High pressure combustion is one aspect of emerging strategies for fuel efficient, low emission combustion devices. Among many advantages, high pressure permits lower temperature, lean combustion while maintaining load requirements. Many of the reaction rates and reaction pathways between fuel/oxidizer and products are uncertain at combustion temperatures and high pressure, especially for radical intermediates. These reaction rates and reaction pathways are measured using highly sensitive tools, typically at very low pressure, and are then extrapolated to high pressure. Recent theoretical predictions suggest highly energetic radical reactions can quickly pass through different pathways in low pressure conditions. Even though most combustion systems are at higher pressures, the measurements at very low pressure underpin the models of these systems, which may lead to great uncertainty in predictive models. Some of the goals of this work are to experimentally test these effects at low pressure and estimate their contribution to mechanism uncertainty at higher pressure. Another goal is to extend highly sensitive measurement techniques to high pressure. Over the course of five years, the PI will use advanced experimental and modeling tools to measure the rates and products of energetic radical reactions spanning combustion relevant temperatures and pressures. The work contributes to cleaner combustion device designs, via improved understanding of chemical reactions relevant to soot formation and high pressure pre-ignition chemistry. Information relating to the abstractions by halogens at high temperature, one aspect of the work, also contributes to fire suppression models. Additionally, efforts in this proposal include development of a combustion themed module for high school AP Physics curricula, which also incorporates pedagogic improvements through student-directed, inquiry-based investigations.
In order to test the effects of energetic pathways at low pressure, reaction rates and products of radical abstractions from hydrocarbons will be measured over wide pressure ranges. Various radicals will be employed, including halogens, in order to vary the enthalpy of reaction and the heat capacity of products at similar levels of collisional stabilization. The miniature high repetition rate shock tube (HRRST), will be used in this study. The HRRST is a tool for generating highly repeatable, highly controlled combustion conditions suitable for measurements using advanced analytical techniques. Species concentrations will be measured with a combination of tunable diode laser absorption for selected time-resolved species and mass spectrometry for products, including synchrotron sourced photoionization mass spectrometry in collaboration with researchers at Argonne National Laboratory and at the Advanced Light Source at Lawrence Berkeley National Laboratory. Reaction rates and product channels can be ascertained from these experiments with detailed chemical modeling. Besides the radical reactions which are the focus of this study, the work develops a nascent diagnostic technique, the HRRST, which can be adopted by a variety of disciplines.
