Linda G. Blevins

Modeling of Bare and Aspirated Thermocouples in Compartment Fires

As part of an effort to characterize the uncertainties associated with temperature measurements in fire environments, models of bare bead, single-shielded aspirated, and double-shielded aspirated thermocouples were developed and used to study the effects of varying the gas and average effective surroundings temperatures on the thermocouple error of each configuration. The models were developed for steady-state conditions and hence provide information about error trends rather than about absolute error values. The models indicate that thermocouples respond differently to changes in effective surroundings temperature in a hot upper-layer than in a relatively cooler lower layer of a room fire. In an upper-layer, for a given gas temperature, the thermocouple error is relatively insensitive to surroundings temperature. In a lower layer, errors which increase rapidly with surroundings temperature are possible. The most extreme errors occur in a lower layer when the gas temperature is low and the surroundings temperature is high. Aspirated thermocouples reduce the errors in both the upper and lower layers of a room fire, but do not eliminate them entirely. The present study is intended to provide fire researchers with a methodology for developing working models of thermocouples which are tailored to their own configurations.

The existence of young soot in the exhaust of inverse diffusion flames

Knowledge of the chemical and physical structure of early soot is useful in the development of sootparticle inception models. This paper examines the hypothesis that soot exiting an inverse diffusion flame is similar in chemical and morphological structure to (1) soot precursor particles and (2) soot that exits underventilated flames. Experiments in volving soot collection from the exhaust of laminar ethylene inverse diffusion flames were performed. Soot samples were analyzed for morphology using transmission electron microscopy, for carbon-to-hydrogen ratio using elemental analysis, for organic fraction using thermaloptical analysis, and for polycyclic aromatic hydrocarbon content using laser microprobe mass spectrometry and gas chromatography/mass spectrometry. Results of these analyses support the validity of the above hypothesis. This finding is significant because exhaust collection from the inverse flame provides an opportunity to gather large samples of young soot without invading the flame with an intrusive probe (a necessary task when collecting precursors low in the center of a normal diffusion flame). Larger samples can then be subjected to more detailed analysis than previously possible. An identification of specific polycyclic aromatic hydrocarbon isomers present in young soot from diffusion flames is reported. The data are available for comparison with polycyclic aromatic hydrocarbon growth, soot inception, and soot growth models.

Measurement of Soot Morphology, Chemistry, and Optical Properties in the Visible and Near-Infrared Spectrum in the Flame Zone and Overfire Region of Large JP-8 Pool Fires

Abstract The dimensionless extinction coefficient, K e , was measured for soot produced in 2 m JP-8 pool fires. Light extinction and gravimetric sampling measurements were performed simultaneously at 635 and 1310 nm wavelengths at three heights in the flame zone and in the overfire region. Measured average K e values of 8.4 ± 1.2 at 635 nm and 8.7 ± 1.1 at 1310 nm in the overfire region agree well with values from 8–10 recently reported for different fuels and flame conditions. The overfire K e values are also relatively independent of wavelength, in agreement with recent findings for JP-8 soot in smaller flames. K e was nearly constant at 635 nm for all sampling locations in the large fires. However, at 1310 nm, the overfire K e was higher than in the flame zone. Chemical analysis of physically sampled soot shows variations in carbon-to-hydrogen (C/H) ratio and polycyclic aromatic hydrocarbon (PAH) concentration that may account for the smaller K e values measured in the flame zone. Rayleigh–Debye–Gans theory of scattering for polydisperse fractal aggregate (RDG-PFA) was applied to measured aggregate fractal dimensions and found to under-predict the extinction coefficient by 17–30% at 635 nm using commonly accepted refractive indices of soot, and agreed well with the experiments using the more recently published refractive index of 1.99–0.89i. This study represents the first measurements of soot chemistry, morphology, and optical properties in the flame zone of large, fully-turbulent pool fires, and emphasizes the importance of accurate measurements of optical properties both in the flame zone and overfire regions for models of radiative transport and interpretation of laser-based diagnostics of soot volume fraction and temperature.

