Search

Chemical-mechanical polishing (CMP) is commonly used in semiconductor fabrication for flattening wafer surfaces. The study uses 3-D computational fluid dynamics (CFD) to investigate slurry flow dynamics during CMP in a wafer-to-pad zone. Three fluids, used slurry, tracer and fresh incoming slurry, are marked in a wafer-to-pad zone to reveal the fluid dynamics influenced by the operational parameters including pad speed of 30–120 RPM, wafer speed of 30–120 RPM, pad to wafer center-to-center distance of 100–200 mm and wafer size of 100–200 mm and pad size of 610 mm. After validation, the computational model is applied to investigate slurry flow fields with comprehensive operational conditions that were not previously studied but are of significant concern in energy saving and CMP process yields. • Computational approach is proposed to analyze the slurry flow dynamics in CMP. • Tracer is used to examine the residence time of slurry flow. • Pad and wafer spinning speeds influence the CMP performance dramatically. • Recirculation zone is found to trap used slurry. • Large wafer size and short pad to wafer center-to-center distance hinder the slurry replacement. [ABSTRACT FROM AUTHOR]

2-D hybrid lattice-Boltzmann Lagrangian simulations are conducted to predict the experimental results for nanoparticle filtrations in mesh screens. Comparisons between simulations and experimental data are made for pressure drops across the screens and screen-collection efficiencies. The penertration of 10–300 nm particles through a screen is evaluated for three screen opening sizes (60, 100 and 180 µm) at different flow rates (2.5, 4 and 6 LPM). The computational results are obtained in a 2-D cross-sectional domain that represents a semi-infinite array of clean fibers. In order to simplify the 3-D configuration where two fibers interweave, three 2-D models that take into account the physical parameters of mesh screens are investigated: opening-size-, open-area- and volume-fraction-based modeling. Regarding the prediction accuracy, the results identify the most suitable mesh parameters for being used in a 2-D model to predict particle filtration. In addition, the spatial distributions of particle deposition on a single fiber are in accordance with structural growth during fiber loading in the regimes of diffusion and interception. [ABSTRACT FROM AUTHOR]

The study aims at proposing a skeletal mechanism of propane oxidation that describes low-temperature combustion and predicts major hydrocarbon product formation in nonpremixed flames. Utilizing a combination of a sensitivity analysis and path flux analysis, we refine and minimize a recently proposed detailed mechanism of UC San Diego (Prince et al., 2017) without empirical adjustments of rate constants for elementary reactions. The skeletal mechanism with 33 species and 122 reactions improves accuracy for autoignition calculations in the negative temperature coefficient region and its size is commensurate to numerical grids used in solutions of computational fluid dynamics with detailed kinetics. For the first time, the previously measured temperature, major products and non-fuel hydrocarbons in diffusion flames of propane associated with counterflow and coflow configurations are, respectively, verified by means of 1-D kinetic modeling and 2-D computational fluid dynamics. A comprehensive analysis of decomposition pathways connected with preserved and removed reactions provides a clear foundation for mechanism developers to build mechanisms of other alkane fuel oxidation. The rate of production analysis interprets how the experimentally measured propene and 1-butene are formed earlier than acetylene and propyne in the coflow flame. Moreover, the present skeletal mechanism, compared with published detailed mechanisms, features a significant reduction in computational cost. [ABSTRACT FROM AUTHOR]

Towards the goal of establishing kinetic database for low-temperature combustion ( T < 1000 K) of mid-sized biodiesel surrogates, the study uses quantum chemistry and statistical kinetic methods to investigate three primary unimolecular reaction pathways of methyl octanoate peroxy radicals, including dissociation, isomerization and concerted elimination. We calculate kinetic barriers and pressure-dependent rate constants at 500–1000 K. The comparison between our computed and previously estimated rate constants offers further insight into how transition state structures and molecular mechanics are correlated with reaction kinetics. In the branching ratio analysis, we investigate the proposed unimolecular reactions and factors affecting these kinetic characteristics. For the first time, the previously measured oxidation rates of methyl octanoate under the cool flame regime (560–1000 K) are computationally verified by kinetic modeling that reflects the contribution of the present submodel to an existing detailed mechanism of methyl octanoate. Consequently, the rate-of-production analysis reveals the significance of newly proposed reaction pathways. [ABSTRACT FROM AUTHOR]

