Large Eddy Simulations of complex turbulent flows

In this dissertation a solution methodology for complex turbulent flows of industrial interests is developed using a combination of Large Eddy Simulation (LES) and Immersed Boundary Method (IBM) concepts. LES is an intermediate approach to turbulence simulation in which the onus of modeling of “universal” small scales is appropriately transferred to the resolution of “problem-dependent” large scales or eddies. IBM combines the efficiency inherent in using a fixed Cartesian grid to compute the fluid motion, along with the ease of tracking the immersed boundary at a set of moving Lagrangian points. Numerical code developed for this dissertation solves unsteady, filtered Navier-Stokes equations using high-order accurate (fourth order in space) finite difference schemes on a staggered grid with a fractional step approach. Pressure Poisson equation is solved using a direct solver based on a matrix diagonalization technique. Second order accurate Adams-Bashforth scheme is used for temporal integration of equations. Dynamic mixed model (DMM) is used to model subgrid scale (SGS) terms. It can represent large scale anisotropy and back-scatter of energy from small-to-large scale through scale-similar term and maintain the energy drain through eddy viscosity term whose coefficient is allowed to change with in the computational domain. This code is validated for several bench-mark problems and is demonstrated to solve complex moving geometry problem such as stator-rotor interaction. A number of parametric studies on jets-in-crossflow are performed to understand complex fluid dynamics issues pertaining to film-cooling. These studies included effects of variation of hole-aspect ratio, jet injection angle, free-stream turbulence intensity and free-stream turbulence length scales on the coherent structure dynamics for jets-in-crossflow. Fundamental flow physics and heat transfer issues are addressed by extracting coherent structures from time-dependent three dimensional flow fields of film-cooling by inclined jet and studying their influence on the film-cooled surface heat transfer. A direct method to perform heat transfer calculations in periodic geometries is proposed and applied to internal cooling in rotating ribbed duct. Immersed boundary method is used to render complex geometry of trapped vortex combustor on Cartesian grid and fluid mixing inside trapped vortex cavity is studied in detail

Aero-Thermal Numerical Predictions of Trailing Edge and Leading Edge Cooling Channels

State-of-art gas turbine are designed to operate at turbine inlet temperatures higher than 2000[K]. Such temperature levels are sustainable only by means of aggressive and efficient cooling of the components exposed to the hot gas path. It should be pointed out that not only must the maximum metal temperature be kept below safety limits, but the thermal field must be reasonably uniform too, in order to limit thermal stresses. Moreover, mod- ern blade cooling systems consist of a combination of internal cavities with cross-sections specifically developed for each different blade portion; therefore, specific studies are essen- tial to describe their performances in detail in order to provide designers with the most accurate knowledge. The need for such detailed information is in conflict with some common practices in cool- ing system design: most of the studies deal with square or rectangular channels cross- section (hence resembling ducts in the central body of the blade), the link between coolant flow field and heat transfer rates is seldom analyzed; finally, the coupling of rotation and different channel orientations is rarely taken into account. Over the last few years CFD (Computational Fluid Dynamic) has been exploited to pro- vide valuable information on complex flow fields and heat transfer in internal cooling passages; indeed, it is already used as an engineering tool in design and optimization processes of gas turbine cooling. On the other hand, the reliability of the numerical tools available at present is not sufficiently high and, hence, detailed experimental analyses are still required for numerical validation purposes. The present thesis focuses on the aspects pertaining to the suitability of CFD for the prediction of the aero-thermal performances inside cooling channels designed for two es- sential portions of the blade, namely trailing edge and leading edge, whose sizes and shapes are quite different from those resembling cooling channel in the central portion of the blade. The trailing edge cooling model is characterized by a trapezoidal cross-section of high aspect-ratio and coolant discharge at the blade tip and along the wedge-shaped trailing side, where seven lengthened pedestals are also installed. Three different configurations are taken into account, namely the smooth channel and two others characterized by the use of ribs in different portions of the duct. Firstly, an extensive comparison with detailed experimental data including local flow velocities, turbulence proprieties and local heat transfer coefficient in static (Ro = 0) and orthogonal rotating conditions (Ro = 0.23) is carried out using the Shear Stress Transport (SST) turbulence model. Moreover, for one rib-roughened configuration in static condition (Ro = 0) different turbulence models are tested in order to enhance all computational results. Finally, the CFD code is exploited to analyzed more engine-like conditions, namely Ro = 0.46 and \u3b3 = 22.5 12 45[\u25e6]. The results show that rotation and channel orientation produce contrasting effects which are more significant in the rib-roughened configuration. In fact, on the radial central portion rotation/orientation generates an increase/decrease in the heat transfer; conversely, on the trailing side region, rotation/orientation has a negative/positive effect on the thermal field. The leading edge cooling model consists of a straight, smooth channel with an equi- lateral triangle cross-section. Geometry and test conditions resemble those pertaining to the passages used for the internal cooling of gas turbine blades leading edge. On the same geometry and at comparable working conditions, heat transfer data are also available from literature. Experimental data are used for CFD validation purposes at Re = 20000 Ro = 0.2 and Re = 10000 Ro = 0.4. Consequently, a wide range of work- ing conditions, namely Re = 10000 12 40000 Ro = 0.2 12 0.6 are numerically explored by the SST turbulence model. The results show that the rotation-induced flow structure is rather complicated showing relevant differences compared to the flow models that have been supposed by the research community so far. Indeed, the secondary flow turned out to be characterized by the presence of two or more vortex cells depending on channel location and Ro number. No separation or reattachment of these structures is found on the channel walls but they are observed at the channel apexes. The stream-wise velocity distribution shows a velocity peak close to the lower apex and the overall flow structure does not reach a steady configuration along the channel length. This evolution is has- ten (in space) if the rotation number is increased while changes of the Re number have no effects. Moreover, thanks to the understanding of the flow mechanisms associated to rotation, it was possible to provide a precise justification for the channel thermal behav- ior. Finally, different channel orientations (namely \u3b3 = 22.5 12 45[\u25e6]) are numerically investigated. The results further demonstrate that the variation of the channel orien- tation to more engine-like conditions significantly affects the flow field and, hence, the aero-thermal behavior

