Influence of cavity flow on turbine aerodynamics

In order to deal with high temperatures faced by the components downstream of the combustion chamber, some relatively cold air is bled at the compressor. This air feeds the cavities under the turbine main annulus and cool down the rotor disks ensuring a proper and safe operation of the turbine. This thesis manuscript introduces a numerical study of the effect of the cavity flow close to the turbine hub on its aerodynamic performance. The interaction phenomena between the cavity and main annulus flow are not currently fully understood. The study of these phenomena is performed based on different numerical approaches (RANS, LES and LES-LBM) applied to two configurations for which experimental results are available. A linear cascade configuration with an upstream cavity and various rim seal geometries (interface between rotor and stator platform) and cavity flow rate available. A rotating configuration that is a two stage turbine including cavities close to realistic industrial configurations. Additional losses incurred by the cavity flow are measured and studied using a method based on exergy (energy balance in the purpose to generate work)

Delineating loss sources within a linear cascade with upstream cavity and purge flow

Purge air is injected in cavities at hub of axial turbines to prevent hot mainstream gas ingestion into inter-stage gaps. This process induces additional losses for the turbine due to an interaction between purge and mainstream flow. This paper investigates the flow in a low-speed linear cascade rig with upstream hub cavity at a Reynolds number commonly observed in modern low pressure turbine stages by the use of numerical simulation. Numerical predictions are validated by comparing against experimental data available. Three different purge mass flow rates are tested using three different rim seal geometries. Numerical simulations are performed using a Large Eddy Simulation (LES) solver on structured grids. An investigation of the different mechanisms associated to turbine flow including cavity and purge air is intended through this simplified configuration. The underlying mechanisms of loss are tracked using an entropy formulation. Once described for a baseline case, the influence of purge flow and rim seal geometry on flow mechanisms and loss generation are described with the emphasis to obtain design parameters for losses reduction. The study quantifies loss generation due to boundary layer on wetted surfaces and secondary vortices developing in the passage. The analysis shows different paths by which purge flow and rim seal geometry can change loss generation including a modification of the shear layer between purge and mainstream, interaction with secondary vortices and a modification of the flow behavior close to hub compared to a smooth configuration. The study shows the influence of purge flow rate and swirl on the strengthening of secondary vortices in the passage and the ability of axial overlapping rim seal to delay the development of secondary vortices compared to simple axial gaps

Numerical simulation of a counter-rotative open rotor using phase-lagged conditions. Initial validation on a single rotor case

This paper presents the Large Eddy Simulation (LES) of a propeller representative of the first rotor of a Counter Rotative Open Rotor (CROR) configuration based on a multiple frequency phase-lagged approach in conjunction with a Proper Orthogonal Decomposition (POD) data storage. This method enables to perform unsteady simulations on multistage turbomachinery configurations including multiple frequency flows with a reduction of the computational domain composed of one single blade passage for each row. This approach is advantageous when no circumferential periodicity occurs in the blade rows of the configuration and a full 360° simulation would be required. The data storage method is based on a POD decomposition replacing the traditional Fourier Series Decomposition (FSD). The inherent limitation of phase-shifted periodicity assumption remains with POD data storage but this compression method alleviates some issues associated to the Fourier transform, especially spectrum issues. The paper is first dedicated to compare the flow field obtained with the LES with phaselagged condition against full-matching URANS, LES simulations and experimental data available around the blade and in the wake of the rotor. The study shows a close agreement of the phase-lagged LES simulation with other simulations performed and a thicker wake compared to the experiments with lower turbulent activity. The analysis of the losses generated in the configuration, based on an entropy formulation and a splitting between boundary layer and secondary flow structures, shows the strong contribution of the blade boundary layer in the losses generated

Reynolds, Mach, and Freestream Turbulence Effects on the Flow in a Low-Pressure Turbine

