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Energy storage using lithium-ion cells dominates consumer electronics and is rapidly becoming predominant in electric vehicles and grid-scale energy storage, but the high energy densities attained lead to the potential for release of this stored chemical energy. This article introduces some of the paths by which this energy might be unintentionally released, relating cell material properties to the physical processes associated with this potential release. The selected paths focus on the anode–electrolyte and cathode–electrolyte interactions that are of typical concern for current and near-future systems. Relevant material processes include bulk phase transformations, bulk diffusion, surface reactions, transport limitations across insulating passivation layers, and the potential for more complex material structures to enhance safety. We also discuss the development, parameterization, and application of predictive models for this energy release and give examples of the application of these models to gain further insight into the development of safer energy storage systems.

Medium scale (30 cm diameter) methanol pool fires were simulated using the latest fire modeling suite implemented in Sierra/Fuego, a low Mach number multiphysics reacting flow code. The sensitivity of model outputs to various model parameters was studied with the objective of providing model validation. This work also assesses model performance relative to other recently published large eddy simulations (LES) of the same validation case. Two pool surface boundary conditions were simulated. The first was a prescribed fuel mass flux and the second used an algorithm to predict mass flux based on a mass and energy balance at the fuel surface. Gray gas radiation model parameters (absorption coefficients and gas radiation sources) were varied to assess radiant heat losses to the surroundings and pool surface. The radiation model was calibrated by comparing the simulated radiant fraction of the plume to experimental data. The effects of mesh resolution were also quantified starting with a grid resolution representative of engineering type fire calculations and then uniformly refining that mesh in the plume region. Simulation data were compared to experimental data collected at the University of Waterloo and the National Institute of Standards and Technology (NIST). Validation data included plume temperature, radial and axial velocities, velocity temperature turbulent correlations, velocity velocity turbulent correlations, radiant and convective heat fluxes to the pool surface, and plume radiant fraction. Additional analyses were performed in the pool boundary layer to assess simulated flame anchoring and the effect on convective heat fluxes. This work assesses the capability of the latest Fuego physics and chemistry model suite and provides additional insight into pool fire modeling for nonluminous, nonsooting flames.

Thermal runaway of lithium-ion batteries is a risk that is magnified when stacks of lithium-ion cells are used for large scale energy storage. When limits of propagation can be identified so that systems can be designed to prevent large scale cascading failure even if a failure does occur, these systems will be safer. This work addresses the prediction of cell-to-cell failure propagation and the propagation limits in lithium-ion cell stacks to better understand and identify safe designs. A thermal-runaway model is presented based on recent developments in thermochemical source terms. It is noted that propagating failure is characterized by temperatures above which calorimetry data is available. Results show high temperature propagating failure predictions are too rapid unless an intra-particle diffusion limit is included, introducing a Damkohler number limiter into the rate expression. This new model form is evaluated against cell-to-cell failure propagation where the end cell of a stack is forced into thermal runaway through a nail-induced short circuit. Limits of propagation for this configuration are identified. Results show cell-to-cell propagation predictions are consistent with measurements over a range of cell states of charge and with the introduction of metal plates between cells to add system heat capacity representative of structural members. This consistency extends from scenarios where propagation occurs through scenarios where propagation is prevented.

Thermal runaway of lithium-ion batteries is a risk that is magnified when stacks of lithium-ion cells are used for large scale energy storage. When limits of propagation can be identified so that systems can be designed to prevent large scale cascading failure even if a failure does occur, these systems will be safer. This work addresses the prediction of cell-to-cell failure propagation and the propagation limits in lithium-ion cell stacks to better understand and identify safe designs. A thermal-runaway model is presented based on recent developments in thermochemical source terms. It is noted that propagating failure is characterized by temperatures above which calorimetry data is available. Results show high temperature propagating failure predictions are too rapid unless an intra-particle diffusion limit is included, introducing a Damkohler number limiter into the rate expression. This new model form is evaluated against cell-to-cell failure propagation where the end cell of a stack is forced into thermal runaway through a nail-induced short circuit. Limits of propagation for this configuration are identified in terms of the energy density and in terms of the thermal contact between cells. Results show cell-to-cell propagation predictions are consistent with measurements over a range of cell states of charge and with the introduction of metal plates between cells to add system heat capacity representative of structural members. This consistency extends from scenarios where propagation occurs through scenarios where propagation is prevented. The authors gratefully acknowledge funding from the Department of Energy, Office of Electricity's Energy Storage Program, under the direction of Dr. Imre Gyuk. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.

