Catálogo de publicaciones - libros

The film cooling technique is often used to protect the hot components in gas turbines engines by introducing cold air through small holes drilled in the wall. The hot products are mixed with the injected gas and the temperature in the vicinity of the wall is reduced. Classical wall functions developed for impermeable walls and used in Reynolds-Averaged Navier-Stokes methods cannot predict momentum/ heat transfer on perforated plates because the flow is drastically modified by effusion. In order to obtain a better understanding of the flow structure and predominant effects, accurate simulations of a turbulent flow around an effusion plate are reported. Large-Eddy Simulations of the flow created by an infinite multi-perforated plate are presented. The plate is perforated with short staggered holes inclined at an angle of 30 deg to the main flow, with a length-to-diameter ratio of 3.46. Injection holes are spaced 6.74 diameters apart in the spanwise direction and 11.68 diameters apart in the streamwise direction. Results for mean velocity and velocity fluctuations are compared with measurements made on the LARA large-scale isothermal experiment [1].

The focus of the paper is on the performance of approximate RANStype near-wall treatments applied within LES strategies to the simulation of flow separation from curved surfaces at high Reynolds numbers. Two types of combination are considered: a hybrid RANS-LES scheme in which the LES field is interfaced, dynamically, with a full RANS solution in the near-wall layer; and a zonal scheme in which the state of the near-wall layer is described by parabolized Navier-Stokes equations which only return the wall shear stress to the LES domain as a wall boundary condition. In both cases, the location of the interface can be chosen freely. The two methods are applied to a flow separating from the trailing edge of a hydrofoil. A second flow considered is one separating from a three-dimensional hill, for which the performance of the zonal method is contrasted with a fine-grid LES and simulations in which the near-wall layer is treated with log-law-based wall functions.

Recent advances in computer science and highly parallel algorithms make Large Eddy Simulation (LES) an efficient tool for the study of complex flows. The available resources allow today to tackle full complex geometries that can not be installed in laboratory facilities. The present paper demonstrates that the state of the art in LES and computer science allows simulations of combustion chambers with one, three or all burners and that results may differ considerably from one configuration to the other. Computational needs and issues for such simulations are discussed. A single burner periodic sector and a triple burner sector of an annular combustion chamber of a gas turbine are investigated to assess the impact of the periodicity simplification. Cold flow results validate this approach while reacting simulations underline differences in the results. The acoustic response of the set-up is totally different in both cases so that full geometry simulations seem a requirement for combustion instability studies.

Combustion instability studies require the identification of the combustion chamber response. In non-premixed devices, the combustion processes are influenced by oscillations of the air flow rate but may also be sensitive to fluctuations of the fuel flow rate entering the chamber. This paper describes a numerical study of the mechanisms controlling the response of a swirled non-premixed combustor burning natural gas and air. The flow is first characterized without combustion and LDV results are compared to Large Eddy Simulation (LES) data. The non-pulsated reacting regime is then studied and characterized in terms of fields of heat release and equivalence ratio. Finally the combustor fuel flow rate is pulsated at several amplitudes and the response of the chamber is analyzed using phase-locked averaging and first order acoustic analysis.

In this contribution a Large Eddy Simulation together with the Artificially Thickened Flame approach is used to study a well known experimental set-up consisting of a rectangular dump combustor - ORACLES. The major drawback of artificially thickening the flame is that the interaction between turbulence and flame is altered. To compensate for the inability of small vortices to wrinkle the flame a subgrid scale wrinkling model has to be introduced. In this contribution the influence of the subgrid scale wrinkling on the flame front in a high Reynolds number flow is investigated. Moreover the influence of different approximations for the subgrid scale velocity on the prediction of the flow field and flame structure is studied.

The present study discusses results from a number of DNS simulations of turbulent flame kernels. The description of the chemistry in these calculations was based on the Flamelet Generated Manifolds (FGM) technique. The differences are imposed by varying the turbulence intensity and length scale within the thin reaction zones regime. This results in changes in the flame-turbulence interaction. The goal of the study is to see if the presently used reduced chemistry is able to properly deal with the turbulent modulations defined by stretch and curvature of the local flamelets. Especially the influences of the given turbulence effects to the local mass burning rate is investigated. Also global flame dynamics are described and an interpretation of the latter is given in terms of local quantities.

As combustion generated nano-organic particles (NOC) may pose significant health and environmental problems, there is great scientific interest in studying their formation and evolution in turbulent combustion systems. Traditional approaches to turbulent combustion numerical modeling apply Reynolds averaging techniques (RANS) to predict the behavior of the mean values of the reacting flow properties. In this way, unsteady effects are not taken into account in the formation of nanoparticles. Large Eddy Simulation represents an attractive methodology for studying turbulent reacting flows and this approach is becoming possible as computational resources are increasing.

A spectral analysis of the energy cascade in magnetohydrodynamics (MHD) is presented using high resolution direct numerical simulations of both forced and decaying isotropic turbulence. The triad interactions between velocity and magnetic field modes are averaged into shell interactions between similar length scales phenomena. This is achieved by combining all the velocity Fourier modes that correspond to wave vectors with similar amplitude into a shell velocity variable. The same procedure is adopted for the magnetic field. The analysis of the interactions between these shell variables gives a global picture of the energy transfers between different length scales, as well as between the velocity and the magnetic fields. Also, two different attempts to separate the shell-to-shell interactions into forward and backward energy transfers are proposed. They provide diagnostics that can be used in order to assess subgrid-scale modelling in large-eddy simulation for turbulent MHD systems.