Numerical simulation of flow and heat transfer of internal cooling passage in gas turbine blade

A computational study of three-dimensional turbulent flow and heat transfer was performed in four types of rotating channels. The first type is a rotating rectangular channel with V-shaped ribs. The channel aspect ratio (AR) is 4:1, the rib height-to-hydraulic diameter ratio (e/Dh) is 0.078 and the rib pitch-to-height ratio (P/e) is 10. The rotation number and inlet coolant-to-wall density ratio were varied from 0.0 to 0.28 and from 0.122 to 0.40, respectively, while the Reynolds number was varied from 10,000 to 500,000. Three channel orientations (90 degrees, -135 degrees, and 135 degrees from the rotation direction) were also investigated. The second type is a rotating rectangular channel with staggered arrays of pinfins. The channel aspect ratio (AR) is 4:1, the pin length-to-diameter ratio is 2.0, and the pin spacing-to-diameter ratio is 2.0 in both the stream-wise and span-wise directions. The rotation number and inlet coolant-to-wall density ratio varied from 0.0 to 0.28 and from 0.122 to 0.20, respectively, while the Reynolds number varied from 10,000 to 100,000. For the rotating cases, the rectangular channel was oriented at 150 degrees with respect to the plane of rotation. In the rotating two-pass rectangular channel with 45-degree rib turbulators, three channels with different aspect ratios (AR=1:1; AR=1:2; AR=1:4) were investigated. Detailed predictions of mean velocity, mean temperature, and Nusselt number for two Reynolds numbers (Re=10,000 and Re=100,000) were carried out. The rib height is fixed as constant and the rib-pitch-to-height ratio (P/e) is 10, but the rib height-to-hydraulic diameter ratios (e/Dh) are 0.125, 0.094, and 0.078, for AR=1:1, AR=1:2, and AR=1:4 channels, respectively. The channel orientations are set as 90 degrees, the rotation number and inlet coolant-to-wall density ratio varied from 0.0 to 0.28 and from 0.13 to 0.40, respectively. The last type is the rotating two-pass smooth channel with three aspect ratios (AR=1:1; AR=1:2; AR=1:4). Detailed predictions of mean velocity, mean temperature and Nusselt number for two Reynolds numbers (Re=10,000 and Re=100,000) were carried out. The rotation number and inlet coolant-to-wall density ratio varied from 0.0 to 0.28 and from 0.13 to 0.40, respectively. A multi-block Reynolds-averaged Navier-Stokes (RANS) method was employed in conjunction with a near-wall second-moment turbulence closure

Large-eddy simulation of turbulent flows with heat transfer in simple and complex geometries