A low-Mach, unstructured, large-eddy-simulation-based, unsteady flamelet approach with a generalized heat loss combustion methodology (including soot generation and consumption mechanisms) is deployed to support a large-scale, quiescent, 5-m JP-8 pool fire validation study. The quiescent pool fire validation study deploys solution sensitivity procedures, i.e., the effect of mesh and time step refinement on capturing key fire dynamics such as fingering and puffing, as mesh resolutions approach O(1) cm. A novel design-order, discrete-ordinate-method discretization methodology is established by use of an analytical thermal/participating media radiation solution on both low-order hexahedral and tetrahedral mesh topologies in addition to quadratic hexahedral elements. The coupling between heat losses and the flamelet thermochemical state is achieved by augmenting the unsteady flamelet equation set with a heat loss source term. Soot and radiation source terms are determined using flamelet approaches for the full range of heat losses experienced in fire applications including radiative extinction. The proposed modeling and simulation paradigm are validated using pool surface radiative heat flux, maximum centerline temperature location, and puffing frequency data, all of which are predicted within 10% accuracy. Simulations demonstrate that under-resolved meshes predict an overly conservative radiative heat flux magnitude with improved comparisons as compared to a previously deployed hybrid Reynolds-averaged Navier–Stokes/eddy dissipation concept-based methodology.

Turbulent fluctuations of the scalar dissipation rate have a major impact on extinction in non-premixed combustion. Recently, an unsteady extinction criterion has been developed (Hewson, 2013) that predicts extinction dependent on the duration and the magnitude of dissipation rate fluctuations exceeding a critical quenching value; this quantity is referred to as the dissipation impulse. The magnitude of the dissipation impulse corresponding to unsteady extinction is related to the difficulty with which a flamelet is exintguished, based on the steady-state S-curve. In this paper we evaluate this new extinction criterion for more realistic dissipation rates by evolving a stochas- tic Ornstein–Uhlenbeck process for the dissipation rate. A comparison between unsteady flamelet evolution using this dissipation rate and the extinction criterion exhibit good agreement. The rate of predicted ex- tinction is examined over a range of Damkohler and Reynolds numbers and over a range of the extinction difficulty. The results suggest that the rate of extinction is proportional to the average dissipation rate and the area under the dissipation rate probability density function exceeding the steady-state quenching value. It is also inversely related to the actual probability that this steady-state quenching dissipation rate is observed and the difficulty of extinction associated with the distance between the upper and middle branches of the S-curve.

This work develops a new modeling framework for decomposition of layered metal oxides in non-aqueous electrolytes. Models based on this framework are appropriate for predictions of thermal runaway in lithium-ion battery systems. It accounts for phase changes with oxygen generation and transport limitations. Effects of particle size, limited electrolyte and state of charge are included. Calorimetry measurements from a variety of literature studies are used to calibrate kinetic rate parameters for nickel manganese cobalt oxide cathodes (LixNi0.33Mn0.33Co0.33O2 or NMC 1:1:1), while other measurements are used to validate performance of the resulting model. This model utilizes a recently published method for predicting the heat of reaction from tabulated thermodynamic data. Acknowledgements: This work was supported by the US Department of Energy Office of Electricity, Energy Storage Program. The authors wish to thank Dr. Imre Gyuk for his support of research advancing safety in stationary energy storage. Sandia National Laboratories is a multi-mission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC., a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government. SAND No: SAND2019-14403 C

© 2018 Elsevier Ltd This paper provides a report of the discussions held at the first workshop on Measurement and Computation of Fire Phenomena (MaCFP) on June 10–11 2017. The first MaCFP workshop was both a technical meeting for the gas phase subgroup and a planning meeting for the condensed phase subgroup. The gas phase subgroup reported on a first suite of experimental-computational comparisons corresponding to an initial list of target experiments. The initial list of target experiments identifies a series of benchmark configurations with databases deemed suitable for validation of fire models based on a Computational Fluid Dynamics approach. The simulations presented at the first MaCFP workshop feature fine grid resolution at the millimeter- or centimeter-scale: these simulations allow an evaluation of the performance of fire models under high-resolution conditions in which the impact of numerical errors is reduced and many of the discrepancies between experimental data and computational results may be attributed to modeling errors. The experimental-computational comparisons are archived on the MaCFP repository [1]. Furthermore, the condensed phase subgroup presented a review of the main issues associated with measurements and modeling of pyrolysis phenomena. Overall, the first workshop provided an illustration of the potential of MaCFP in providing a response to the general need for greater levels of integration and coordination in fire research, and specifically to the particular needs of model validation. ispartof: Fire Safety Journal vol:101 pages:1-17 ispartof: location:Switzerland status: published

