RANS MODELING OF FLOW IN ROTATING CAVITY SYSTEM

The accurate prediction of fluid flow within rotating systems has a primary role for the reliability and performance of gas turbine engine. The selection of a suitable turbulence model for the study of such complex flows remains an open issue in the literature. This paper reports a numerical benchmark of the most used eddy viscosity RANS models available within the commercial CFD solvers Fluent and CFX together with an innovative Reynolds Stress Model closure. The predictions are compared to experimental data and previous numerical calculations available in the open literature for three test cases. Test case 1 corresponds to a rotating cavity with a radial outflow, considered experimentally by Owen and Pincombe. In that case, the main difficulty arises from the choice of the boundary conditions at the outlet. Several types of boundary conditions have been then considered. All models fail to predict the radial velocity distribution. Nevertheless, the RSM offers the best agreement against the experimental data in terms of the averaged tangential velocity in the core. Test case 2 corresponds to a Taylor-Couette system with an axial Poiseuille flow studied experimentally by Escudier and Gouldson. Even if the two-equation models provide reliable data for the mean velocity field, they strongly underestimate the turbulence intensities everywhere. The agreement between the RSM and the measurements is rather satisfactory for the mean and turbulent fields, though this second-order closure does not predict the asymmetry of the normal stresses. The main discrepancies appear indeed very close to the stator. Test case 3 is a rotor-stator system with throughflow, corresponding to the test rig of Poncet et al. All the models catch the main features of rotor-stator flows, such as the value of the entrainment coefficient or the location of the transition from the Stewartson to the Batchelor flow structures. The RSM improves especially the predictions of the shear stress

NUMERICAL CHARACTERIZATION OF AERODYNAMIC LOSSES OF JET ARRAYS FOR GAS TURBINE APPLICATIONS

ABSTRACT Jet array is an arrangement typically used to cool several gas turbine parts. Some examples of such applications can be found in the impingement cooled region of gas turbine airfoils or in the turbine blade tip clearances control of large aero-engines. In order to correctly evaluate the impinging jet mass flow rate, the characterization of holes discharge coefficient is a compulsory activity. In this work an aerodynamic analysis of jet arrays for active clearance control was performed; the aim was the definition of a correlation for the discharge coefficient (C d ) of a generic hole of the array. The data were taken from a set of CFD RANS simulations, in which the behaviour of the cooling system was investigated over a wide range of fluid-dynamics conditions. More in detail, several different holes arrangements were investigated with the aim of evaluating the influence of the hole spacing on the discharge coefficient distribution. Tests were conducted by varying the jet Reynolds number in a wide range of effective engine operative conditions (Re=2000-12000,Pressure-Ratio=1.01-1.6). To point out the reliability of the CFD analysis, some comparisons with experimental data, measured at the "Department of Energy Engineering" of the University of Florence , were drawn. An in depth analysis of the numerical data set has underlined the opportunity of an efficient reduction through the mass velocity ratio of hole and feeding pipe: the dependence of the discharge coefficients from this parameter is roughly logarithmic

Numerical Benchmark of Turbulence modelling in Gas Turbine Rotor-Stator System

Accurate design of the secondary air system is one of the main tasks for reliability and performance of gas turbine engines. The selection of a suitable turbulence model for the study of rotor-stator cavity flows, which remains an open issue in the literature, is here addressed over a wide range of operating conditions. A numerical benchmark of turbulence models is indeed proposed in the case of rotor-stator disk flows with and without superimposed throughflow. The predictions obtained by the means of several two equation turbulence models available within the CFD solver Ansys CFX 12.0 are compared with those previously evaluated by Poncet et al. through the Reynolds Stress Model (RSM) of Elena and Schiestel implemented in a proprietary finite volume code. The standard k-eps and k-w SST models including high and low Reynolds approaches, have been used for all calculations presented here. Further more, some tests were conduced using the innovative k-w SST-CC and k-w SST-RM models that take into account the curvature effects via the Spalart-Shur correction term and the reattachement modification proposed by Menter respectively. The numerical calculations have been compared to extensive velocity and pressure measurements performed on the test rig of the IRPHE's laboratory in Marseilles. Several configurations, covering a wide range of real engine operating conditions, were considered. The influence of the typical non dimensional flow parameters (Reynolds number and flowrate coefficient) on the flow structure is studied in detail. In the case of an enclosed cavity, the flow exhibits a Batchelor-like structure with two turbulent boundary layers separated by a laminar rotating core. When an inward axial throughflow is superimposed, the flow remains of Batchelor type with a core rotating faster than the disk because of conservation of the angular momentum. In this case, turbulence intensities are mainly confined close to the stator. Turbulence models based on a low Reynolds approach provide better overall results for the mean and turbulent fields especially within the very thin boundary layers. The standard k-w SST model offers the best trade-off between accuracy and computational cost for the parameters considered here. In the case of an outward throughflow, the k-w SST in conjunction with a low Reynolds approach and RSM models provide similar results and predict quite well the transition from the Batchelor to the Stewartson structures

