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

Experimental and numerical flow analysis of low-speed fans at highly loaded windmilling conditions

This paper aims for the analysis of experimental and numerical results of windmilling flow topologies far from freewheeling condition. Two fans were investigated: a baseline design and an innovative one meant to reach good performance in both compressor and turbine modes. Experiments are conducted with global and local characterizations to determine energy recovery potential and local loss mechanisms. The numerical study is carried out with mixing plane steady simulations, the results of which are in fair agreement with experimental data. The difference of local topology between freewheeling and highly loaded windmill demonstrates that classical deviation rules such as Carter’s are not well-suited to highly loaded windmilling flows. Finally, under certain conditions, the minor influence of the stator on the rotor topology indicates that non rotating elements can be considered as loss generators

Numerical analysis of secondary flow topologies of low-speed axial fans from compressor to load-controlled windmill

mode to loaded windmill. A first objective is to identify generic patterns of rotor separation topology at windmill and bring light on the mechanisms responsible for it. This study also aims at numerically providing new elements of understanding on tip leakage flows and curvature effects while shifting from compressor to turbine operation. Two machines were investigated : a conventional fan and an innovative design meant to reach both high compressor and turbine efficiencies. In compressor or turbine operation, the tip leakage flow is located on the blade suction side. However, the location of the latter is inverted from one mode to the other. The inversion of the pressure gradient leads, near the hub boundary layer of a blade passage, to an inversion of the cross-flow direction

Experimental validation of the continuous evolution of flow topology from compressor mode to highly loaded windmill of low-speed axial machines

In aeronautics, onboard axial fans, only used on the ground to cool heat exchangers, can be regarded as potential electrical generation devices when operating at freewindmill in flight. In this case, the appropriate windmilling configuration is the load-controlled one, from which energy recovery is possible. This paper presents a detailed experimental analysis, on a conventional axial compressor, of the continuous local topology evolution on both rotor and stator rows from compressor mode to highly loaded windmill. The objective is to deepen the understanding of windmilling flows and validate experimentally numerical results obtained on a previous paper from the present authors. In this latter, the innovative concept of a dualmachine, meant to reach good performances in both compressor and turbinemodes, was presented and numerically validated. The present paper experimentally confirms the relevance of the new design

Comparaison des méthodes numériques stationnaires et instationnaires dans la prédiction d'écoulements décollés - Application à un ventilateur subsonique en autorotation

Les phénomènes de décollement autour de profils portants apparaissent lorsque l’incidence de l’écoulement s’éloigne trop fortement de sa valeur de conception. L’importante désadaptation de l’aubage entraîne alors un décollement (total ou partiel) de la couche limite ainsi que le décrochage du profil. Dans le domaine des turbomachines, les décollements sont à l’origine des phénomènes de pompage des compresseurs et font, à ce titre, l’objet de nombreuses études, majoritairement des simulations numériques. Ces dernières sont effectuées dans le but d’en comprendre l’origine physique afin de déboucher sur des méthodologies de contrôle. La littérature propose pour des études d’écoulements décollés des méthodologies LES, DES et DNS en se limitant, le plus souvent, à des géométries académiques 2D (cylindre, profil Naca). Dans cet article, la prédictivité relative des méthodes numériques stationnaires (RANS) et instationnaires (NLH, Phase Lagged) est analysée en situation d’écoulements massivement décollés sur un étage rotor-stator industriellement représentatif d’équipements embarqués sur avions. Le code de calcul utilisé est FineTurbo (NUMECA Int). Le modèle de Spalart-Allmaras assure la fermeture des équations turbulentes sur un maillage bas Reynolds de 5 millions de cellules. Les résultats obtenus caractérisent le fonctionnement en autorotation (situation de forte hors adaptation) du ventilateur axial étudié. Les méthodes sont évaluées sur leur capacité à calculer correctement les performances des points de fonctionnements (expérimentalement connus) très éloignés du point de conception. Ces résultats mettent en évidence le mauvais comportement de l’approche RANS dans la prédiction des performances globales. Les écarts de débit masse entre les valeurs expérimentales et calculées sont maximaux pour les très faibles débits (20% environ) et minimaux pour les forts débits (5% environ). Les simulations instationnaires montrent que la prise en compte des interactions rotor-stator améliore la prédictivité du code de calcul et autorisent ainsi une analyse locale plus fine

Theoretical Analysis of the Aerodynamics of Low-Speed Fans in Free and Load-Controlled Windmilling Operation

