Yudaya R. Sivathanu

Coupled radiation and soot kinetics calculations in laminar acetylene/air diffusion flames☆

Abstract Radiation heat transfer from flames depends on the instantaneous soot volume fractions and temperatures. A coupled radiation and soot kinetics calculation in laminar acetylene/air and acetylenemethane/air diffusion flames is described. Transport equations for mass, momentum, gas-phase mixture fraction, enthalpy (sensible + chemical), soot mass fraction, and soot number density are solved. A simplified soot kinetics model incorporating nucleation, growth, oxidation, and agglomeration processes is used. The reaction rates in the simplified kinetics model depend on the temperature and the local concentrations of acetylene and oxygen. The major gas species concentrations are obtained from state relationships. The local temperature is obtained by solving the energy equation, taking radiation loss and gain and the energy exchanges associated with soot formation and oxidation into consideration. The radiative source/sink term in the energy equation is obtained using a multiray method. Since these flames radiate a substantial part of their energy, the kinetic rates associated with soot processes are strongly coupled to the energy equation. This strong coupling between radiation, and soot formation and oxidation processes is modeled for the first time. The results of the soot kinetics model are compared with measurements of soot volume fractions obtained using laser tomography. The agreement between measurements and predictions of soot volume fractions supports the present method. The predicted temperature profiles support the structure of strongly radiating flames discovered earlier.

Total radiative heat loss in jet flames from single point radiative flux measurements

Abstract A method for estimating total radiant output of turbulent jet flames based on the measurement of radiative heat flux at a single location is reported. The radiative flux from a variety of jet flames was measured and plotted in normalized coordinates to establish the feasibility of this approach. In addition, the radiative flux from acetylene-air diffusion flames to representative detector locations, for two different burner geometries and flow conditions, was calculated using the Planck-averaged equation of transfer coupled with a multiray technique. The local temperature and soot volume fractions for the calculations were obtained from emission/absorption measurements. The normalized calculated heat flux for the two flames also collapse with the experimental data. This result shows that scalar property distributions combined with the appropriate view factor for the single location are the basis for the single point technique.

Coupled Structure and Radiation Analysis of Acetylene/Air Flames

A coupled radiation-structure analysis of turbulent, non-premixed, strongly radiating acetylene/air flames is described. The analysis extends the laminar flamelet concept to include the effects of local radiative heat loss/gain. A new method for the calculation of the radiative source term is presented. New measurements of mean and fluctuating emission temperatures and radiation intensities, and previous data concerning flame structure are used to evaluate the predictions. Results show good agreement between measurements and predictions of flame structure similar to past uncoupled calculations. The mean emission temperatures and the mean visible radiation intensities are substantially underpredicted by the uncoupled analysis. The coupled calculations provide reasonable estimates of both quantities.

THERMAL RADIATION PROPERTIES OF TURBULENT LEAN PREMIXED METHANE AIR FLAMES

Thermal radiation properties of turbulent premixed flames have received little attention in the past perhaps because of the lower radiative heat loss compared with that for non-premixed flames. However, the high-temperature sensitivity of NO kinetics and the importance of radiation in near-limit laminar premixed flames provide fundamental reasons for studies of radiation properties of turbulent premixed flames. Reduced cooling airflows in lean premixed combustors, miniaturization of combustors, and the possible use of radiation sensors in combustion control schemes are some of the practical reasons for studying radiation heat transfer in these flames. Motivated by this, we report the first (to our knowledge) study of spectral radiation properties of turbulent premixed flames. Measurements of mean, root mean square (rms) and probability density functions (PDFs) of spectral radiation intensities leaving diametric paths at five heights in two turbulent lean premixed methane/air jet flames stabilized using small H2/air pilot flames in a coflow of air were completed. Measurements of spectral radiation intensities leaving three laminar flames were also completed. These data were used to evaluate narrowband radiation calculations independent of the treatment of turbulent fluctuations. Stochastic spatial series analysis was used to estimate instantaneous distributions of temperature. The analysis requires the specification of mean and rms temperature distributions, integral length scale distributions, and an assumption of exponential spatial correlation function. We specified the mean and rms temperature distributions measured by calibrated narrowband thin filament pyrometry. A simple flame and mixing model was used to relate the concentrations of CO2 and H2O to the temperature. We used scalar spatial series in conjunction with a radiation model to calculate the mean, rms, and PDFs of spectral radiation intensities. Overall, the model predictions are in reasonable agreement with the data. The only improvement needed is in the area of capturing correlated occurrences of high temperatures along the radiation path.

Simultaneous emission absorption measurements in toluene-fueled pool flames: Mean and RMS Properties

Abstract Local measurements of mean and RMS emission intensities, transmittances, emission temperatures, and soot volume fractions based on emission and absorption in toluence flames burning in a pool configuration are reported. Radial profiles of these quantities at six axial stations within the flame are selected for discussion. The results show large fluctuations in temperatures and soot volume fractions at all locations including those near the liquid surface. Differences between the soot volume fractions based on emission and those based on absorption indicate the presence of large quantities of relatively cold soot at all positions.

Transient structure and radiation properties of strongly radiating buoyant flames

Measurements of instantaneous temperature and soot volume fractions based on absorption and emission in highly buoyant turbulent acetylene/air and propylene/air flames are reported. These measurements are used to predict mean, rms, probability density functions, and power spectral densities of spectral radiation intensities along a representative horizontal chord in the flame. The results show the presence of large quantities of relatively cold soot in the vicinity of smaller amounts of hot soot particles. The resulting inhomogeneity in the temperature of soot in the flame leads to negative cross correlations between temperature and soot volume fractions. The treatment of such correlations was found necessary for predicting the observed probability density functions and the power spectral densities of spectral radiation intensities.

