Isabelle Trébinjac

Analysis of the Interrow Flow Field Within a Transonic Axial Compressor: Part 1—Experimental Investigation

This paper relates to two-dimensional laser two-focus (L2F) anemometry measurements, conducted in a transonic research high-pressure compressor. Curved glass, which conforms to the shroud contour, was used. The resulting optical distortions of the control volume were corrected using an original and inexpensive optical assembly. Synchronized measurements were performed on two surfaces of the IGV-rotor interrow region. Particular care was taken in evaluating the capabilities of our L2F technique to accurately describe a flow field with strong gradient. The results presented show that the flow field is dominated by the moving oblique shock and its interaction with the IGV wake.

Analysis of the Unsteady Flow Field in a Centrifugal Compressor from Peak Efficiency to Near Stall with Full-Annulus Simulations

This study concerns a 2.5 pressure ratio centrifugal compressor stage consisting of a splittered unshrouded impeller and a vaned diffuser. The aim of this paper is to investigate the modifications of the flow structure when the operating point moves from peak efficiency to near stall. The investigations are based on the results of unsteady three-dimensional simulations, in a calculation domain comprising all the blade. A detailed analysis is given in the impeller inducer and in the vaned diffuser entry region through time-averaged and unsteady flow field. In the impeller inducer, this study demonstrates that the mass flow reduction from peak efficiency to near stall leads to intensification of the secondary flow effects. The low momentum fluid accumulated near the shroud interacts with the main flow through a shear layer zone. At near stall condition, the interface between the two flow structures becomes unstable leading to vortices development. In the diffuser entry region, by reducing the mass flow, the high incidence angle from the impeller exit induces a separation on the diffuser vane suction side. At near stall operating point, vorticity from the separation is shed into vortex cores which are periodically formed and convected downstream along the suction side.

Effect of Unsteadiness on the Performance of a Transonic Centrifugal Compressor Stage

Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. The characteristic curves of the compressor stage resulting from the unsteady simulations and the experiments show a good agreement over the whole operating range. On the contrary, the total pressure ratio resulting from the steady simulations is clearly overestimated. A detailed analysis of the flow field at design operating point led to identify the physical mechanisms involved in the blade row interaction that underlie the observed shift in performance. Attention was focused on the deformation in shape of the vane bow shock wave due its interaction with the jet and wake flow structure emerging from the impeller. An analytical model is proposed to quantify the time-averaged effects of the associated entropy increase. The model is based on the calculation of the losses across a shock wave at various inlet Mach numbers corresponding to the moving of the jet and wake flow in front of the shock wave. The model was applied to the compressor stage performance calculated with the steady simulations. The resulting curve of the overall pressure ratio as a function of the mass flow is clearly shifted toward the unsteady results. The model, in particular, enhances the prediction of the choked mass flow.

Analysis of the interrow flow field within a transonic axial compressor : Part 2-unsteady flow analysis

An analysis of the experimental data, obtained by laser two-focus anemometry in the IGV-rotor interrow region of a transonic axial compressor, is presented with the aim of improving the understanding of the unsteady flow phenomena. A study of the IGV wakes and of the shock waves emanating from the leading edge of the rotor blades is proposed. Their interaction reveals the increase in magnitude of the wake passing through the moving shock. This result is highlighted by the streamwise evolution of the wake vorticity. Moreover, the results are analyzed in terms of a time-averaging procedure and the purely time-dependent velocity fluctuations that occur are quantified. It may be concluded that they are of the same order of magnitude as the spatial terms for the inlet rotor flow field. That shows that the temporal fluctuations should be considered for the three-dimensional rotor time-averaged simulations.

Analysis of the Inter-Row Flow Field Within a Transonic Axial Compressor: Part 2 — Unsteady Flow Analysis

An analysis of the experimental data, obtained by laser two-focus anemometry in the IGV-rotor inter-row region of a transonic axial compressor, is presented with the aim of improving the understanding of the unsteady flow phenomena. A study of the IGV wakes and of the shock waves emanating from the leading edge of the rotor blades is proposed. Their interaction reveals the increase in magnitude of the wake passing through the moving shock. This result is highlighted by the streamwise evolution of the wake vorticity. Moreover, the results are analyzed in terms of a time averaging procedure and the purely time-dependent velocity fluctuations which occur are quantified. It may be concluded that they are of the same order of magnitude as the spatial terms for the inlet rotor flow field. That shows that the temporal fluctuations should be considered for the 3D rotor time-averaged simulations.Copyright

Study and Control of a Radial Vaned Diffuser Stall

The aim of the present study is to evaluate the efficiency of a boundary layer suction technique in case of a centrifugal compressor stage in order to extend its stable operating range. First, an analysis of the flow pattern within the radial vaned diffuser is presented. It highlights the stall of the diffuser vanes when reaching a low massflow. A boundary layer separation in the hub-suction side corner grows when decreasing the massflow from the nominal operating point to the surge and finally leads to a massive stall. An aspiration strategy is investigated in order to control the stall. The suction slot is put in the vicinity of the saddle that originates the main separating skin-friction line, identified thanks to the analysis of the skin-friction pattern. Several aspiration massflow rates are tested, and two different modelings of the aspiration are evaluated. Finally, an efficient control is reached with a removal of only 0,1% of the global massflow and leads—from a steady-state calculations point of view—to an increase by 40% of the compressor operating range extent.

