Gregory J. Smallwood

The chemical effects of carbon dioxide as an additive in an ethylene diffusion flame: implications for soot and NOx formation

Abstract A numerical study of the chemical effects of carbon dioxide addition on both the fuel side and the oxidizer side of a diffusion flame was conducted in an attempt to explore the chemistry mechanism of the experimentally observed soot suppression by carbon dioxide addition. The laminar ethylene diffusion flame established in a counterflow configuration was considered by using detailed chemistry and transport properties. A novel strategy was developed that is able to isolate the chemical effects of a species added on either the fuel or the oxidizer side. Numerical results show that carbon dioxide, added either on the fuel side or the oxidizer side, indeed participates in chemical reactions. The specific aspects of the chemical effects of carbon dioxide addition that have direct implications for chemical suppression of soot formation are reduced concentration of acetylene and flame temperature and conversion of carbon dioxide by hydrogen atom to hydroxyl which prompts oxidation of soot precursors in the soot formation region. Reactions CO 2 +H → CO+OH and CO 2 +CH → HCO+CO were found to be responsible for the chemical effects of carbon dioxide addition. The chemical effects of CO 2 addition on the fuel side are small but becomes significant when introduced on the oxidizer side. The chemical effects of CO 2 addition were also found to suppress NO x formation.

The chemical effect of CO2 replacement of N2 in air on the burning velocity of CH4 and H2 premixed flames

When CO2 is added to air or is used to replace N2 in air, it is anticipated that the burning velocity of fresh fuel mixtures may be affected through the following three mechanisms: i) the variation of the transport and thermal properties of the mixture, ii) the possible direct chemical effect of CO2, and iii) the enhanced radiation transfer by CO2. Experimental measurements of the burning velocity of CH4/O2/ CO2 mixtures at various equivalence ratios and pressures have been conducted by Zhu et al. [1] using double flames in the counterflow configuration. The radiative effect of CO2 on the burning velocity of CH4/O2/N2/CO2 mixtures has been recently studied by Ju et al. [2] and Ruan et al. [3]. Moreover, it has also been pointed out in several studies that CO2 is not inert but directly participates in chemical reactions primarily through CO OH 7 CO2 H [4-6]. The objective of this study is to numerically investigate the chemical effects of CO2 replacement of N2 in air on the burning velocity of lean to stoichiometric CH4/O2/N2/ CO2 and H2/O2/N2/CO2 mixtures at 1 atm.

Spectrally Resolved Measurement of Flame Radiation to Determine Soot Temperature and Concentration

A multiwavelength flame emission technique is developed for high spatial resolution determination of soot temperature and soot volume fraction in axisymmetric laminar diffusion flames. Horizontal scans of line-integrated spectra are collected over a spectral range of 500-945 nm. Inversion of these data through one-dimensional tomography using a three-point Abel inversion yields radial distributions of the soot radiation from which temperature profiles are extracted. From an absolute calibration of the flame emission and by use of these temperature data, absorption coefficients are calculated, which are directly proportional to the soot volume fractions. The important optical parameters are discussed. It is shown that a uniform sampling cross section through the flame must be maintained and that variations in sampling area produce inconsistencies between measurements and theory, which cannot be interpreted as spatial averaging of the property field. The variations in cross-sectional sampling area have the largest influence on the measurements at the edges of the flame, where the highest resolution is required. Emission attenuation by soot has been shown to have minor influence on the soot temperature and soot volume fraction for the soot loading of the axisymmetric flame tested. An emission correction scheme is outlined, which could be used for more heavily sooting flames. For a refractive index absorption function E(m) = Im[(m 2 - 1)/(m 2 + 2)] that is independent of wavelength, the soot temperatures and soot volume fractions measured with this technique are in excellent agreement with data obtained by coherent anti-Stokes Raman scattering nitrogen thermometry and two-dimensional soot extinction in the same ethylene coflow diffusion flame. The agreement of the results suggests a limit of the slope of the spectral response of E(m) to be between 0 and 20% over the spectral range examined.

