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

Fundamental Studies Of Flame Structure Through Laser Plasma Diagnostics

Increasing concerns about air pollution and global climate change are drawing attention to the need for efficiency improvements and emission reductions for combustion processes, which account for more than 85% of energy production in United States. Combustion efficiency and emissions are affected by the mixing and reacting of fuel and oxidizer. Understanding such behavior plays a critical role in flame structure studies and combustion optimization. However, experimentally obtaining mixture fraction, which is a widely used quantity to describe the mixing behavior, has proven to be a challenge, especially for heavier hydrocarbon fuels or fuel rich flames. Moreover, measuring flame temperature simultaneously with mixture fraction adds complexity into the experimental setup. In this dissertation, laser plasma diagnostics techniques were developed to provide a straightforward method to simultaneously obtain composition and temperature measurements. The capability of these novel techniques is applicable to more complex fuels and a broader range of equivalence ratios than has heretofore been possible, and facilitates a better understanding of flame structure. Laser-induced breakdown spectroscopy (LIBS) is proposed as an alternative method of measuring mixture fraction. A back-scattering setup is utilized to mitigate the beam steering effects in non-uniform and unsteady flames. The calibration for the LIBS system was completed in an ethylene-air premixed flame under a broad range of equivalence ratios. The elemental species distributions for H, C, N, O were measured in a counter-flow diffusion flame. The measured mixture fraction compared favorably with the numerical results from OPPDIF flame code. On the basis of LIBS measured elemental species profile, the preferential diffusion effect was analyzed. Utilizing the sound emission from laser-induced plasmas, acoustic-based laser induced breakdown thermometry (LIBT) was developed as a novel method for flame temperature measurement. The established correlation between the optical emission and acoustic emission in a premixed flame demonstrated that the acoustic signal can serve as an internal standard in the gas phase LIBS measurement. The influences of flame temperature and composition on the acoustic signal were investigated independently. The composition effect was found to be second order comparing to the temperature effect. The statistics of the LIBT measurement were also analyzed to better understand the distribution of samples. Furthermore, the temperature and gas density distributions in a counter-flow diffusion flame were measured using LIBT and were found to compare favorably with numerical results. To evaluate the possibility of simultaneous composition and temperature measurement using laser plasma diagnostics, the spatial and temporal resolutions of LIBS and LIBT were carefully examined. The accuracy of LIBT technique was analyzed as a function of sample size from a statistical perspective. The results demonstrated that LIBT has spatial and temporal resolutions comparable to that of LIBS. Finally, a preliminary study using a Burke- Schumann flame and a Hencken burner was performed to understand the influence of turbulent flow. Measuring composition and temperature simultaneously using laser plasma diagnostics provides substantial benefits over traditional measurement technique. However, in exchange for such benefits, information on major species concentrations can no longer be directly measured. To infer the molecular species profile from the elemental species profile, the underlying partial-equilibrium assumption was examined. Among partially-equilibrated reactions, the water-gas shift (WGS) reaction is most often assumed to be in equilibrium because of its important role in the high temperature zone. Thus, the equilibrium domain of WGS reaction was systematically studied in different hydrocarbon flames under varying strain rates to evaluate the validity of partial equilibrium assumption. The underlying mechanism for WGS-equilibrium was also examined. The results suggested that even though the WGS reaction has a broad partial-equilibrium domain in syngas, methane, ethylene and propane flames, the mechanisms responsible for partial equilibrium were very different. In hydrocarbon flames, the water-gas-shift reaction can achieve partial equilibrium even though the two elementary reactions behind it are not equilibrated

