Shock Tube Experimental Data Research Articles

Semiclassical modeling of state-specific dissociation rates in diatomic gases

A nonempirical, containing no adjustable parameters, theoretical model is suggested for calculations of state-specific dissociation rates in diatomic gases. Effects of molecular rotation and three dimensionality of collisions are consistently accounted for. The model is based upon a modified forced harmonic oscillator (FHO) scaling, with anharmonic frequency correction and energy symmetrization. The FHO scaling allows close-coupled calculations of multiquantum transitions between vibrational states, and it requires evaluation of collisional energy transfer to classical oscillator. Three-dimensional classical energy transfer models in both free-rotation and impulsive (sudden) approximations were used in conjunction with the FHO quantum scaling. The new theory describes the role of various degrees of freedom in dissociation both qualitatively and quantitatively. One of the predictions is that at low and moderate temperatures, dissociation is strongly preferential, with state-specific rates sharply increasing with vibrational energy; however, at high temperatures, the rate dependence on vibrational energy becomes less steep, turning into a virtually nonpreferential. Calculated thermal (equilibrium) and nonequilibrium dissociation rates of oxygen and nitrogen show a very good agreement with shock-tube experimental data taken from the literature.

An extension of CVDV model to Zeldovich exchange reactions for hypersonic non-equilibrium air flows

A new preferential vibration-dissociation-exchange reactions coupling model – labelled CVDEV – resulting from an extension of the well-known Treanor and Marrone CVDV model, has been derived to take into account the coupling between the vibrational excitation of the $N_{2}$ and $O_{2}$ molecules and the two Zeldovich exchange reactions. Analytical expressions for the exchange reactions coupling factor and for the average vibrational energy lost – or gained – by a molecule through an exchange reaction have been developed. The influence of such a coupling has been shown by means of numerical simulations of hypersonic air flows through normal and bow shock waves. Code-to-code comparisons between our model and other recent approaches have been conducted. The infrared radiation of nitric oxide behind a normal shock wave resulting from computations with the CVDEV model has been compared with other coupling model results and to recent shock tube experimental data. These comparisons have shown a good agreement of our model results with the experimental data. In this context, the results show the prominent influence of $N_{2}$ vibration coupling on the first Zeldovich reaction, and the absence of $O_{2}$ vibration coupling effects on the second Zeldovich reaction.

Evaluation of Ignition Improvers for Methane Autoignition

The objective of this study was to estimate the efficiency of methane autoignition promotion by testing different ignition improvers including hydrogen, H2, hydrogen peroxide, H2O2, ozone,O3 and dimethyl ether, DME, (CH3)2O. This was accomplished by computing ignition delays for CH4/O2/Ar or N2 mixtures of various compositions, concentrations of the promoters, pressures and temperatures. Ignition delay times for additive-free mixtures were used for tuning a methane oxidation mechanism consisting of 185 reversible elementary reactions between 32 species. A selection of the reaction rate parameters available in the standard databases was made to optimize the agreement between simulation and experimental results for one particular set of test conditions (reference mixture) by refining the rate parameters of the most sensitive stages revealed by sensitivity analysis. The agreement achieved between model predictions and shock tube experimental data is very good. To investigate the effect of dimethyl ether on methane autoignition, the mechanism was extended up to 301 reactions and 53 species to predict the autoignition delay at high pressures. Methane autoignition promotion is proven to be much stronger for H2O2, O3 and (CH3)2O additives than for H2. The mechanism of ignition acceleration is attributed mainly to the dominant role of the O and OH-radicals generated by the rapid decomposition of hydrogen peroxide and ozone, and to the peroxide intermediates and low temperature branching, in the case of dimethyl ether.

Comprehensive Mechanism for Methanol Oxidation

Abstract A detailed chemical kinetic mechanism, involving 26 chemical species and 84 elementary reactions, is proposed for the oxidation of methanol. Within the scope of this mechanism, turbulent flow reactor and shock tube experimental data are used to determine rate expressions for several of the important reactions involving CH3OH and its intermediate product species, CH2OH. Calculations using the proposed mechanism and elementary reaction rates accurately reproduce experimental results over a combined temperature range of 1000-2180K, for fuel-air equivalence ratios between 0.05 and 3.0 and for pressures between 1 and 5 atmospheres. The resulting chemical kinetic model is then employed, together with an unsteady, one-dimensional numerical model for flame propagation, to predict the laminar flame speed of a stoichiometric methanol-air mixture. The calculated laminar flame speed is 44+ 2 cm/ sec and is in good agreement with experimentally observed values.

On Difference Methods for Shock Wave Propagation in Variable Area Ducts Including Wall Friction

Results are presented of an analytical study concerned with the prediction of the propagation of shock waves through air ducting systems. The solution is one-dimensional but is appropriate for ducts which have a variable cross-sectional area and includes attenuation due to viscous effects at the wall of the duct. Finite-difference methods are utilized to obtain an approximate solution to the basic fluid dynamic equations. Comparisons are given between analytical results and shock tube experimental data which validate the capabilities of the methods used to predict shock wave attenuation and the effect of duct area variation on shock strength.

Numerical Study of Strong Plasma Shock Waves Produced in an Electromagnetic Shock Tube

A set of two-fluid Navier-Stokes equations with classical physical transport coefficients is used to compute the evolution and structure of collisional plasma shock waves in an electromagnetic shock tube. Shock speeds up to 200 cm/μsec and shocked-plasma temperatures of the order of kilovolts are studied. A strong transverse bias magnetic field is employed, which significantly alters the size and shape of the shock profiles, when compared with the zero bias field case. The wave structure is different from that of a two-fluid gas-dynamic shock without transverse magnetic field. Especially significant is the effect of the small ion Larmor radius in reducing the gasdynamic shock thickness by at least an order of magnitude in the transverse shock case, which permits collisional shocks to have thicknesses much smaller than the post-shock mean free path. These collisional shock waves produce very hot ions, the ion temperature increasing with shock speed, although the formation time and distances also increase substantially with shock speed. Generally, the results compare favorably with electromagnetic shock tube experimental data.