M. Coussirat

Computational Fluid Dynamics Modeling of Impinging Gas-Jet Systems: I. Assessment of Eddy Viscosity Models

Computational fluid dynamics plays an important role in engineering design. To gain insight into solving problems involving complex industrial flows, such as impinging gas-jet systems (IJS), an evaluation of several eddy viscosity models, applied to these IJS has been made. Good agreement with experimental mean values for the field velocities and Nusselt number was obtained, but velocity fluctuations and local values of Nusselt number along the wall disagree with the experiments in some cases. Experiments show a clear relation between the nozzle-to-plate distance and the Nusselt number at the stagnation point. Those trends were only reproduced by some of the numerical experiments. The conclusions of this study are useful in the field of heat transfer predictions in industrial IJS devices, and therefore for its design.

Fluid Added Mass Effect in the Modal Response of a Pump-Turbine Impeller

In this paper, the reduction in the natural frequencies of a pump-turbine impeller prototype when submerged in water has been investigated. The impeller, with a diameter of 2.870m belongs to a pump-turbine unit with a power of around 100MW. To analyze the influence of the added mass, both experimental tests and numerical simulations have been carried out. The experiment has been performed in air and in water. From the frequency response functions the modal characteristics such as natural frequencies and mode shapes have been obtained. A numerical simulation using FEM (Finite Elements Model) was done using the same boundary conditions as in the experiment (impeller in air and surrounded by a mass of water). The modal behaviour has also been calculated. The numerical results were compared with the available experimental results. The comparison shows a good agreement in the natural frequency values both in air and in water. The reduction in frequency due to the added mass effect of surrounding fluid has been calculated. The physics of this phenomenon due to the fluid structure interaction has been investigated from the analysis of the mode-shapes.Copyright

Computational Fluid Dynamics Modeling of Impinging Gas-Jet Systems: II. Application to an Industrial Cooling System Device

A numerical analysis of the flow behavior in industrial cooling systems based on arrays of impinging jets has been performed, using several eddy viscosity models to determine their modeling capabilities. For the cooling system studied, and in terms of mean Nusselt number values, the best agreement between experimental results and numerical predictions was obtained with the realizable k-e model. On the other hand, numerical predictions of the local Nusselt number and its spatial variations along the wall are better adjusted to the experiments when using either the standard k-e or the standard k-? models. The results obtained also show that the predicted thermal field depends strongly on the combination of near-wall treatment and selected turbulence model.

Detailed study of the rotor-stator interaction phenomenon in a moving cascade of airfoils

In turbomachinery the Rotor-Stator Interaction (RSI) is an important phenomenon that has a strong influence on the machine behavior. These interactions can have a significant impact on the vibrational and acoustical characteristics of the machine. Unsteadiness and turbulence play a fundamental role in complex flow structure and the use of Computational Fluid Dynamics (CFD) is becoming a usual requirement in design in turbomachinery due to the difficulties and high cost of the necessary experiments needed to identify RSI phenomena. The flow inside a turbomachinery working under design condition is complex but apparently, when working under off-design conditions, it becomes more complex due to the boundary layer separation phenomena. Therefore, the choice of an appropriate turbulence model is far from trivial and a suitable turbulence modeling plays a very important role for successful CFD results. In this work the RSI generated between a moving cascade of blades and fixed flat plate located downstream were studied by means of CFD modeling and compared against experimental results. Design and off-design conditions were modeled and a detailed comparison between them has been made. To analyze in detail the flow pattern, mean velocities in the boundary layer were obtained and compared against experimental results. Furthermore, results concerning to turbulence intensity were compared against an experimental database. It was observed that for each operating condition, the flow in the cascade show special features. For flow inside the turbomachine under design conditions there is no separation, the wake is thin and the characteristic length of the eddies is small. For off-design conditions, there is a large separation and the wake is thick with large eddies. The results obtained can be used to obtain a deeper insight into the RSI phenomena.