The suitability of crude and purified struvite (MgNH4PO4), a major precipitate in wastewater streams, was investigated for renewable replacement of conventional nitrogen and phosphate resources for cultivation of microalgae. Bovine effluent wastewater stone, the source of crude struvite, was characterized for soluble N/P, trace metals, and biochemical components and compared to the purified mineral. Cultivation trials using struvite as a major nutrient source were conducted using two microalgae production strains, Nannochloropsis salina and Phaeodactylum tricornutum, in both lab and outdoor pilot-scale raceways in a variety of seasonal conditions. Both crude and purified struvite-based media were found to result in biomass productivities at least as high as established media formulations (maximum outdoor co-culture yield ∼20±4gAFDW/m(2)/day). Analysis of nutrient uptake by the alga suggest that struvite provides increased nutrient utilization efficiency, and that crude struvite satisfies the trace metals requirement and results in increased pigment productivity for both microalgae strains.

The Luttinger Theorem, which relates the electron density to the volume of the Fermi surface in an itinerant electron system, is taken to be one of the essential features of a Fermi liquid. The microscopic derivation of this result depends on the vanishing of a certain integral, the Luttinger integral $I_{\rm L}$, which is also the basis of the Friedel sum rule for impurity models, relating the impurity occupation number to the scattering phase shift of the conduction electrons. It is known that non-zero values of $I_{\rm L}$ with $I_{\rm L}=\pm\pi/2$, occur in impurity models in phases with non-analytic low energy scattering, classified as singular Fermi liquids. Here we show the same values, $I_{\rm L}=\pm\pi/2$, occur in an impurity model in phases with regular low energy Fermi liquid behavior. Consequently the Luttinger integral can be taken to characterize these phases, and the quantum critical points separating them interpreted as topological., Comment: 5 pages 7 figures

Accurate models of thermal runaway in lithium-ion batteries require quantitative knowledge of heat release during thermochemical processes. A capability to predict at least some aspects of heat release for a wide variety of candidate materials a priori is desirable. This work establishes a framework for predicting staged heat release from basic thermodynamic properties for layered metal-oxide cathodes. Available enthalpies relevant to thermal decomposition of layered metal-oxide cathodes are reviewed and assembled in this work to predict potential heat release in the presence of alkyl-carbonate electrolytes with varying state of charge. Cathode delithiation leads to a less stable metal oxide subject to phase transformations including oxygen release when heated. We recommend reaction enthalpies and show the thermal consequences of metal-oxide phase changes and solvent oxidation within the battery are of comparable magnitudes. Heats of reaction are related in this work to typical observations reported in the literature for species characterization and calorimetry. The methods and assembled databases of formation and reaction enthalpies in this work lay groundwork a new generation of thermal runaway models based on fundamental material thermodynamics, capable of predicting accurate maximum cell temperatures and hence cascading cell-to-cell propagation rates.

The heat generated during a single cell failure within a high energy battery system can force adjacent cells into thermal runaway, creating a cascading propagation effect through the entire system. This work examines the response of modules of stacked pouch cells after thermal runaway is induced in a single cell. The prevention of cascading propagation is explored on cells with reduced states of charge and stacks with metal plates between cells. Reduced states of charge and metal plates both reduce the energy stored relative to the heat capacity, and the results show how cascading propagation may be slowed and mitigated as this varies. These propagation limits are correlated with the stored energy density. Results show significant delays between thermal runaway in adjacent cells, which are analyzed to determine intercell contact resistances and to assess how much heat energy is transmitted to cells before they undergo thermal runaway. A propagating failure of even a small pack may stretch over several minutes including delays as each cell is heated to the point of thermal runaway. This delay is described with two new parameters in the form of gap-crossing and cell-crossing time to grade the propensity of propagation from cell to cell.

