Filippo Rubechini

Real Gas Effects in Turbomachinery Flows: A Computational Fluid Dynamics Model for Fast Computations

A numerical model was included in a three-dimensional viscous solver to account for real gas effects in the compressible Reynolds averaged Navier-Stokes (RANS) equations. The behavior of real gases is reproduced by using gas property tables. The method consists of a local fitting of gas data to provide the thermodynamic property required by the solver in each solution step. This approach presents several characteristics which make it attractive as a design tool for industrial applications. First of all, the implementation of the method in the solver is simple and straightforward, since it does not require relevant changes in the solver structure. Moreover, it is based on a low-computational-cost algorithm, which prevents a considerable increase in the overall computational time. Finally, the approach is completely general, since it allows one to handle any type of gas, gas mixture or steam over a wide operative range. In this work a detailed description of the model is provided. In addition, some examples are presented in which the model is applied to the thermo-fluid-dynamic analysis of industrial turbomachines.

A Redesign Strategy to Improve the Efficiency of a 17-Stage Steam Turbine

A three-dimensional Reynolds averaged Navier–Stokes solver was applied to the aerodynamic redesigning of a 17-stage steam turbine. The redesign procedure was divided into three steps. In the first one, a single embedded stage was considered, and an optimization of stator lean and rotor twist was carried out by applying suitable repeating inlet/outlet boundary conditions. In the second step, a proper geometrical transformation between the original reference stage and the optimized one was identified and then applied to all other turbine stages, thus leading to a first approximation of the redesigned turbine. Finally, a neural-network-based refinement of the stator and rotor twist of each stage was performed to account for its actual position and operating conditions within the meridional channel. In this work, a detailed description of the redesign procedure is provided, and the aerodynamic characteristics of the optimized geometry are discussed and compared with the original ones.

Numerical Investigation of a Transonic Centrifugal Compressor

A three-dimensional Navier-Stokes solver is used to investigate the flow field of a high-pressure ratio centrifugal compressor for turbocharger applications. Such a compressor consists of a double-splitter impeller followed by a vaned diffuser. The inlet flow to the open shrouded impeller is transonic, thus giving rise to interactions between shock waves and boundary layers and between shock waves and tip leakage vortices. These interactions generate complex flow structures which are convected and distorted through the impeller blades. Detailed laser Doppler velocimetry flow measurements are available at various cross sections inside the impeller blades highlighting the presence of low-velocity flow regions near the shroud. Particular attention is focused on understanding the physical mechanisms which govern the flow phenomena in the near shroud region. To this end numerical investigations are performed using different tip clearance modelizations and various turbulence models, and their impact on the computed flow field is discussed.

Numerical Analysis of the Vaned Diffuser of a Transonic Centrifugal Compressor

A three-dimensional Navier–Stokes solver is used to investigate the flow field of a high pressure ratio centrifugal compressor for turbocharger applications. Such a compressor consists of a double-splitter impeller followed by a vaned diffuser. Particular attention is focused on the analysis of the vaned diffuser, designed for high subsonic inlet conditions. The diffuser is characterized by a complex three-dimensional flow field and influenced by the unsteady interaction with the impeller. Detailed particle image velocimetry flow measurements within the diffuser are available for comparison purposes.

The Impact of Gas Modeling in the Numerical Analysis of a Multistage Gas Turbine

In this work a numerical investigation of a four stage heavy-duty gas turbine is presented. Fully three-dimensional, multistage, Navier-Stokes analyses are carried out to predict the overall turbine performance. Coolant injections, cavity purge flows, and leakage flows are included in the turbine modeling by means of suitable wall boundary conditions. The main objective is the evaluation of the impact of gas modeling on the prediction of the stage and turbine performance parameters. To this end, four different gas models were used: three models are based on the perfect gas assumption with different values of constant c p , and the fourth is a real gas model which accounts for thermodynamic gas properties variations with temperature and mean fuel/air ratio distribution in the through-flow direction. For the real gas computations, a numerical model is used which is based on the use of gas property tables, and exploits a local fitting of gas data to compute thermodynamic properties. Experimental measurements are available for comparison purposes in terms of static pressure values at the inlet/outlet of each row and total temperature at the turbine exit.

