Development and validation of a pressure based CFD methodology for acoustic wave propagation and damping

Combustion instabilities (thermo-acoustic pressure oscillations) have been recognised for some time as a problem limiting the development of low emissions (e.g., lean burn) gas turbine combustion systems, particularly for aviation propulsion applications. Recently, significant research efforts have been focused on acoustic damping for suppression of combustion instability. Most of this work has either been experimental or based on linear acoustic theory. The last 3-5 years has seen application of density based CFD methods to this problem, but no attempts to use pressure-based CFD methods which are much more commonly used in combustion predictions. The goal of the present work is therefore to develop a pressure-based CFD algorithm in order to predict accurately acoustic propagation and acoustic damping processes, as relevant to gas turbine combustors. The developed computational algorithm described in this thesis is based on the classical pressure-correction approach, which was modified to allow fluid density variation as a function of pressure in order to simulate acoustic phenomena, which are fundamentally compressible in nature. The fact that the overall flow Mach number of relevance was likely to be low ( mildly compressible flow) also influenced the chosen methodology. For accurate capture of acoustic wave propagation at minimum grid resolution and avoiding excessive numerical smearing/dispersion, a fifth order accurate Weighted Essentially Non-Oscillatory scheme (WENO) was introduced. Characteristic-based boundary conditions were incorporated to enable accurate representation of acoustic excitation (e.g. via a loudspeaker or siren) as well as enable precise evaluation of acoustic reflection and transmission coefficients. The new methodology was first validated against simple (1D and 2D) but well proven test cases for wave propagation and demonstrated low numerical diffusion/dispersion. The proper incorporation of Characteristic-based boundary conditions was validated by comparison against classical linear acoustic analysis of acoustic and entropy waves in quasi-1D variable area duct flows. The developed method was then applied to the prediction of experimental measurements of the acoustic absorption coefficient for a single round orifice flow. Excellent agreement with experimental data was obtained in both linear and non-linear regimes. Analysis of predicted flow fields both with and without bias flow showed that non-linear acoustic behavior occurred when flow reversal begins inside the orifice. Finally, the method was applied to study acoustic excitation of combustor external aerodynamics using a pre-diffuser/dump diffuser geometry previously studied experimentally at Loughborough University and showed the significance of boundary conditions and shear layer instability to produce a sustained pressure fluctuation in the external aerodynamic

A modeling technique for STOVL ejector and volume dynamics

New models for thrust augmenting ejector performance prediction and feeder duct dynamic analysis are presented and applied to a proposed Short Take Off and Vertical Landing (STOVL) aircraft configuration. Central to the analysis is the nontraditional treatment of the time-dependent volume integrals in the otherwise conventional control-volume approach. In the case of the thrust augmenting ejector, the analysis required a new relationship for transfer of kinetic energy from the primary flow to the secondary flow. Extraction of the required empirical corrections from current steady-state experimental data is discussed; a possible approach for modeling insight through Computational Fluid Dynamics (CFD) is presented

Advanced Numerical Simulations of Two-phase CO2 Ejectors

Over the last decade, Carbon dioxide (R744) a natural fluid has gained significant interest as a potential substitute for synthetic refrigerants commonly used in refrigeration, air-conditioning, and heat pump systems. Because of CO2 properties, such cycles generally operate in transcritical conditions. Moreover, their Coefficient Of Performance (COP) is relatively low compared to conventional cycles using synthetic refrigerants, because of higher entropy production of C02 along an isenthalpic expansion from a supercritical state to a subcritical state. Integrating a two-phase ejector, as an expansion device, is a promising technology to significantly improve the system efficiency, which would make CO2 adequate for HVAC applications. For example, in a CO2 ejector-expansion system, an ejector replaces the classical throttling valve to partly recover the throttling losses and provides a compression work reducing the compressor load. As a result, the COP and cooling capacity can be improved. However, many complex physical phenomena occur within a two-phase CO2 ejector and they are not yet fully understood, such as the turbulent mixing between the primary and the secondary flows, the flashing in the primary nozzle, shock waves-shear layer interactions, as well as phase-change processes. In this thesis, a numerical approach was developed by combining an efficient look-up table method for CO2 properties, density-based solvers, and characteristic Navier-Stokes boundary conditions (NSCBC) in order to correctly predict those complex flow features. This look-up table method allows to compute vapor, liquid, supercritical and two-phase properties of CO2 from 217K to 1000K and pressures up to 50MPa. It was coupled to three density-based solvers which allow to perform inviscid simulations, Reynolds-Averaged Navier-Stokes Simulations (RANS), and Large-Eddy Simulations (LES). Validations and verifications were performed for these three solvers. Then, Converging-diverging nozzles and ejectors were investigated by using RANS simulations. The developed solver was used to conduct an exergy tube analysis for a two-phase CO2 ejector and the sensibility of the method to the numerics was discussed. Finally, the compound-choking theory was extended for real gas flows and it was used to check the choking condition of the investigated ejector.L’objectif principal de ce travail de thèse est de développer une approche numérique complète capable de simuler de manière précise et rapide l’écoulement et les transferts exergétiques au sein d’éjecteurs transcritiques au CO2. Tout d’abord, une méthode tabulée basée sur l’équation d’état de Span-Wagner (SW) est développée pour calculer les propriétés du CO2 [59] à l’état de vapeur, liquide, supercritique et diphasique. Cette approche est précise et efficace. Les écarts relatifs maximaux par rapport à l’équation d’état de SW sont de 0.23% et 1.2% pour la pression et la vitesse du son, respectivement et l’écart absolu maximal pour la température est de 0.06 K. Dans le cas d’un tube à choc 1D, cette approche s’avère de 66.6 à 90 fois plus rapide que si on utilise l’équation d’état de SW. Deuxièmement, cette méthode tabulée est couplée à trois solveurs basés sur la densité : CLAWPACK pour les simulations d’écoulements inviscides, rhoCentralFoam pour des modélisations RANS (Reynolds-Average Navier-Stokes) essentiellement et AVBP pour des simulations des grandes échelles. Différents cas tests en 1D et 2D sont effectués pour valider l’implémentation de la méthode dans ces trois solveurs. Ces cas incluent les problèmes du tube à choc, de la dépressurisation et de la cavitation. Troisièmement, afin de se rapprocher des éjecteurs, les tuyères convergentes-divergentes de Nakagawa et al. dans des conditions supercritiques et sous-critiques sont simulées à l’aide des solveurs CLAWPACK et rhoCentralFoam. On constate que le modèle de turbulence a une influence significative sur les résultats numériques, en particulier pour les tuyères ayant un petit angle divergent. La tuyère de Berana et al. est étudiée également. Un choc épais est prédit, ce qui correspond bien aux mesures expérimentales. Quatrièmement, l’éjecteur de Li et al. est examiné via le solveur rhoCentralFoam pour une condition on-design. L’analyse des tubes d’exergie proposée par Lamberts et al. pour un éjecteur à air est appliquée. La sensibilité de la méthode est discutée. La résolution des gradients a une influence significative sur les termes de destruction. Par conséquent, le maillage et les schémas numériques peuvent affecter fortement l’analyse des tubes d’exergie. Enfin, la théorie de “compound-choking” est étendue à l’écoulement diphasique au CO2. Elle prédit que l’écoulement est choqué au début de la section de mélange, tandis que selon la ligne sonique, l’écoulement est choqué à la fin de cette section. Finalement, des calculs RANS d’un éjecteur complet sont comparées à de nouvelles mesures faites sur le banc expérimental développé au Laboratoire des Technologies de l’Énergie (LTE, Shawinigan). Un bon accord est obtenu pour le profil de pression pariétale. Les tubes de transport de quantité de mouvement et d’énergie cinétique sont analysés et révèlent une zone de recirculation à l’entrée du flux secondaire. Cependant, la condition de fonctionnement n’est pas appropriée pour les cycles d’éjecteur à expansion (régime off-design)

