Publications

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Planar laser-induced incandescence (LII) imaging is reported at repetition rates up to 100 kHz using a burst-mode laser system to enable studies of soot formation dynamics in highly turbulent flames. To quantify the accuracy and uncertainty of relative soot volume fraction measurements, the temporal evolution of the LII field in laminar and turbulent flames is examined at various laser operating conditions. Under high-speed repetitive probing, it is found that LII signals are sensitive to changes in soot physical characteristics when operating at high laser fluences within the soot vaporization regime. For these laser conditions, strong planar LII signals are observed at measurement rates up to 100 kHz but are primarily useful for qualitative tracking of soot structure dynamics. However, LII signals collected at lower fluences allow sequential planar measurements of the relative soot volume fraction with a sufficient signal-to-noise ratio at repetition rates of 10-50 kHz. Guidelines for identifying and avoiding the onset of repetitive probe effects in the LII signals are discussed, along with other potential sources of measurement error and uncertainty.

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Final burnout of char particles from practical fuels such as coal and biomass occurs in the presence of a large ash component. Also, newly utilized coal resources, such as those from India, often contain much larger ash fractions than have traditionally been utilized. In the past, the inhibitory influence of ash on pulverized coal particle combustion has been most frequently modeled using an ash film model, though such films are rarely found when examining partially combusted particles. Conversely, some measurements have suggested that mineral components exposed on the surface of burning pulverized coal (pc) particles may diffuse back into the char matrix, the effect of which can be modeled as an ash dilution effect. To explore the implications of these different ash inhibition models on the temporal evolution of char combustion during burnout, we have developed a new computational model that considers the possibility of an ash film effect, an ash dilution effect, or some arbitrary combination of the two effects acting in tandem, which is the most realistic scenario. This new model predicts that restricted diffusion through the ash film has a significant impact on the char burnout rate throughout its lifetime, whereas char dilution only inhibits combustion significantly when most of the char has been consumed and the combustion mode shifts from predominantly external diffusion control to mixed diffusion control, with sensitivity to both external and internal diffusion resistance. The comparison of the model predictions with experimental results also confirms the previously suggested need to include gasification reaction steps when modeling coal char combustion.

Non-premixed oxy-fuel combustion of natural gas is used in industrial applications where highintensity heat is required, such as glass manufacturing and metal forging and shaping. In these applications, the high flame temperatures achieved by oxy-fuel increases radiative heat transfer to the surfaces of interest and soot formation within the flame is desired for further augmentation of radiation. However, the high energy consumption and cost of traditional methods of oxygen production have limited the penetration of oxy-fuel combustion technologies. New approaches to oxygen production, using ion transport membranes or metal organic frameworks (MOFs), are being developed that may reduce the oxygen production costs associated with conventional cryogenic air separation, but which are likely to be more economical for intermediate levels of oxygen enrichment of air, rather than for the high-purity oxygen that is produced by conventional cryogenic air separation. To determine the influence of oxygen enrichment on soot formation and radiation, we developed a non-premixed coannular burner in which oxygen concentrations and flow rates can be independently varied, to distinguish the effects of turbulent mixing intensity, characteristic flame residence time, and oxygen enrichment on soot formation and flame radiation intensity. Local radiation intensities and soot concentrations have been measured using a thin-film thermopile and planar laser-induced incandescence (LII), respectively. Results show that turbulence intensity has a marked effect on soot formation and thermal radiation. Somewhat surprisingly, soot formation is found to increase as the oxygen concentration decreases from 100% to 50%, for flames in which the turbulence intensity remains constant. At the same time, the thermal radiation from these flames only decreases gradually for an extended range of oxygen concentrations. These results suggest that properly designed oxygen-enriched burners that enhance soot formation for intermediate levels of oxygen purity may be able to achieve similar thermal radiation intensities as traditional oxy-fuel burners utilizing high-purity oxygen.

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In CFD models of pulverized coal combustion, which often have complex, turbulent flows with millions of coal particles reacting, the char combustion sub-model needs to be computationally efficient. There are several common assumptions that are made in char combustion models that allow for a compact, computationally efficient model. In this work, oft used single- and double-film simplified models are described, and the temperature and carbon combustion rates predicted from these models are compared against a more accurate continuous-film model. Both the single- and double-film models include a description of the heterogeneous reactions of carbon with O2, CO2, and H2O, along with a Thiele based description of reactant penetration. As compared to the continuous-film model, the double-film model predicts higher temperatures and carbon consumption rates, while the single-film model gives more accurate results. A single-film model is therefore preferred to a double-film model for a simplified, yet fairly accurate description of char combustion. For particles from 65 to 135μm, in O2 concentrations ranging from 12 to 60vol.%, with either CO2 or N2 as a diluent, particle temperatures from the single-film model are expected to be accurate within 270K, and carbon consumption rate predictions should be within 16%, with greater accuracies for a CO2 diluent and at lower bulk oxygen concentrations. A single-film model that accounts for reactant penetration and both oxidation and gasification reactions is suggested as a computationally efficient sub-model for coal char combustion that is reasonably accurate over a wide range of gas environments. © 2013 The Combustion Institute.

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