R.W. Dibble

Effect of Damköhler number on superequilibrium OH concentration in turbulent nonpremixed jet flames

Abstract Simultaneous, spatially resolved measurements of mixture fraction and absolute hydroxyl radical concentration are obtained for the first time in nonpremixed, turbulent, hydrogenair flames. This is accomplished by combining spontaneous Raman scattering with linear, laser-induced fluorescence (LIF). The Raman scattering data define the instantaneous, local collisional quenching environment of the OH molecules, allowing quenching corrections to be applied for each laser shot and making the linear LIF measurements quantitative. The effect of Damkohler number on OH superequilibrium is determined by performing measurements at selected locations in two argon-diluted hydrogen flames (Reynolds numbers 8,500 and 17,000). Results demonstrate that departures from chemical equilibrium in these flames are a consequence of the fact that time scales for turbulent transport are competitive with time scales for three-body radical recombination reactions. Due to the slow characteristic time for the radical recombination, convective histories, as well as instantaneous local conditions, determine hydroxyl concentrations. Damkohler numbers are not sufficiently low for the rapid bimolecular reactions to be strongly affected. Comparison of turbulent flame data with results from strained laminar flame calculation incicates that partial equilibrium of the bimolecular reactions is a good approximation near the stoichiometric mixture fraction. Comparisons of the experimental data with predictions by Monte Carlo simulations using a partial equilibrium chemistry model show good overall agreement. However, simulations predict a smaller variance of OH concentration than is measured for a given value of mixture fraction, lower OH concentrations at off-stoichiometric conditions, and a more rapid decay toward equilibrium with streamwise distance.

The spontaneous raman scattering technique applied to nonpremixed flames of methane

Abstract Simultaneous space- and time-resolved measurements of the concentrations of CH 4 , O 2 , N 2 , H 2 O, H 2 , CO, and CO 2 have been made using spontaneous Raman scattering, in the blue regions of CH 4 turbulent nonpremixed flames. The temperature is measured from the Rayleigh scattered signal. A “fluorescence” interference, which is broadband and contaminates in varying degrees the Rayleigh and all the Raman lines, is believed to be due to a number of molecules or flame radicals, including C 2 and CN or even incandescence of small particle nuclei. The “fluorescence” has been monitored at a bandhead of C 2 (516.5 nm) and its effect reduced by placing a Polaroid filter at the entrance slit of the spectrometer. The remaining “fluorescence” has been corrected for, using correction curves generated from measurements made in a laminar counterflow CH 4 diffusion flame and a diluted CH 4 N 2 = 1 2 (by vol.) laminar diffusion flame. Measurements of CO and CO 2 are not reliable in the rich regions of the flame where the “fluorescence” is intense. With minor modifications to the optical system, CO and CO 2 could also be measured with acceptable accuracy in regions of intense “fluorescence” and the “fluorescence” correction further refined. This work is considered to be an important extension of the applications of spontaneous Raman scattering as a measurement technique in flames.

Pdf Modeling of Turbulent Nonpremixed Methane Jet Flames

Abstract Anexpanded model of turbulent nonpremixed combustion is herein presented. In the model, the scalar mixing and reactions are described by a probability density function (pdf) submodel capable or handling five scalars, while the turbulent velocity field is described by a second-order moment closure. Two plausible chemical reaction models are considered: a five-scalar, four-step, reduced reaction mechanism and a four-scalar constrainted equilibrium model. Detailed comparisons of model predictions with laser Raman experimental data provide a valuable evaluation of the models ability in predicting nonequilibrium chemistry in turbulent nonpremixed flames. Overall, the model fails to predict greater departure from chemical equilibrium as mixing rates are increased. Interestingly, this failure is not due to the chemical model, both of which perform satisfactorily. Instead, the failure to predict greater departure from chemical equilibrium is a subtle artifact of the current Monte Carlo simulation of tur...

Turbulent nonpremixed flames of methane near extinction: Probability density functions☆

Abstract Space- and time-resolved measurements of major species concentrations and temperature have been made using the Raman-Rayleigh scattering technique in the blue (visibly soot free) regions of turbulent nonpremixed flames of methane close to extinction. The data are presented in this paper in the form of single variate probability density functions (pdfs) for the mixture fraction, temperature, and the mass fractions of CH4, O2, H2O, H2, CO2, and CO. Representative instantaneous scatter plots and joint pdfs are also shown. When the mixing rates are low, the data show mostly fully burnt mixtures indicating that the chemistry is relatively fast. As the flame approaches extinction, most local mixtures become either partially burnt or simply mixed. The joint pdfs shown bimodality for mixture fractions less than ∼.1 and centered distributions for richer mixtures. When close to extinction, fully burnt pockets of fluid are still encountered and these may be responsible for keeping the flame alight. Questions relating to the local, instantaneous flame structure near extinction are discussed in light of existing theoretical models.

