M. Capitelli

Collision integrals of high-temperature air species

Collision integrals (transport cross sections ) of air species in the temperature range 50 ‐100,000 K have been calculated by using experimental and theoretical informations on potential energy curves and cross sections. The results for the different interactions (neutral‐neutral, neutral ‐parent ion, electron ‐neutral) have been compared with old and recent calculations obtaining, in general, a satisfactory agreement. Finally analytical forms for e tting and interpolating the present numerical results are reported.

Heavy metal concentrations in soils as determined by laser-induced breakdown spectroscopy (LIBS), with special emphasis on chromium

Soil is unanimously considered as one of the most important sink of heavy metals released by human activities. Heavy metal analysis of natural and polluted soils is generally conducted by the use of atomic absorption spectroscopy (AAS) or inductively coupled plasma optical emission spectroscopy (ICP-OES) on adequately obtained soil extracts. Although in recent years the emergent technique of laser-induced breakdown spectroscopy (LIBS) has been applied widely and with increasing success for the qualitative and quantitative analyses of a number of heavy metals in soil matrices with relevant simplification of the conventional methodologies, the technique still requires further confirmation before it can be applied fully successfully in soil analyses. The main objective of this work was to demonstrate that new developments in LIBS technique are able to provide reliable qualitative and quantitative analytical evaluation of several heavy metals in soils, with special focus on the element chromium (Cr), and with reference to the concentrations measured by conventional ICP spectroscopy. The preliminary qualitative LIBS analysis of five soil samples and one sewage sludge sample has allowed the detection of a number of elements including Al, Ca, Cr, Cu, Fe, Mg, Mn, Pb, Si, Ti, V and Zn. Of these, a quantitative analysis was also possible for the elements Cr, Cu, Pb, V and Zn based on the obtained linearity of the calibration curves constructed for each heavy metal, i.e., the proportionality between the intensity of the LIBS emission peaks and the concentration of each heavy metal in the sample measured by ICP. In particular, a triplet of emission lines for Cr could be used for its quantitative measurement. The consistency of experiments made on various samples was supported by the same characteristics of the laser-induced plasma (LIP), i.e., the typical linear distribution confirming the existence of local thermodynamic equilibrium (LTE) condition, and similar excitation temperatures and comparable electron number density measured for all samples. An index of the anthropogenic contribution of Cr in polluted soils was calculated in comparison to a non-polluted reference soil. Thus, the intensity ratios of the emission lines of heavy metal can be used to detect in few minutes the polluted areas for which a more detailed sampling and analysis can be useful.

Self-Consistent Model of Chemical, Vibrational, Electron Kinetics in Nozzle Expansion

A self-consistent model to describe vibrational, electronically excited states (master equations) and free electron kinetics (Boltzmann equation) has been applied to study N 2 expansion through a converging-diverging conic nozzle. Strong departures from equilibrium can be observed for both vibrational, electronically excited states and electron energy distributions. In particular, the role of electronically excited states of nitrogen molecules and free electrons has been investigated. The strong interaction between these two systems, by means of inelastic and superelastic collisions, influences not only the internal state kinetics, but also the macroscopic quantities such as Mach number and gas temperature profile

Nonequilibrium vibrational kinetics in the boundary layer of re-entering bodies

Nonequilibrium vibrational distributions of N2 in the boundary layer surrounding a blunt body in hypersonic flow have been calculated by coupling the nonequilibrium vibrational kinetics, the dissociation and recombination processes, and the boundary-layer equations. The role of different energy exchange processes [vibration-vibration (V-V), vibration-translation (V-T), and recombination-dissociation] in affecting vibrational kinetics has been studied by considering each process. Then the complete kinetics are taken into account, obtaining a global view of the interplay of the different microscopic processes. The model used for the recombination/dissociation processes is such to selectively pump levels v = 25 and 45 of the N2 vibrational manifold. This vibrational energy is then redistributed by V-V and V-T processes. As a result, strongly nonequilibrium vibrational distributions are obtained, despite the thermalizing action of the V-T processes by nitrogen atoms.

