Giovanni Brignole

Time Resolved Simulation and Experimental Validation of the Flow in Axial Slot Casing Treatments for Transonic Axial Compressors

Building on the experience of previous investigations, a casing treatment was developed and applied to an axial transonic compressor stage, in literature referred to as Darmstadt Rotor 1. The aerodynamics of the experimental compressor stage was improved by applying axially orientated semicircular slots to the original plain casing, which both enhanced the operating range and design point efficiency. A gain in total pressure ratio along the entire design speed line was also observed. Within the scope of this study four different axial casing treatments were designed. Their effect on the flow in a transonic axial compressor stage was investigated parametrically using time-resolved 3D-FANS simulations with a mesh of approximately 4.8 · 106 grid points. This research aims to identify correlations between the geometrical cavity design and the changed channel flow. The findings help to formulate parameters for evaluating the performance of casing treatments. These criteria can further be used as target functions in the design optimisation process. The predicted behaviour of the transonic compressor was validated against experiments as well as an alternative numerical model, the non-linear harmonic method. Both confirmed the effect of the slots in raising efficiency as well as moving the design speed line towards higher pressure ratios. In the experiments, the addition of the slots increased the total pressure ratio at stall conditions by more than 5% and reduced mass flow from 87.5% of the design mass flow to less than 77.5% compared to the original geometry.© 2008 ASME

Experimental and Numerical Analysis of Axial Skewed Slot Casing Treatments for a Transonic Compressor Stage

An investigation of a single-stage transonic compressor with axial skewed slot casing treatments is presented. The studied compressor stage is characterised by a design mass flow rate of 16 kg/s at a total pressure ratio of 1.5 and a rotor tip speed of 400m/s. The research comprises experimental measurements as well as time-resolved simulations at full and part speed. Total pressure ratio measurements and efficiency speedlines are complemented by traversing downstream of the stator and static pressure measurements at the rotor end wall. The experimental work is supported by unsteady computational fluid dynamics analysis to provide further insight into the ruling flow phenomena. The simulations were carried out fully three-dimensionally in a computational domain with approximately 4.8 million grid points including the cavity mesh. The application of the axial skewed slots led to both, an enhanced operating range and an increased design point efficiency. Rises in total pressure ratio along the entire speed lines were observed.Copyright

Aerodynamic Design of Abradable Liners With Integrated Endwall Treatments for Axial Compressor Rotors

In this study endwall treatments, which are integrated into an abradable liner, are used to reduce the liner solidity, defined by the volumetric proportion between endwall treatments and solid casing. Consequently the milled off amount of liner material during the rubbing process is decreased. The mechanical stresses in the rotor blades are thus supposed to be reduced, so that liner materials with higher strength can be used or additional blade tip coatings are dispensable. Accordingly, the purpose of the present study was to develop geometries of endwall treatments, which reduce the liner solidity as much as possible without degrading the stage performance of the test compressor. The focus of the work lies exclusively on the aerodynamics. Investigations were made by steady and unsteady computational fluid dynamics on a transonic single stage axial compressor with two different tip clearance sizes (0.64%/1.28% span). The developed configurations resemble casing treatments, comparable to axial slots and circumferential grooves, which are adapted to the specific tasks of liners. Solidity could be reduced by as much as 29% with negligible efficiency degradation for small tip gaps and increased efficiencies for large tip clearances.Copyright

A Numerical Model for Casing Treatment Applications in Axial Flow Compressors

A numerical model has been developed to reproduce the effects of complex casing treatments (CT) in steady RANS simulations of multistage compressors. While some CTs, such as circumferential grooves, can be described by a rotation surface and can thus easily be included in conventional steady simulations, the CFD analysis of other casing treatments like axial slot or recessed vanes, currently requires a time-resolving analysis of the interaction between such structures and rotating parts. At present unsteady simulations are still too time consuming to be used in the early phase of a compressor design. In the presented study a numerical model was developed for casing treatment applications, to introduce the unsteady effects caused by such casing treatments into steady CFD-simulations. With the help of the model, non-axisymmetric elements can be eliminated from the geometry allowing a steady simulation to be used. The flow acceleration and redirection caused by these geometrical elements is replaced with adequate source terms introduced into the three-dimensional Navier-Stokes equations. These source terms, derived from a consecutive time- and circumferential averaging of the three-dimensional unsteady Reynolds-averaged Navier-Stokes-equations, arise from the momentum and energy equations. Using these additional terms, the CT-model simulates both the pressure forces that the walls of the real casing treatment exert on the flow, and the effects of the mean blockage induced by the omitted geometry. Furthermore, the deterministic stresses, caused by a circumferentially inhomogeneous flow within the CT-structure, are modeled. The source terms consist of geometrical data that can be derived directly from the real geometry of the casing treatment as well as physical quantities of the time-averaged flow in the real casing treatment. The latter terms can be obtained from a reference unsteady simulation. In the presented case one unsteady simulation was sufficient to set up the model for a complete speed line. The model was implemented into the three-dimensional Navier-Stokes-code TRACE [5][12]. By using steady instead of unsteady CFD simulations, the time required for a speedline computation was reduced by a factor of 10. At the same time, the numerical results of the CT-model showed good alignment with the reference data. The model was evaluated for several different styles of compressors. In this paper various results are presented, including speedlines as well as radial inflow- and outflow-profiles.© 2013 ASME