A model of reduced oxidation kinetics using constituents and species: Iso-octane and its mixtures with n-pentane, iso-hexane and n-heptane

A previously described methodology for deriving a reduced kinetic mechanism for alkane oxidation and tested for n-heptane is here shown to be valid, in a slightly modified version, for iso-octane and its mixtures with n-pentane, iso-hexane and n-heptane. The model is still based on partitioning the species into lights, defined as those having a carbon number smaller than 3, and heavies, which are the complement in the species ensemble, and mathematically decomposing the heavy species into constituents which are radicals. For the same similarity variable found from examining the n-heptane LLNL mechanism in conjunction with CHEMKIN II, the appropriately scaled total constituent molar density still exhibits a self-similar behavior over a very wide range of equivalence ratios, initial pressures and initial temperatures in the cold ignition regime. When extended to larger initial temperatures than for cold ignition, the self-similar behavior becomes initial temperature dependent, which indicates that rather than using functional fits for the enthalpy generation due to the heavy species’ oxidation, an ideal model based on tabular information extracted from the complete LLNL kinetics should be used instead. Similarly to n-heptane, the oxygen and water molar densities are shown to display a quasi-linear behavior with respect to the similarity variable, but here their slope variation is no longer fitted and instead, their rate equations are used with the ideal model to calculate them. As in the original model, the light species ensemble is partitioned into quasi-steady and unsteady species; the quasi-steady light species mole fractions are computed using the ideal model and the unsteady species are calculated as progress variables using rates extracted from the ideal model. Results are presented comparing the performance of the model with that of the LLNL mechanism using CHEMKIN II. The model reproduces excellently the temperature and species evolution versus time or versus the similarity variable, with the exception of very rich mixtures, where the predictions are still very good but the multivalued aspect of these functions at the end of oxidation is not captured in the reduction. The ignition time is predicted within percentages of the LLNL values over a wide range of equivalence ratios, initial pressures and initial temperatures.     

Computer simulated rate processes in copper vapor lasers

A computer model for metal vapor lasers has been developed which places emphasis on the change of excited state populations of the lasant through inelastic collisions and radiative interaction. Also included are an energy equation for the pumping electrons and rate equations for laser photon densities. Presented are results of calculations for copper vapor with a neon buffer over a range of conditions. General agreement with experiments was obtained.     

Regulation of neurokinin-1 receptor expression by GABA(B) receptor agonists

Activation of GABA(B) receptors produces analgesia in acute and chronic pain models. Data indicate that a possible mechanism for this effect is a GABA(B) receptor-induced blockade of neurokinin-1 (NK-1) receptor gene expression in the spinal cord. While much more potent GABA(B) receptor agonists (CGP 44532) have been developed, there is no information on their antinociceptive properties or their ability to influence NK-1 receptors. To address these issues, rats were treated with baclofen or CGP 44532 and tested for sedation, ataxia, and pain-related behaviors in a chronic pain model (formalin hindpaw injection). In a separate group of experiments the analgesic response to a single dose of CGP 44532 was tested prior, and subsequent to, its chronic administration. The results indicate that CGP 44532 is a substantially more potent analgesic than baclofen. In addition, after chronic administration baclofen was no longer capable of inducing analgesia or of inhibiting the increased expression of NK-1R mRNA and CGP 44532 was still fully effective in both regards. The results suggest that GABA(B) agonists could be clinically useful analgesics.     

Technical note: assessment of recovery site of mobile nylon bags for measuring ileal digestibility of starch in dairy cows

The objective of this study was to evaluate recovery site of mobile nylon bags for measuring ileal digestibility of ruminally undegraded starch in dairy cows. Eight feed samples of untreated and treated concentrates were examined. Three lactating cows equipped with rumen fistula and duodenal and ileal cannulas were used in the experiment. The mobile nylon bags containing intact feeds or residues after a 12-h ruminal incubation were pretreated using a 2-step procedure to simulate abomasal digestion before insertion through the duodenal cannula. To assess the effect of hindgut fermentation on starch digestibility, approximately half of the bags were collected from the ileum and half from the feces. The results indicate that feed samples should be preincubated in rumen before insertion into duodenum, and that samples with relatively high fractions of rumen-undigestible starch should be collected from the ileum instead of from feces.     

Chemoenzymatic Catalysis: Cooperativity Enables Opportunity

The application of enzymes in synthetic organic chemistry has emerged as a powerful means to generate molecular complexity in a highly selective, efficient, and sustainable manner. While enzymes have increasingly been incorporated into synthetic sequences for numerous academic and industrial applications on their own and in sequential processes, their utility in cooperative catalysis with small molecule catalytic platforms has recently drawn increased attention across the field of organic synthesis. In this review, we present a selection of notable accomplishments in cooperative chemoenzymatic catalysis and provide a perspective on its future directions.     