In this study, Chlorella zofingiensis harvesting by dissolved air flotation (DAF) was critically evaluated with regard to algal concentration, culture conditions, type and dosage of coagulants, and recycle ratio. Harvesting efficiency increased with coagulant dosage and leveled off at 81%, 86%, 91%, and 87% when chitosan, Al3+, Fe3+, and cetyl trimethylammonium bromide (CTAB) were used at dosages of 70, 180, 250, and 500 mg g(-1), respectively. The DAF efficiency-coagulant dosage relationship changed with algal culture conditions. Evaluation of the influence of the initial algal concentration and recycle ratio revealed that, under conditions typical for algal harvesting, it is possible that the number of bubbles is insufficient. A DAF algal harvesting model was developed to explain this observation by introducing mass-based floc size distributions and a bubble limitation into the white water blanket model. The model revealed the importance of coagulation to increase floc-bubble collision and attachment, and the preferential interaction of bubbles with larger flocs, which limited the availability of bubbles to the smaller sized flocs. The harvesting efficiencies predicted by the model agree reasonably with experimental data obtained at different Al3+ dosages, algal concentrations, and recycle ratios. Based on this modeling, critical parameters for efficient algal harvesting were identified. (C) 2014 Wiley Periodicals, Inc.

Autoflocculation is triggered by the coordination of cationic surface sites on mineral precipitates, or Ca + 2 and Mg + 2 , to anionic algal carboxylate groups. The solution composition controls on autoflocculation can be quantitatively described using surface complexation models of algal, Mg(OH) 2 , hydroxylapatite, and calcite interfaces. Cation bridging of adjacent algae triggers autoflocculation at lower pH than does Mg(OH) 2 formation. Autoflocculation in response to lime addition reaches a maximum at pH ~ 11 because of Mg(OH) 2 formation, but then plateaus with further lime addition because of the decrease in Mg(OH) 2 surface charge with increasing pH. Hydroxylapatite surfaces are positively charged, hence able to autoflocculate algae, under most conditions. Calcite surfaces are positively charged, and autoflocculation potential is high, when dissolved Ca + 2 and Mg + 2 levels are high; calcite has low autoflocculation potential when Ca + 2 and Mg + 2 levels are low and sulfate levels are high, and the calcite surface is negatively charged.

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/H 2 /N 2 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.

We study the higher-order Fermi-liquid relations of Kondo systems for arbitrary impurity-electron fillings, extending the many-body quantum theoretical approach of Yamada-Yosida. It includes partly a microscopic clarification of the related achievements based on Nozi\`{e}res' phenomenological description: Filippone, Moca, von Delft, and Mora [Phys.\ Rev.\ B {\bf 95}, 165404 (2017)]. In our formulation, the Fermi-liquid parameters such as the quasi-particle energy, damping, and transport coefficients are related to each other through the total vertex $\Gamma_{\sigma\sigma';\sigma'\sigma} (\omega, \omega'; \omega', \omega)$, which may be regarded as a generalized Landau quasi-particle interaction. We obtain exactly this function up to linear order with respect to the frequencies $\omega$ and $\omega'$ using the anti-symmetry and analytic properties. The coefficients acquire additional contributions of three-body fluctuations away from half-filling through the non-linear susceptibilities. We also apply the formulation to non-equilibrium transport through a quantum dot, and clarify how the zero-bias peak evolves in a magnetic field., Comment: 5 pages, 2 figures, Introduction has been revised

We show that when there is a sudden transition from a small to a large Fermi surface at a field-induced quantum critical point, similar to what may have been observed in some heavy-fermion compounds, an additional term has to be taken into account in the Luttinger-Friedel sum rule.We calculate this additional term for a local model which has a field-induced quantum critical point (QCP) and show that it changes abruptly at the transition, such that it satisfies a generalized Luttinger-Friedel sum rule on each side of the transition, and characterizes the two Fermi-liquid phases separated by the QCP as a discrete (topological) index., Comment: 5 pages, 6 figures