A calibration-independent laser-induced incandescence technique for soot measurement by detecting absolute light intensity

Laser-induced incandescence (LII) has proved to be a useful diagnostic tool for spatially and temporally resolved measurement of particulate (soot) volume fraction and primary particle size in a wide range of applications, such as steady flames, flickering flames, and Diesel engine exhausts. We present a novel LII technique for the determination of soot volume fraction by measuring the absolute incandescence intensity, avoiding the need for ex situ calibration that typically uses a source of particles with known soot volume fraction. The technique developed in this study further extends the capabilities of existing LII for making practical quantitative measurements of soot. The spectral sensitivity of the detection system is determined by calibrating with an extended source of known radiance, and this sensitivity is then used to interpret the measured LII signals. Although it requires knowledge of the soot temperature, either from a numerical model of soot particle heating or experimentally determined by detecting LII signals at two different wavelengths, this technique offers a calibration-independent procedure for measuring soot volume fraction. Application of this technique to soot concentration measurements is demonstrated in a laminar diffusion flame.

Two-dimensional imaging of soot volume fraction in laminar diffusion flames

A technique for acquiring two-dimensional soot-volume-fraction measurements in laminar flames has been demonstrated. The technique provides a map of very low noise concentration over a range of wavelengths (250-1100 nm). A noise level of 0.0007 in extinction and a spatial resolution of 30-40 microm for soot concentration were achieved with an arc lamp source that was filtered to provide greater spatial coherence and a CCD detector. The broadband arc lamp source also allowed us to avoid the added noise resulting from speckle with coherent laser sources. Beam steering, due to refractive-index gradients in the flame, was measured and compared with theoretical predictions. The optical arrangement to minimize the effect of beam steering is described. As a result the beam steering had no effect on the soot measurements in the flames examined. Flame-transmission maps obtained with this system in an ethylene/air laminar diffusion flame are presented. Tomographic analysis from use of an Abel inversion of the line-of-sight data to obtain radial profiles of soot concentration is described.

Effects of Gas and Soot Radiation on Soot Formation in a Coflow Laminar Ethylene Diffusion Flame

Abstract A computational study of soot formation in an undilute axisymmetric laminar ethylene-air coflow jet diffusion flame at atmospheric pressure was conducted using a detailed gas-phase reaction mechanism and complex thermal and transport properties. A simple two-equation soot model was employed to predict soot formation, growth, and oxidation with interactions between the soot chemistry and the gas-phase chemistry taken into account. Both the optically thin model and the discrete-ordinates method coupled with a statistical narrow-band correlated-K based wide band model for radiative properties of CO, CO2, H2O, and soot were employed in the calculation of radiation heat transfer to evaluate the adequacy of using the optically thin model. Several calculations were performed with and without radiative transfer of radiating gases and/or soot to investigate their respective effects on the computed soot field and flame structure. Radiative heat transfer by both radiating gases and soot were found to be important in this relatively heavily sooting flame studied. Results of the optically thin radiation model are in good agreement with those obtained using the wide band model except for the flame temperature near the flame tip.

Flame front surface characteristics in turbulent premixed propane/air combustion

The characteristics of the flame front surfaces in turbulent premixed propane/air flames were investigated. Flame front images were obtained using laser-induced fluorescence (LIF) of OH and Mie scattering on two Bunsen–type burners of 11.2-mm and 22.4-mm diameters. Nondimensional turbulence intensity, u′/SL, was varied from 0.9 to 15, and the Reynolds number, based on the integral length scale, varied from 40 to 467. Approximately 100 images were recorded for each experimental condition. Fractal parameters (fractal dimension, inner and outer cutoffs) and corresponding standard deviations were determined by analysis of the flame front images using the caliper technique. The fractal dimensions derived from OH and Mie scattering images are almost identical. However, inner and outer cutoffs from OH images are consistently higher than those obtained from Mie scattering. The self-similar region of the flame front wrinkling is about a decade for all flames studied. In the nondimensional turbulence intensity range from 1 to 15, it was found that the mean fractal dimension is about 2.2 and it does not show any dependence on turbulence intensity. This contradicts the findings of the previous studies that showed that the fractal dimension asymptotically reaches to 2.35–2.37 when the nondimensional turbulence intensity u′/SL exceeds 3. It is shown that the reason for this discrepancy is the image analysis method used in the previous studies. Examples are given to show the inadequacy of the circle method used in previous studies for extraction of fractal parameters from flame front images. The fractal parameters obtained so far, in this and previous studies, are not capable of correctly predicting the turbulent burning velocity using the available fractal area closure model.

