A direct numerical simulation of a turbulent channel flow is performed. The unsteady Navier-Stokes equations are solved numerically at a Reynolds number of 3300, based on the mean centerline velocity and channel half-width, with about 4 million grid points. All essential turbulence scales are resolved on the computational grid and no subgrid model is used. A large number of turbulence statistics are computed and compared with the existing experimental data at comparable Reynolds numbers. Agreements as well as discrepancies are discussed in detail. Particular attention is given to the behavior of turbulence correlations near the wall. A number of statistical correlations which are complementary to the existing experimental data are reported for the first time.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0022112087000892

Turbulence statistics in fully developed channel flow at low reynolds number

Turbulent combustion modeling

Dynamics and stability of lean-premixed swirl-stabilized combustion

Oxy-fuel combustion of solid fuels

Large-eddy simulation (LES) of turbulent combustion is a relatively new research field. Much research has been carried out over the past years, but to realize the full predictive potential of combustion LES, many fundamental questions still have to be addressed, and common practices of LES of nonreacting flows revisited. The focus of the present review is to highlight the fundamental differences between Reynolds-averaged Navier-Stokes (RANS) and LES combustion models for nonpremixed and premixed turbulent combustion, to identify some of the open questions and modeling issues for LES, and to provide future perspectives.

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Large-eddy simulation of turbulent combustion

It is important that combustion models capture the effects of turbulent mixing on reaction zone structure in non-premixed and partially premixed flames. A more complete understanding of the response of species mass fractions to turbulent mixing is needed to improve predictive capabilities, particularly with regard to combustion intermediates and minor species. Using the combination of Raman scattering. Rayleigh scattering, and laser-induced fluorescence, simultaneous measurements of CO, OH, H2, and NO are obtained along with the major species, temperature, and mixture fraction in a series of six piloted methane/air jet flames. Flame conditions vary from laminar to turbulent with significant localized extinction. Two-photon laser-induced fluorescence (TPLIF) is used to determine instantaneous CO concentrations, providing an improvement over Raman scattering measurements of CO in methane flames. Conditional probability density functions (cpdf's) of species mass fractions in the six flames are compared. Significant changes are observed in the mass fraction cpdf's of several species. Results for H2O, CO2, H2, and OH are consistent with the concept that turbulent transport becomes dominant over molecular diffusion within the range of Reynolds numbers and axial locations considered in these experiments. The cpdf's of CO mass fraction are broadened in the turbulent flames relative to the laminar flame. However, there is not an increase in the maximum conditional mean value of the CO mass fraction as suggested by some previously reported measurements in methane flames. The cpdf's of NO mass fraction at a given streamwise location in the turbulent flames show NO levels decreasing significantly as jet velocity increases.

Effects of turbulence on species mass fractions in methane/air jet flames

Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames

Moderate and intense low oxygen dilution combustion is a newly implemented and developed concept to achieve high thermal efficiency and fuel savings while maintaining emission of pollutants at very low levels. It utilizes the concept of heat and exhaust gas recirculation to achieve combustion at a reduced temperature, a flat thermal field, and low turbulence fluctuations. An experimental burner is used in this study to simulate the heat and exhaust gas recirculation applied to a simple jet in a hot coflow. Temporally and spatially resolved measurements of reactive scalars are conducted on three different turbulent nonpremixed flames of a H2/CH4 fuel mixture at a fixed-jet Reynolds number, and different oxygen levels in the hot oxidant stream. The data were collected using the single-point Raman-Rayleigh-laser-induced fluorescence technique. The results show substantial variation in the flame structure and appearance with the decrease of the oxygen level. By reducing the oxygen level in the hot coflow, the flame becomes less luminous, the temperature increase in the reaction zone can get as low as 100 K, and the levels of CO and OH are substantially lowered. The levels of NO also decrease with decreasing the oxygen levels and at 3% by mass, it is less that 5 ppm. For this case, a widely distributed NO profile is found which is not consistent with profiles for other oxygen levels.

Structure of turbulent non-premixed jet flames in a diluted hot coflow

An experiment and numerical investigation is presented of a lifted turbulent H2/N2 jet flame in a coflow of hot, vitiated gases. The vitiated coflow burner emulates the coupling of turbulent mixing and chemical kinetics exemplary of the reacting flow in the recirculation region of advanced combustors. It also simplifies numerical investigation of this coupled problem by removing the complexity of recirculating flow. Scalar measurements are reported for a lifted turbulent jet flame of H2/N2 (Re = 23,600, H/d = 10) in a coflow of hot combustion products from a lean H2/Air flame ((empty set) = 0.25, T = 1,045 K). The combination of Rayleigh scattering, Raman scattering, and laser-induced fluorescence is used to obtain simultaneous measurements of temperature and concentrations of the major species, OH, and NO. The data attest to the success of the experimental design in providing a uniform vitiated coflow throughout the entire test region. Two combustion models (PDF: joint scalar Probability Density Function and EDC: Eddy Dissipation Concept) are used in conjunction with various turbulence models to predict the lift-off height (H(sub PDF)/d = 7,H(sub EDC)/d = 8.5). Kalghatgi's classic phenomenological theory, which is based on scaling arguments, yields a reasonably accurate prediction (H(sub K)/d = 11.4) of the lift-off height for the present flame. The vitiated coflow admits the possibility of auto-ignition of mixed fluid, and the success of the present parabolic implementation of the PDF model in predicting a stable lifted flame is attributable to such ignition. The measurements indicate a thickened turbulent reaction zone at the flame base. Experimental results and numerical investigations support the plausibility of turbulent premixed flame propagation by small scale (on the order of the flame thickness) recirculation and mixing of hot products into reactants and subsequent rapid ignition of the mixture.

/pdf/simultaneous-laser-raman-rayleigh-lif-measurements-and-cqx43ulkgt.pdf

Simultaneous laser raman-rayleigh-lif measurements and numerical modeling results of a lifted turbulent H2/N2 jet flame in a vitiated coflow

Fifty years after the foundation of the Combustion Institute and almost 150 years after Michael Faraday's famous lectures on the combustion of a candle, combustion diagnostics have come a long way from visual inspection of a flame to detailed analysis of a combustion process with a multitude of sophisticated techniques, often using lasers. The extended knowledge on combustion phenomena gained by application of these diagnostic techniques, combined with equally advanced numerical simulation of the process, has been instrumental in designing modern combustion devices with efficient performance and reduced pollutant emission. Also, similar diagnostic techniques are now employed to develop sensors for process control in combustion. This article intends to give a perspective on the potential of combustion diagnostics by highlighting selected application examples and by guiding the reader to recent literature. In particular, techniques are emphasized, which permit measurement of important features of the chemical composition, sometimes in conjunction with flow field parameters. Although a complete image of present research and applications in combustion diagnostics and control is beyond the scope of this article, this overview may be a starting place where ideas may be found to solve specific combustion problems with the aid of diagnostics. (Less)

Robert S. Barlow

Papers

Effects of turbulence on species mass fractions in methane/air jet flames

Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames

Structure of turbulent non-premixed jet flames in a diluted hot coflow

Simultaneous laser raman-rayleigh-lif measurements and numerical modeling results of a lifted turbulent H2/N2 jet flame in a vitiated coflow

Combustion at the focus: laser diagnostics and control