The present work is a contribution to understanding the windmilling operation of low-speed fans. Such an operating situation is described in the literature, but the context (mainly windmilling of aero-engines) often involves system dependence in the analysis. Most of the time, only regimes very close to the free-windmilling are considered. A wider range is analyzed in the present study, since the context is the examination of the energy recovery potential of fans. It aims at detailing the isolated contribution of the rotor, which is the only element exchanging energy with the flow. Other elements of the system (including the stator) can be considered as loss generators and be treated as such in an integrated approach. The evolution of the flow is described by the use of theoretical and experimental data. A theoretical model is derived to predict the operating trajectories of the rotor in two characteristic diagrams. A scenario is proposed, detailing the local evolution of the flow when a gradual progression toward free and load-controlled windmilling operation is imposed. An experimental campaign exerted on two low-speed fans aims at the analysis of both the local and global aspects of the performance, for validation. From a global point of view, the continuity of the operating trajectory is predicted and observed across the boundary between the quadrants of the diagrams. The flow coefficient value for the free-windmilling operation is fairly well predicted. From a local point of view, the local co-existence of compressor and turbine operating modes along the blade span is observed as previously reported. It is further demonstrated here that this configuration is not exclusive to free-windmilling operation and occurs inside a range that can be theoretically predicted. It is shown that for a given geometry, this local topology strongly depends on the value of the flow coefficient and is very sensitive to the inlet spanwise velocity distribution

Aeroacoustic Analysis of a Counter Rotating Open Rotor based on the Harmonic Balance Method

The Counter Rotating Open Rotor (CROR) powerplant is an interesting architecture for future regional aircraft propulsion since it offers higher propulsive efficiency and thereby lower fuel consumption than the conventional Turbofan engine. The noise levels generated are however potentially larger compared to a Turbofan due in part to the absence of a ducting nacelle. This raises the need for efficient, high fidelity tools that can be used for the design and evaluation of new blade concepts capable of meeting strict noise regulations. In this paper, a Computational Aeroacoustics (CAA) platform for CRORs based on the Harmonic Balance method is presented. The method is formulated in the time domain and solves for the dominant frequencies of the flow by expressing the solution as a truncated Fourier series in time. Coupling between the resolved frequencies is furthermore possible since the nonlinear URANS equations are solved for. The far field acoustic signature is obtained by solving a convective form of the Ffowcs Williams-Hawkings equations for permeable surfaces. The CAA platform is applied to a generic, full scale, pusher type CROR operating at cruise conditions

Generic Properties of Flows in Low-Speed Axial Fans Operating at Load-Controlled Windmill

This paper aims at underlining the existence of some generic properties of windmilling flows, partially spread in the literature but never clearly stated. Two kinds of axial machines are investigated from compressor mode to highly loaded windmill: a conventional fan with poor turbine performance and an optimized fan able to reach high efficiencies in both compressor and windmilling operations. Both simulations and experiments are used to perform the analysis. Three particular behaviours were identified as typical of fans operating at windmill: the inverse stacking of the speed lines visible in (P, Qm) turbine maps, the appearance of a slope change on the loading-to-flow coefficient diagram at windmill and a threshold effect occurring at highly loaded windmill

Assessment of Steady and Unsteady Full Annulus Simulations Predictivity for a Low-Speed Axial Fan at Load-Controlled Windmill

A steady mixing plane approach is compared with the time-averaged solution of an unsteady full annulus calculation for a conventional fan operating at load-controlled windmill. The objective is to assess the added value of a complete unsteady calculation compared with a more classical approach, especially concerning the effect of the spatial and temporal periodicity release in such an unusual operation as windmill. Experiment with global steady measurements and rotor radial characterizations was conducted. Numerical analysis demonstrates that windmilling global performances obtained with the time-averaged solution of the unsteady simulation are not far different from the steady case, especially in the rotor. Some differences arise in the stator, particularly regarding the velocity field. Temporal periodicity release in this row has clearly a significant effect on the flow unsteady response. A detailed analysis highlights that generic patterns of windmilling flows recorded on a steady approach are also reported on the unsteady case

Experimental and Numerical Flow Analysis of Low-Speed Fans at Highly Loaded Windmilling Conditions

This paper aims for the analysis of experimental and numerical results of windmilling flow topologies far from reewheeling condition. Two major cooling fans were investigated: a baseline design and an innovative one meant to reach good performance in both compressor and turbine modes. Experiments are conducted with global and local characterizations to determine energy recovery potential and local loss mechanisms. Also, tests were performed on a turbofan engine to confirm some trends observed on the cooling fans. The numerical study is carried out with mixing plane steady simulations, the results of which are in fair agreement with experimental data. The difference of local topology between freewheeling and highly loaded windmill demonstrates that classical deviation rules such as Carter’s are not well-suited to highly loaded windmilling flows. Finally, under certain conditions, the minor influence of the stator on the rotor topology indicates that nonrotating elements can be considered as loss generators