Turbulent Couette-Taylor flows with endwall effects: a numerical benchmark

International audienceThe accurate prediction of fluid flow within rotating systems has a primary role for the reliability and performance of rotating machineries. The selection of a suitable model to account for the effects of turbulence on such complex flows remains an open issue in the literature. This paper reports a numerical benchmark of different approaches available within commercial CFD solvers together with results obtained by means of in-house developed or open-source available research codes exploiting a suitable Reynolds Stress Model (RSM) closure, Large Eddy Simulation (LES) and a direct numerical simulation (DNS). The predictions are compared to the experimental data of Burin et al. (2010) in an original enclosed Couette-Taylor apparatus with endcap rings. The results are discussed in details for both the mean and turbulent fields. A particular attention has been turned to the scaling of the turbulent angular momentum G with the Reynolds number Re. By DNS, G is found to be proportional to Rea, the exponent a = 1.9 being constant in our case for the whole range of Reynolds numbers. Most of the approaches predict quite well the good trends apart from the k-w SST model, which provides relatively poor agreement with the experiments even for the mean tangential velocity profile. Among the RANS models, even though no approach appears to be fully satisfactory, the RSM closure offers the best overall agreement

Aerothermal Analysis of a Turbine Casing Impingement Cooling System

Heat transfer and pressure drop for a representative part of a turbine active cooling system were numerically investigated by means of an in-house code. This code has been developed in the framework of an internal research program and has been validated by experiments and CFD. The analysed system represents the classical open bird cage arrangement that consists of an air supply pipe with a control valve and the present system with a collector box and pipes, which distribute cooling air in circumferential direction of the casing. The cooling air leaves the ACC system through small holes at the bottom of the tubes. These tubes extend at about 180° around the casing and may involve a huge number of impinging holes; as a consequence, the impinging jets mass flow rate may vary considerably along the feeding manifold with a direct impact on the achievable heat transfer levels. This study focuses on the performance, in terms of heat transfer coefficient and pressure drop, of several impinging tube geometries. As a result of this analysis, several design solutions have been compared and discussed

Benchmark numérique des écoulements de Couette-Taylor turbulents

Les résultats issus de modèles de turbulence disponibles dans CFX et STAR CCM+ sont comparés aux mesures de Burin et al. (2010) ainsi qu'à de nouveaux calculs issus du modèle RSM d'Elena et Schiestel (1996) et d'un code LES basé sur le modèle de Smagorinsky dynamique développé au M2P2. Les paramètres géométriques (rapport d'aspect = 2.12, rapport des rayons = 0.35) sont ceux considérés dans l'expérience originale de Burin et al. Une comparaison détaillée entre les différentes approches est effectuée sur une large gamme du nombre de Reynolds (10000-400000)

Numerical Simulation of Oil Jet Lubrication for High Speed Gears

The Geared Turbofan technology is one of the most promising engine configurations to significantly reduce the specific fuel consumption. In this architecture, a power epicyclical gearbox is interposed between the fan and the low pressure spool. Thanks to the gearbox, fan and low pressure spool can turn at different speed, leading to higher engine bypass ratio. Therefore the gearbox efficiency becomes a key parameter for such technology. Further improvement of efficiency can be achieved developing a physical understanding of fluid dynamic losses within the transmission system. These losses are mainly related to viscous effects and they are directly connected to the lubrication method. In this work, the oil injection losses have been studied by means of CFD simulations. A numerical study of a single oil jet impinging on a single high speed gear has been carried out using the VOF method. The aim of this analysis is to evaluate the resistant torque due to the oil jet lubrication, correlating the torque data with the oil-gear interaction phases. URANS calculations have been performed using an adaptive meshing approach, as a way of significantly reducing the simulation costs. A global sensitivity analysis of adopted models has been carried out and a numerical setup has been defined

Thermo-Structural Analysis Of Steam Tracing Arrangements Applied To Pump Barrels

LecturePumps steam tracing is widely used in Oil&Gas industry for critical services in which the process fluid requires a minimum temperature to avoid its crystallization during stand-by. This paper describes the process of utilizing Computational Fluid Dynamics to perform a thermo-structural analysis of a barrel pump to determine the optimal steam tracing arrangement to maintain a minimum internal temperature. The most critical part of the analysis was to define the Heat Transfer Coefficient of the entire system. The computations consisted in conjugate Computational Fluid Dynamics solutions involving the ambient temperature and wind distribution, the skid dimensions and arrangement (barrels materials), the tracing system (carbon steel piping), the insulation (Mineral Wool) and the fluid compartments, both steam inside the piping and air in the gaps. The steam was modelled as a single-phase fluid with properties defined to consider the latent heat of condensatio