Conserved Scalar Probability Density Functions in a Turbulent Jet Diffusion Flame

The first four moments of conserved scalar probability density functions (p. d. f. 's) measured by Raman scattering in an H//2 turbulent jet diffusion flame are analysed and compared with those found in a non-reacting CH//4 jet. The measurements are in good agreement, indicating that heat release and combustion have little effect on p. d. f. shapes. However, the measured p. d. f. 's are not qualitatively similar to the simple forms often assumed in combustion modelling. A three-zone model was used to separate the experimental p. d. f. 's into a delta function (non-turbulent zone), a Gaussian (turbulent zone) and the remainder (interface zone). The interface zone contributed as much as 90% of the total p. d. f. in both the H//2 flame and the non-reacting CH//4 jet. A physical interpretation for the existence of broad interface zones in reacting and non-reacting turbulent jet flows is suggested based upon large-scale structures

Rayleigh/Raman/LIF measurements in a turbulent lean premixed combustor

Much of the industrial electrical generation capability being added worldwide is gas-turbine engine based and is fueled by natural gas. These gas-turbine engines use lean premixed (LP) combustion to meet the strict NO{sub x} emission standards, while maintaining acceptable levels of CO. In conventional, diffusion flame gas turbine combustors, large amount of NO{sub x} forms in the hot stoichiometric zones via the Zeldovich (thermal) mechanism. Hence, lean premixed combustors are rapidly becoming the norm, since they are specifically designed to avoid these hot stoichiometric zones and the associated thermal NO{sub x}. However, considerable research and development are still required to reduce the NO{sub x} levels (25-40 ppmvd adjusted to 15% O{sub 2} with the current technology), to the projected goal of under 10 ppmvd by the turn of the century. Achieving this objective would require extensive experiments in LP natural gas (or CH{sub 4}) flames for understanding the combustion phenomena underlying the formation of the exhaust pollutants. Although LP combustion is an effective way to control NO{sub x}, the downside is that it increases the CO emissions. The formation and destruction of the pollutants (NO{sub x} and CO) are strongly affected by the fluid mechanics, the finite-rate chemistry, and their (turbulence-chemistry) interactions. Hence, a thorough understanding of these interactions is vital for controlling and reducing the pollutant emissions. The present research is contributing to this goal by providing a detailed nonintrusive laser based data set with good spatial and temporal resolutions of the pollutants (NO and CO) along with the major species, temperature, and OH. The measurements reported in this work, along with the existing velocity data on a turbulent LP combustor burning CH{sub 4}, would provide insight into the turbulence-chemistry interactions and their effect on pollutant formation

Prediction and measurement of a non-equilibrium turbulent diffusion flame

Superequilibrium radical concentrations in a turbulent CO/H 2/N 2 jet diffusion flame are computed using a two-scalar pdf model and directly measured using single pulse laser saturated OH fluorescence. The model is based on the averaged Navier-Stokes equations and the k∈l turbulence model. Non-equilibrium chemistry is accounted for by including CO in the partially equilibrated oxyhydrogen radical pool. Two scalars (mixture fraction and eaction progress suffice to describe the thermochemical system. Laser saturated fluorescence is used to directly measure the mean and fluctuating components of OH concentrations and thus the radical pool. Measurements and model both find mean OH concentrations which are four to six times larger than equilibrium with rms values of OH concentration also reasonably predicted. Superequilibrium effects are predicted to lower the mean temperature by as much as 250 K in agreement with experiments. Evidence of the breakdown of partial equilibrium was found in cool fuel-rich zones where predictions of temperature and OH concentration were too high. Extensions of the model to predict thermal NO formation and CO burnout are discussed. © 1985 Combustion Institute

Superequilibrium and thermal nitric oxide formation in turbulent diffusion flames

Measurements and modeling of the formation of superequilibrium radicals and nitric oxide in atmospheric pressure turbulent jet diffusion flames are presented which quantify the influence of superequilibrium on thermal NOx formation. Variation of fuel gas compositions (CO/H2/N2, CO/H2/CO2, and CO/H2/Ar) permits partial separation of chemical and fluid mechanical effects. Superequilibrium OH radical concentrations are measured by single-pulse laser saturated fluorescence and NO and NO2 concentrations by probe sampling and chemiluminescent detection. Four different types of probes were used to quantify probe sampling effects. In turbulent reaction zones, virtually all of the NOx in the flame occurred in the form of NO but far downstream of the flame nearly half of the NOx occurred as NO2. Thermal NOx maximized near stoichiometric flame zones; the rich shift observed by others may be a probe sampling artifact. In turbulent CO/H2/N2 jet diffusion flames, both measurements and a nonequilibrium turbulent combustion model show that superequilibrium decreases average temperatures by 250K, increases average OH concentrations by a factor of 4-6, and increases thermal NOx formation principally by broadening the range of mixture fraction (both rich and lean) where thermal NOx is formed. Calculated increases in thermal NOx due to superequilibrium in turbulent CO/H2/N2 jet diffusion flames are factors of 2.5 at 1 atm and 1.4 at 10 atm. The two-scalar pdf model predicts that thermal NOx yield is independent of Reynolds number in disagreement with previous experimental reports. © 1987