Browsing by Subject "Chemical Kinetics"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Consistent Chemical Kinetics and Continuum Models for High Temperature Air
    (2020-02) Singh, Narendra
    High-fidelity nonequilibrium reaction models for hypersonic air flow are developed. Hypersonic flows create shock waves, which compress and heat the surrounding gas to high-temperatures. Strong shock waves cause dissociation of nitrogen and oxygen molecules. Predicting the extent of dissociation and recombination of atomic species is important since the state of the gas near the vehicle surface determines heating rates and gas-surface chemistry that damages the heat shield. Since experimental data is difficult to obtain under such extreme conditions, numerical simulation plays an important role. Predictive numerical simulations require accurate reaction chemistry models. Computational models developed thus far range from simple empirical models fit to limited experimental data to models with millions of input parameters that track individual quantized energy state transitions. The level of model fidelity required for accurate engineering analysis remains an open question of active research. Models coupling internal energy and dissociation chemistry tend to be developed at either the kinetic scale or the continuum scale. In this dissertation, we develop new nonequilibrium models for shock heated flows that are analytically consistent between kinetic and continuum formulations, and are based on recent ab-initio data.
  • Thumbnail Image
    Item
    Modeling and Analysis of Chemical Kinetics for Hypersonic Flows in Air
    (2018-11) Chaudhry, Ross
    Gas-phase chemical kinetics are relevant for hypersonic flows, but they are currently modeled in CFD using empirical assumptions and decades-old experimental data. Recent advances in quantum chemistry have enabled the construction of accurate potential energy surfaces (PESs) for diatom-diatom interactions in air. Using these PESs, a database of simulated interactions is generated and analyzed; N2 + N2, N2 + N, N2 + O2, O2 + O2, and O2 + O reactions are considered. The conditions studied range from 4000 K to 30,000 K and include thermal equilibrium and nonequilibrium test sets. The nitrogen dissociation rate is found to be similar for collision partners N2, N, and O2. The oxygen dissociation rate, in contrast, is moderately dependent on partner species; O2 is approximately 2 to 3 times more effective than partner N2. Oxygen dissociation with partner N2 is therefore found to be substantially overpredicted by current CFD models, which is consistent with the limited experimental data available for this reaction. The presence of N is known from experiments to promote nitrogen dissociation; this augmentation is found to be due to increased vibrational relaxation, rather than an increased dissociation rate as described by current CFD models. Similar observations are made for oxygen dissociation with partner O, due to a combination of vibrational and electronic energy relaxation. Using only the shock tube data that informed popular CFD models, it was impossible to isolate the effect of increased relaxation from increased dissociation. The change in vibrational energy per dissociation, a necessary input to CFD, is found to be very sensitive to the degree of thermal nonequilibrium. This dependence is not well predicted by any existing chemical kinetics models; correctly describing this term fundamentally changes the thermal evolution of a gas in CFD. The mechanics of dissociation are similar for all reactions studied, so a series of aggregate analyses on all dissociation reactions is performed. Vibration is found to have a more pronounced effect on dissociation than rotation, due to rotation increasing the centrifugal barrier. The classic Marrone-Treanor preferential dissociation model is found to accurately describe all data in the nonequilibrium test sets, but it neglects the effect of rotational energy on dissociation. A modified model is proposed that describes rates to within 22% and vibrational energy changes to within 4% of the dissociation energy, for all dissociation reactions and conditions. For this work, we have considered Boltzmann or approximately Boltzmann distributions, but the population of high-energy molecules is known to be depleted in a dissociating gas ensemble. Various kinetics models based on Boltzmann distributions are implemented in US3D, a production CFD solver designed for hypersonic flows. As expected, the dissociation rate is overpredicted compared to the benchmark data. Work remains, therefore, to account for the non-Boltzmann distributions that exist in reality. These data and insights about dissociation can form the basis for next-generation chemical kinetics models for CFD.