Turbulent transport and length scale measurement experiments with comfined coaxial jets

A three phase experimental study of mixing downstream of swirling and nonswirling confined coaxial jets was conducted to obtain data for the evaluation and improvement of turbulent transport models currently employed in a variety of computational procedures. The present effort was directed toward the acquisition of length scale and dissipation rate data that provide more accurate inlet boundary conditions for the computational procedures and a data base to evaluate the turbulent transport models in the near jet region where recirculation does not occur, and the acquisition of mass and momentum turbulent transport data for a nonswirling flow condition with a blunt inner jet inlet configuration rather than the tapered inner jet inlet. A measurement technique, generally used to obtain approximate integral length and microscales of turbulence and dissipation rates, was computerized. Results showed the dissipation rate varied by 2 1/2 orders of magnitude across the inlet plane, by 2 orders of magnitude 51 mm from the inlet plane, and by 1 order of magnitude at 102 mm from the inlet plane for a nonswirling flow test conditions

A computational fluid dynamics investigation of turbulent swirling burners

This thesis presents detailed numerical calculations of the Unsteady, Reynolds- Averaged Navier-Stokes (URANS) equations to simulate isothermal, single-phase flow in the geometries of realistic swirl burners at large Reynolds numbers. Simulations are run with two different turbulence closures, viz., the standard k-epsilon and Reynolds stresses (RSM) models. The numerical method is validated concerning convergence, grid density and far-field influence. Results describe a flow that is in any case periodic or pseudo-periodic, and exhibits quite convincing time-dependent features: bubble- and spiral-type vortex breakdowns and vortex core precession. Some simulations are validated by comparison with corresponding experiments. Good agreement with the experiments has been obtained for mean flow, and frequency peaks of the power spectral density of pressure fluctuations. In order to asses the reliability of URANS methods within this context, calculated time-averaged flow and coherent structures are documented via 2D graphs, spectral analysis, 3D isosurfaces and advanced, vortex-related visualization methods and 2D snapshot proper orthogonal decomposition (S-POD). Differences arising from the nature of the turbulence model (k-epsilon vs. RSM) are very relevant indeed, given the cost factor involved and the apparent verisimilitude of the predicted flow; they are thoroughly analyzed

Influence of large-scale motion on turbulent transport for confined coaxial jets. Volume 2: Navier-Stokes calculations of swirling and nonswirling confined coaxial jets

The existence of large scale coherent structures in turbulent shear flows has been well documented. Discrepancies between experimental and computational data suggest a necessity to understand the roles they play in mass and momentum transport. Using conditional sampling and averaging on coincident two-component velocity and concentration velocity experimental data for swirling and nonswirling coaxial jets, triggers for identifying the structures were examined. Concentration fluctuation was found to be an adequate trigger or indicator for the concentration-velocity data, but no suitable detector was located for the two-component velocity data. The large scale structures are found in the region where the largest discrepancies exist between model and experiment. The traditional gradient transport model does not fit in this region as a result of these structures. The large scale motion was found to be responsible for a large percentage of the axial mass transport. The large scale structures were found to convect downstream at approximately the mean velocity of the overall flow in the axial direction. The radial mean velocity of the structures was found to be substantially greater than that of the overall flow

Numerical simulation of a non-reactive turbulent flow inside a cyclonic industrial boiler using LES and URANS