Some basic aspects of turbulence, transition to turbulence, and turbulence modelling, are summarized in Chapter 1 (Introduction). Emphasis is put on the increasing understanding of turbulent phenomena made possible by recent advances in the theory of dynamical systems; on the concept of "coherent structure"; and on the parallel evolution of computing power and computational fluid dynamics. Chapter 2 is a survey of the turbulence modelling technique known as Large Eddy Simulation (LES). The equations governing fluid flow and scalar transport are introduced; the direct simulation of turbulent flows and its limitations are briefly outlined; and the concept of LES, with the related topics of decomposition, filtering and subgrid modelling, is discussed. The state of the art in LES is reviewed in the last three sections, under the separate headings of proposed subgrid models; wall boundary conditions; and applications presented in the literature. An attempt is made to give the most complete and updated possible account of the subject; work carried out from the early 'Seventies up to now is considered. Emphasis is put more on physical models and corresponding performances than on numerical methods and computational details. In Chapter 3, the finite-volume numerical techniques used in the present work are presented and discussed. Emphasis is put on those aspects and options which bear more relevance for the accuracy and quality of the results, such as the pressure-velocity coupling algorithm, the discretization of advective terms and the treatment of centred (co-located), body-fitted grids. The architecture and the basic features of the computer code Harwell-FL0W3D, Release 2, are outlined, while the modifications introduced in order to implement the Smagorinsky subgrid model and the appropriate boundary conditions for LES are described in detail. In Chapters 4-6 results are presented and discussed for the basic geometries studied. Chapter 4 deals with the flow between indefinite parallel plates (plane channel), one of which heated with a uniform heat flux. For this basic geometry, a detailed study is presented on the influence of numerical options (grid size, time step and time-stepping method, pressure-velocity coupling, discretization of the advective terms); model parameters (Smagorinsky constant, subgrid Prandtl number, near-wall damping); Reynolds number; and alternative wall boundary conditions. The issues of initial conditions, numerical transients and statistical processing of the results are also discussed with some depth. In Chapter 5, computations are presented for a plane channel having one of the walls roughened by transverse square ribs. An extensive literature review of experimental and numerical studies on this geometry is included. The parametrical study is limited here to the influence of grid size and Reynolds number; LES results are presented in detail for a reference case, and are compared with experimental flow and heat transfer data. Chapter 6 is dedicated to the geometry of cross-corrugated ducts, representative of storage-type air preheaters for fossil-fuelled power stations. Flow and heat transfer predictions from direct simulation and LES are presented; they are compared with experimental results and with numerical predictions obtained by a standard and a low-Reynolds number version of the k-s turbulence model. Finally, Chapter 7 summarizes the main conclusions which can be drawn from the above studies. Emphasis is put on the basic issue of LES applicability to engineering problems of practical interest, and of its feasibility using a commercial, general-purpose (though highly sophisticated) computer code. A critical comparison with more conventional turbulence modelling approaches is outlined, and 'weak spots', or issues requiring further clarifications, are pointed out for future studies. The work includes an extensive bibliography with almost 400 references, and an appendix on the tensorial formulation of the governing equations of fluid dynamics in general domains

Aeronautical engineering: A continuing bibliography with indexes (supplement 282)

This bibliography lists 623 reports, articles, and other documents introduced into the NASA scientific and technical information system in Aug. 1992. The coverage includes documents on the engineering and theoretical aspects of design, construction, evaluation, testing, operation, and performance of aircraft (including aircraft engines) and associated components, equipment, and systems. It also includes research and development in aerodynamics, aeronautics, and ground support equipment for aeronautical vehicles

Advanced Computational Thermal Fluid Physics (CTFP) and Its Assessment for Light Water Reactors and Supercritical Reactors

Background: The ultimate goal of the study is the improvement of predictive methods for safety analyses and design of Generation IV reactor systems such as supercritical water reactors (SCWR) for higher efficiency, improved performance and operation, design simplification, enhanced safety and reduced waste and cost. The objective of this Korean / US / laboratory / university collaboration of coupled fundamental computational and experimental studies is to develop the supporting knowledge needed for improved predictive techniques for use in the technology development of Generation IV reactor concepts and their passive safety systems. The present study emphasizes SCWR concepts in the Generation IV program

Combustor-turbine interactions: Hot spot migration and thermal environment prediction for a better understanding and design of helicopter engines

This PhD thesis, funded by SAFRAN Helicopter Engines, focuses on Large Eddy Simulation (LES) of the FACTOR test rig to investigate combustor-turbine interactions in the context of next generation lean combustion engines. The FACTOR test rig is a full annular non-reactive lean combustion simulator with a single staged high-pressure turbine located at the DLR in G�ottingen. Another test rig featuring three sectors or 54_ of the full annular DLR test rig is available at the University of Florence. Both rigs provide a huge amount of validation data. In this thesis, certain aspects of LES in turbomachinery are investigated in detail and the manuscript is divided into two parts dealing respectively with the modeling of cooling systems and an analysis of the ow _eld in the combustion chamber and high-pressure vane passage. First, a heterogeneous and a homogeneous coolant injection model for multiperforated plates in combustion chambers are tested against experimental results. From this _rst study it is shown that the heterogeneous model allows for a more realistic coolant jet representation and should be retained for future simulations. In gas turbine engines the application of coolant systems is not only mandatory in the combustion chamber, but also in the _rst stages of the high-pressure turbine. The next section therefore investigates the previously presented heterogeneous injection model as a mean to model the e_ects of the NGV cooling system on the main ow and compares the simulation to a second one with a fully resolved coolant system. The second part deals with simulations that extend over combustion chamber and high-pressure vanes and speci_cally addresses the impact of the ow _eld in the combustor on the high-pressure vanes. The main objective here is to better understand wall temperature distribution on the turbine blade wall which is obtained by use of higher order statistics analysis to highlight thermally critical areas. Based on such coupled multiple component LES, a discussion is initiated to identify a path allowing to take into consideration the impact of the combustion chamber on isolated high-pressure vane simulations using di_erent reconstructed unsteady inlet conditions

Aeronautical engineering: A continuing bibliography with indexes (supplement 253)

This bibliography lists 637 reports, articles, and other documents introduced into the NASA scientific and technical information system in May, 1990. Subject coverage includes: design, construction and testing of aircraft and aircraft engines; aircraft components, equipment and systems; ground support systems; and theoretical and applied aspects of aerodynamics and general fluid dynamics