Thermodynamically-based Model Research Articles

On the determination of cyclic plastic strain energy with the provision for microplasticity

A statistical method for the estimation of plastic strain energy per cycle in metals experiencing fatigue is presented. It is shown that the plastic strain distribution under cyclic loading follows a two-parameter Weibull probability function, where monotonic stress-strain data is used as input. The second probability function is incorporated to account for different crystallographic directions. To verify the results, a thermodynamically-based model is used to measure the PSE per cycle and A finite element model was developed to predict plastic strain energy per cycle. The results via the presented method corroborate the predictions of the numerical and thermodynamically-based models.

A general model for predicting the solubility behavior of H2O–CO2 fluids in silicate melts over a wide range of pressure, temperature and compositions

In this study, a general thermodynamically-based model is presented for the solubility of H2O–CO2 in silicate melts over a wide range of temperatures (from 933K to 2003K), pressures (up to 30kbar) and melt compositions (from ultramafic to silicic, from metaluminous to peraluminous, and from subalkaline to peralkaline). Using an equation of state for the fluid phase that differs from previous models and deriving activity coefficients of species in silicate melts based on a specific interaction theory, this model can not only reproduce the data used for calibrations, but also predict the solubility well beyond the data range. The accuracies of solubilities predicted from this model are shown to be 7.2% for H2O and 9.7% for CO2, respectively, which are well within the experimental uncertainty. In addition, this model can be applied to the determination of equilibrium saturation pressure at which silicate melts are saturated with a fluid phase, modeling the evolution of ascending magmas through degassing processes, and elucidating the fluid-melt partitioning behavior of CO2 and H2O. An online version of this model is available at the website: http://calc.geochem-model.org/Test/Services/WebForm1.aspx and the program can be downloaded at: www.geochem-model.org/archives/programs.htm.

A thermodynamically-based model for predicting microbial growth and community composition coupled to system geochemistry: Application to uranium bioreduction

‘Bioimmobilization’ of redox-sensitive heavy metals and radionuclides is being investigated as a way to remediate contaminated groundwater and sediments. In one approach, growth-limiting substrates are added to the subsurface to stimulate the activity of targeted groups of indigenous microorganisms and create conditions favorable for the microbially-mediated reductive precipitation (‘bioreduction’) of targeted contaminants. We present a theoretical framework for modeling this process that modifies conventional geochemical reaction path modeling to include thermodynamic descriptions for microbial growth and may be called biogeochemical reaction path modeling. In this approach, the actual microbial community is represented by a synthetic microbial community consisting of a collection of microbial groups; each with a unique growth equation that couples a specific pair of energy yielding redox reactions. The growth equations and their computed standard-state free energy yields are appended to the thermodynamic database used in conventional geochemical reaction path modeling, providing a direct coupling between chemical species participating in both microbial growth and geochemical reactions. To compute the biogeochemical reaction paths, growth substrates are reacted incrementally with the defined geochemical environment and the coupled equations are solved simultaneously to predict reaction paths that display changing microbial biomass, community composition (i.e. the fraction of total biomass in each microbial group), and the aqueous and mineral composition of the system, including aqueous speciation and oxidation state of the targeted contaminants. The approach, with growth equations derived from the literature using well-known bioenergetics principles, was used to predict the results of a laboratory microcosm experiment and an in situ field experiment that investigated the bioreduction of uranium. Predicted effects of ethanol or acetate addition on uranium concentration and speciation, major ion geochemistry, mineralogy, microbial biomass and community composition were in qualitative agreement with experimental observations although the available data precluded rigorous model testing. While originally developed for use in better understanding of bioimmobilization of heavy metals and radionuclides, the modeling approach is potentially useful for exploring the coupling of microbial growth and geochemical reactions in a variety of other basic and applied biotechnology research settings.