State-to-state approach in the kinetics of air components under re-entry conditions

AIAA, Fluid Dynamics Conference, 27th, New Orleans, LA, June 17-20, 1996 A state to state vibrational kinetics for air components including recombination-dissociation processes as well as the formation of NO through the reaction between vibrationally excited nitrogen molecules and atomic oxygen has been inserted in a monodimensional fluid dynamic code, describing the boundary layer surrounding a body under reentry conditions. The results show that the formation of NO is strongly enhanced by the nonequilibrium vibrational distribution of N2 formed during the recombination process. This kind of distribution is responsible of a non-Arrhenius behavior of dissociation constants of N2 and O2 as well of the NO formation rate as a function of instantaneous temperature. (Author)

Modelling of LIBS plasma expansion

Abstract A one-dimensional time-dependent fluid dynamic model has been developed to describe the expansion of the plume produced by laser ablation. The model includes chemical reactions considered in local thermodynamic equilibrium to describe the expansion of a TiO plasma. The results are discussed in connection with LIBS plasmas.

Reduction of State-to-State Kinetics to Macroscopic Models in Hypersonic Flows

The state-to-state chemical kinetic model, which considers a kinetic equation for each vibrational state of diatomic molecules, has been applied to some supersonic flow regimes and in particular in boundary layer, nozzle expansion, and shock wave. Nonequilibrium vibrational distribution obtained in the calculations shows strong departure from equilibrium-inducing non-Arrhenius global chemical rates, which differ substantially from macroscopic rates commonly used in fluid-dynamic codes. The evolution properties of the distribution have been investigated by a zero-dimensional numerical code in controlled conditions. We are trying to obtain from zero-dimensional results the approach to find appropriate macroscopic rate models to be used in fluid-dynamic codes accounting for the vibrational nonequilibrium. A comparison of analytical fitting of the zero-dimensional data and fluid dynamic global rates has been performed. Nomenclature ci = coefficients for the solution of the master equation Ev = energy of the vth vibrational level k = Boltzmann constant k d = dissociation rate constant k p = rates of the process p

The influence of atomic and molecular metastable states in high-enthalpy nozzle expansion nitrogen flows

The role of atomic and molecular electronically excited states on the whole kinetics of an high-enthalpy nozzle flow has been examined by using a self-consistent model which couples Euler equations with appropriate master equations and with the Boltzmann equation for the electron energy distribution function (eedf). The results show that in high-enthalpy flows metastable atomic nitrogen can form structures in the eedf through superelastic collisions, partially smoothed by electron-electron Coulomb collisions.

Electron and vibrational kinetics in the boundary layer of hypersonic flow

A simplified fluid dynamic model of the boundary layer in hypersonic flow has been coupled with complete vibrational kinetics for pure nitrogen. We focus our attention on the electron-molecule collisions and calculate the rate coefficients of such processes by solving the stationary Boltzmann equation for electron kinetics. The role of ionization degree in affecting vibrational and electron energy distributions has also been investigated.

Vibrationally relaxing flow of N2 past an infinite cylinder

The results of a numerical study aimed to verify the accuracy of the thermodynamic models based on the Boltzmann distribution in the hypersonic regime and to detect discrepancies, if any, with the analysis based on the more rigorous method of vibrational kinetics are presented and discussed. The test case considered in the study is the steady, two-dimensional, chemically inert, inviscid e ow of diatomic nitrogen past an ine nite cylinder in a uniform stream at M = 6.5. The vibrational relaxation has been calculated from the standard vibrational energy rate equation, assuming a harmonic ‐ oscillator behavior of N 2, and from a set of vibrational master equations that account for the e rst 10 vibrational energy levels of the N2 molecule, assumed as an anharmonic oscillator. The e ow patterns produced by the two calculation methods are thoroughly discussed and the critical ine uence of the vibrational relaxation time, associated with the harmonic ‐ oscillator models, in determining agreement or disagreement with the results from the vibrational kinetics method is evidenced.