We study the low-energy behavior of the vertex function of a single Anderson impurity away from half-filling for finite magnetic fields, using the Ward identities with careful consideration of the anti-symmetry and analytic properties. The asymptotic form of the vertex function $\Gamma_{\sigma\sigma';\sigma'\sigma}^{}(i\omega,i\omega';i\omega',i\omega)$ is determined up to terms of linear order with respect to the two frequencies $\omega$ and $\omega'$, as well as the $\omega^2$ contribution for anti-parallel spins $\sigma'\neq \sigma$ at $\omega'=0$. From these results, we also obtain a series of the Fermi-liquid relations beyond those of Yamada-Yosida. The $\omega^2$ real part of the self-energy $\Sigma_{\sigma}^{}(i\omega)$ is shown to be expressed in terms of the double derivative $\partial^2\Sigma_{\sigma}^{}(0)/\partial \epsilon_{d\sigma}^{2}$ with respect to the impurity energy level $\epsilon_{d\sigma}^{}$, and agrees with the formula obtained recently by Filippone, Moca, von Delft, and Mora in the Nozi\`{e}res phenomenological Fermi-liquid theory [Phys.\ Rev.\ B {\bf 95}, 165404 (2017)]. We also calculate the $T^2$ correction of the self-energy, and find that the real part can be expressed in terms of the three-body correlation function $\chi_{\uparrow\downarrow,-\sigma}^{[3]} = \partial \chi_{\uparrow\downarrow}/\partial \epsilon_{d,-\sigma}^{}$. We also provide an alternative derivation of the asymptotic form of the vertex function. Specifically, we calculate the skeleton diagrams for the vertex function $\Gamma_{\sigma\sigma;\sigma\sigma}^{}(i\omega,0;0,i\omega)$ for parallel spins up to order $U^4$ in the Coulomb repulsion $U$. It directly clarifies the fact that the analytic components of order $\omega$ vanish as a result of the cancellation of four related Feynman diagrams which are related to each other through the anti-symmetry operation., Comment: 15 figures, 19pages, and supplemental material (15 pages), typo found in (6.15) has been corrected

We calculate the renormalized parameters for the quasiparticles and their interactions for the Hubbard model in the paramagnetic phase as deduced from the low energy Fermi liquid fixed point using the results of a numerical renormalization group calculation (NRG) and dynamical mean-field theory (DMFT). Even in the low density limit there is significant renormalization of the local quasiparticle interaction $\tilde U$, in agreement with estimates based on the two-particle scattering theory of Kanamori (1963). On the approach to the Mott transition we find a finite ratio for $\tilde U/\tilde D$, where $2\tilde D$ is the renormalized bandwidth, which is independent of whether the transition is approached by increasing the on-site interaction $U$ or on increasing the density to half-filling.The leading $\omega^2$ term in the self-energy and the local dynamical spin and charge susceptibilities are calculated within the renormalized perturbation theory (RPT) and compared with the results calculated directly from the NRG-DMFT. The dynamic ${\bf q},\omega$ spin susceptibility $\chi({\bf q},\omega)$ is also estimated from repeated quasiparticle scattering with a local renormalized scattering vertex., Comment: 13 pages 19 figures

We analyze conditions for the observation of a low energy SU(4) fixed point in capacitively coupled quantum dots. One problem, due to dots with different couplings to their baths, has been considered by Tosi, Roura-Bas and Aligia (2015). They showed how symmetry can be effectively restored via the adjustment of individual gates voltages, but they make the assumption of infinite on-dot and inter-dot interaction strengths. A related problem is the difference in the magnitudes between the on-dot and interdot strengths for capacitively coupled quantum dots. Here we examine both factors, based on a two site Anderson model, using the numerical renormalization group to calculate the local spectral densities on the dots and the renormalized parameters that specify the low energy fixed point. Our results support the conclusions of Tosi et al. that low energy SU(4) symmetry can be restored, but asymptotically achieved only if the inter-dot interaction $U_{12}$ is greater than or of the order of the band width of the coupled conduction bath $D$, which might be difficult to achieve experimentally. By comparing the SU(4) Kondo results for a total dot occupation $n_{\rm tot}=1$ and $n_{\rm tot}=2$ we conclude that the temperature dependence of the conductance is largely determined by the constraints of the Friedel sum rule rather than the SU(4) symmetry and suggest that an initial increase of the conductance with temperature is a distinguishing characteristic feature of an $n_{\rm tot}=1$ universal SU(4) fixed point., 14 pages and 20 figures