A model of reduced kinetics for alkane oxidation using constituents and species: Proof of concept for n-heptane

A methodology for deriving a reduced kinetic mechanism for alkane oxidation is described and applied to n-heptane. The model is based on partitioning the species of the skeletal kinetic mechanism into lights, defined as those having a carbon number smaller than 3, and heavies, which are the complement in the species ensemble. For modeling purposes, the heavy species are mathematically decomposed into constituents, which are similar but not identical to groups in the group additivity theory. From analysis of the LLNL skeletal mechanism in conjunction with CHEMKIN II, it is shown that a similarity variable can be formed such that the appropriately scaled global constituent molar density exhibits a self-similar behavior over a very wide range of equivalence ratios, initial pressures and initial temperatures that is of interest for predicting n-heptane oxidation. Furthermore, the oxygen and water molar densities are shown to display a quasi-linear behavior with respect to the similarity variable. The light species ensemble is partitioned into quasi-steady and unsteady species. The concept is tested by using tabular information from the LLNL skeletal mechanism in conjunction with CHEMKIN II. The test reveals that the similarity concept is indeed justified and that the combustion temperature is well predicted, but that the ignition time is overpredicted. To palliate this deficiency, functional modeling is incorporated into our conceptual reduction. Due to the reduction process, models are also included for the global constituent molar density, the kinetics-induced enthalpy evolution of the heavy species, the contribution to the reaction rate of the unsteady lights from the heavies, the molar density evolution of oxygen and water, the mole fractions of the quasi-steady light species and the mean molar heat capacity of the heavy species. The model is compact in that there are only nine species-related progress variables. Results are presented comparing the performance of the model for predicting the temperature and species evolution with that of the skeletal mechanism. The model reproduces the ignition time over a wide range of equivalence ratios, initial pressure and initial temperature.     

Interpulse kinetics in copper and copper halide lasers

The various rate processes that govern the interpulse relaxation in metal vapor and metal halide vapor lasers are considered. Computer calculations indicate that the rapid metastable levels relaxation observed in copper and copper halide laser experiments requires the existence of a relatively small resonance in the cross section for metastable excitation or deexcitation near threshold. The accurate calculation of interpulse relaxation requires knowledge of rate constants presently not well known; this is especially so for metal halide lasers.     

Modeling of multicomponent homogeneous nucleation using continuous thermodynamics

A theory of homogeneous nucleation in a multicomponent vapor is developed by combining classic nucleation and continuous thermodynamics concepts. The perfect gas equation of state is used in conjunction with this theory to obtain a model valid at low vapor pressures. The theory is applied to kerosenes used for fuels in aeronautics (Jet A, JP-7, and RP-1) at temperatures from 220 to 360 K and at vapor pressures up to 1 bar. The results show that although the overall nucleation trends regarding the dependency on vapor pressure and temperature are similar for all kerosenes, Jet A has a distinct behavior compared with JP-7 and RP-1. For all kerosenes, as pressure increases, the nucleus size rapidly decreases and is smallest at low temperatures.     

Prediction of premixed, n-heptane and iso-octane unopposed jet flames using a reduced kinetic model based on constituents and light species

A model of steady, quasi one-dimensional premixed laminar jet flame developing unopposed into a uniform flow has been formulated using a previously successful reduced chemical-kinetics model  and . A detailed derivation of the steady quasi one-dimensional conservation equations revealed that it is only under very restrictive conditions – probably very difficult to achieve experimentally and the validity of which is not reported in detail in experimental studies – that the quasi one-dimensional concept is meaningful. The governing equations have been mathematically manipulated to be consistent with the framework of the reduced chemical-kinetics model which relied on constituents representing the heavy species, and on quasi-steady light species and unsteady light species. The flame model includes accurate transport property calculation for high-pressure conditions and a real-gas equation of state. Based on a found self-similarity  and  which deteriorates at increasingly rich conditions, the chemistry model consists of tables of kinetic rates, quasi-steady species molar fractions and the heavy species mean molar mass extracted from the LLNL model in the framework of the reduced kinetics. The progress variables are only the mass fractions of the unsteady light species and the temperature. The values of the dependent variables are specified at the inflow location and null gradients are specified at the outflow. Simulations were performed for both n-heptane and iso-octane air oxidation over a wide range of pressures and equivalence ratios. The limited documentation of experimental conditions not specifying the inflow velocity (or flux) made it impossible to use this data for detailed comparison. In the one case where the inflow velocity was available for a burner experiment, those conditions were adopted for the simulation and the configuration was changed to a constant-area jet to approach the burner configuration. Results from this simulation compared favorably with the data, considering the different configurations. Results from parametric studies not associated with experimental data showed that at stoichiometric conditions the flame temperature, flame velocity and strain rate are not sensitive to the pressure, although flames become increasingly thinner with increasing pressure and the yield of the unsteady light species is different. Computations conducted at 40 bar for various equivalence ratios and for velocities differing with the equivalence ratio showed that the maximum flame velocity, flame strain and flame temperature were obtained at stoichometric conditions. Finally, we discuss the limitations of utilizing a priori obtained reduced chemical-kinetic models in flames calculations.