Publications

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The existence of a critical dissipation rate, above which a steady nonpremixed flame is extinguished, is well known. Recent advances in modeling have allowed the simulation of turbulent nonpremixed flames that include local extinction as a consequence of the stochastic variation in dissipation rates. In this paper we present an extinction criterion for flames subject to unsteady dissipation rates. This criterion is expressed in terms of the critical dissipation-impulse magnitude, which depends on the time-integrated excess dissipation rate and stoichiometric factors. Limiting behaviors for large and small fluctuations of the dissipation rate above the critical value are identified. For large dissipation-rate fluctuations, the critical dissipation-impulse magnitude is independent of the details of the temporal dissipation-rate evolution. This critical dissipation-impulse magnitude is found to depend only on the steady-state characteristics of the particular fuel-oxidizer mixture present, namely the shape of the steady-state S-curve. In this way, a useful extinction criterion is developed that defines conditions for which unsteady mixing dynamics lead to extinction based on information available from steady-state flames. This criterion is found applicable for a diverse set of flames including n-heptane, diluted n-heptane, methane, partially premixed methane and CO/H2/N2 mixtures when dissipation-rate fluctuations are large. As the magnitude of the dissipation rate fluctuations approaches zero, the critical impulse approaches zero, which corresponds to the well-known steady extinction limit. Thus, this work extends the prediction of extinction from the steady limit to the unsteady. © 2013.

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The existence of a critical dissipation rate, above which a nonpremixed flame is extinguished, has been known for decades. Recent advances in modeling have allowed the simulation of turbulent nonpremixed flames that include local extinction as a consequence of the stochastic variation in mixing rates. In this paper we present the critical dissipation impulse magnitude that will lead to extinction even if the mean dissipation rate is well below the criteria for a steady flame. This critical impulse magnitude depends on the time-integrated excess dissipation rate, stoichiometric factors and the form of the S-curve describing the steady-state flame. This criteria is evaluated in a diverse set of flames including n-heptane, diluted n-heptane and CO/H2/N2 mixtures.

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This document summarizes a three year Laboratory Directed Research and Development (LDRD) program effort to improve our understanding of algal flocculation with a key to overcoming harvesting as a techno-economic barrier to algal biofuels. Flocculation is limited by the concentrations of deprotonated functional groups on the algal cell surface. Favorable charged groups on the surfaces of precipitates that form in solution and the interaction of both with ions in the water can favor flocculation. Measurements of algae cell-surface functional groups are reported and related to the quantity of flocculant required. Deprotonation of surface groups and complexation of surface groups with ions from the growth media are predicted in the context of PHREEQC. The understanding of surface chemistry is linked to boundaries of effective flocculation. We show that the phase-space of effective flocculation can be expanded by more frequent alga-alga or floc-floc collisions. The collision frequency is dependent on the floc structure, described in the fractal sense. The fractal floc structure is shown to depend on the rate of shear mixing. We present both experimental measurements of the floc structure variation and simulations using LAMMPS (Large-scale Atomic/Molecular Massively Parallel Simulator). Both show a densification of the flocs with increasing shear. The LAMMPS results show a combined change in the fractal dimension and a change in the coordination number leading to stronger flocs.

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Expert panels comprised of subject matter experts identified at the U.S. National Laboratories (SNL, ANL, INL, ORNL, LBL, and BNL), universities (University of Wisconsin and Ohio State University), international agencies (IRSN, CEA, JAEA, KAERI, and JRC-IE) and private consultation companies (Radiation Effects Consulting) were assembled to perform a gap analysis for sodium fast reactor licensing. Expert-opinion elicitation was performed to qualitatively assess the current state of sodium fast reactor technologies. Five independent gap analyses were performed resulting in the following topical reports: (1) Accident Initiators and Sequences (i.e., Initiators/Sequences Technology Gap Analysis), (2) Sodium Technology Phenomena (i.e., Advanced Burner Reactor Sodium Technology Gap Analysis), (3) Fuels and Materials (i.e., Sodium Fast Reactor Fuels and Materials: Research Needs), (4) Source Term Characterization (i.e., Advanced Sodium Fast Reactor Accident Source Terms: Research Needs), and (5) Computer Codes and Models (i.e., Sodium Fast Reactor Gaps Analysis of Computer Codes and Models for Accident Analysis and Reactor Safety). Volume II of the Sodium Research Plan consolidates the five gap analysis reports produced by each expert panel, wherein the importance of the identified phenomena and necessities of further experimental research and code development were addressed. The findings from these five reports comprised the basis for the analysis in Sodium Fast Reactor Research Plan Volume I.

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The effects of algae concentration, ferric chloride dose, and pH on the flocculation efficiency of the freshwater algae Chlorella zofingiensis can be understood by considering the nature of the electrostatic charges on the algae and precipitate surfaces. Two critical conditions are identified which, when met, result in flocculation efficiencies in excess of 90% for freshwater algae. First, a minimum concentration of ferric chloride is required to overcome the electrostatic stabilization of the algae and promote bridging of algae cells by hydroxide precipitates. At low algae concentrations, the minimum amount of ferric chloride required increases linearly with algae concentration, characteristic of flocculation primarily through electrostatic bridging by hydroxide precipitates. At higher algae concentrations, the minimum required concentration of ferric chloride for flocculation is independent of algae concentration, suggesting a change in the primary flocculation mechanism from bridging to sweep flocculation. Second, the algae must have a negative surface charge. Experiments and surface complexation modeling show that the surface charge of C. zofingiensis is negative above a pH of 4.0±0.3 which agrees well with the minimum pH required for effective flocculation. These critical flocculation criteria can be extended to other freshwater algae to design effective flocculation systems. © 2011 Wiley Periodicals, Inc.

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