Off-design considerations through the properties of some pressure-ratio line of radial inflow turbines

Radial turbines are commonly used in applications involving operation through severe off-design conditions. The emergence of variable-geometry systems leads to the distinction between two off-design concepts: operational and geometric off-designs. Both of these operating constraints should be integrated in the design procedure. Recent developments in prediction and optimization methods allowed such an integration, but involving complex algorithms is coupled with semiempiric loss models. This paper provides a basis to obtain simple information from an existing or predesigned machine, for both operational and geometric offdesign conditions. An alternative turbine map is defined using loading and flow coefficients. A one-dimensional analysis shows that the constant pressure-ratio lines are straight lines whose slope is remarkably correlated with the pressure-ratio value and geometrical characteristics. This theoretical approach is validated against the experimentation of two machines, the linearity is observed in both cases. The direct influence of the stator configuration on the pressure-ratio lines confirms the applicability of this work to variable-geometry stages. A dimensionless cross-section of the stator is thus defined. However, the unexpected displacement of the intercept of the pressure-ratio lines limits the application field of this method. Nevertheless, a simple performance prediction analysis is proposed for blocked mass flow operation

Performance Back-deduction from a Loading to Flow Coefficient Map: Application to Radial Turbine

Radial turbine stages are often used for applications requiring off-design operation, as turbocharging for instance. The off-design ability of such stages is commonly analyzed through the traditional turbine map, plotting the reduced mass-flow against the pressure-ratio, for reduced-speed lines. However, some alternatives are possible, such as the flow-coefficient (Ψ ) to loading-coefficient (φ) diagram where the pressure-ratio lines are actually straight lines, very convenient property to perform prediction. A robust method re-creating this map from a predicted Ψ−φ diagram is needed. Recent work has shown that this back-deduction quality, without the use of any loss models, depends on the knowledge of an intermediate pressure-ratio. A modelization of this parameter is then proposed. The comparison with both experimental and CFD results is presented, with quite good agreement for mass flow rate and rotational speed, and for the intermediate pressure ratio. The last part of the paper is dedicated to the application of the intermediate pressure-ratio knowledge to the improvement of the deduction of the pressure ratio lines in the Ψ−φ diagram. Beside this improvement, the back-deduction method of the classical map is structured, applied and evaluated

Two-equation modeling of turbulent rotating flows

The possibility to take into account the effects of the Coriolis acceleration on turbulence is examined in the framework of two-equation eddy-viscosity models. General results on the physical consistency of such turbulence models are derived from a dynamical-system approach to situations of time-evolving homogeneous turbulence in a rotating frame. Application of this analysis to a (k,epsilon) model fitted with an existing Coriolis correction [J. H. G. Howard, S. V. Patankar, and R. M. Bordynuik, "Flow prediction in rotating ducts using Coriolis-modified turbulence models", ASME Trans. J. Fluids Eng. 102, (1980)] is performed. Full analytical solutions are given for the flow predicted with this model in the situation of homogeneously sheared turbulence subject to rotation. The existence of an unphysical phenomenon of blowup at finite time is demonstrated in some range of the rotation-to-shear ratio. A direct connection is made between the slope of the mean-velocity profile in the plane-channel flow with spanwise rotation, and a particular fixed point of the dynamical system in homogeneously sheared turbulence subject to rotation. The general analysis, and the understanding of typical inaccuracies and misbehavior observed with the existing model, are then used to design a new model which is free from the phenomenon of blowup at finite time and able to account for both of the main influences of rotation on turbulence: the inhibition of the spectral transfer to high wave numbers and the shear/Coriolis instability

Potential of power recovery of a subsonic axial fan in windmilling operation

During the last decades, efforts to find efficient green energy solutions have been widely increased in response to environmental concerns. Among all renewable energies, this paper is focused on wind power generation. To this end, a windmilling axial fan in turbine operation is experimentally and numerically investigated. Under specific conditions, the studied fan is naturally freewheeling. Consequently, the main objective of this analysis is to determine whether or not this intrinsic windmilling behavior can be optimized for power generation. A preliminary study of the fan is dedicated to the knowledge of the fan characteristics in normal operating conditions. Then, two windmilling configurations (direct and reverse flow direction) are tested and compared on the basis of the output power. An analysis of the velocity triangle gives the opportunity to evaluate the energy recovery potential of both solutions. Of the two, the reversed configuration showed a higher level of output power than the direct one

Description of the unsteady flow pattern from peak efficiency to near surge in subsonic centrifugal compressor stage

This paper aims to describe the flow structure modifications when the operating point moves from peak efficiency to near stall condition in a moderate pressure ratio centrifugal compressor stage consisted of a splittered unshrouded impeller and a vaned diffuser. The investigations are based on three-dimensional U-RANS simulation results. The flow is described in the impeller and in the vaned diffuser through time-averaged flow quantities and unsteady fluctuations. Results show that at low mass flow rate, the effects of secondary flow in the impeller are more pronounced, inducing both, high time-averaged values and temporal fluctuations of the flow angle near the shroud at the diffuser inlet, leading to vane suction side boundary layer separation. Pressure waves due to impeller diffuser interaction spread through the vaned diffuser generating unsteadiness which intensifies at near surge condition

Numerical error evaluation for tip clearance flow calculations in a centrifugal compressor

