Pascale Kulisa

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.

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.

Heat transfer prediction on transonic turbine blades

The design and optimization of turbine blades submitted to high-temperature flows require the prediction of aerodynamic and thermal flow characteristics. A computation method for aerothermal viscous flows has been developed. The method is based on a compressible boundary layer approach. Tests were performed on turbine blade configurations. These tests include most difficulties that can be encountered in reality: laminar-turbulent transition, separation bubbles, strong accelerations, shock waves. Predictions of the wall heat transfer prove to be satisfactory.

Potential Flow Field Generated by Downstream Moving Bars and Propagated on a Turbine Blade at Low Reynolds Number

During the last few decades, the size and weight of turbo-machinery have been continuously reduced. However, by decreasing the distance between rows, rotor-stator interaction is strengthened. Two interactions now have the same magnitude: wake interaction and potential effect. Studying this effect is essential to understand rotor-stator interactions. Indeed, this phenomenon influences the whole flow, including the boundary layer of the upstream and downstream blades, ergo the stability of the flow and the efficiency of the machine. A large scale turbine cascade followed by a specially designed rotating cylinder system is used. Synchronised velocity LDA measurements on the vane profile show the flow and boundary layer behavior due to the moving bars. To help the general understanding and to corroborate our experimental results, numerical investigations are carried out with an unsteady three dimensional Navier-Stokes code. Moreover, the numerical study informs about the potential disturbance to the whole flow of the cascade.Copyright

Numerical Investigation of Film Cooling Flow Induced by Cylindrical and Shaped Holes

Abstract: The present study is the second half of a two part work carried out in collaboration with SNECMA which tends to investigate a shaped hole film cooling experimentally and numerically. The aim of this paper is the numerical study of 3D phenomena induced by cylindrical and shaped hole film cooling on a flat wall.

Heat Transfer Computations for Turbine Blade Airfoils

The design and optimization of turbine blades subjected to high temperature flows require the prediction of aerodynamic and thermal flow characteristics. A computation of aerothermal viscous flow model has been developed suitable for the turbine blade design process. The computational time must be reduced to allow intensive use in an industrial framework. The physical model is based on a compressible boundary layer approach, and the turbulence is a one-equation model. Special attention has been paid to the influence of wall curvature on the turbulence modelling. Tests were performed on convex wall flows to validate the turbulence model. Turbine blade configurations were then computed. These tests include most difficulties that can be encountered in practice : laminar-turbulent transition, separation bubble, strong accelerations, shock wave. Satisfactory predictions of the wall heat transfer are observed.Copyright