Branched-chain fatty acid methyl esters (FAME), known for cold flow improvers for biodiesel, have received less attention to their chemical kinetics in flames. This study proposes a detailed mechanism reduced to be a compact form to predict the formation of non-fuel small hydrocarbons and light-weight polycyclic aromatic hydrocarbons (PAH) from the oxidation of methyl isobutanoate [MIB, (CH 3) 2 CHC(=O)OCH 3 ], a C5-branched methyl ester. Using the path flux analysis, the newly constructed MIB oxidation mechanism with 344 species and 2147 reactions is shrunk and refined to 63 species and 288 reactions, a minimum size for its application to computational fluid dynamics. The minimized MIB mechanism incorporated into a 2D axisymmetric coflow model is used to validate mass-spectrometric data obtained from a laminar coflow diffusion flame of methane doped with MIB. For the first time, ten experimentally measured centerline profiles in the flame are computationally interpreted by nine C3–C6 unsaturated hydrocarbons, three carbonyls and four aromatics. Furthermore, we analyze the spatial distribution of the investigated intermediates and reveal their corresponding reaction pathways linked to the MIB decomposition. • Compact kinetic mechanism for methyl isobutanote oxidation is proposed. • CFD coupled with the kinetic mechanism describes chemistry in a nonpremixed flame. • Non-fuel hydrocarbons measured by mass-spectrometry are modeled for the first time. • Reaction pathway analyses elucidate abundant formation of aromatics correlated with the fuel. [ABSTRACT FROM AUTHOR]

A chemical kinetic submechanism proposed here is an essential prerequisite to model autoignition of methyl esters at low temperature region (less than ∼900 K), where reactions of methyl-ester peroxy radicals (ROO∙) and hydroperoxy methyl-ester radicals (Q∙OOH) are crucial and relatively unexplored. The potential energy surfaces of the methyl butanoate peroxy radicals + O2 systems are computed by the G3MP2B3 composite approach. 114 pathways are identified leading to the formation of key radicals in the ignition kernel such as OH and HO2. Particular attention is focused on: (1) intramolecular H-migration of ROO∙, (2) unimolecular dissociations of ROO∙ and Q∙OOH and (3) reactions of ROO∙ + HO2. Using the canonical transition state theory, the high-pressure limit rate constants for reactions in the kinetic submechanism are calculated. Standard enthalpy of formation, entropy and heat capacities are evaluated for intermediates and products formed during combustion by means of the standard statistical mechanics formulae. The agreement and disagreement between our calculated kinetic parameters and previous estimates offer further insight into the uncertainty associated with theoretical estimation. We perform the branching ratio analysis for the competing channels between the reverse dissociation of ROO∙ (ROO∙ → R∙ + O2) and explored unimolecular reactions decomposing ROO∙. Additionally, we also quantify the similarity and dissimilarity between the rate constants determined here and those previously calculated for normal alkanes. Finally, the effect of transition state size on the rate constants for the isomerizations of methyl-ester peroxy radicals is systematically analyzed. [ABSTRACT FROM AUTHOR]

Abstract: Growth of the hydrogen market has motivated increased study of hydrogen production. Understanding how biomass is converted to hydrogen gas can help in evaluating opportunities for reducing the environmental impact of petroleum-based fuels. Using an atmospheric-pressure microwave plasma reactor coupled with species-selective analysis, experiments are conducted at microwave power levels of 800, 900 and 1000 W, a reactant flow rate of 12 slm, and 1 g of dry Spirulina algae in nitrogen. At the absorbed microwave power levels used in this experiment, hydrogen gas produced is in the range of 36.75–45.13% volume fraction, 13.42–15.48 mg per minute, and 12.37–31.46 mg per gram of Spirulina algae consumed. Moreover, the selection of power levels demonstrates that 20.62–52.43% hydrogen atom mass content in dry algae is converted to hydrogen gas. In general, the effect of reaction temperatures on the gas product formation is qualitatively consistent with those produced from other biomass materials reported in literature. Overall, these results will help to expand our knowledge concerning Spirulina algae and hydrogen yield on the basis of microwave-assisted pyrolysis and reaction temperatures, which will inform the study and design of hydrogen production technologies. [Copyright &y& Elsevier]