Applications of Various Methods of Analysis to Combustion Instabilities in Solid Propellant Rockets

Instabilities of motions in a combustion chamber are consequences of the coupled dynamics of combustion processes and of the flow in the chamber. The extreme complexities of the problem always require approximations of various sorts to make progress in understanding the mechanisms and behavior of combustion instabilities. This paper covers recent progress in the subject, mainly summarizing efforts in two areas: approximate analysis based on a form of Galerkin's method, particularly useful for understanding the global linear and nonlinear dynamics of combustion instabilities and numerical simulations intended to accommodate as fully as possible fundamental chemical processes in both the condensed and gaseous phases. One purpose of current work is to bring closer together these approaches to produce more comprehensive and detailed realistic results applicable to the interpretation of observations and for design of new rockets for both space and military applications. Particularly important are the goals of determining the connections between chemical composition and instabilities; and the influences of geometry on nonlinear behavior

Center for Modeling of Turbulence and Transition (CMOTT). Research briefs: 1990

Brief progress reports of the Center for Modeling of Turbulence and Transition (CMOTT) research staff from May 1990 to May 1991 are given. The objectives of the CMOTT are to develop, validate, and implement the models for turbulence and boundary layer transition in the practical engineering flows. The flows of interest are three dimensional, incompressible, and compressible flows with chemistry. The schemes being studied include the two-equation and algebraic Reynolds stress models, the full Reynolds stress (or second moment closure) models, the probability density function models, the Renormalization Group Theory (RNG) and Interaction Approximation (DIA), the Large Eddy Simulation (LES) and Direct Numerical Simulation (DNS)

Velocity and energy relaxation in two-phase flows

In the present study we investigate analytically the process of velocity and energy relaxation in two-phase flows. We begin our exposition by considering the so-called six equations two-phase model [Ishii1975, Rovarch2006]. This model assumes each phase to possess its own velocity and energy variables. Despite recent advances, the six equations model remains computationally expensive for many practical applications. Moreover, its advection operator may be non-hyperbolic which poses additional theoretical difficulties to construct robust numerical schemes |Ghidaglia et al, 2001]. In order to simplify this system, we complete momentum and energy conservation equations by relaxation terms. When relaxation characteristic time tends to zero, velocities and energies are constrained to tend to common values for both phases. As a result, we obtain a simple two-phase model which was recently proposed for simulation of violent aerated flows [Dias et al, 2010]. The preservation of invariant regions and incompressible limit of the simplified model are also discussed. Finally, several numerical results are presented.Comment: 37 pages, 10 figures. Other authors papers can be downloaded at http://www.lama.univ-savoie.fr/~dutykh

Institute for Computational Mechanics in Propulsion (ICOMP)

The Institute for Computational Mechanics in Propulsion (ICOMP) is operated by the Ohio Aerospace Institute (OAI) and the NASA Lewis Research Center in Cleveland, Ohio. The purpose of ICOMP is to develop techniques to improve problem-solving capabilities in all aspects of computational mechanics related to propulsion. This report describes the accomplishments and activities at ICOMP during 1993