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Title: Methods for accurate determination of mixture fraction dissipation in turbulent flames.

Abstract

No abstract prepared.

Authors:
Publication Date:
Research Org.:
Sandia National Laboratories
Sponsoring Org.:
USDOE
OSTI Identifier:
908079
Report Number(s):
SAND2007-1815C
TRN: US200722%%188
DOE Contract Number:
AC04-94AL85000
Resource Type:
Conference
Resource Relation:
Conference: Proposed for presentation at the European Combustion Meeting, ECM2007 held April 13-13, 2007 in Chania, Greece.
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; COMBUSTION KINETICS; FLAMES; MIXTURES; TURBULENCE; ACCURACY

Citation Formats

Wang, Guanghua. Methods for accurate determination of mixture fraction dissipation in turbulent flames.. United States: N. p., 2007. Web.
Wang, Guanghua. Methods for accurate determination of mixture fraction dissipation in turbulent flames.. United States.
Wang, Guanghua. Thu . "Methods for accurate determination of mixture fraction dissipation in turbulent flames.". United States. doi:.
@article{osti_908079,
title = {Methods for accurate determination of mixture fraction dissipation in turbulent flames.},
author = {Wang, Guanghua},
abstractNote = {No abstract prepared.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Mar 01 00:00:00 EST 2007},
month = {Thu Mar 01 00:00:00 EST 2007}
}

Conference:
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  • Three different experimental approaches for making instantaneous, two-dimensional (2D) measurements of the mixture fraction in turbulent nonpremixed hydrocarbon flames are presented. The availability of spatial information on the mixture fraction provided by these techniques make determination of the scalar dissipation feasible. Each of the techniques to be presented is based on simultaneously imaging fuel concentration and Rayleigh scattering. Measurement of the fuel concentration can be accomplished in a number of ways, including (1) Raman scattering from the fuel, (2) fluorescence from the fuel, or (3) fluorescence from a molecular tag added to the fuel. Preliminary results for experiments utilizing eachmore » of these approaches will be presented. In one experiment, joint Raman/Rayleigh measurements were performed in a turbulent methane flame. This approach worked quite well and was able to provide valuable mixture fraction statistics. However, the weak Raman signal required a tradeoff between signal/noise and spatial resolution. A second experiment involved imaging fuel fluorescence from an acetaldehyde flame. The acetaldehyde fluorescence/Rayleigh images obtained in this experiment were of high quality. On the fuel-rich side of the flame, however, the calculated mixture fraction shows a dip and plateau. These irregularities may be the result of fuel pyrolysis and could represent a limitation of this approach. A third method of mixture fraction measurement was to introduce acetone as a fuel tracer and image its fluorescence. Acetone fluorescence proved to be problematic when excited at a wavelength of 320 nm. Measurements show indications of a temperature dependence of the fluorescence yield, making it difficult to interpret the signal.« less
  • A computational study of a nonisenthalpic premixed turbulent jet flame is described. The flame burns a homogeneously premixed stoichiometric methane-air mixture injected into a coflow of air. The enthalpy (chemical + sensible) varies because of mixing between the jet fluid and the coflow. The performance of the Bray-Moss (BM) model and three flame surface density (FSD) models is evaluated by comparing the predictions of mean velocity and temperature profiles with recent experimental data. The reaction progress variable approach, which is established for isenthalpic flames, is extended to the present nonisenthalpic flames by including mean and mean square mixture fracture equations.more » The joint probability density function (PDF) of the reaction progress variable and the mixture fraction is modeled in terms of two statistically independent PDFs. The time-averaged reaction rate term is modeled using the BM and the FSD models. The effects of mixing with the coflow air were found to be unimportant in the evaluation of the flame speed required for modeling the mean reaction rate term. All models yielded reasonable predictions of mean velocity. Predictions of time-averaged temperatures agree better with the thin filament pyrometry data than those of Favre-averaged temperatures. The BM and MB models provided the best agreement with the mean temperature data but the other FSD models with slight tuning of the constants could provide similar agreement as well. The results show that a simple extension of the FSD models is promising for the treatment of nonisenthalpic flames. It appears that the differences in the conceptual framework of the FSD models disappear in their implementation using basically the same turbulence properties of kinetic energy and dissipation rates.« less
  • A new diagnostic system has been developed to obtain spatially and temporally resolved simultaneous multiple point measurements of species concentrations and temperatures in turbulent flames. The diagnostic approach is based on pulsed Raman scattering induced by a KrF excimer laser. The system has been applied in the reaction zones of turbulent nonpremixed H[sub 2]-air flames to obtain statistics of species concentrations, temperature, and mixture fraction (f). Results of means, and fluctuations of the measured quantities do not differ markedly from the published point measurements in similar flames, validating the diagnostic approach. Instantaneous, one-dimensional (radial) scalar dissipation rates (X) are measuredmore » in the reaction zones of the nonpremixed H[sub 2]-air flames using the multipoint, multispecies detectivity of the diagnostic system. Radial profiles and probability density functions (pdfs) of scalar dissipation are reported. The pdfs of scalar are similar to f[sub rms][sup 3], consistent with the k-[epsilon] models of turbulent combustion. Measurements of X are compared to laminar flamelet model calculations. Laminar flame strain rate parameters (a) estimated from the scalar dissipation measurements range from a = 50--1,000 s[sup [minus]1], much lower than the extinction strain rates (a [approx] 12,000 s[sup [minus]1]) of H[sub 2]-air flames.« less
  • The velocity-dissipation-composition probability density function (pdf) method is used to model a turbulent CO/H[sub 2]/N[sub 2]-air-piloted jet diffusion flame in the regime of extinction. The thermochemistry is modeled by a three-scalar simplified formulation obtained by the intrinsic low-dimensional manifold (ILDM) method. Calculations are performed for five different jet velocities, and the scalar pdfs are compared with experimental data. Overall good agreement is obtained between the calculations and the experimental results, with the only significant difference being the high level of scatter in the experimental data compared with the pdf results: reasons for this difference are discussed. The pdf method ismore » found to predict flame extinction at approximately the same jet velocity as that of the experiment. A small amount of local extinction is observed in the pdf results for the high-jet-velocity cases.« less