Matthew J. A. Rickard

A facility for gas- and condensed-phase measurements behind shock waves

A shock-tube facility consisting of two, single-pulse shock tubes for the study of fundamental processes related to gas-phase chemical kinetics and the formation and reaction of solid and liquid aerosols at elevated temperatures is described. Recent upgrades and additions include a new high-vacuum system, a new gas-handling system, a new control system and electronics, an optimized velocity-detection scheme, a computer-based data acquisition system, several optical diagnostics, and new techniques and procedures for handling experiments involving gas/powder mixtures. Test times on the order of 3 ms are possible with reflected-shock pressures up to 100 atm and temperatures greater than 4000 K. Applications for the shock-tube facility include the study of ignition delay times of fuel/oxidizer mixtures, the measurement of chemical kinetic reaction rates, the study of fundamental particle formation from the gas phase, and solid-particle vaporization, among others. The diagnostic techniques include standard differential laser absorption, FM laser absorption spectroscopy, laser extinction for particle volume fraction and size, temporally and spectrally resolved emission from gas-phase species, and a scanning mobility particle sizer for particle size distributions. Details on the set-up and operation of the shock tube and diagnostics are given, the results of a detailed uncertainty analysis on the accuracy of the test temperature inferred from the incident-shock velocity are provided, and some recent results are presented.

Comparison of characteristic time diagnostics for ignition and oxidation of fuel/oxidizer mixtures behind reflected shock waves

ABSTRACT Various methods for determining characteristic times of shock-tube ignition and oxidation are compared. Onset and peak times were obtained from time histories for four different species (CH, CH*, OH, OH*) as predicted by a modern detailed kinetics mechanism. Appropriate submechanisms for CH* and OH* formation and quenching were added to the existing mechanism to differentiate the excited-state species from the ground-state molecules. The modeling focused on mixtures of acetylene or ethane with oxygen highly diluted in argon at high temperatures (1200–2050 K) and nearly atmospheric pressures. Using a detailed mechanism known to accurately simulate the shock-tube chemistry, emphasis was placed on cohesion of characteristic times among the species and the extent to which one may be used to predict another. Generally, ignition onset times were found to be more consistent than peak times, with OH peaking at times least typical of the group. Onset time versus inverse temperature curves based on any one species agree with those of the other three species to within 25% for the hydrocarbon mixtures and given mechanism utilized herein. Results suggest that ignition onset time should be used for greater consistency, and kinetics modeling of excited-state species such as OH* and CH* should be included if comparing to data obtained using chemiluminescence diagnostics.

Effects of capillary spacing on EHD spraying from an array of cone jets

Abstract Electrohydrodynamic (EHD) sprays are fundamentally characterized by low liquid flows that are atomized into relatively small, monodisperse droplets. Since previous studies have shown that droplet size increases with increasing flow rate, the current work employs an array of capillaries intending to increase fluid throughput without increasing the size of the droplets produced. To do this, an array of four-capillary nozzles is examined by measuring the required potential needed for stable EHD spraying in the cone-jet mode as a function of capillary separation. In addition, a simple electrostatic model is used to support the experimental results, and to predict the behavior of a larger, 5×5 square array. Results show that the potential required for cone-jet spraying in a two-dimensional array of capillaries generally increases as the capillary spacing decreases (due to electrical shielding), but at very close spacing the potential can decrease if the neighboring capillaries are dry. This result suggests that EHD arrays can benefit from fine wire electrodes interspersed among the capillaries.

Reflected Shock Ignition of SiH4/H2/O2/Ar andSiH4/CH4/O2/Ar Mixtures

High-temperature experiments were performed behind reflected shock waves with H 2 /O 2 , SiH 4 /H 2 /O 2 , CH 4 /O 2 , and SiH 4 /CH 4 /O 2 mixtures highly diluted in argon. Reflected-shock temperatures ranged from 1000-2250 K at a pressure near 1 atm. Reaction progress was monitored by observation of the time histories of several species by the use of emission techniques, including OH*, SiH4, and CH*. The oxidation and ignition data are reported in the form of species concentration profiles and plots of characteristic times as a function of temperature. The presence of silane in the fuel/O 2 mixtures markedly reduced the ignition delay time of all H 2 /O 2 and CH 4 /O 2 mixtures, usually by a factor of two or more, even at molar SiH 4 concentrations as low as 1% of the fuel concentration. Although the decrease in ignition delay time was, in some cases, quite significant, the activation energy of ignition when plotted on an Arrhenius diagram remained virtually unchanged; this result indicates that the chain-branching behavior that takes place when silane is present, although faster and through different elementary reactions, is similar to the behavior when silane is not present. The addition of silane to H 2 /O 2 mixtures also appears to extend the chain-branching kinetics to lower temperatures, eliminating the classic chain termination seen near the second explosion limit of H 2 /O 2 ignition.