Microgravity Combustion Diagnostics Workshop

Through the Microgravity Science and Applications Division (MSAD) of the Office of Space Science and Applications (OSSA) at NASA Headquarters, a program entitled, Advanced Technology Development (ATD) was promulgated with the objective of providing advanced technologies that will enable the development of future microgravity science and applications experimental flight hardware. Among the ATD projects one, Microgravity Combustion Diagnostics (MCD), has the objective of developing advanced diagnostic techniques and technologies to provide nonperturbing measurements of combustion characteristics and parameters that will enhance the scientific integrity and quality of microgravity combustion experiments. As part of the approach to this project, a workshop was held on July 28 and 29, 1987, at the NASA Lewis Research Center. A small group of laser combustion diagnosticians met with a group of microgravity combustion experimenters to discuss the science requirements, the state-of-the-art of laser diagnostic technology, and plan the direction for near-, intermediate-, and long-term programs. This publication describes the proceedings of that workshop

A multiple mapping conditioning model for differential diffusion

This work introduces modeling of differential diffusion within the multiple mapping conditioning (MMC) turbulent mixing and combustion framework. The effect of differential diffusion on scalar variance decay is analyzed and, following a number of publications, is found to scale as Re. The ability to model the differential decay rates is the most important aim of practical differential diffusion models, and here this is achieved in MMC by introducing what is called the side-stepping method. The approach is practical and, as it does not involve an increase in the number of MMC reference variables, economical. In addition we also investigate the modeling of a more refined and difficult to reproduce differential diffusion effect - the loss of correlation between the different scalars. For this we develop an alternative MMC model with two reference variables but which also makes use of the side-stepping method. The new models are successfully validated against DNS results available in literature for homogenous, isotropic two scalar mixing

A Higher-Order Flamelet Model for Turbulent Combustion Simulations.

Current projection of energy consumption trends has shown that combustion of fossil fuel will continue to play an important role in industrial thermal processes, power generation, and transportation for a substantial period. In order for these sectors to sustain under the finite fossil fuel reserves, improvements in existing devices and development of novel concepts that emphasize on energy efficiency are necessary. Numerical simulations can be used to address this need, in particular by complementing experiments with extensive and quick parametric studies. However, this is only viable if numerical predictions of the combustion processes are accurate, which requires adequate modeling of the multi-physics phenomena in turbulent reacting flows. In this work, the flamelet-type combustion model, one of the most widely used approaches for turbulent reacting flow simulations, is thoroughly analyzed in terms of the validity of its underlying assumptions and limitations in its description of different combustion regimes. Diagnostic tools that account for the flamelet formulation are developed and applied to two different direct numerical simulation (DNS) results. These analyses show that the omission of the higher-order and unsteady flamelet effects by most conventional flamelet models is not valid in realistic configurations that are characterized by complex vortical structures, flame extinction and reignition, and turbulence-chemistry interactions. Following these findings, a higher-order flamelet model that describes the conventionally omitted flamelet effects is developed for large-eddy simulations (LES) applications. This model is based on the physical interpretation of flamelets as quasi one-dimensional structures in the turbulent flow, and the consideration of the effects that the spatial-filtering in LES methodology has on these structures. The model is applied in LES of a turbulent counterflow diffusion flame configuration, demonstrating improved agreement with the reference DNS solutions of the same case than the steady flamelet/progress variable (FPV) and laminar approximation models.PhDAerospace EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133466/1/chanyli_1.pd

Large eddy simulation of partially premixed turbulent combustion

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Application of LES-PDF methods on turbulent reacting flows