Fuel characteristics of processed, high-fiber sugarcane

A study of treatment methods to improve the fuel characteristics of sugarcane variety B52298 was conducted. Two parent materials, whole cane (WC) and stripped cane (SC), were included in the study. The whole cane material was subjected to three treatments: (1) no treatment, WC-U; (2) a single milling, WC-M; and (3) an initial milling followed by leaching and a secondary milling, WC-MLM. Treatments (1) through (3) are in order of increasing severity. The stripped cane material was subjected to treatment (3) and designated as SC-MLM. Regardless of parent material, milling produced moisture contents of ∼50% wet basis and fiber bulk densities of ∼97 kg m−3 in the treated fuels and produced a shift in particle distributions toward smaller sizes. Geometric mean diameters (by weight) of the WC-U, WC-M, WC-MLM, and SC-MLM materials were 2.3, 1.8, 1.3, and 1.3 mm, respectively. Ash generated from the fuel was reduced by roughly 1% (absolute) for each milling operation, resulting in reductions of ∼2% for the WC-MLM and SC-MLM treatments. Ash reduction was primarily due to the removal of K, Cl, and S by the treatment operations. Ash removal, in addition to reductions in the O content of the treated fuels, contributed to an increase in the energy content of the fuels from ∼17.6 MJ kg−1 in the parent materials to 18.4 and 19.2 MJ kg−1 for the WC-MLM and SC-MLM treatments, respectively. K, Cl, S, and N concentrations were all reduced in the fuel by the treatments. K comprised ∼1.3% of the parent materials and Cl accounted for 0.65% and 0.83% of dry matter for the whole cane and stripped cane parent materials, respectively. Reductions in K concentration relative to the parent materials for the WC-M, WC-MLM, and SC-MLM treatments were 50%, 86%, and 91%, respectively. Cl was reduced 62% by the WC-M treatment relative to the unprocessed whole cane, and removal was essentially complete for the two leached treatments. Sulfur in the two parent materials accounted for ∼0.22% of plant dry matter. Compared to the parent materials, the WC-M, WC-MLM, and SC-MLM treatments removed 36%, 82%, and 86% of the S, respectively. Nitrogen concentrations in the stripped cane and whole cane parent materials were 0.48% and 0.37%, respectively. Nitrogen reduction by the WC-M, WC-MLM, and SC-MLM treatments was 12%, 27%, and 57%, respectively. Ash deformation temperatures (oxidizing atmosphere) increased in the treated fuels compared to parent materials. Ash from the WC-MLM treatment did not attain the initial stage of deformation at the maximum test temperature, 1482 °C. Ash of the WC-M and SC-MLM treatments became fluid at ∼1350 °C. Experimentally determined fluid temperatures for the more severely treated fuels compared well with values predicted by a ternary phase diagram for the SiO2–K2O–CaO system. Slagging and fouling indices were computed for each of the fuel treatments. Values for WC-U and WC-M exceeded a benchmark of 0.34 kg (K2O+Na2O) GJ−1 and would be expected to cause ash deposition in boiler use. Values for the WC-MLM and SC-MLM treatments were 0.13 and 0.08 kg (K2O+Na2O) GJ−1, respectively, and are good candidates for boiler fuels. Concomitant reductions in S and Cl for these two fuels further reduce the likelihood of ash deposition, as well as improve environmental performance by reducing criteria and acid gas pollutant emissions. Mass balances for K and Cl were conducted for the treatment operations. Closure for the balances ranged from 112% to 122% over all treatments, and was viewed as validating the consistency of the results.

Laser-induced breakdown spectroscopy of alkali metals in high-temperature gas

Laser-induced breakdown spectroscopy (LIBS) measurements of alkali in the high-temperature exhaust of a glass furnace show an attenuation of the Na and K LIBS signals that correlates with the stoichiometry of the bath gas surrounding the spark. The results are explained as being due to (1) a strong increase in the concentration of atomic Na and K, resulting in neutral line signal absorption by these atoms, and to (2) a change of phase of the major Na- and K-containing species from an aerosol to a gaseous phase when the gas mixture becomes fuel rich, resulting in a reduced LIBS emission intensity. LIBS sampling at lower temperatures, or in a consistently oxidizing environment, or both are suggested strategies for circumventing these difficulties.