A CFD-based throughflow method with an explicit body force model and an adaptive formulation for the S2 streamsurface

The paper describes the development and validation of a novel CFD-based throughflow model. It is based on the axisymmetric Euler equations with tangential blockage and body forces and inherits its numerical scheme from state-of-the-art CFD solver (TRAF code), including real-gas capabilities. A crucial aspect of the numerical procedure is represented by an adaptive approach for the meridional flow surface, which employs a new time-dependent equation to accommodate incidence and deviation effects, and which allows the explicit calculation of the blade body force. A realistic distribution of entropy along the streamlines is proposed in order to compute dissipative forces on the basis of a distributed loss model. The throughflow code is applied to the investigation of the NASA rotor 67 transonic fan and of a four stage low-pressure steam turbine at design conditions. The performance of the method is evaluated by comparing predicted operating characteristics and spanwise distributions of flow quantities with the results of CFD, steady, viscous calculations and experimental data.

The Influence of Roughness on a High-Pressure Steam Turbine Stage: An Experimental and Numerical Study

This work deals with the influence of roughness on high-pressure steam turbine stages. It is divided in three parts. In the first one, an experimental campaign on a linear cascade is described, in which blade losses are measured for different values of surface roughness and in a range of Reynolds numbers of practical interest. The second part is devoted to the basic aspects of the numerical approach and consists of a detailed discussion of the roughness models used for computations. The fidelity of such models is then tested against measurements, thus allowing their fine-tuning and proving their reliability. Finally, comprehensive computational fluid dynamics (CFD) analysis is carried out on a high-pressure stage, in order to investigate the influence of roughness on the losses over the entire stage operating envelope. Unsteady effects that may affect the influence of the roughness, such as the upcoming wakes on the rotor blade, are taken into account, and the impact of transition-related aspects on the losses is discussed.

Aerodynamic Redesigning of an Industrial Gas Turbine

This paper deals with the aerodynamic redesigning of a four-stage heavy-duty gas turbine. Traditional design tools, such as through-flow methods, as well as more sophisticated tools, such as three-dimensional RANS computations, were applied in subsequent steps according to a given hierarchical criterion. Each design or analysis tool was coupled with modern optimization techniques, and the overall redesign procedure relies on a neural-network-based approach aimed at maximizing the turbine’s power output while satisfying geometrical and mechanical constraints. A detailed description of the redesign procedure is provided, and the aerodynamic characteristics of the optimized geometry are discussed and compared to the original ones.Copyright

Design and Off-Design Numerical Investigation of a Transonic Double-Splitter Centrifugal Compressor

The flow field of a high pressure ratio centrifugal compressor for turbocharger applications is investigated using a three-dimensional Navier-Stokes solver. The compressor is composed of a double-splitter impeller followed by a vaned diffuser. The flow field of the transonic open-shrouded impeller is highly three-dimensional, and it is influenced by shock waves, tip leakage vortices and secondary flows. Their interactions generate complex flow structures which are convected and distorted through the impeller blades. Both steady and unsteady computations are performed in order to understand the physical mechanisms which govern the impeller flow field while the operation ranges from choke to surge. Detailed Laser Doppler Velocimetry (LDV) flow measurements are available at various cross-sections inside the impeller blades at both design and off-design operating conditions.© 2008 ASME

Real Gas Effects in Turbomachinery Flows: A CFD Model for Fast Computations

A numerical model was included in a three-dimensional viscous solver to account for real gas effects in the compressible Reynolds Averaged Navier-Stokes (RANS) equations. The behavior of real gases is reproduced by using gas property tables. The method consists of a local fitting of gas data to provide the thermodynamic property required by the solver in each solution step. This approach presents several characteristics which make it attractive as a design tool for industrial applications. First of all, the implementation of the method in the solver is simple and straightforward, since it does not require relevant changes in the solver structure. Moreover, it is based on a low-computational-cost algorithm, which prevents a considerable increase in the overall computational time. Finally, the approach is completely general, since it allows one to handle any type of gas, gas mixture or steam over a wide operative range. In this work a detailed description of the model is provided. In addition, some examples are presented in which the model is applied to the thermo-fluid-dynamic analysis of industrial turbomachines.© 2003 ASME