Chemical kinetics plays a crucial role in many high-temperature flows of engineering and scientific interest, particularly including combustion and propulsion systems. Despite significant progress in the fundamental theories associated with chemical kinetics, this field still relies very heavily on experimentation, and shock tubes have become a primary source of fundamental and applied chemical kinetic data, and virtually the only source of data at temperatures above about 1500K. Most shock tube kinetics experiments are conducted behind reflected shock waves, where the reacting gases are effectively stagnated, and utilize some form of optical diagnostics with fast time resolution to characterize the rate of the chemical reactions under study. Of particular value are laser-based measurements of species and temperature using absorption of light in quantum-specific atomic or molecular transitions. Here we present an overview of the state-of-the-art of shock tube methodologies for generating uniform, accurately known reaction conditions, and we discuss the range of laser absorption diagnostics now available for monitoring species time histories. Several examples will be given of recent applications of shock tube/laser methods for studies of chemical kinetics, primarily aimed at advancing fundamental knowledge of detailed reaction mechanisms, applicable to combustion.
There are three main areas where recent advances in shock tube methods are providing wider and more accurate test conditions. The first is an effort to achieve significantly longer test times, particularly at lower temperatures where kinetics processes are slower. This can be achieved using a combination of long driver geometries and suitable driver gas tailoring; test times of the order of 100 ms can now be achieved. The second is an effort to compensate for changes in reflected shock conditions due to non-idealities in shock tubes such as boundary layers and shock attenuation. This can be achieved by the use of driver inserts; non-reactive pressure profiles can now be generated with near-zero dP5/dt. Without this compensation, the changes in test conditions in both temperature and pressure strongly affect the rate of kinetics processes in ways that are difficult to model. The third is an effort to generate constant-pressure reflected shock conditions during reactive, energetic experiments using a constrained-reaction-volume strategy. This driven-section filling strategy has enabled experimenters to provide near-constant-pressure test conditions during exothermic and endothermic processes and greatly facilitates the accurate zero-dimensional modeling of these processes.
Laser absorption diagnostics provide quantitative, non-intrusive, high-bandwidth monitoring of many important kinetics species. These measured species concentration time-histories provide a significantly stronger constraint on the development of detailed reaction models than global behavior constraints, such as ignition delay times. In addition, high-sensitivity laser absorption diagnostics can be used in simple kinetics systems (particularly at low concentrations) to isolate and measure the reaction rate constants of individual elementary reactions. Currently available laser systems provide diagnostics for fuel and fuel components (e.g. JP-8 and alkanes), transient radicals (e.g. OH, CH3, NH2), stable intermediates (e.g. C2H4, CH4, CO, NO) and combustion products (e.g. H2O, CO2). Recent availability of solid-state infrared tunable diode lasers has expanded the set of measurable species to include larger alkenes (propene and iso-butene), alkynes (acetylene) and oxygenated species (methanol, aldehydes). Laser absorption of CO2 can also be used to monitor temperature, which in the constant pressure systems described above (in constrained reaction volume experiments) can be used to monitor heat release rates.
Shock tube kinetics research at Stanford University has been supported by the U.S. Air Force Office of Scientific Research under the Basic Research Initiative Program Grant Number FA9550-12-1-0472 with Dr. Chiping Li as Program Manager, the U.S. Army Research Office under contract/grant number W911NF1310206 with Dr. Ralph Anthenien as Program Manager, and the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-FG02-88ER-13857, with Dr. Wade Sisk as contract monitor.
Schematic of shock tube/laser absorption facility at Stanford University.