This article presents the large-eddy simulation (LES) of a low-pressure turbine (LPT) nozzle guide vane (NGV) for different Reynolds (Re) and Mach (Ma) numbers with or without inlet turbulence prescribed. The analysis is based on a slice of an LPT blading representative of a midspan flow, where secondary flows, hub, and shroud effects are lower. The characteristic Re of the LPT can vary by a factor of four between take-off and cruise conditions. In addition, the LPT operates at different Ma values, and the incident flow can have significant levels of turbulence due to upstream blade wakes. This article investigates numerically using LES the flow around an LPT blading with three different Reynolds number Re= 175,000 (cruise), 280,000 (mid-level altitude), and 500,000 (take-off) keeping the same characteristic Mach number Ma = 0.2 and three different Mach number Ma= 0.2, 0.5, and 0.8 keeping the same Reynolds number Re= 280,000. These different simulations are performed with 0% freestream turbulence (FST) followed by inlet turbulence (6% FST). The study focuses on the influence of these three parameters (Re, Ma, and upstream turbulence) on different flow characteristics: pressure distribution around the blade, near-wall flow behavior, loss generation, and turbulent kinetic energy (TKE) budget. The results show an earlier boundary layer separation on the aft region of the blade suction side when the Re is increased, while the increase of the Ma delays separation, similar to freestream turbulence. The TKE budget led on the different cases shows the predominant effect of the turbulent production and diffusion in the wake, the axial evolution of these different terms being relatively insensitive to Re and Ma

Loss assessment of a counter rotating open rotor using URANS/LES with phase-lagged assumption

This paper presents the study of the losses generated in a Counter Rotating Open Rotor (CROR) configura- tion at three different operating conditions (approach, cutback and sideline). Unsteady Reynolds Averaged Navier-Stokes (URANS) and Large-Eddy Simulation (LES) approaches are used and compared to describe the flow field and the mechanisms of loss. Since no common circumferential periodicity occurs in the two blade rows of the configuration (11 blades for the front rotor and 9 for the rear rotor), a full 360 ◦simu- lation would be required. In order to reduce the related computational cost, a phase-lagged assumption approach is used. This method enables to perform unsteady simulations on multi-stage propulsive con- figurations including multiple frequency flows with a computational domain reduced to one single blade passage for each row. The phase-lagged approach requires a large data storage reduced in the study by a data compression method. The data compression method is based on a Proper Orthogonal Decomposition (POD) replacing the traditional Fourier Series Decomposition (FSD). The inherent limitation of the phase- shifted periodicity assumption remains with the POD data storage but this compression method alleviates some issues associated with the FSD, especially spectrum content issues. The analysis of the losses gen- erated in the configuration is based on an entropy formulation. In particular, the losses are split between boundary layer contributions and the remaining domain where wakes and secondary flows occur. The study shows the influence of the leading edge vortex on the suction side boundary layer transition of the front and rear rotor blades at high rotational speed (cutback and sideline). The main source of losses is associated with the suction side boundary layer over the front and rear rotor blades with a main peak of loss production at around 75% of the blade chord

Characterization of a highly efficient chevron‑shaped anti‑contamination device

This paper is devoted to the characterization of an optimized chevron-shaped anti-contamination device (ACD). This device can prevent efficiently the propagation of turbulence from the fuselage along the attachment line (hypothetical streamline that spreads the flow going to suction side and the one going to pressure side) of swept wings and enables the development of a new laminar boundary layer downstream. More specifically, the aim is to prevent boundary-layer transition along the attachment line by a contamination process. This process is characterized by the typical Reynolds number R and the associated Poll’s criterion. Thus, ACD efficiency will be expressed in terms of R values. Some experiments performed on a new numerically optimized ACD have shown its ability to prevent leading- edge contamination up to R values close to the natural transition process of the laminar boundary layer along the attachment line. The corresponding stability analysis of the laminar boundary layer is made using the Görtler–Hämmerlin stability approach. The study is completed with the different transition processes that can occur downstream the attachment line, around the airfoil, especially with crossflow analysis

Loss assessment of the NASA SDT configuration using LES with phase-lagged assumption

This paper presents the numerical study of the Source Diagnostic Test fan rig of the NASA Glenn (NASA SDT). Large-Eddy Simulations (LES) based on a finite volume approach are performed for the three different Outlet Guide Vane (OGV) geometries (baseline, low-count and low-noise) and three rotational speeds corresponding to approach, cutback and sideline operating conditions respectively. The full stage and nacelle geometries are considered in the numerical simulations, and results are compared to available measurements. The NASA SDT configuration is equipped respectively with 22 fan blades and either 26 of 54 vanes depending on the OGV geometry. The simulation domain could only be reduced to half of the full annulus and would still be a significant cost for the LES. In order to reduce computational cost, an LES with phase-lagged assumption approach is used. This method allows to perform unsteady simulations of multistage turbomachinery configurations including multiple frequency flows with a reduced computational domain composed of one single blade passage for each row. The large data storage required by the phase-lagged approach is handled by a compression method based on a Proper Orthogonal Decomposition replacing the traditional Fourier series decomposition. This compression method improves the signal spectral content especially at high frequency. Based on the numerical simulations, the flow field is described and used to assess the losses generated in the turbofan configuration based on an entropy approach. The results show different flow topologies for the fan depending on the rotational speed with a leading edge shock at high rotational speed. The fan boundary layer contributes strongly to losses with the majority of the losses being generated close to the leading edge for the dissipation due to mean strains and close to the recirculation zone occurring on the suction side for the turbulent kinetic energy production