Conserved scalar fluxes measured in a turbulent nonpremixed flame by combined laser Doppler velocimetry and laser Raman scattering

Abstract This paper presents a new combined laser Doppler velocimetry-laser Raman scattering (LDV-Raman) apparatus which simultaneously measures velocity and scalars. Measurements in a ducted nonpremixed turbulent flame using this apparatus are presented and compared with previous LDV-Mie and LDV-Rayleigh measurements. A partial equilibrium numerical model of the hydrogen flame herein investigated predicts the major species to be at their equilibrium concentration while radical species, such as hydroxyl, can have mean concentrations that are three times greater than their equilibrium concentrations.

Differential Molecular Diffusion Effects in Turbulent Mixing

Abstract The differential diffusion of two species in a turbulent mixing flow is considered. In flows of moderate Reynolds number molecular diffusivities can be a subtantial fraction of the effective turbulent diffusivity and significant differential diffusion can result for species with widely different molecular diffusivities. A theory is presented for the mean and second moments of concentration difference and results are reported for computations for a jet of hydrogen and propane mixing with air. An experiment using Rayleigh scattering to detect the differential diffusion is suggested. Differential diffusion is considered to be very significant in turbulent diffusion flames where molecular and thermal diffusivities are large at high temperatures.

Finite chemical kinetic effects in a subsonic turbulent hydrogen flame

Departures from chemical equilibrium appear in nonpremixed turbulent flames at very high mixing rates, as shown by dimensional analysis based on Damkohler number (characteristic time of mixing over characteristic time of chemical reaction). This paper presents an experimental study that shows departures from chemical equilibrium in a hydrogen-air flame, which is often erroneously considered to have an infinitely fast chemical rate and therefore to be at chemical equilibrium.

Conditional sampling of velocity and scalars in turbulent flames using simultaneous LDV-Raman scattering

The laser Doppler velocimeter (LDV) measures the velocity distribution of particles which is often an acceptable representation of the distribution of gas velocities. However, in turbulent two stream mixing flows, the particle velocity distribution will differ from the gas velocity distribution when the particle densities in the two streams are unequal. This bias is explored in a reacting and nonreacting turbulent jet which is surrounded by coflowing air. By adding seed particles to only the coflow air and then to only the jet fluid, the limits of this bias are established. Additional measurements with an LDV triggered laser Raman scattering system demonstrate that the bias in the LDV sampling is propagated to the Raman measurements. An analytical equation is presented which will generate unbiased velocity and scalar distributions from measurements obtained from seeding only one stream at a time.

''Fluorescence'' interference with Raman measurements in nonpremixed flames of methane

What appears to be fluorescence has been measured in the blue (visibly soot free) regions of laminar and turbulent nonpremixed flames of methane at atmospheric pressure. The measurements are part of an experiment to measure the spontaneous Raman scattering of eight species in the flame. The flames studied range from ones with low mixing rates to flames close to extinction. The “fluorescence” is nonresonantly excited by a pulsed dye laser and has been routinely monitored at λ = 516.5 nm which is a bandhead of diatomic carbon, C2. It is Stokes “fluorescence” that interferes with the Raman signal detection. It is found that the “fluorescence” is broadband covering all the visible spectrum, and is observed only on the rich side of stoichiometry with noticeable intensities for fuel number densities in the range 0.1 to ⋍ 1.5 × 1018 cm−3. The “fluorescence” intensity is of the order of the Raman signals, increases with the fourth power of the laser flux, is unaffected by flame stretch, and decreases with dilution of the unburned methane. No “fluorescence” is detected at temperatures below ⋍ 1000°C. Although the sources of this “fluorescence” are not fully known, it is likely that C2 is only one of many contributing radicals and molecules. The possible contribution of incandescence from vaporizing soot particle is also discussed. Irrespective of the sources of the “fluorescence,” its interference with the Raman lines of stable species in the flame can be corrected for using Stokes and anti-Stokes “fluorescence” monitors, and the Raman scattering technique can therefore be used for measurements in the blue regions of methane flames and may be extendable to higher hydrocarbon flames.

Conditional probability density functions measured in turbulent nonpremixed flames of methane near extinction

Abstract Spontaneous Raman-Rayleigh measurements of temperature and species concentrations have been made in turbulent nonpremixed flames of methane close to extinction. For each data point, five different values of reactedness are calculated from the measured temperature and the mass fractions of CH4, O2, H2O and CO2. These data are then sampled with respect to five ranges of mixture fraction and the conditional single and joint probability density functions (PDFs) of reactedness calculated from the various reactive scalars are presented for each range. Conditional means and root mean squares (rms) of fluctuations of reactedness, temperature, and species mass fractions are also presented. It is found that for mixture fraction ranges lying below the rich reaction limit, ξ ξR, the majority of mixtures are partially reacted and the conditional PDFs have centered distributions. The correlation between the conditional PDFs of various reactive scalars indicate that more than a single-step reaction mechanism is required to adequately describe the chemistry. Close to extinction, the flame was kept alight by fully burned pockets of fluid that provide a source of reignition for mixtures with slower mixing rates. The flame blows off when the frequency of occurrence of these pockets becomes small.