Publications

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Reliable prediction of char conversion, heat release, and particle temperature during heterogeneous char oxidation relies upon quantitative calculation of the CO2/CO production ratio. This ratio depends strongly on the surface temperature, but also on the local partial pressure of oxygen and thus becomes more important in simulations of oxy-fuel or pressurized combustion systems. Existing semi-empirical intrinsic kinetic models of char combustion have been calibrated against the temperature-dependence of the CO2/CO production ratio, but have neglected the effect of the local oxygen concentration. In this study we employ steady-state analysis to demonstrate the limitations of the existing 3-step semi-global kinetics models and to show the necessity of using a 5-step model to adequately capture the temperature- and oxygen-dependence of the CO2/CO production ratio. A suitable 5-step heterogeneous reaction mechanism is developed and its rate parameters fit to match CO2/CO production data, global reaction orders, and activation energies reported in the literature. The model predictions are interrogated for a broad range of conditions characteristic of pressurized, oxy-fuel, and conventional high-temperature char combustion, for which essentially no experimental information on the CO2/CO production ratio is available. The results suggest that the CO2/CO production ratio may be considerably lower than that estimated with existing power-law correlations for oxygen partial pressures less than 10 kPa and surface temperatures higher than 1600 K. To assist with implementation of the mechanistic CO2/CO production ratio results, an analytical procedure for calculating the CO2/CO production ratio is presented. © 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

One of the characteristics of CO2 that influences the oxy-fuel combustion of pulverized coal char is its low diffusivity, in comparison to N2. To further explore how the gas diffusivity influences the apparent rate of pulverized char combustion, experiments were conducted in a laminar, optical flow reactor that has been extensively used to quantify char particle combustion rates. Helium, nitrogen, and CO2 diluent gases were employed as diluent gases. The diffusivity of oxygen through helium is 3.5 times higher than through nitrogen, tending to supply more oxygen to the particle and accelerating the particle combustion rate and heat release. However, the thermal conductivity of helium is 5 times larger than that of nitrogen, tending to keep the burning char particle temperature close to that of the surrounding gas. The combination of these two factors makes char combustion in helium atmospheres significantly more kinetically controlled than combustion of char particles in nitrogen atmospheres. The char particle combustion temperatures were highest for combustion in N2 environments, with combustion in CO2 and He environments producing nearly identical char combustion temperatures, despite much more rapid particle burnout in helium. Preliminary analysis of the apparent char kinetic burning rate in He yields a rate that is approximately three times greater than the rate in N2, likely reflecting the greater internal penetration of oxygen into char particles burning in helium. Analysis with intrinsic kinetic models is being applied to better understand the data and therefore the role of gas diffusivity on apparent kinetic rates of char combustion.

Soot emissions from internal combustion engines and aviation gas turbine engines face increasingly stringent regulation, but available experimental datasets for sooting turbulent combustion model development and validation are largely lacking, in part due to the difficulty of making quantitative space- and time-resolved measurements in this type of flame. To address this deficiency, we have performed a number of different laser and optical diagnostic measurements in sooting, nonpremixed jet flames fueled by ethylene or a prevaporized JP-8 surrogate. Most laser diagnostic techniques inherently lose their quantitative rigor when significant laser beam and signal attenuation occur in sooting flames. However, the '3-line' approach to simultaneous measurement of soot concentration (on the basis of laser extinction) and soot temperature (on the basis of 2-color pyrometry) actually relies on the presence of significant laser attenuation to yield accurate measurements. In addition, the 3-line approach yields complete time-resolved information. In the work reported here, we have implemented the 3-line diagnostic in well-controlled non-premixed ethylene and JP-8 jet flames with a fuel exit Reynolds number of 20,000 using tapered, uncooled alumina refractory probes with a 10 mm probe end separation. Bandpass filters with center wavelengths of 850 nm and 1000 nm were used for the pyrometry measurement, with calibration provided by a hightemperature blackbody source. Extinction of a 635 nm red diode laser beam was used to determine soot volume fraction. Data were collected along the flame centerline at many different heights and radial traverses were performed at selected heights. A data sampling rate of 5 kHz was used to resolve the turbulent motion of the soot. The results for the ethylene flame show a mean soot volume fraction of 0.4 ppm at mid-height of the flame, with a mean temperature of 1450 K. At any given instant, the soot volume fraction typically falls between 0.2 and 0.6 ppm with a temperature between 1300 and 1650 K. At greater heights in the flame, the soot intermittency increases and its mean concentration decreases while its mean temperature increases. In the JP-8 surrogate flame, the soot concentration reaches a mean value of 1.3 ppm at mid-height of the flame, but the mean soot temperature is only 1270 K. Elevated soot concentrations persist for a range of heights in the JP-8 flame, with a rise in mean temperature to 1360 K, before both soot volume fraction and temperature tail off at the top of this smoking flame.

