Experimental Investigations of the Aerodynamics of an Annular Compressor Cascade at Reversed Flow Conditions

Compressor unsafe operating regimes yield unsteady high speed turbulent flows in which complex aeroelastic phenomena occur. If the blade flutter and forced response behavior (i.e. aeroelastic stability) can be predicted reliably for normal flow conditions, its assessment at severe-off design conditions remains a critical task for compressor development programs. Due to complex flow fields and highly transient phenomena, reversed flow conditions are still difficult to predict. Within this frame, the understanding of the physical mechanisms of surge onset and events is essential to improve surge occurrence prediction and control. This paper presents steady-state results of an experimental investigation performed on an annular compressor cascade subjected to constant subsonic backflow inlet conditions. The investigations were carried out at EPF Lausanne, in the annular test facility for non-rotating cascades. With an upstream swirled flow corresponding to real axial turbomachine conditions, an axisymmetric flow can be achieved in the test section. This paper constitutes an introduction and a basis to an associated forthcoming experimental study, dedicated to aeroelastic investigations. The steady-state flow conditions are measured upstream and downstream of the test section, with 5-hole aerodynamic probes. The compressor cascade consists of 20 blades, mounted on 20 independent elastic springs and masses to enable the excitation of different IBPA during the aeroelastic measurements. A pair of blades was instrumented with respectively pressure and suction side pressure taps along the airfoil chord length in order to measure the surface steady-state pressure. Static pressure taps were also inserted in the casing of the test section to assess flow characteristics in the blade tip area. To provide a comparison tool to the measurements, associated CFD calculations at offdesign conditions were performed. The comparison between CFD and measurements shows a high level of confidence. Since not many experiments exist at severe off-design conditions, these experimental results are a precious data source for CFD validation.

Aeroelasticity at Reversed Flow Conditions – Part 1: Numerical and Experimental Investigations of a Compressor Cascade with Controlled Vibration

Virginie Anne Chenaux, Peter Ott

The prediction of flutter and forced response at normal flow conditions has become a standard procedure during the design of compressor airfoils. But at severe off-design conditions, the flow field becomes very complex, especially during the surge blow-down phase where reversed flow conditions occur. The correct prediction of the unsteady pressures and the resulting aerodynamic excitation or damping at these conditions remains an extremely challenging task. In the first part of the paper, basic investigations for these flow conditions are presented. Aeroelastic calculations during compressor surge are shown in the second part. Experimental investigations were performed in the Annular Test Facility for non-rotating cascades at EPF Lausanne. The test cascade was exposed to flow conditions as expected during the surge blow-down phase which is characterized by large separation regions. Measurements of the steady-state flow conditions on the blade surface, at the outer wall, upstream and downstream of the cascade provided detailed information about the steady flow conditions. The cascade was then subjected to controlled vibration of the blades with constant amplitudes and inter-blade phase angles. Unsteady pressure measurements on the blade surface and at the casing wall provided information about the resulting unsteady flow conditions. Analytical CFD calculations were performed. The steady flow field was calculated using a RANS code. Based on the steady-state flow field, unsteady calculations applying a linearized code were carried out. The agreement between measurements and calculations shows that the steady flow as well as the unsteady flow phenomena can be predicted quantitatively. In addition, knowing the blade vibration mode shape, which in this case is a torsion mode, the aerodynamic damping can be determined for the corresponding flow conditions.

2011

Numerical and Experimental Investigations of an Annular Transonic Compressor Cascade Considering Leakage Flows

Virginie Anne Chenaux, Peter Ott

The accuracy of flutter or forced response analyses of turbomachinery blade assemblies strongly depends on the correct prediction of the unsteady aerodynamic loads acting on the vibrating blades. For unsteady linearized CFD solvers, the quality of the steady-state flow solution constitutes the basis for efficient and accurate CFD unsteady computations. This paper presents the steady-state numerical and experimental results of an annular transonic compressor cascade dedicated to aeroelastic investigations. The measurements were performed in an annular test facility for non-rotating cascades. For both a subsonic and a transonic flow condition, the steady-state flow field measured was simulated and the steady-state static pressure distributions measured on the blade surface were compared to the computational predictions. Especially for the transonic flow condition, results show that the nonlinear effect induced by the presence of a shock in the blade channel requires a detailed computational model taking into account the geometrical features of the experiment. The simulations were performed with two different meshing setups. For a first setup omitting the geometrical complexity of the experimental model and only including the blade passage, results highlight that the steady-state blade loading is not predicted correctly. With a second computational setup including the cascade’s detailed geometry, the relevant physics of the experiment are captured and the prediction accuracy is improved. The presence of leakage flows that arise due to several cascade’s slits and cavities was identified and their impact on the main flow field is discussed. For both flow regimes, the computational setup, the boundary conditions imposed as well as the characterization of the physics associated to the experiment are analyzed to define the optimal level of modeling required to ensure a robust mean flow solution to the initialization of the unsteady CFD procedure.

