Lars L. Andersson

Quantized vibrational densities of states and ergodic energy transfer in molecular collisions

Abstract A simple method of generating quantized ro-vibrational densities of states based only on the thermodynamic data is presented. The result is a smooth function accurate over a wide energy range. Its simple power-law form allows the ergodic collision theory limits of energy-transfer moments to be calculated analytically, revealing the nature and magnitude of the quantum effects.

The interaction strength dependence of diatomic collisional energy transfer : a molecular dynamics study

Abstract Molecular dynamics simulation is used to study the activation/deactivation mechanism in the unimolecular decomposition of a diatomic gas. The strength parameter ϵ in the Lennard-Jones representation of the intermolecular potential is varied to reveal the dependence of the energy transfer rate on the strength of interaction in the collision complex. A dramatic increase is found with increasing ϵ and the energy transferred per collision approaches the ergodic collision limit at high ϵ.

A molecular dynamics investigation of energy transfer efficiency in collisions of diatomic molecules

Abstract Diatomic collisional energy transfer has been studied with the aim of aiding the development of accurate representation of the activation-deactivation mechanism in gas phase reaction rate theory. We seek to determine the dependence of the energy transfer efficiency on the parameters describing the colliding molecules, i.e. mass, vibrational frequency, intermolecular potential and initial energy or temperature. While weak van der Waals type interactions give rise to very poor energy transfer efficiency scaling up the interactions to the strength of weak chemical bonds increases the energy transfer efficiency dramatically up to nearly statistical limit values. The dependence on mass and vibrational frequency is found to be weaker. The interaction strength is resolved into two factors, hardness of encounter and attractive strength. Varying these factors independently by a modification of the intermolecular atomic pair potential, both are found to contribute strongly to the observed energy transfer efficiency. Two temperatures, 160 and 1500 K, are considered and the sensitivity to the intermolecular potential is found to be distinctly greater at the lower temperature. The average energy transferred per collision 〈Δ E 〉 is obtained as a function of the internal energy of the target molecule at normal and high interaction strength.

On the use of moments for describing the molecular orientation distribution

Abstract The posibilities of drawing conclusions about angular distribution functions F (ω) from measured moments [cos l ω] = ∫ cos l ω F (ω)d(ω) are discussed. F (ω) is expanded as a sum of Legendre polynomials with the moments in the coefficients. In practice only the first few moments are experimentally available. The limited use of a truncated series is demonstrated for distributions in polymers, electric fields and liquid crystals.

Time-dependent solution of pre-mixed laminar flames with a known temperature profile

Abstract A computer program designed for the evaluation of molecular flows interacting through chemical kinetics and molecular diffusion is described. Measured values of temperature profile and mass flow are used. The starting profiles and the hot boundary values are calculated by a kinetics approximation found by neglecting diffusion. A time-dependent method is used together with successive grid refinements. The successive grid refinements reduced the execution times by a factor of 5 for a H2/air flame at a pressure of 1 atm. For a CH4/O2 flame at 0.05 atm the reduction due to grid refinements was a factor 50 or more according to the estimations. The execution times for the test flames were a factor 4 slower than a current implementation of the steady state method. Possible optimizations of the present time-dependent version can decrease that difference significantly. The computed concentration profiles agreed with published computed results within 1 %.

Apparatus for studying premixed laminar flames using mass spectrometry and fiber‐optic spectrometry

An integrated flat‐flame/ microprobe sampling quadrupole mass spectrometer system, complemented by optical spectrometry based on optical fibers, is presented. The short microprobe sampling line (total 25 cm) is directly connected to an open ion source closely flanked by two nude cryopumps (900 l/s) yielding a background pressure of 10−9 Torr and a sampling pressure of about 10−5 Torr. Due to this improved microprobe system, mass spectrometry can be used for analysis of stable species (including fuel, O2, H2O, CO2, CO, and Ar) with less disturbance of the sample than with a conventional microprobe with a back pressure of about 1 Torr. Optical spectrometry is used for the study of emission from important radical species (such as C2, CH, and OH). The system is proposed as a complement to more conventional flat‐flame/MBMS systems in which the sampling cone can effect the experimental system. Details are provided concerning the configuration of the whole system ranging from gas delivery to data evaluation. Tes...

A Fast Time-Dependent Code for Evaluation of Experimental Premixed Laminar Flames

Abstract A time-dependent flame code designed for the evaluation of experimental premixed laminar flames is described. Measured values of temperature profile and mass flow are used. The calculations start from a kinetic approximation on a coarse grid, which is successively refined, A local implicit chemistry and explicit transport approach, incorporating a multi-component diffusion model, is used. Compared to our simpler earlier code, based on a trace diffusion model, the execution time is reduced by at least a factor of 10. Characteristic features in this new code are use of a time-independent approximation of the transport coefficients, automatic selection of the time-step, and a fast linear equation solver. For two test flames the execution times were a factor 2-3 shorter than the corresponding execution times published using a fast implementation of the steady state method

Pressure dependence of unimolecular reactions: Collision efficiencies in mixtures of weak and strong colliders

Abstract In practical applications such as combustion studies, prediction of the pressure dependence of unimolecular reaction steps is made more difficult by the great variety of conditions (pressure, temperature and composition) arising in the medium. A general scheme for the treatment of pressure dependence is needed. Here we investigate the pressure dependence of the collision efficiencies of weak colliders in a multicomponent medium. An extrapolation method is presented which can account for such pressure dependence by reference to single-component data. The scheme is tested on a model for the decomposition of ethane in a mixture of a strong and a weak collider.

Theoretical analysis of collisional energy transfer in unimolecular reactions: Collision efficiencies in binary mixtures

Abstract The unimolecular decomposition of ethane and acetyl is studied by weak collision RRKM theory. The reactions take place in a binary mixture of weak and strong colliders represented in the master equation by exponential, shifted Gaussian or strong collision contributions to the energy transfer kernel. The pressure dependence of the rate coefficients is studied as a function of mixture composition over the full range from the low-pressure limit to the high-pressure limit. Attention is focused on the validity of linear mixture rules which predict mixture dependence in terms of single component rate coefficients. A recently proposed iterative extrapolation method for the pressure- and composition-dependent collision efficiencies of the different medium species is tested and found to reproduce the falloff of the rate coefficient to good accuracy. Deviations due to nonlinear composition dependence are seen in the low-pressure limit.