The effects of changes in cellular characteristics and dissolved organic matter (DOM) on dissolved air flotation (DAF) harvesting of Chlorella zofingiensis at the different growth phases were studied. Harvesting efficiency increased with Al(3+) dosage and reached more than 90%, regardless of growth phases. In the absence of DOM, the ratio of Al(3+) dosage to surface functional group concentration determined the harvesting efficiency. DOM in the culture medium competed with algal cell surface functional groups for Al(3+), and more Al(3+) was required for cultures with DOM than for DOM-free cultures to achieve the same harvesting efficiency. As the culture aged, the increase of Al(3+) dosage due to increased DOM was less than the decrease of Al(3+) dosage associated with reduced cell surface functional groups, resulting in overall reduced demand for Al(3+). The interdependency of Al(3+) dosage and harvesting efficiency on concentrations of cell surface functional groups and DOM was successfully modeled.

Within the UK, local and regional government drives to reduce the quantity of waste being sent to landfill have led to an increase in small-scale composting schemes, instigated by local councils and not-for-profit organisations. The composting process relies upon the proliferation of microorganisms, leading to their emission into the ambient environment. In this investigative study, total bacteria and Aspergillus fumigatus emitted from a small-scale composting facility in central London were measured in different spatial and temporal dimensions. Bioaerosols did not disperse in concentrations significantly higher than those measured at 'background' locations, where maximum geometric mean was 55 x 10(2) Colony Forming Units (CFU) per m(-3). Concentrations on-site and at the nearest potential receptor were comparable to those found at commercial facilities, reaching 25 x 10(4) and 29 x 10(3) CFU m(-3) for total bacteria and A. fumigatus respectively. The room housing the facility was contaminated by moulds; likely to result from high relative humidity of the air (consistently above 80% during this study), building material, and the generation of organic dust. The complex diurnal meteorological variations of urban environments are likely to influence bioaerosol dispersal, and consequent exposure risk for sensitive receptors. Site planning tools including Geographical Information Systems (GIS) mapping with buffer zones around schools and hospitals, and use of computerised models for the design of rooms housing urban composting facilities are proposed as methods for reducing the risk of occupational and off-site receptor exposure. (C) 2010 Elsevier Ltd. All rights reserved.

We present broadband, dual-pump coherent anti-Stokes Raman scattering (CARS) measurements of N2 and O2 in 2 m diameter methanol pool fires. The design of the fiber-optically coupled CARS instrument for fire measurements is described. Single-shot temperatures and O2/N2 ratios were obtained simultaneously from a single measurement point at the center of the fire plume, and the measured temperature–oxygen statistics are compared to results of a time-domain-filtered Reynolds-averaged Navier-Stokes simulation. The measured and simulated mean fire temperatures agree to within 2–4%, with larger turbulent fluctuations observed in the measured temperatures. The behavior of the mean temperature conditioned on the O2/N2 ratio is similar for both simulation and experiment, but with simulated temperatures that are up to 10% lower than measured values for O2/N2 below 0.18. The uncertainty in the CARS measurements is described. A single-shot detection limit of O2/N2 = 0.06 was determined from the observed signal to nois...

The interaction between soot and enthalpy evolution in a buoyant turbulent flame, exhibiting key attributes of a fire, is studied using a novel approach. This approach is based on a spatially evolving form of the one-dimensional turbulence (ODT) model that resolves the full range of scales, in a single spatial dimension, from the scale of the plume evolution to that of the soot layers. The model is compared with limited experimental data available. The evolved flow field of a meter-scale flame is then analyzed both in terms of conventional spatial averages and conditional averages. The conditional moments of the terms in the soot and enthalpy evolution equations are analyzed to elucidate the balance between the large-scale evolution (advective terms), the source terms, and the turbulent mixing (dissipative terms). The results show the significant influence of the mixture evolution on the transport of soot relative to the flame (in mixture-fraction space).