Inner cutoff scale of flame surface wrinkling in turbulent premixed flames

The inner cutoff represents the smallest scale for the interaction of turbulent eddies with the premixed flame front. The nondimensional inner cutoff has been shown to scale with Karlovitz number, Kaβ, where β is −12. Three experimental data sets, with available statistical information, strongly agree with this scaling. However, the five data sets, two without experimental error statistic, seem to favor β = −13, in accordance with the direct numerical simulation results. The inner cutoff expressions reported in literature, obtained through experimental data, numerical data, or physical reasoning, have been evaluated by comparing with five experimental data sets reported recently. It has been shown that the Gibson scale, which was proposed as the inner cutoff and asserts an inverse square dependence on Karlovitz number, does not show any agreement with the experimental data or direct numerical simulation results.

Characterization of flame front surfaces in turbulent premixed methane/air combustion

Abstract A detailed experimental investigation of the application of fractal geometry concepts in determining the turbulent burning velocity in the wrinkled flame regime of turbulent premixed combustion was conducted. The fractal dimension and cutoff scales were determined for six different turbulent flames in the wrinkled flame regime, where the turbulence intensity, turbulent length scale, and equivalence ratio were varied. Unlike previous reports, it has proved possible to obtain the fractal dimension and inner and outer cutoffs from individual flame images. From this individual data, the pdf distributions of all three fractal parameters, along with the distribution of th predicted increase in surface area, may be determined. The analysis of over 300 flame images for each flame condition provided a sufficient sample size to accurately define the pdf distributions and their means. However, the predicted S T S L , calculated using fractal parameters, was significantly below the measured values. For conical flames, a geometrical modification factor was employed to predict S T S L , however, this did little to improve the predictions. There appeared to be no dependence of the predicted S T S L on the approach flow turbulence. The cutoffs did not seem to vary significantly with any of the length scales in the approach flow turbulence, although the fractal dimension did appear to have a weak dependence on u′ S L and ReΛ. The probable reasons that fractal geometry does not correctly predict S T S L are that S T S L = A w A 0 does not hold in wrinkled turbulent premixed flames, that the flame front surface cannot be described by a single scaling exponent, or that these are not wrinkled flames even though they are within conventional definitions of the wrinkled flame regime.

Numerical modelling of soot formation and oxidation in laminar coflow non-smoking and smoking ethylene diffusion flames

A numerical study of soot formation and oxidation in axisymmetric laminar coflow non-smoking and smoking ethylene diffusion flames was conducted using detailed gas-phase chemistry and complex thermal and transport properties. A modified two-equation soot model was employed to describe soot nucleation, growth and oxidation. Interaction between the gas-phase chemistry and soot chemistry was taken into account. Radiation heat transfer by both soot and radiating gases was calculated using the discrete-ordinates method coupled with a statistical narrow-band correlated-k based band model, and was used to evaluate the simple optically thin approximation. The governing equations in fully elliptic form were solved. The current models in the literature describing soot oxidation by O2 and OH have to be modified in order to predict the smoking flame. The modified soot oxidation model has only moderate effects on the calculation of the non-smoking flame, but dramatically affects the soot oxidation near the flame tip in the smoking flame. Numerical results of temperature, soot volume fraction and primary soot particle size and number density were compared with experimental data in the literature. Relatively good agreement was found between the prediction and the experimental data. The optically thin approximation radiation model significantly underpredicts temperatures in the upper portion of both flames, seriously affecting the soot prediction.