Stephen Voelkel

Quasi-state-specific QCT method for calculating the dissociation rate of nitrogen in thermal non-equilibrium

The dissociation of nitrogen was studied using a quasi-classical trajectory (QCT) analysis in the context of calculating the dissociation rate surface for a dense range of temperatures for use in computational fluid dynamics (CFD) applications. By sampling rovibrational states from a Boltzmann distribution but uniformly sampling the relative speed, the dissociation rate was calculated for translational and rovibrational temperatures between 8000 K and 20000 K. The justification for this approach was verified by analyzing different sampling techniques. It was found that uniformly sampling the relative speed increased the uncertainty of the thermally averaged dissociation rate, but the same QCT results could be used for a large range of temperatures. This is in contrast to Monte Carlo sampling techniques, where a new batch of trajectories must be simulated for each desired temperature. To generate the dissociation rate surface, 500 million trajectories were simulated, and the non-equilibrium rates were compared to other models and experimental data, generally showing good agreement.

Vibrational non-equilibrium effects in supersonic jet mixing

A joint experimental and computational study is being conducted to investigate the effects of vibrational non-equilibrium on supersonic combustion, although the focus of this paper is on mixing between a supersonic jet and a subsonic coflow. A new facility has been constructed that consists of a Mach 1.5 turbulent jet issuing into an electrically heated coflow. In the preliminary experiments reported here, air is used in both the jet and the coflow. The degree of non-equilibrium in the jet shear layers is quantified by using high-spectral resolution timeaverage spontaneous Raman scattering. The Raman scattering is complemented with planar temperature imaging using Rayleigh scattering. Much of the current work is focused on the extent to which vibrational non-equilibrium can be assessed by using time-averaged Raman scattering in a turbulent flow with large-scale temperature fluctuations. The experimental work is supported by direct numerical simulation of related jet flows. Preliminary DNS of turbulent jets in coflow with imposed vibrational non-equilibrium shows that vibrational relaxation effects have a first-order effect on the jet temperature field and mixing physics.

Multitemperature dissociation rate of N2 + N2 → N2 + N + N calculated using selective sampling quasi-classical trajectory analysis

The dissociation rate of nitrogen for the reaction N2+N2→N2+N+N was calculated as a function of a translational, vibrational, and rotational temperature, each ranging from 6000 to 60,000xa0K. The rat...

Analysis of hydrogen-air detonation waves with vibrational nonequilibrium

The impact of vibrational nonequilibrium on induction zone and cell-structures in hydrogen-air detonations was modeled and analyzed. To this end, simulations assuming thermal equilibrium were compared to those in which vibrational nonequilibrium was modeled. 1-D simulations showed that modeling vibrational nonequilibrium has only a marginal effect of on the induction zone length. However, 2-D simulations showed that vibrational nonequilibrium plays a critical role in determining the detonation cell size. The average cell width increased from 5.5 × 10−3u2005m for the simulation assuming thermal equilibrium to 1.0 × 10−2u2005m for the simulation with vibrational nonequilibrium.

Numerical investigation of vibrational relaxation coupling with turbulent mixing

In flows where the relaxation rate of vibrational motion of the molecules to equilibrium is comparable to the flow through time scales, the presence of turbulence can alter the mixing and equilibration process. To understand the coupling between mixing and vibrational relaxation, a novel state-specific species model is solved in a background turbulent flow. The method is applied to mixing of two nitrogen streams at different static temperatures. The relaxation rates for each state are computed using quasi-classical trajectory analysis. For the flow conditions considered, the first ten vibrational levels are computed in the flow solver.The direct numerical simulation shows that population in different vibrational levels are significantly affected by turbulence and that the local distribution becomes nonBoltzmann. In certain locations in the jet, the population from the direct calculation can be several orders of magnitude different than the local-temperature based Boltzmann level. Last, while the bulk vibrational energy is inferior to its local equilibrium value throughout the mixing layer, the high energy level populations (levels 3 to 8) are on the opposite always over-populated. As chemical reactions are affected by these high vibrational energy populations, a simple temperature model would under-estimate the impact of nonequilibrium on combustion.