Since globally mesh independent solution are still beyond available computer resources for industrial cases, a method to quantify locally the numerical error is proposed. The design of experiments method helps selecting mesh parameters that influence the tip clearance solution, so that additional meshes are computed to evaluate the numerical error on the shroud friction coefficient. In the field of CFD applied to turbomachinery, this study results from a partnership between ENSICA, Liebherr-Aerospace Toulouse and Numeca International. This paper focuses on numerical error evaluation for RANS simulations, applied to centrifugal compressor flow field calculations. CFD is now commonly used for centrifugal compressor design optimization, but, as Hutton and Casey develop in [1], there is an urging demand for improved quality and trust in industrial CFD. Indeed, this stresses the need for comprehensive and thorough numerical error evaluation, namely the process of verification, as defined for example by Oberkampf and Trucano in [2]. Unfortunately, 3D turbulent calculations for turbomachinery components are still very demanding in computational resources and, to the knowledge of the author, there is no published result concerning comprehensive verification of the entire flow field in centrifugal compressors. As a first step on the way to achieve that, this paper presents a method aiming at the obtention of a numerical solution that can be regarded as locally mesh-independent. In other words, the objective is to compute the flow field on a grid such that the solution obtained has a specific region where the numerical error is negligible. It has long been recognized that the tip clearance of a centrifugal compressor is of paramount importance for aerodynamic performances, which means that accurately predicting the flow field in this region is crucial for accurate prediction of performances by means of CFD codes. Numerous studies have been published that compare numerical and experimental results in the tip region. However, in these studies, numerical error still remains an issue; for instance Basson and Lakshminarayana [3] show excellent comparisons with experiments, but they attribute the remaining discrepancies to insufficient grid resolution. Indeed, accurate predictions of global effects, such as efficiency, require a fine description of flow details. Therefore, friction at the shroud endwall is the concern of the study, since it is a very sensitive indicator of the quality of the velocity profile’s prediction at the wall

Unified classification and characterization of axial turbomachines and propellers

Reaching the mid/long-term air transport emission reduction goals imposed by both European and American standards impose increasing the propulsive systems’ adaptability to various operating conditions, in order to maximize the aircraft overall efficiency all along the flight mission. This implies the enlargement of the design space of propulsive systems such that it can even be operated equally as a compressor or turbine, which leads to rethink the paradigm of designing turbomachines. The continuity in the definition and characterization of different types of turbomachines should be restored which is proposed through this contribution. Analytical relationships allowing to switch between compressor map, propeller map and map are developed. To minimize the inputs of the maps’ conversion relations, a methodology to extract mean flow features from any characterization map is presented, namely the rotor outlet relative flow angle and mean streamline radius. The application of the characterization maps’ conversion relations on a turbofan’s single-stage axial fan and on a propeller allowed their validation through the physical coherence of the results. The flow features extraction methodology also showed very satisfying results with comparison to experiments. Eventually, the ability of the formalism as a powerful performance analysis tool for all kind of turbomachines is stressed out, which makes it the best candidate for the unified treatment of turbomachines

Mise en relation analytique de la cinématique de l’écoulement et des performances d’un rotor caréné à Mach de vol intermédiaire

Reaching the European long-term air transport emission reduction goals impose shifting by 2050 toward hybridation/electrification of airplanes as well as global optimization of flight missions. This implies developping distributed propulsion systems through the multiplication of small shrouded rotors operating at high efficiency within a wide range. Present litterature lacks a low-order shrouded rotor model specifically designed to work on such wide operating range. In this paper, the working range of the existing models is benchmarked on the basis of similarity factors built on an innovative expression of the shrouded rotors problem. It is established here that the model of Jardin et al. [1] presents the wider working range thanks to the "homokinetic" surface definition but lacks robustness and needs an additional input that is related to the "homokinetic" surface. The extension of this model to the compressible domain increases its robustness and an empirical model of the "homokinetic" surface helps droping the additional input

Performance of a Thrust Vectoring Solution for UAV

This paper focuses on the performance of a thrust-vectoring system tted to UAV/UCAV applications. It consists on the implementation of a cover-nozzle at the rear of the engine nozzle, which can rotate in the pitching and yawing planes. An experimental investigation is developed. A rst part of this investigation focuses on the consequences of the presence of the system on the engine behavior. Those consequences can prove drastic if the outlet cross-section of the cover-nozzle is not designed properly. A second part is dedicated to the performance of the system itself. The intensity of the lateral forces created are checked for dierent geometric conguration of the cover-nozzle. Some linearity is also expected between the deviation of the cover nozzle and the creation of lateral force, to ease the handling quality implementation on the nal aircraft. Those considerations are also analyzed as a function of the geometric parameters. The conclusions lead to some design recommendation for the cover nozzle geometry

Innovative fan design for both high compressor and windmilling performance

controlledwindmilling conditions is numerically investigated. Initially, the ram air is equipped with a classic fan, only conceived to work as a compressor. The purpose of the new fan is to be able to work in antagonist functioning modes with high efficiencies: a compressor mode during which energy is given to the flow and a turbine mode (windmilling conditions) where energy is extracted from it. A numerical study was conducted to check the results predicted by the design tool on the new geometry. Good efficiencies were observed in both compressor and turbine modes, confirming the relevance of the design method. Afterwards, a thorough local comparison was achieved between the two fans to better understand the flow topology of the new design