Thermodynamic Model for Fluid–Fluid Interfacial Areas in Porous Media for Arbitrary Drainage–Imbibition Sequences

Fluid–fluid interfacial areas are important in controlling the rate of mass and energy transfer between fluid phases in porous media. We developed a modified thermodynamically based model (TBM) to predict fluid–fluid interfacial areas in porous media for arbitrary drainage–imbibition sequences. The TBM explicitly distinguishes between interfacial areas associated with continuous (free) and isolated (entrapped) nonwetting fluids. The model is restricted to two‐fluid systems in which (i) no significant conversion of mechanical work into heat occurs, (ii) the wetting fluid completely wets the porous medium's solid surfaces, and (iii) no changes in interfacial area due to mass transfer between phases occur. We show example calculations for two different drainage–imbibition sequences in two porous media: a highly uniform silica sand and a well‐graded silt. The TBM's predictions for interfacial area associated with free nonwetting fluid are identical to those of a previously published geometry‐based model (GBM); however, predictions for interfacial area associated with entrapped nonwetting fluid are consistently larger in the TBM than in the GBM. Although a comparison of model predictions with experimental data is currently only possible to a limited extent, good general agreement was found for the TBM. As required model parameters are commonly used as inputs for or tracked during multifluid‐flow simulations, the modified TBM may be easily incorporated in numerical codes.

Effects of Electrolyte Concentration on Surfactant Adsorption to a QCM Immersed in Surfactant + Electrolyte Solutions

Surfactant adsorption from aqueous electrolyte solutions onto metal surfaces was characterized through the use of a Quartz Crystal Microbalance (QCM). The need for a better understanding of the surfactant adsorption process became apparent in previous studies by Morton et al., directed toward the development of a thermodynamically-based model of oil removal from metal surfaces. These modeling efforts utilized existing data on surfactant adsorption data, yet required the estimation of surfactant adsorption phenomena, such as the transition between monolayer adsorption and multi-layer adsorption and the location of the critical micelle concentration (CMC). Experimental techniques utilizing the quartz crystal microbalance (QCM) have been shown to be highly reliable for measuring slight changes, on the order of a nanogram, in the adsorption behavior of surfactants from aqueous solutions. The current study demonstrates that the addition of low concentrations (< 100 mM) of sodium chloride to aqueous solutions of ionic surfactant can have a significant effect on the adsorption of the surfactant to a gold surface. An analysis of the QCM measurements as well as a discussion of the effects of salt addition on various solution properties will be presented. Impacts of the current results to previously published work as well as potential applications will be discussed.

An extended mechanical threshold stress plasticity model: modeling 6061-T6 aluminum alloy

The mechanical threshold stress plasticity model of Follansbee and Kocks was designed to predict the flow stress of metals and alloys in the regime where thermally activated mechanisms are dominant and high temperature diffusion effects are negligible. In this paper we present a model that extends the original mechanical threshold stress to the high strain-rate regime (strain rates higher than 10 4 s 1 ) and attempts to allow for high temperature effects. We use a phonon drag model for moderate strain rates and an overdriven shock model for extremely high strain rates. A temperature dependent model for the evolution of dislocation density is also presented. In addition, we present a thermodynamically-based model for the evolution of temperature with plastic strain. Parameters for 6061-T6 aluminum alloy are determined and compared with experimental data. The strain-rate dependence of the flow stress of 6061T6 aluminum is found to be in excellent agreement with experimental data. The amount of thermal softening is underestimated at high temperatures (greater than 500K) but still is an improvement over the original model. We also find that the pressure dependence of the shear modulus does not completely explain the pressure dependence of the flow stress of 6061-T6 aluminum alloy.

Modeling the roles of Ca uniporter, Na/Ca exchanger and Na/H exchanger in regulating Ca, Na and pH flux in cardiac mitochondria using in vitro spectrofluorometry

Ca mediates mitochondrial signaling pathways and modulates energy metabolism. Alteration of Ca homeostasis can lead to mitochondrial dysfunction. To gain a biophysical understanding of mitochondrial Ca regulation, we developed a thermodynamically-based model of mitochondrial energetics and cation balance by integrating kinetic models of Ca uniporter (CU), Na/Ca exchanger (NCE) and Na/H exchanger (NHE) across mitochondrial inner membrane to an existing model of oxidative phosphorylation (Beard, PLoS Comp Biol 1:, 2005). Kinetic flux expressions for CU, NCE and NHE were developed and parameterized using published data on fluxes measured in isolated cardiac mitochondria. We used the integrated model to understand the integrated roles of CU, NCE and NHE in regulating trans-membrane Ca, Na and H fluxes, and to determine the consequences of these fluxes on redox state and respiration. We measured Ca, Na and H in isolated cardiac mitochondria (by ion selective fluorescence spectrophotometry) and buffer (by ion selective electrodes) in response to changing buffer levels of these ions, ADP, and inhibitors of CU, NCE and NHE. Measurements of mitochondrial Ca dynamics match model predictions and support the hypothesis that NCE is electrogenic with stoichiometry of 3:1. This integrated model will help us to understand mechanisms by which Ca plays a role in mitochondrial signaling and energetics.

