Abstract:An experimental study of the erosion of two graphites (CFZ and PO3) by water and CO 2 at elevated temperatures was performed using a liquid rocket simulator with H2-O2-N2 feed for the water reaction study, and CO-O 2 -N 2 for the CO2 study. Specimen wall T w and reactant partial pressure P z were varied systematically to determine their effects on erosion rate JR. In addition, the effect of excess H2 on the water-graphite reaction was studied. The graphite-CO2 reaction was found to be first order with respect … Show more
“…The CO 2 -graphite complexes CO 2 − P 2 , CO 2 − P 3 , and CO 2 − P 7 are found to dissociate easily to yield CO + O-graphite, and the CO molecules are irreversibly lost to the vacuum within 10 fs. This finding is consistent with our CO 2 dissociative adsorption PES studies and previous experimental observations. − 7 The dynamics structures for the QM/MD simulations of dissociation products of CO 2 on S at 5000 K using the DFTB-D method. Three trajectories are shown corresponding to different initial geometries.…”
We present predictions of reaction rate constants for dissociative adsorption reactions of CO(x) (x = 1, 2) and NO(x) (x = 1, 2) molecules on the basal graphite (0001) surface based on potential energy surfaces (PES) obtained by the integrated ONIOM(B3LYP:DFTB-D) quantum chemical hybrid approach with dispersion-augmented density functional tight binding (DFTB-D) as low level method. Following an a priori methodology developed in a previous investigation of water dissociative adsorption reactions on graphite, we used a C(94)H(24) dicircumcoronene graphene slab as model system for the graphite surface in finite-size molecular structure investigations, and single adsorbate molecules reacting with the pristine graphene sheet. By employing the ONIOM PES information in RRKM theory we predict reaction rate constants in the temperature range between 1,000 and 5,000 K. We find that among CO(x) and NO(x) adsorbate species, the dissociative adsorption reactions of CO(2) and both radical species NO and NO(2) are likely candidates as a cause for high temperature oxidation and erosion of graphite (0001) surfaces, whereas reaction with CO is not likely to lead to long-lived surface defects. High temperature quantum chemical molecular dynamics simulations (QM/MD) at T = 5,000 K using on-the-fly DFTB-D energies and gradients confirm the results of our PES study.
“…The CO 2 -graphite complexes CO 2 − P 2 , CO 2 − P 3 , and CO 2 − P 7 are found to dissociate easily to yield CO + O-graphite, and the CO molecules are irreversibly lost to the vacuum within 10 fs. This finding is consistent with our CO 2 dissociative adsorption PES studies and previous experimental observations. − 7 The dynamics structures for the QM/MD simulations of dissociation products of CO 2 on S at 5000 K using the DFTB-D method. Three trajectories are shown corresponding to different initial geometries.…”
We present predictions of reaction rate constants for dissociative adsorption reactions of CO(x) (x = 1, 2) and NO(x) (x = 1, 2) molecules on the basal graphite (0001) surface based on potential energy surfaces (PES) obtained by the integrated ONIOM(B3LYP:DFTB-D) quantum chemical hybrid approach with dispersion-augmented density functional tight binding (DFTB-D) as low level method. Following an a priori methodology developed in a previous investigation of water dissociative adsorption reactions on graphite, we used a C(94)H(24) dicircumcoronene graphene slab as model system for the graphite surface in finite-size molecular structure investigations, and single adsorbate molecules reacting with the pristine graphene sheet. By employing the ONIOM PES information in RRKM theory we predict reaction rate constants in the temperature range between 1,000 and 5,000 K. We find that among CO(x) and NO(x) adsorbate species, the dissociative adsorption reactions of CO(2) and both radical species NO and NO(2) are likely candidates as a cause for high temperature oxidation and erosion of graphite (0001) surfaces, whereas reaction with CO is not likely to lead to long-lived surface defects. High temperature quantum chemical molecular dynamics simulations (QM/MD) at T = 5,000 K using on-the-fly DFTB-D energies and gradients confirm the results of our PES study.
“…This correction is accomplished by scaling with the ratio of the transfer coefficients, as predicted by the Bartz equation 22 for the ablation and calorimeter tests. The mass-transfer coefficient is determined from the Spalding relationship (7) where Le is the Lewis number. This mass-transfer coefficient and the experimental surface ablation rate and surface temperature are then used in an open system ACE calculation to determine the species partial pressures at the ablating surface.…”
An experimental program using an arc plasma generator was conducted to determine the kinetic mass consumption rate of pyrolytic graphite in simulated propellant atmospheres. These results, which include a wide range of gas species partial pressures and ablation rates, were successfully correlated using an equation with a functional form determined from phenomenological considerations. These correlations were in turn incorporated as an integral part of a prediction procedure which includes the simultaneous consideration of boundary-layer diffusion, equilibrium gas-phase chemistry, kinetically controlled surface chemistry, sublimation, indepth heat conduction and, surface mass transfer. The end product of this program was a procedure for predicting pyrolytic graphite ablation rates in rocket motor nozzles under conditions where surface kinetic effects are important.
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