2014

Numerical and experimental investigation of the shock-vane interaction in a transonic compressor

Stefan Kristukat

A numerical and experimental investigation of the shock-IGV (inlet guide vane) interaction in a transonic compressor is conducted at the Laboratoire de Thermique Appliquée et de Turbomachines (LTT) at the Ecole Polytechnique Fédérale de Lausanne (EPFL). The objective of the presented research work is to determine the impact of the upstream propagating shock pattern of an axial transonic compressor rotor on an aerodynamically loaded IGV and the influence of this interacting process on the compressor flow field for different operating points and aerodynamic setups. For the approach to the subject, a literature review shed light on the different research efforts over the past years. A large numerical study is conducted including the work with two different flow solvers. A numerical 3D transient analysis (commercial code) was conducted in order to provide information about the influence of the axial spacing on the overall mass flow through the stage. A numerical Q-3D transient analysis (industry owned code) on one streamline at a constant rotor inlet Mach number is performed in order to compare the shock-IGV interaction for different vane types similar to what is known in literature. A semi-analytical model was developed in order to evaluate in a fast approach the impact of a rotor bow shock on an upstream situated vane. The results of the numerical investigation can be summarized as following: The overall results of this investigation show that the upstream vane of a transonic compressor rotor transforms the rotor forcing function into an excitation with vane shape depending orientation and force. The time averaged flow field of the IGV does not show a measurable change of the exit flow angle due to the shock-IGV interaction for the studied operating points and axial distances. The 3D transient calculations do not show any influence of the examined axial spacing between vane and blade on the mass flow rate through the stage. The results of the Q3D analysis are compared with the semi-analytic approach and the measurements, both showing with some restrictions good accordance to the model. The results of the numerical investigation give an accurate prediction of the transient aerodynamic load of the upstream vane. An analytic tool was developed which uses three input parameters, the rotor relative inlet Mach number, the stagger angle of the rotor and the axial spacing between vane and rotor in order to provide an estimation of the amplitude of the pressure variation on the upstream situated vane. An extensive modification of the existing test facility of the LTT precedes the experimental study. The main steps of the approach are: A subsonic axial compressor test facility was modified to allow test runs at transonic rotor inlet conditions. A perfect leak tightness of the closed circuit was accomplished to enable its operation in heavy gas. Heavy gases like refrigerants have been used in many studies for compressor testing. While respecting certain restrictions during the use of heavy gas, a reduction of operation and manufacturing costs of the facility is beneficial. A high molecular weight of the gas reduces the sonic speed to the half of that of air thus reducing the dimension and the rotational speed of the rotor significantly and consequently reduces the demand in the material quality of the blading. A 1.5 stage transonic compressor had been designed and manufactured for the heavy gas (R134a) in a first step. Preliminary measurements showed that the influence of the downstream stator makes it difficult to separate the influence of shock-IGV interaction on the compressor flow field from loss generation mechanisms downstream of the rotor. Thus, the stator was removed and measurements were conducted in a one stage transonic compressor. Modifications on the construction of the testing setup are done in a way that the distance between IGV and rotor can be varied by means of spacers. Thus the flow can be determined for up to four different axial distances at varying rotational speed for different operating points. For transonic operation the time-dependent load of the IGV (inlet guide vane) due to the forcing function of the rotor bow shock was recorded with a high speed data acquisition system. The results of the experimental investigation can be summarized as following: No influence of the axial spacing on the efficiency and the mass flow was observed in the investigation. The operation characteristics remained equal between the minimum and the maximum vane-blade spacing. The measured global values of the stage are close to the values the design was made for, although the overall efficiency was generally lower than predicted. The experimental results of the pressure variation on the upstream vane agree well with the predicted values. The study in general showed a good matching between the numerical and the experimental investigation taking into account a rotor blade shape which is not optimized to the extend. The shock-vane interaction is responsible for an excitation of the entire vane blade. With decreasing distance between the rotor leading edge and vane trailing edge the excitation increases. The strength and orientation of the reflected shock/pressure wave part depends upon the vane shape. A more sophisticated design of the rotor blade shape should be in the focus of further investigations using this test facility at transonic rotor inlet conditions. Within this investigation a tool was developed in order to predict the load variation on the vane driven by the transient rotor forcing function. The tool maps the fact that the pressure on the vane surface oriented against rotational direction of the rotor exceeds the pressure of the passage shock due to the amplification effect of the reflection of the shock on a surface. Its possible application not only for compressors but also for turbine related topics should be an objective for similar investigations.

EPFL2008

Numerical and experimental investigation of the shock-vane interaction in a transonic compressor

Stefan Kristukat

EPFL2008

Aeroelasticity at Reversed Flow Conditions – Part 1: Numerical and Experimental Investigations of a Compressor Cascade with Controlled Vibration

Virginie Anne Chenaux, Peter Ott

2011

Numerical and Experimental Investigations of an Annular Transonic Compressor Cascade Considering Leakage Flows

Virginie Anne Chenaux, Peter Ott

2014