Mechanical Engineering Ph.D. Dissertation

Abstract

This Ph.D. thesis presents high-temperature mid-infrared absorption spectra and reaction kinetics of a wide range of oxygenated species. This work mainly employs a recently developed broadband rapidly scanning laser and a shock tube. The range of oxygenated compounds under study encompasses a diverse array of molecules, including C1-C4 alkyl acetates, C2-C6 symmetric ethers, and some linear (dimethoxymethane and 1,2-dimethoxyethane) and cyclic (1,3-dioxolane and 2-methyl-1,3-dioxolane) diethers, which hold paramount significance in fields ranging from environmental science to industrial processes.

The mid-IR region of the electromagnetic spectrum plays a pivotal role in molecular spectroscopy due to its ability to probe unique rovibrational modes of molecules. High-temperature (up to1000 K) absorption spectra were measured over 905 – 1190 cm^-1 and 1160 – 1330 cm^-1 as these regions correspond to the strongest rovibrational bands of the investigated oxygenates. A commonly observed behavior in the absorption spectra of these oxygenates is that increasing the temperature results in broadened spectra, which is explained by the widening of the Boltzmann distribution of molecules by populating higher-J transitions at elevated temperatures. Another observed behavior is the red shift of the band peak as temperature increases, which indicates the emergence of hot-band transitions at elevated temperatures.

As a complement to the reported spectra over a wide wavelength range, fixed-wavelength diagnostics have been proposed for all investigated molecules for high-temperature transient reactive environments. Literature fixed-wavelength laser diagnostic work is limited to small molecules such as CO, CO₂, and H₂O. Laser-based diagnostics of large molecules are quite scarce mainly because of the limited availability of spectroscopic information at high temperatures. High-temperature spectra of oxygenates reported in this thesis provide the data required for selecting the optimum wavelength for sensitive and selective measurements. These fixed-wavelength diagnostics are demonstrated in this work by measuring fuel decay profiles during high-temperature pyrolysis studies in a shock tube.

Understanding the kinetic behavior of oxygenated species at elevated temperatures is of paramount importance as these oxygenates are proposed as future renewable fuels. Furthermore, the study of their kinetics and reactivity under high-temperature conditions provides valuable insights into the reaction mechanisms and pathways that aid in designing efficient chemical processes and reducing the environmental impact of combustion. We experimentally derived unimolecular decomposition rate coefficients for many of these oxygenated fuels and compared with the literature and/or theory. These measured unimolecular decomposition rate coefficients provide valuable insights into the reaction kinetics of fuels and allow modelers to improve the predictive capability of their kinetic models

Bio: Mohammad Adil is a Ph.D. student in the mechanical engineering department in the PSE division at KAUST, supervised by Prof. Aamir Farooq. He received his Master of Science (MS) degree from the Indian Institute of Technology Guwahati (IITG), in India. His current research focuses on laser absorption spectroscopy in high-temperature applications.