This research concerns the application of the Probability Density Function (PDF) on Large Eddy Simulations (LES) of turbulent reacting flows in a wide range of open flame configurations spanning between the premixed and non-premixed regime. The aim is to validate the applicability of the PDF model on a wide range of flames without any special treatment. Additionally, the \textit{a-posteriori} Chemical Exposive Mode Analysis (CEMA) has been applied to the results in order to examine the flame structure and identify locations of extinction, re-ignition, etc. Four different series of flames are studied, each one of them belonging to a completely different combustion regime. The F1-F3 premixed turbulent flames is the first family of flames where the PDF method is applied. The LES-PDF model is shown to accurately predict the flow field and the scalar field even on a very coarse grid. The simulations were performed on a personal computer, so the computational power was severely restricted. Nevertheless, the PDF model was able to give accurate predictions, so one of the flames was chosen for a further sensitivity analysis. A large number of modelling parameters were studied and the results show little sensitivity to them in contrast to RANS-PDF approaches in premixed flames. Finally, the model is able to capture large scale quenching at qualitatively the correct extinction speed. The Cambridge-Sandia series of swirling stratified flames was also examined. It encompasses a wide range of flames with various combinations of swirl and stratification ratio levels. Four distinct cases were selected and tested. For the most simple flames (SwB1 and SwB5), the model gives excellent prediction for both the flow field and the scalar distribution. The introduction of the additional fields improves slightly the results, especially at locations further away from the nozzle exit. For the flames which exhibit more complex flow fields and complex characteristics (SwB6 and SwB11), the model gives reasonable results, given the complexity of the flow field. The introduction of differential diffusion and heat losses towards the ceramic cap was studied independently on the SwB11 flame and was found to have counteracting effects. Therefore, their combination was tested and was found to give a significant improvement. The next series of flames is the Sydney Swirl flames. The SM1 and SM2 flames are two complex swirling flames with a difficult flow field to capture. The field is composed of recirculating zones and vortex break-down bubble areas. The SM2 has not been tested in the literature and this work is the first modelling approach. The flow field simulation results are reasonable, given the complexity of the flame. The biggest discrepancies are observed close to the nozzle exit. The Chemical Explosive Mode Analysis is also performed to give information about the flame structure. The flame is divided into three distinct zones with the second one being a very large quenching region. The CEMA analysis explains why the flame does not quench, but re-ignites further down. Finally, the Delft III premixed flame is studied, a difficult flame to model as it shows quenching with large extinction pockets despite the moderately low Reynolds number. The major flow characteristics were accurately captured by the simulation and the introduction of the additional stochastic fields improves the results close to the nozzle exit. Contrary to most researchers that model the pilot flow as a single heat source close to the nozzle exit, in this work the pilot flow is modelled as a separate flow stream, something that increased the complexity of the simulations due to the extremely thin pilot rim which was comparable to the cell size. Nevertheless, the model was able to accurately capture the localized extinction throughout the flame and the application of the Chemical Explosive Mode Analysis gave further insight into the structure of the flame.Open Acces

Chemical Structure and Dynamics in Laminar Flame Propagation

The objective of this dissertation is to investigate fundamental aspects of premixed flame structures as well as flame dynamics that arise due to conjugate heat transfer in narrow channels. Laminar premixed combustion simulations in narrow 2D channels show that conjugate heat transfer allows for combustion of mixtures at small scales that are not flammable at normal conditions. To investigate the impact of conjugate heat transfer, preheated 1D cases with premixed H2/Air fuel are simulated for a wide range of operating conditions based on inlet temperature and equivalence ratio. For post-processing, Chemical Explosive Mode Analysis (CEMA, an eigen-analysis technique) is used as a computational diagnostic tool. Classical CEMA is refined to introduce directional information to track dominant promoting and counteracting chemical modes that are linked to specific species and reactions. A major result of this analysis is that flame structures are shown to follow the same trend if they have similar flame temperature, regardless of the inlet conditions. Laminar premixed combustion in narrow channels is known to produce a range of dynamic flame phenomena (stationary/non-stationary and symmetric/asymmetric flames) that depend on operating conditions. Mechanisms that lead to different dynamics are investigated by tracking flame fronts and related metrics for laminar premixed CH4/air and syngas/air flames. Flow re-directions because of local extinctions and corresponding flame edges are found to be the main causes for such dynamics. Synthesized gases (syngas) have been recently considered to be used at small-scale combustion systems because a) they can be produced from cheap heavy fuels such as glycerol and b) they have better combustion characteristics compared to the initial heavy fuel. Therefore, syngas production from glycerol, which is available in high volumes and low costs has been studied. By investigating glycerol reforming processes at a wide range of intermediate temperatures and stoichiometries, optimum operating conditions for producing syngas are explained