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Experimental measurements in laboratory-scale turbulent burners with well-controlled boundary and flow configurations can provide valuable data for validating models of turbulence-chemistry interactions applicable to the design and analysis of practical combustors. This paper reports on the design of two canonical nonpremixed turbulent jet burners for use with undiluted gaseous and liquid hydrocarbon fuels, respectively. Previous burners of this type have only been developed for fuels composed of H2, CO, andor methane, often with substantial dilution. While both new burners are composed of concentric tubes with annular pilot flames, the liquid-fuel burner has an additional fuel vaporization step and an electrically heated fuel vapor delivery system. The performance of these burners is demonstrated by interrogating four ethylene flames and one flame fueled by a simple JP-8 surrogate. Through visual observation, it is found that the visible flame lengths show good agreement with standard empirical correlations. Rayleigh line imaging demonstrates that the pilot flame provides a spatially homogeneous flow of hot products along the edge of the fuel jet. Planar imaging of OH laser-induced fluorescence reveals a lack of local flame extinction in the high-strain near-burner region for fuel jet Reynolds numbers (Re) less than 20 000, and increasingly common extinction events for higher jet velocities. Planar imaging of soot laser-induced incandescence shows that the soot layers in these flames are relatively thin and are entrained into vortical flow structures in fuel-rich regions inside of the flame sheet. © 2011 American Institute of Physics.

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This report documents the results of a project funded by DoD's Strategic Environmental Research and Development Program (SERDP) on the science behind development of predictive models for soot emission from gas turbine engines. Measurements of soot formation were performed in laminar flat premixed flames and turbulent non-premixed jet flames at 1 atm pressure and in turbulent liquid spray flames under representative conditions for takeoff in a gas turbine engine. The laminar flames and open jet flames used both ethylene and a prevaporized JP-8 surrogate fuel composed of n-dodecane and m-xylene. The pressurized turbulent jet flame measurements used the JP-8 surrogate fuel and compared its combustion and sooting characteristics to a world-average JP-8 fuel sample. The pressurized jet flame measurements demonstrated that the surrogate was representative of JP-8, with a somewhat higher tendency to soot formation. The premixed flame measurements revealed that flame temperature has a strong impact on the rate of soot nucleation and particle coagulation, but little sensitivity in the overall trends was found with different fuels. An extensive array of non-intrusive optical and laser-based measurements was performed in turbulent non-premixed jet flames established on specially designed piloted burners. Soot concentration data was collected throughout the flames, together with instantaneous images showing the relationship between soot and the OH radical and soot and PAH. A detailed chemical kinetic mechanism for ethylene combustion, including fuel-rich chemistry and benzene formation steps, was compiled, validated, and reduced. The reduced ethylene mechanism was incorporated into a high-fidelity LES code, together with a moment-based soot model and models for thermal radiation, to evaluate the ability of the chemistry and soot models to predict soot formation in the jet diffusion flame. The LES results highlight the importance of including an optically-thick radiation model to accurately predict gas temperatures and thus soot formation rates. When including such a radiation model, the LES model predicts mean soot concentrations within 30% in the ethylene jet flame.