A Study of Coupled Turbulent Mixing, Soot Chemistry, and Radiation Effects Using the Linear Eddy Model

Transient simulations of strongly radiating, acetylene-air, nonpremixed flames in stationary, homogeneous turbulence are conducted in order to study coupled turbulence, soot chemistry, and radiation interactions. The linear eddy model is used to simulate turbulent advection. A laminar flamelet state relationship combustion model is employed along with two different soot models. The first soot model involves an extension of the laminar flamelet concept to soot using a soot volume fraction state relationship. The second soot model involves transport equations for soot mass fraction and soot number density, which include finite rate source terms to account for soot nucleation, surface growth, agglomeration, and oxidation. Radiation effects are accounted for by including the appropriate source/sink terms in the conservation of energy equation. The effects of a presumed surrounding large scale field which radiates with the spectral properties of soot at an assumed effective temperature are also included. Simulations are conducted for two values of the surrounding temperature and the model large eddy turnover time. The results capture several unique aspects of strongly radiating turbulent flames. In particular, an inflection point in the temperature versus mixture fraction profile is observed near the soot region which highlights the effects of radiative cooling. The large difference between radiation source terms calculated using mean properties and those calculated using instantaneous properties highlights the important interactions between turbulence and radiation.

Effects of gas-band radiation on soot kinetics in laminar methane / air diffusion flames

Abstract A coupled radiation and soot kinetics calculation of laminar methane/air diffusion flame properties is described. Transport equations for mass, momentum, mixture fraction, enthalpy (sensible + chemical) including gas-band radiation, soot mass fraction, and soot number density are solved. A simplified soot kinetics model incorporating nucleation, growth, oxidation, and agglomeration processes is used. The reaction rates in the simplified kinetics model depend on the temperature and the local concentrations of C 2 H 2 , O 2 , and OH. The major gas species and the C 2 H 2 and OH concentrations are obtained using state relationships. The local temperature is obtained by solving the energy equation, taking radiation loss and gain from gas species and soot particles into consideration. The radiative source/sink term in the energy equation is obtained using a multiray method in conjunction with the narrow-band algorithm RADCAL. The results of the soot kinetics model are compared with existing laser-induced incandescence (LII) measurements of soot volume fractions. Reasonable comparison can be obtained only with an arbitrary downstream shift of 20 mm in the origin of the predictions from the burner exit. This highlights the need for improved chemical kinetics, but does not affect the following conclusions: 1) the contribution of participating gas (CO 2 and H 2 O) radiation dominates that of soot radiation by an order of magnitude in the present methane/air flames, and 2) even for the present weakly radiating flames, the local radiative heat loss/gain strongly influences the soot nucleation, formation, and oxidation rates.

Measurements and stochastic time and space series simulations of spectral radiation in a turbulent non-premixed flame

Experimental data are essential for the validation of radiation submodels, which have been found to be important for predicting pollutant formation in turbulent flames. Instantaneous radiation signals also provide fundamental information about scalar properties in turbulent combustion. Motivated by this, we report measurements of line-of-sight spectral radiation intensities from a non-premixed CH 4 /H 2 /N 2 turbulent jet flame. The burner and the operating conditions are selected to take advantage of extensive scalar property and velocity measurements available in the literature. At three axial locations in the flame, a fast IR array spectrometer was used to capture the instantaneous radiation intensities for diametric radiation paths. Radiation intensities for the chord-like paths along various radial positions at one of the axial locations were also measured. By using stochastic time and space series (TASS) analysis, the instantaneous emission spectra were also simulated accounting for the turbulence/radiation interactions. In the simulations, the measurements of scalar statistics and mean velocity data were adopted to avoid uncertainties of a combustion model. The calculated mean and root mean square spectral radiation intensities are within 10% of the experimental data. Since the calculated root mean square values are strongly dependent on the integral length scales used in the TASS, these scales were estimated by fitting the calculation to the data. A tomography-like technique was also adopted to simulate the radiation intensities for chord-like paths from the flame edge to the center to examine the radial variation of the integral length scale. The results show factors of 3 variations in the integral length scale that have been ignored in the past work.

A Discrete Probability Function Method for the Equation of Radiative Transfer

Abstract A discrete probability function (DPF) method for the equation of radiative transfer is derived. The DPF is defined as the integral of the probability density function (PDF) over a discrete interval. The derivation allows the evaluation of the DPF of intensities leaving desired radiation paths including turbulence-radiation interactions without the use of computer intensive stochastic methods. The DPF method has a distinct advantage over conventional PDF methods since the creation of a partial differential equation from the equation of transfer is avoided. Further, convergence of all moments of intensity is guaranteed at the basic level of simulation unlike the stochastic method where the number of realizations for convergence of higher order moments increases rapidly. The DPF method is described for a representative path with approximately integral-length scale-sized spatial discretization. The results show good agreement with measurements in a propylene/air flame except for the effects of intermittency resulting from highly correlated realizations. The method can be extended to the treatment of spatial correlations as described in the Appendix. However, information regarding spatial correlations in turbulent flames is needed prior to the execution of this extension.