Modeling soot formation in turbulent nonpremixed combustion is a difficult problem. Unlike most gaseous combustion species, soot lacks a strong state relationship with the mixture fraction due to unsteady formation rates which overlap transport timescales, and strong differential diffusion between gaseous species and soot. The conditional moment closure model (CMC) has recently been applied to the problem of turbulent soot formation. A challenge in CMC modeling is the treatment of differential diffusion. Three-dimensional direct numerical simulation (DNS) of a nonpremixed ethylene jet flame with soot formation has been performed using a 19 species reduced ethylene mechanism and a four-step, three-moment, semi-empirical soot model. The DNS provides full resolution of the turbulent flow field and is used to perform a-priori analysis of a recent CMC model derived from the joint scalar PDF transport equation. Unlike other approaches, this CMC model does not require additional transport equations to treat differentially diffusing species. A budget of the terms of the CMC equation for both gaseous species and soot is presented. In particular, exact expressions for unclosed terms are compared to typical closure models for scalar dissipation, cross-dissipation, differential diffusion, and reactive source terms. The differential diffusion model for gaseous species is found to be quite accurate, while that for soot requires an additional model for the residual term.

Improvements to the existing Eulerian-Lagrangian two-phase dilute spray model, referred to as Vulcan, are required in order to handle high mass loadings or very small solid particles. Such flows are relevant to modern fire-suppression techniques among other applications. These improvements include developing a new treatment for efficiently integrating small particles, a new subcycling time-step selection algorithm for the time-splitting solution technique, and a new treatment for placement of the two-way coupling source terms on the fluid grid. Despite the added complexity of these modifications, performance tuning of the code was also performed so that the solution speed is either equivalent to or faster than the previous code, depending on particle size. The new algorithms are applied to predicting suppressant distribution from a Goodrich-244 fire-suppression system in a simulated aircraft cargo bay. Results using these new algorithms indicate that the larger particles found in the Goodrich-244 suppress...

The characteristics of auto-ignition in HCCI engines in the presence of charge stratification is studied theoretically using activation energy asymptotics. The analysis accounts for the effects of pressure rise due to piston motion and heat release, and the consequent compression-heating of the charge. The chemical kinetics are approximated using a single reaction step with a large activation energy and the compression-heating effects are accounted for through an entropy variable. The resultant model is simple and suitable for extensive parametric studies of HCCI engine combustion. Some preliminary results on the effects of mixture and composition stratification on the duration of burning are presented using a presumed Gaussian PDF for the initial temperature distribution. The effects of mixture stratification on the feasible operating conditions is demonstrated using simple criteria to delineate the knocking and incomplete end-gas combustion regimes.

Parametric studies were conducted for a coalescing and evaporating turbulent water spray using a stochastic separated flow technique that includes submodels for droplet dynamics, heat and mass transfer, and droplet–droplet binary collisions. While the initial droplet size distribution, in general, is not known due to the difficulty in the optical access to the nozzle exit region, the size distribution is modeled using the analytical PDFs (probability density functions) such as log-normal, Rosin–Rammler, Gaussian, and Nukiyama–Tanasawa distribution model. Standard deviation of the PDFs is varied and their effects on droplet size and speed distribution in the downstream are reported. The arithmetic mean droplet size at the nozzle exit, which is used as input for simulations, was extrapolated using the existing experimental data obtained at downstream locations.

One new technology for fire suppressant distribution in total-flooding applications is the solid-propellant gas-generator (SPGG) technology. This article presents experimental and modeling studies of one such prototype system in order to better understand observations in the testing of this system. This particular SPGG system generates fine particles that act to suppress any fire in conjunction with inert gases also generated in the SPGG system. Initial conditions for the simulations are obtained from the available measurements of the prototype system. The modeling provides key information related to the distribution of the particles and their potential effectiveness as a fire suppressant. The primary variable in the SPGG design as identified in the initial measurements, also presented here, was the particle size, with typical particle sizes being measured at 2 and 15 µm. The key modeling result is that there is a tradeoff between the most uniform distribution of particles and the available surface-to-vol...

We analyze the properties of the quasiparticle excitations of metallic antiferromagnetic states in a strongly correlated electron system. The study is based on dynamical mean field theory (DMFT) for the infinite dimensional Hubbard model with antiferromagnetic symmetry breaking. Self-consistent solutions of the DMFT equations are calculated using the numerical renormalization group (NRG). The low energy behavior in these results is then analyzed in terms of renormalized quasiparticles. The parameters for these quasiparticles are calculated directly from the NRG derived self-energy, and also from the low energy fixed point of the effective impurity. They are found to be in good agreement. We show that the main low energy features of the $\bf k$-resolved spectral density can be understood in terms of the quasiparticle picture. We also find that Luttinger's theorem is satisfied for the total electron number in the doped antiferromagnetic state., 15 pages, 17 figures; some minor changes and additional figure