagrangian model which integrates a macromixing and a micromixing scheme. Comparison between numerical and experimental results allows us to discuss the performance of the model and to define its sensitivity to e .

Simulation of near surge instabilities onset in a turbocharger compressor

This study focuses on numerical simulations of a small automotive turbocharger compressor stage. Two medium speed characteristics were reproduced from nominal operating points to surge and were compared with experimental measurements. The aim of this study is to analyze the flow unsteadiness occurring near the surge line. The complete geometry of the impeller is meshed; hence, no spatio-temporal hypothesis is done during simulations. The main flow patterns are investigated to identify structures that might be responsible of surge inception. Results show that both impeller and diffuser are affected by stall. Blade channels are affected by a complete shroud recirculation extending from upstream of the impeller inlet to the diffuser inlet. Three radial recirculation zones are detected on the vaneless diffuser walls, strongly influenced by the two-pike asymmetric pressure field induced by the volute tongue. Its influence is observed at the inlet of the compressor, increasing the inter-blade flow unsteadiness.

Experimental Investigations of Corner Stall in a Linear Compressor Cascade

In applied research, a lack of understanding of corner stall, i.e. the three-dimensional (3D) separation in the juncture of the endwall and blade corner region, which has limited the efficiency and the stability of compressors. Both Reynolds-averaged Navier-Stokes (RANS) and large eddy simulation (LES) still need to be calibrated for turbomachinery applications. In the fundamental research of the turbulent boundary layer (TBL), there are a lot of findings of the effects of curvature and pressure gradients, which also play an important role in physics of corner stall. The purpose of this thesis is (i) to carry out an experiment in a cascade, (ii) to gain a database that could be used to calibrate both RANS and LES, and (iii) to give some basic explanations of corner stall through investigating the TBL on the suction side at the mid-span which is more complex than those in the basic investigations but simpler than those in a real engine. A detailed and accurate experiment of 3D flow field through a linear compressor cascade has been set up. Experimental data were acquired for a Reynolds number of 3.82×10 ^5 based on blade chord and inlet flow conditions. Measurements have been achieved by hot-wire anemometry, pressure taps on blade and endwall, five-hole pressure probe, oil visualization, 2D particle image velocimetry (PIV),and two-component laser Doppler anemometry (LDA). An original and complete database was thus obtained. The TBL on the suction side at mid-span was investigated. The wall-normal negative pressure gradient restrains the separation, on the contrary to its influence in the corner stall. The streamwise adverse pressure gradient can be responsible for the development of Reynolds stresses. The remarkable phenomenon at measurement stations near the trailing edge of blade is that an inflection point occurs in each profile of the mean streamwise velocity. At this inflection point, the magnitudes of the Reynolds stresses reach their maximum values, and the direction of energy diffusion also changes. The velocity field in the corner stall was presented. Bimodal histograms of velocity exist in the experiment. The bimodal points mainly appear in the region around the mean interface of separated flow and non-separated flow. At a bimodal point the local two velocity components are non-independent from each other, due to the aperiodic interplay of two basic modes in the flow field. Two modes were proposed to interpret the physics of bimodal behaviour.

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

Abstract 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.

The Detailed Structure and Behavior of Discrete Cooling Jets in a Turbine

Abstract: Three‐dimensional jets are an efficient way of cooling the walls of modern high‐pressure turbines. Introduced in the external flow that develops around the turbine blade, they are associated with a set of vortex structures. The purpose of this paper is to underline the respective origin and importance of these structures, with reference to both experimental and numerical results. Steady and unsteady vortices will be analyzed. Recommendations for numerical simulations will be proposed from these observations.

Numerical Analysis of Three-Dimensional Corner Separation in a Linear Compressor Cascade

(1)Laboratoire de Mecanique des Fluides et d’Acoustique, Ecole Centrale de Lyo´ n, 69130 Ecully, France(2)School of Jet Propulsion, Beijing University of Aeronautics and Astronautics, 100191 Beijing, China∗ Feng.Gao@ec-lyon.frABSTRACTNowadays, the internal flow in aircraft engine compressorscan be quite accurately reproduced at design condition by theCFD tools. However, CFD generally fails to simulate some sin-gular 3D phenomena, near off-design conditions, such as thecorner separation. Studies have pointed out that the separationregions are often over-estimated when the flow state is far fromdesign condition, owing to the turbulence model. Much workis devoted to improving the capability of the turbulence modelin capturing the onset and the extent of the corner separation,which is desired in the designing procedures.In this paper, steady RANS simulations are carried out in thesame configuration as an experiment of Ma et al. These simula-tions are obtained with a high-precision in-house Navier-Stokessolver (Turb’Flow). With the same mesh, an unsteady simulation(URANS) is subsequently presented, in order to investigate theinfluence of a fluctuating inflow.Attention is focused on a specific angle of attack of 4 de-grees, for which the three-dimensional corner separation isclearly observed. For the unsteady simulation, unsteadiness isimposed through perturbations of the angle of attack at the inlet.The results ofthe steady andunsteadycomputationsareanalyzedand compared with those of the experiment. The time-averagedURANS results agree well with the RANS results. The fluctuati nginflow does not show much influence on the mean performanceof the compressor cascade. The onset of the corner separationoccurs earlier in the simulations than in the experiment, consid-ering the blade surface pressure and the passage velocity pro-files. However, the cross-stream extent of the corner separationappears slightly under-estimated by CFD, according to the out-let total pressure losses andthe passagevelocityprofiles. Finally,the URANS simulation allows to recover bi-modal PDFs, as ob-served in the experiment.NOMENCLATUREc Chord lengthc

Numerical Investigation of High-Pressure Turbine Environment Effects on the Prediction of Aerothermal Performances

Aerothermal prediction for the high-pressure turbine is challenging because of the complex environment that interacts with the turbine: hot-streak migration, unsteady flow phenomena, fluid/solid thermal coupling and technological details (squealer tip, coolant ejections, fillets, etc.). There is a need to compare their relative impacts on the blade temperature and turbine efficiency prediction. This is the main purpose of this paper. URANS simulations of the flow have been performed with a structured flow solver in a one stage high-pressure turbine. The baseline simulation takes into account the squealer tip and an inlet condition representative of a hot streak generated by the combustion chamber. Other technological details (coolant ejections and fillets) and fluid/solid thermal coupling on the rotor blade are alternatively considered in the simulation in order to quantify their relative contribution. The Chimera technique is used to ease the integration of technological details. The conjugate heat transfer (CHT) problem is solved by means of a code coupling where fluxes and temperatures are exchanged at the blade surface between the fluid dynamics solver and the solid thermal code. Results shows that rotor blade fillets have a little impact on both the blade temperature and the turbine efficiency (less than 1%). On the contrary, taking into account external cooling leads to a modification of radial distribution of loss and loading coefficients and reduces the efficiency by 2%. The blade temperature is also impacted, mainly on the suction side where differences of several per cent with the baseline case are observed. Fluid/solid coupling mainly affects the blade temperature prediction by homogenizing it which induces differences around 3% with the baseline case. To complete the analysis, a post-processing that includes a computation of local entropy production terms is used. It shows that the entropy production is mainly due to turbulent dissipation and allows to identify the reduction of efficiency of the case with cooling as an additional production of entropy where the cooling flow mixes with the main flow.Copyright