Abstract: Growth of the biodiesel industry has motivated increased study of the combustion characteristics of its constituent molecules and building combustion modeling capability. Understanding how these characteristics differ between bio-derived and conventional diesel fuels can help in evaluating biodiesel performance. A kinetic modeling comparison of methyl butanoate and n-butane, its corresponding alkane, contrasted the combustion of methyl esters and normal alkanes, towards understanding the effect of the methyl ester moiety. Utilizing a combined n-heptane and methyl butanoate kinetic mechanism in shock tube simulations, the results predicted no region of negative temperature coefficient (NTC) behavior for methyl butanoate, compared to a well defined NTC region for n-butane. We observed that oxidation pathways associated with the methyl ester moiety inhibited NTC behavior, through increased production of hydroperoxy radicals (HO2) instead of hydroxyl radicals (OH). In addition, we compared the evolution of carbon monoxide, carbon dioxide, ethylene and acetylene. The early formation of CO and CO2, directly from methyl butanoate, revealed unique reaction pathways that also influenced a reduction in soot precursor formation. Overall, these results will help to understand how combustion processes change with the inclusion of oxygenated fuels, which will inform the study and design of combustion technologies. [Copyright &y& Elsevier]

Abstract: Burgeoning global demand for energy has increased concerns about the fuel security issues and deleterious environmental impacts that result from the ubiquitous use of fossil fuels to meet these needs. This article is a review of completed work towards the goal of creating chemical kinetic mechanisms for biodiesel, which will aid in the development of clean and efficient combustors that utilize alternative fuels. As the composition of biodiesel is too complex to directly model, efforts have instead focused on the development of mechanisms for surrogates, simpler molecules that can produce the primary characteristics of biodiesel combustion. Research initially targeted smaller molecules like methyl butanoate to investigate the role of the characteristic ester group that is present in the fatty acid alkyl esters that comprise biodiesel. The study of isomers and similar unsaturated compounds elucidated the effects of molecular structure on combustion. Subsequent efforts involved the study of larger molecules that are close in scale to biodiesel molecules, such as methyl decanoate, as well as molecules that are present in biodiesel, such as methyl stearate. Applications of kinetic modeling demonstrate its utility in the study of combustion through, for example, revealing the chemistry in the early formation of CO2 in biodiesel and its soot reduction tendencies. The results of this review illustrate key limitations in kinetic modeling, namely a need for high-pressure kinetic methodology and a need for continuous improvement of kinetic mechanisms through theory and experiment. These limitations suggest direction for future research; further experimental and theoretical work will produce accurate mechanisms for appropriate biodiesel surrogates. All of these efforts represent significant advances in kinetic modeling that are important towards the goal of building a predictive capability for biodiesel combustion. Such predictive capability will aid the development of combustion technologies that will help society meet its energy needs in an environmentally conscious manner. [Copyright &y& Elsevier]

Abstract: This paper analyzes the heat transfer and fluid flow of natural convection in a cavity filled with Al2O3/water nanofluid that operates under differentially heated walls. The Navier–Stokes and energy equations are solved numerically, coupling Xu’s model () for calculating the effective thermal conductivity and Jang’s model () for determining the effective dynamic viscosity, with the slip mechanism in nanofluids. The heat transfer rates are examined for parameters of non-uniform nanoparticle size, mean nanoparticle diameter, nanoparticle volume fraction, Prandtl number, and Grashof number. Enhanced and mitigated heat transfer effects due to the presence of nanoparticles are identified and highlighted. Based on these insights, we determine the impact of fluid temperature on the heat transfer of nanofluids. Decreasing the Prandtl number results in amplifying the effects of nanoparticles due to increased effective thermal diffusivity. The results highlight the range where the heat transfer uncertainties can be affected by the size of the nanoparticles. [Copyright &y& Elsevier]