A numerical simulation of a non-reactive turbulent flow inside a cyclonic industrial CO boiler was investigated in order to understand the swirling formation, the fluid behavior in different locations inside the domain and the distribution of chemical species. As 80% of the energy matrix in Brazil is generated by combustion processes and government regulations about NOx emissions are becoming more restrict, enhancing combustion efficiency in a CO boiler with a turbulent swirling flow to reduce pollutant emissions has become an engineering research topic. Enhancing mixing processes through turbulent swirling flows might reduce thermal NOx formation. Computational fluid dynamics simulations were realized using the in-house MFSim code with the turbulent closure models LES, URANS Standard k − ", URANS Standard k − " Modified and URANS Realizable k − ". A theoretical basis about turbulence, LES and URANS closure models, mixing and swirling flows was provided. A state of art comprising different authors pointed out that some works with URANS Standard k − " demonstrated a premature solid-body rotation formation due to its eddy viscosity assumption and that swirling flows may reduce pollutant emissions by improving mixing of reactants and decreasing flame temperature. Validations concerning multi-component mixing flows and Immersed Boundary method were presented. From the results, LES and URANS Standard k − " presented similar velocity field results, capable of capturing the swirling formation. When analyzing three URANS closure models, a turbulent kinetic energy graph illustrated that it is relevant to observe the modeled part and the value obtained from velocity field fluctuations. The modified model presented low turbulent viscosity values and an LES-like behavior, with similar results to the standard model. The realizable model presented distant results comparing to the other models studied and there was no reverse flow in its swirling core. Adding different chemical species did not modify the velocity field and the highest mixing level was obtained in the most intense turbulent swirling region, close to the inlets. The data provided may assist in the comprehension of swirling formation, mixture processes inside a boiler and temperature control to reduce pollutant emissionsCAPES - Coordenação de Aperfeiçoamento de Pessoal de Nível SuperiorDissertação (Mestrado)Uma simulação numérica de um escoamento turbulento não reativo em uma caldeira industrial ciclônica de CO foi investigada a fim de se compreender a formação de um escoamento rotativo, o comportamento do fluido em diferentes locais dentro do domínio e a distribuição de espécies químicas. Como 80% da matriz energética no Brasil é gerada por processos de combustão e as regulamentações governamentais sobre as emissões de NOx estão se tornando mais restritas, o aumento da eficiência da combustão em uma caldeira de CO com escoamento turbulento ciclônico para reduzir as emissões de poluentes tornou-se um tema de pesquisa de engenharia. Melhorar os processos de mistura por meio de escoamentos turbulentos rotativos pode reduzir a formação térmica de NOx. Simulações de dinâmica dos fluidos computacional foram realizadas usando o código MFSim com os modelos de fechamento turbulento LES, URANS Standard k − ", URANS Standard k − " Modificado e URANS Realizable k − ". Foi fornecida uma base teórica sobre turbulência, modelos de fechamento LES e URANS, escoamentos com mistura e escoamentos rotativos. Um estado da arte compreendendo diferentes autores apontou que alguns trabalhos com URANS Standard k − " demonstraram uma formação de rotação de corpo sólido prematura devido à sua suposição de viscosidade turbulenta e que escoamentos rotativos podem reduzir as emissões de poluentes, melhorando a mistura de reagentes e diminuindo a temperatura da chama. Foram apresentadas as validações relativas aos escoamentos com mistura de multicomponentes e ao método da Fronteira Imersa. Dos resultados, LES e URANS Standard k − " apresentaram campos de velocidade semelhantes, capazes de capturar a formação de escoamento rotativo. Ao analisar três modelos de fechamento URANS, um gráfico de energia cinética turbulenta ilustrou que é relevante observar a parte modelada e o valor obtido a partir das flutuações do campo de velocidade. O modelo modificado apresentou baixos valores de viscosidade turbulenta e comportamento semelhante a LES, com resultados similares ao modelo Standard. O modelo realizável apresentou resultados distantes em comparação com os outros modelos estudados e não houve escoamento reverso em seu núcleo giratório. A adição de diferentes espécies químicas não modificou o campo de velocidade e o maior nível de mistura foi obtido na região de turbulência mais intensa, próxima às entradas. Os dados fornecidos podem auxiliar na compreensão da formação de escoamento rotativo, processos de mistura dentro de uma caldeira e controle de temperatura para reduzir as emissões de poluentes

Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

Recent developments in numerical schemes, turbulent combustion models and the regular increase of computing power allow Large Eddy Simulation (LES) to be applied to real industrial burners. In this paper, two types of LES in complex geometry combustors and of specific interest for aeronautical gas turbine burners are reviewed: (1) laboratory-scale combustors, without compressor or turbine, in which advanced measurements are possible and (2) combustion chambers of existing engines operated in realistic operating conditions. Laboratory-scale burners are designed to assess modeling and funda- mental flow aspects in controlled configurations. They are necessary to gauge LES strategies and identify potential limitations. In specific circumstances, they even offer near model-free or DNS-like LES computations. LES in real engines illustrate the potential of the approach in the context of industrial burners but are more difficult to validate due to the limited set of available measurements. Usual approaches for turbulence and combustion sub-grid models including chemistry modeling are first recalled. Limiting cases and range of validity of the models are specifically recalled before a discussion on the numerical breakthrough which have allowed LES to be applied to these complex cases. Specific issues linked to real gas turbine chambers are discussed: multi-perforation, complex acoustic impedances at inlet and outlet, annular chambers.. Examples are provided for mean flow predictions (velocity, temperature and species) as well as unsteady mechanisms (quenching, ignition, combustion instabil- ities). Finally, potential perspectives are proposed to further improve the use of LES for real gas turbine combustor designs

Confined turbulent swirling recirculating flow predictions

The capability and the accuracy of the STARPIC computer code in predicting confined turbulent swirling recirculating flows is presented. Inlet flow boundary conditions were demonstrated to be extremely important in simulating a flowfield via numerical calculations. The degree of swirl strength and expansion ratio have strong effects on the characteristics of swirling flow. In a nonswirling flow, a large corner recirculation zone exists in the flowfield with an expansion ratio greater than one. However, as the degree of inlet swirl increases, the size of this zone decreases and a central recirculation zone appears near the inlet. Generally, the size of the central zone increased with swirl strength and expansion ratio. Neither the standard k-epsilon turbulence mode nor its previous extensions show effective capability for predicting confined turbulent swirling recirculating flows. However, either reduced optimum values of three parameters in the mode or the empirical C sub mu formulation obtained via careful analysis of available turbulence measurements, can provide more acceptable accuracy in the prediction of these swirling flows

Aerothermal modeling program, phase 1

The physical modeling embodied in the computational fluid dynamics codes is discussed. The objectives were to identify shortcomings in the models and to provide a program plan to improve the quantitative accuracy. The physical models studied were for: turbulent mass and momentum transport, heat release, liquid fuel spray, and gaseous radiation. The approach adopted was to test the models against appropriate benchmark-quality test cases from experiments in the literature for the constituent flows that together make up the combustor real flow

Fuel injector: Air swirl characterization aerothermal modeling, phase 2, volume 1

A well integrated experimental/analytical investigation was conducted to provide benchmark quality relevant to a prefilming type airblast fuel nozzle and its interaction with the combustor dome air swirler. The experimental investigation included a systematic study of both single-phase flows that involved single and twin co-axial jets with and without swirl. A two-component Phase Doppler Particle Analyzer (PDPA) was used to document the interaction of single and co-axial air jets with glass beads that simulate nonevaporating spray and simultaneously avoid the complexities associated with fuel atomization processes and attendant issues about the specification of relevant boundary conditions. The interaction of jets with methanol spray produced by practical airblast nozzle was also documented in the spatial domain of practical interest. Model assessment activities included the use of three turbulence models (k-epsilon, algebraic second moment (ASM), and differential second moment (DSM)) for the carrier phase, deterministic or stochastic Lagrangian treatment of the dispersed phase, and advanced numerical schemes. Although qualitatively good comparison with data was obtained for most of the cases investigated, the model deficiencies in regard to modeled dissipation rate transport equation, single length scale, pressure-strain correlation, and other critical closure issues need to be resolved before one can achieve the degree of accuracy required to analytically design combustion systems