Lattice-Boltzmann algorithm for simulating thermal two-phase flow

An algorithm is described for incorporating thermal effects into lattice-Boltzmann simulations of two-phase flow. This algorithm is a combination of a two-distribution model for simulating single-phase thermal flow recently proposed by the authors and the thermodynamically-based model for isothermal two-phase flow of Swift et al. [Phys. Rev. E 54, 5041 (1996)]. The algorithm also corrects a problem with the original single-phase thermal flow model, which described the thermal energy flux as proportional to a gradient of the internal energy instead of being proportional to the gradient of the temperature. For ideal-gas systems, these two descriptions are equivalent but for nonideal systems there is a systematic discrepancy between the original thermal model and classical hydrodynamics. The algorithm is tested on several simple problems. These include formation of a free-standing isothermal thin liquid film, evaporation of a thin liquid film from a heated plate, evaporation of an isolated droplet, and condensation of liquid in a channel. Where possible, the simulations are compared against known analytic results.

From Laboratory Experiments to a Geological Disposal Vault: Calculation of Used Nuclear Fuel Dissolution Rates

ABSTRACTCalculation of used nuclear fuel dissolution rates in a geological disposal vault requires a knowledge of the redox conditions in the vault. For redox conditions less oxidizing than those causing UO2 oxidation to the U3O7 stage, a thermodynamically-based model is appropriate. For more oxidizing redox conditions a kinetic or an electrochemical model is needed to calculate these rates. The redox conditions in a disposal vault will be affected by the radiolysis of groundwater by the ionizing radiation associated with the fuel. Therefore, we have calculated the alpha-, beta- and gamma-dose rates in water in contact with the reference used fuel in the Canadian Nuclear Fuel Waste Management Program (CNFWMP) as a function of cooling time. Also, we have determined dissolution rates of UO2 fuel as a function of alpha and gamma dose rates from our electrochemical measurements. These room-temperature rates are used to calculate the dissolution rates of used fuel at 100°C, the highest temperature expected in a container in the CNFWMP, as a function of time since emplacement. It is shown that beta radiolysis of water will be the main cause of oxidation of used CANDU fuel in a failed container. The use of a kinetic or an electrochemical corrosion model, to calculate fuel dissolution rates, is required for a period of ∼ 1000 a following emplacement of copper containers in the geologic disposal vault envisaged in the CNFWMP. Beyond this time period a thermodynamically-based model adequately predicts the fuel dissolution rates. The results presented in this paper can be adopted to calculate used fuel dissolution rates for other used UO2 fuels in other waste management programs.

Influence of salinity upon sediment-water partitioning of aromatic hydrocarbons

Concern over the potential acute and long-term effects of oil pollution on marine environments has lead to a resurgence of investigations of the fate, transport and chemical transformations of aromatic hydrocarbons associated with spilled oil, exploration and operational discharges and produced water discharges in coastal marine and estuarine environmental systems. The nature of the particle reactivity or sorption/desorption equilibria which control the fate, transport and chemical transformations of hydrocarbons has been investigated on various sediment types (a range of organic carbon and particle sizes) over a range of salinities. No significant differences were observed between sorption and desorption equilibrium constants (Kd). Significant increases in sorption/desorption constants were observed with increasing salinity. A thermodynamically-based model is presented which allows the estimation of both the Setchenow constant as well as the chemical activity coefficient of the compound in the sediment organic phase from experimental data. The results of the application of this model suggest that the increase in sorption with salinity is the result of two phenomena: salting out as the result of electrolyte effects on the aqueous activity coefficient of the solute and the salting out of the sediment organic matter resulting in changes in the charge and conformation with increasing ionic strength.