Fine Particulate Formation During Switchgrass/Coal Cofiring

Experiments to examine the effects of bioinass/coal cofiring on fine particle formation were performed in the Sandia Multi-Fuel Combustor using fuels of pure coal, three combinations of switchgrass and coal, and pure switchgrass. For this work, fine particles with aerodynamic diameter between 10 nm and I μm were examined. A constant solid-fuel thermal input of 8 kW was maintained. The combustion products were cooled from 1200 to 420°C during passage through the 4.2 m long reactor to simulate the temperatures experienced in the convection pass of a boiler. Fine particle number densities, mass concentrations, and total integrated number and mass concentrations at the reactor exit were determined using a scanning mobility particle sizer. The fine particle number concentrations for cofiring were much higher than those achieved with dedicated coal combustion However, the total integrated mass concentration of particles remained essentiall constant for all levels of cofiring from 0% coal to 100% coal. The constant mass concentration is significant because pending environmental regulations are likely to be based on particle mass rather than particle size.

Implications of air infiltration in oxygen/fuel fired glass furnaces

AbstractContinuous measurements of exhaust gas in an oxygen/natural gas container glass furnace show that diurnal variations in ambient air temperature produce significant changes in exhaust gas composition. At the highest ambient temperatures, the concentrations of air related species (O2, N2 and NO) are the lowest, while those of glass related species (e.g. SO2) are the highest. A detailed mass balance of the furnace shows that air infiltration may account for 30 wt-% of the total gaseous input to the furnace. The sensible heat associated with the N2 in the infiltrated air represents 6% of the total specific energy input. Therefore, reducing the amount of air infiltration can significantly improve furnace efficiency. As an alternative, when significant reductions in air infiltration cannot be readily achieved, a control strategy is proposed for the O2 flow into the furnace that considers the diurnal variations of infiltrated air flow. This strategy yields improvements in energy efficiency and furnace st...

An Investigation of Extinguishment by Thermal Agents Using Detailed Chemical Modeling of Opposed Jet Diffusion Flames

Abstract : The manufacture of the halons widely used in fire extinguishing systems was banned in 1994 due to their deleterious effect on stratospheric ozone. Since the late 198Os there have been ongoing research efforts to identify replacement agents having comparable properties. This search has proven difficult and continues today with a large directed effort known as the Next Generation Fire Suppression Technology Program (NGP). As part of the NGP, the National Institute of Standards and Technology is investigating whether highly effective thermal agents are feasible. Thermal agents are defined as those that obtain their effectiveness solely by heat extraction and dilution. Excluded from investigation are species that directly or indirectly disrupt the combustion chemistry such as halons, which derive much of their effectiveness by the release of bromine atoms that catalytically remove hydrogen atoms in the flame zone. A great deal is known about the effects of thermal agents on flames.

Fine Particulate Formation During Biomass/Coal Cofiring

Experiments to examine the effects of biomass/coal cofiring on fine particle formation were performed in the Sandia Multi-Fuel Combustor using fuels of pure coal, 3 combinations of switchgrass and coal, and pure switchgrass. A constant thermal input was maintained. The combustion products were cooled during passage through the 4.2 m long reactor to simulate the temperatures experienced in the convection pass of a boiler. Fine particle number densities, mass concentrations, and total number concentrations for particles between 10 nm and 1 μm at the reactor exit were determined using a Scanning Mobility Particle Sizer. The results indicate that the fine particle loading for cofiring is higher than that achieved with dedicated coal combustion but lower than that achieved with dedicated switchgrass combustion.Copyright

Original ArticlesComputed structure of low strain rate partially premixed CH4/air counterflow flames: implications for NO formation

Abstract Results from computations of low strain rate, partially premixed methane/air counterflow flames are reported. The Oppdif computer code was used with GRI-Mech 2.11 to obtain the results. When the fuel-side equivalence ratio (Φ B ) is above 2.5, the present flame structure can be described as a CH 4 /air premixed flame merged with a CO/H 2 /air nonpremixed flame. When Φ B is below 2.5, the two flame zones exist on opposite sides of the stagnation plane, and the CO/H 2 /air nonpremixed flame is characterized by hydrocarbon concentration peaks on its fuel-side edge. Broad NO destruction regions, caused primarily by CH i + NO reactions, exist between the resulting double hydrocarbon concentration peaks. The fuel-side equivalence ratio is the most important indicator of how rapidly NO is destroyed relative to how rapidly it is formed, and NO destruction reactions are more important in pure diffusion flames than in partially premixed flames for the present low strain rate computations.