• Flameless oxidation is used for an ultralow-emission waste treatment process. • A multi-burner system economically achieved the flameless oxidation. • Integrated process can save 24.4–48.7% auxiliary diesel and natural gas. • Regulated pollutant and PAH emissions were obviously reduced by new system. • Bottom ash vitrification took place in combustion unit to save extra energy. • Integrated system solved the corrosion issue found in the previous studies. This is the first study integrate the flameless oxidation (FO) and in-chamber melting (ICM) processes in a primary chamber of a laboratory waste incinerator to improve energy and emission performances. Two liquid burners created a twin-cyclonic fluid field that achieved the FO and ICM in the same chamber. The first cyclone provided a well-mixed and lower temperature FO to reduce auxiliary diesel consumption, NOx and PM emissions by 25.8%, 30.9%, and 79.2%, respectively, from the original system. The hot gases produced by FO enhance the ICM process and transformed the bottom ashes to stabler slags, in turn meeting the regulations for nonhazardous wastes. The other cyclone enhanced the drying and water–gas shift reaction in the drying zone by recirculating the CO and enthalpy from FO and ICM. Eventually, the residual CO, hydrocarbons, and H 2 were sent to the secondary chamber for further oxidation. A computational fluid dynamic simulation supported the fluid field assumption posed in this study. Moreover, advanced scrubbers were employed after thermal treatments to reduce HCl and SO 2 by 81.8% and 38.8% and further retarded the corrosion rate in the baghouse supporting cage by 87.7%. The precursors of condensable particulate matter were reduced by condensation and finally removed in the baghouse. Nevertheless, the emissions of the high- and mid-molecular-weight polycyclic aromatic hydrocarbons were greatly reduced by 60.8–93.1% and 80.2–99.9%, respectively. Consequently, the new system reduced annual emissions by 40.7–87.6% and operating costs by 41.5%, allowing recovery of the remodification investment in 20.5 months. [ABSTRACT FROM AUTHOR]

• The ignition delay times of petro- and renewable fuels were numerically simulated. • HRJ has the shortest ignition delay time in the low temperature range. • The ignition delay time of all fuels was shorter when increasing the pressure. In this research, the composition and proportion of surrogates were first determined according to the composition of the petrochemical and renewable fuels. Then, the CHBR model in CHEMKIN-Pro software was used to verify the ignition delay time of HRJ, JP-5, and HRD. The differences in the ignition delay time of the fuels under an equivalence ratio of 1.0, different pressures (8, 11, 30 bar), and a pressure of 20 bar, and different equivalence ratios (0.5, 1.0, 1.5) are discussed. Among the three types of aviation fuel, the ignition delay time of HRJ in the low-temperature range was the shortest, while that of JP-5 was the longest. The average ignition delay time of HRJ in the low-temperature range under different equivalence ratios and pressures was approximately 59% and 57% lower than that of JP-5, respectively. On the other hand, the average ignition delay time of HRD in the low-temperature range at different equivalence ratios and pressures was 45% and 55% lower than that of petrochemical diesel, respectively. The ignition delay time of all of the fuels was shorter when the pressure was increased. [ABSTRACT FROM AUTHOR]

The study proposes a refined kinetic mechanism to investigate the temperature effect on the formation of the 7 most abundant US-EPA (United States Environmental Protection Agency) PAHs in pyrolysis of acetylene. This work is motivated by the issue that the previously published kinetic mechanisms of C 2 H 2 -PAH exhibit very limited consistency and accuracy to predict C 2 H 2 conversion rate and formation of two- to four-ring aromatics. Based on the previous investigations using a tubular flow reactor, we propose a reactor module consisting of three serially connected 1-D plug-flow reactors, where preheating, heat release and isothermal conditions are considered. The reactor configuration is capable of improving the prediction of C 2 H 2 conversion and PAH formation during the pyrolysis of acetylene at 970–1360 K and 1 atm. In addition, the present mechanism made up of 290 species and 1175 reactions is the refined integration of previous kinetic databases, and its computed results are in satisfactory agreement with experimental values. Using the computed spatial distributions of mole fractions along the serially connected 1-D tubes, we interpret the temperature effect on the outlet mole fractions that were previously detected by gas chromatography–mass spectrometry. Finally, we analyze the temperature effect on reaction pathways leading to the formation of PAHs. [ABSTRACT FROM AUTHOR]