Analysis of the flow in a transonic centrifugal compressor stage from choke to surge

Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. Unsteady three-dimensional simulations were performed with the code elsA that solves the turbulent-averaged Navier–Stokes equations, at three operating points: choked flow, peak efficiency, and near surge. Numerical results were validated with experimental data coming from laser Doppler anemometry and unsteady pressure measurements. This article focuses on the change in flow structures when the operating point moves from choke to surge. The main changes in the impeller consist in an enlargement of the wake (of the jet-wake flow structure) and an increase in the exit time-averaged flow angle. Consequently, in the diffuser passage, the main flow trajectory moves towards the vane pressure side, and the boundary layer separation transfers from pressure side to suction side. The interaction between the vane bow shock wave and the impeller blade leads to pressure waves α+, which propagate in the diffuser passage. These pressure waves generate alternately opposite and favourable pressure gradients, which drive the boundary layers to periodic separation. From choke to surge, the intensity of the pressure waves α+ increases. The interaction also leads to subsonic pockets Г, which are torn out from the vane-leading edge bow shock and swept along the vane suction side. The induced change in the shock shape and location combined with the severe hub/suction side corner separation are thought to be at the origin of the surge inception.

Experimental and numerical investigation of the flow field in a high-pressure centrifugal compressor impeller near surge

Abstract Numerical and experimental investigations were conducted in a transonic centrifugal compressor stage composed of a backswept splittered unshrouded impeller and a vaned diffuser. The present article focuses on the results obtained within the impeller, at an operating condition close to the surge of the compressor. The experimental results were obtained from a laser Doppler anemometry investigation. Unsteady numerical simulations of the compressor stage were performed using a three-dimensional Reynolds-averaged Navier—Stokes code with a phase-lagged technique, at both peak efficiency and close to surge operating conditions. A good agreement between the experiments and simulations were obtained, which justifies the use of the computational fluid dynamics results for the comparison of the flow field at both operating conditions (peak efficiency and near surge). Even if the change in flow field within the impeller from peak efficiency to near surge looked to be gradual, an overall rotation of the whole flow in the blade passages led to a non-homogeneous flow at the impeller exit in terms of angle and velocity level. Therefore, the vaned diffuser has to tolerate upstream flows, which are all the more distorted as the operating point moves towards surge.

Effect of the Unsteadiness on the Diffuser Flow in a Transonic Centrifugal Compressor Stage

The study is focused on the analysis of the flow structure within the vaned diffuser of a transonic high-pressure centrifugal compressor stage. The analyzed time-dependent flow field comes from unsteady computations of the stage using a 3D Navier-Stokes code with a phase-lagged technique, at an operating point close to the design point. A good comparison with available experimental data allowed the use of CFD for investigating the details of the flow in order to assess the effect of the unsteadiness in the diffuser flow development. Applying various data processing techniques, it is shown that the unsteadiness is due to the jet and wake flow structure emerging from the radial impeller and to the pressure waves brought about by the interaction between the vane bow shock wave and the impeller blade. The interaction between the pressure waves and the vane pressure side boundary layer leads to a pulsating behavior of separated bubbles within the diffuser. The pressure waves are similar in shape and strength whatever the blade height. The observed change in the flow field from hub to tip is due to migration of the low momentum fluid contained in the wake toward the pressure side/hub corner.

A Novel Approach to Evaluate the Benefits of Casing Treatment in Axial Compressors

Passive control devices based on casing treatments have already shown their capability to improve the flow stability in axial compressors. However, their optimization remains complex due to a partial understanding of the related physical mechanisms. In order to quantitatively assess the interaction between slots and the blade tip flow, the present paper develops a novel analysis methodology based on a control-volume approach located in the rotor tip region. This methodology may be used for analyzing the casing treatment based on both axi- and non-axisymmetric slots design. The second issue of the paper focuses on the application of the current approach to better understand the effects of axi- and non-axisymmetric grooves in three different axial compressors which differ by the flow regime (subsonic/transonic) and the smooth casing shape (cylindrical/concave). Numerical simulations are performed, and results of the current approach with and without casing treatments are compared.