A Shock-Tube Study of the Oxidation of C2H6/O2/Ar and C 2 H 6 /SiH 4 /O 2 /Ar Mixtures

*† ‡ § ** Ethane ignition and oxidation behind reflected shock waves with and without silane addition were studied using several dilute mixtures of varying concentrations and equivalence ratios (0.5 < φ < 2.0). The C2H6/SiH4/O2/Ar mixtures were studied at temperatures and pressures between 1230-1862 K and 0.9-3.0 atm, respectively. Argon dilution ranged from 91-98%. The reaction process was studied by monitoring the A 2 Σ + → X 2 Π chemiluminescence emission from the hydroxyl radical near 307 nm. In this study and in several previous studies, there have been different ways of obtaining the ignition delay time both in terms of diagnostics and in definition. A summary of the different techniques is given and used to make fair comparisons with other studies. A correlation of ignition delay time with temperature and concentration is proposed and compared with previous studies. This correlation has an r 2 value of over 0.98, mainly due to the inclusion of an argon concentration dependency. The overall activation energy for ethane ignition was found to be 36.0 kcal/mol over the range of conditions studied. Consistency of the data with previous ignition experiments is discussed. The experimental results indicate that silane addition to an ethane mixture at levels as low as 20% of the fuel can create up to a 50% reduction in ignition delay time for fuel-lean mixtures at high temperatures. It also created a reduction of about 22% for stoichiometric mixtures. Comparison of the present ethane-only ignition data with results from the literature highlights some of the differences in ignition definition, diagnostics, and range of conditions amongst the different studies.

EXPERIMENTAL INVESTIGATION OF RADIO FREQUENCY IDENTIFICATION RANGE FOR INTRAOCULAR IMPLANTS

Application of RFID technology to intraocular instrumentation requires an understanding of the reliability of propagation through optical tissues and fluids. In this study, the working distance for passive RFID operation is measured in a simulated intraocular environment using porcine eyes. Results are obtained in three common RFID ranges: low frequency, high frequency and ultra-high frequency. Several media were tested to assess the possible impact of intraocular materials. Variations in the possible working distance were observed for certain media, but the data suggest that reliable RFID operation for intraocular sensors is possible at a range of 2-4 cm.

Silane oxidation behind reflected shock waves

Several shock-tube experiments were performed to study the oxidation of argon- diluted SiH4/O2 mixtures behind reflected shock waves for temperatures between 1080 and 1780 K at pressures near 1 atm. Reaction progress was monitored using infrared emission from SiH4, FM absorption of SiH2, and chemiluminescence from OH*. Because of the difficulty in making premixed mixtures of silane and oxygen, these experiments produced the first shock- tube data on silane oxidation without the presence of another oxidizing species such as H2. A set of reactions was employed to model the OH* time history, and a reaction involving SiH + O2 was required to reproduce the OH* behavior. When compared to an existing chemical kinetics model of high-temperature silane oxidation, the agreement between data and model for the SiH2 and SiH4 measurements is not good, indicating some improvement in the mechanism is needed.

Ignition Delay Time Measurements of C2HX Fuels and Comparison to Several Detailed Kinetics Mechanisms

Recent results from experiments and modeling by the authors are reviewed for the ignition of acetylene, ethylene, and ethane in oxygen/argon mixtures at temperatures between 1000 and 2300 K and pressures near 1 atm. The ignition measurements were obtained behind reflected shock waves using emission from electronically excited OH and CH radicals to monitor the reaction progress. While many discrepancies exist amongst previous studies for these lower-order hydrocarbons, the accuracy afforded by the present experiments provides conclusive evidence verifying the trends seen in certain studies from the literature. Several modern, detailed chemical kinetics mechanisms were compared to the new results with some models showing quite good agreement with both ignition delay times and species profiles, particularly for stoichiometric mixtures. However, improvement is still required to match the entire range of fuel concentrations, temperatures, and mixture ratios, particularly for fuel-rich mixtures.Copyright