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

Description of the Flow in a Two-Stage Low-Pressure Turbine With Hub Cavities

In gas turbine, multi-stage row blading and technological effects can exhibit significant differences for the flow compared with isolated smooth blade rows. Upstream stages promote a non-uniform flow field at the inlet of the downstream rows that may have large effects on mixing or boundary layer transition processes. The rows of current turbines (and compressors) are already very closely spaced. Axial gaps between adjacent rows of approximately 1/4 to 1/2 of the axial blade chord are common practice. Future designs with higher loading and lower aspect ratios, i.e., fewer and bigger blades, and the ever present aim at minimizing engine length or compactness, will aggravate this condition even further. Interaction between cascade rows will therefore keep increasing and need to be taken into account in loss generation estimation. Also the cavities at hub platform induce purge flow blowing into main annulus and additional losses for the turbine. A robust method to account for the loss generated due to these different phenomena needs to be used. The notion of exergy (energy in the purpose to generate work) provides a general framework to deal with the different transfers of energy between the flow and the gas turbine. This study investigates the flow in a two-stage configuration representative of a low-pressure turbine including hub cavities based on large eddy simulation (LES). A description of the flow in the cavities, the main annulus, and at rim seal interface is proposed. The assessment of loss generated in the configuration is proposed based on an exergy analysis. The study of losses restricted to boundary layer contributions and secondary flows show the interaction processes of secondary vortices and wake generated in upstream rows on the flow in downstream row

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

Numerical study of a linear cascade with upstream cavity using various rim-seal geometries and purge rates

This paper introduces numerical investigation of a lowspeed linear cascade rig with upstream cavity at Reynolds number commonly observed in modern low-pressure turbine stage. This configuration was tested experimentally during the EU project MAGPI (2007-2011) focused on the impact of secondary air systems on gas turbine performance. Three different purge mass flow rate have been tested numerically using three different rim-seal geometry (axial clearance, simple and double radial overlap). The mechanisms and influence of these two parameters on loss generation for the main annulus flow is investigated. The ability of high-fidelity numerical methods to deal with such kind of configuration is assessed by comparing several unsteady codes over the axial geometry at three purge mass flow rate available. Two Large-Eddy Simulation (LES) solvers based respectively on structured and unstructured meshes and a LES-LBM approach in which equations discretization is based upon a Lattice-Boltzmann Method (LBM) and a Sub-Grid Scale (SGS) model from LES developments are used. The comparison against MAGPI experiments and previous Reynolds Averaged Navier-Stokes (RANS) simulation show that despite a variety of flow dynamics modelling, discretization and numerical parameters, the different unsteady codes are well able to recover aerodynamic quantities into the mainstream passage in which purge flow blows at various rate and different rim-seal geometry. Further results obtained from such high-fidelity methods exhibit strong interaction of separated hub boundary at rimseal interface with nonuniform pressure field imposed by downstream blade leading to a strong in-depth of mainstream flow into the cavity for the axial clearance. Simple and double overlap damper this phenomenon due to localized recirculation zone into the rim-seal. In addition, hub passage vortex and blade suction side unsteadiness are shown to be strongly related to the vortex shedding process occurring at rim-seal interface

VORTEX LATTICE METHOD FOR THE CALCULATION OF THE TIP LEAKAGE FLOW: EVALUATION ON A SINGLE BLADE

International audienceThis paper investigates the use of a Vortex Lattice Method to simulate tip-leakage flow with small computational effort. The module PyLiSuite presented in the present paper has been completely developed from scratch and is validated on twodimensional and three-dimensional basic cases. The validation lies on the estimate of the lift coefficient computed from the circulation given by the module. The capabilities of PyLiSuite regarding tip-leakage flow are gauged in comparison with novel experimental measurements on a single blade with an adjustable gap. The results show a good prediction of the shape and size of the tip-leakage vortex for large tip gaps. Differences in the position and the deficit of static pressure in the core of the vortex are noted. The future improvements on the module concern the influence of viscosity to be accounted for and the computation time which could be shortened

Comparison of Various CFD Codes for LES Simulations of Turbomachinery: From Inviscid Vortex Convection to Multi-Stage Compressor

peer reviewedSome possible future High Fidelity CFD codes for LES simulation of turbomachinery are compared on several test cases increasing in complexity, starting from a very simple inviscid Vortex Convection to a multistage axial experimental compressor. Simulations were performed between 2013 and 2016 by major Safran partners (Cenaero, Cerfacs, CORIA and Onera) and various numerical methods compared: Finite Volume, Discontinuous Galerkin, Spectral Differences. Comparison to analytical results, to experimental data or to RANS simulations are performed to check and measure accuracy. CPU efficiency versus accuracy are also presented. It clearly appears that the level of maturity could be different between codes and numerical approaches. In the end, advantages and disadvantages of every codes obtained during this project are presented