Broadband noise prediction of a counter rotating open rotor based on LES simulation with phase-lagged assumption

This paper presents the broadband noise analysis of a Counter Rotating Open Rotor (CROR) configuration. The numerical study is based on a hybrid approach: a Large Eddy Simulation (LES) code solves the near-flow field of the CROR configuration and the near-to-far-field propagation is then predicted by a Ffowcs Williams-Hawkings analogy either based on a solid or porous formulation. The LES solver uses a phase-lagged assumption with a proper orthogonal decomposition for the data storage. The numerical approach is validated against wind tunnel experimental data of a CROR configuration (AI-PX7) at three different operating points focused on the broadband noise. The analysis of the numerical simulations shows the predominant effect of the rear rotor suction side on the radiated broadband noise with an increasing contribution with span location. This source is related to the impingement of front rotor wakes and the development of leading edge vortices that induce large pressure fluctuations close to the leading edge on the suction side

Description of the flow in a linear cascade with an upstream cavity part 2: Assessing the loss generated using an exergy formulation (draft)

Purge air is injected in cavities at hub of axial turbines to prevent hot mainstream gas ingestion into in- terstage gaps. This process induces additional losses for the turbine due to an interaction between purge and mainstream flow. To deal with this issue, this paper is devoted to the study of a low speed linear cas- cade with an upstream cavity at a Reynolds number representative of a low-pressure turbine using RANS and LES with inlet turbulence injection. Different rim seal geometries and purge flow rates are studied. Details about numerical methods and comparison with experiments can be found in a companion paper. The analysis here focuses on the loss generation based on the description of the flow and influence of the turbulence introduced in the companion paper. The measure of loss is based on an exergy analy- sis (i.e. energy in the purpose to generate work) that extends a more common measure of loss in gas turbines, entropy. The loss analysis is led for a baseline case by splitting the simulation domain in the contributions related to the boundary layers over the wetted surfaces and the remaining domain (i.e. the complementary of boundary layers domains) where secondary flows and related loss are likely to occur. The analysis shows the strong contribution of the blade suction side boundary layer, secondary vortices in the passage and wake at the trailing edge on the loss generation. The study of different pur ge flow rates shows increased secondary vortices energy and subsequent loss for higher purge flow rates. The rim seal geometry with axial overlapping promotes a delayed development of secondary vortices in the passage compared to simple axial gap promoting lower levels of loss

Description of the flow in a linear cascade with an upstream cavity Part 1: Influence of turbulence (draft)

In gas turbines, transitional flows are likely to occur over many components depending on the geometri- cal arrangement, inlet turbulence and Reynolds number. In the case of a low-pressure turbine, the transi- tion from a laminar to a turbulent boundary layer is generally either a bypass process due to free stream turbulence or a separation-induced transition due to the adverse pressure gradient on the blade. The overall blade losses and the operating point are strongly dependent on the ability to predict this bound- ary layer state, the size and length of the separation bubble. Therefore, turbomachinery designers require tools which accurately predict the laminar-turbulent transition. The Reynolds Averaged Navier–Stokes (RANS) formalism is currently commonly used due a to relatively low computational cost. Except partic- ular developments, this approach is not suited to predict transition processes. The Large Eddy-Simulation (LES) approach is able to predict transition processes at a higher computational cost making it suitable for low-pressure turbine applications in conjunction with inlet turbulence injection since the free-stream turbulence is generally non-negligible and affect near-wall flow behavior. The present study introduces a description of the flow in a linear cascade with an upstream hub cavity at a Reynolds number represen- tative of low-pressure turbines by three different approaches (RANS, LES and LES with inlet turbulence injection). This study shows the influence of turbulence modelling and turbulence injection at the inlet of the domain on the boundary layer state at hub and shroud modifying the secondary vortices radial migration in the blade passage and the cancelling of suction side separation bubble at high free-stream turbulence. The Kevin–Helmholtz instability at the rim seal interface is also cancelled at high free-stream turbulence