Investigation of dual-pulse laser plasmas for ignition of fuel-air mixtures
Date
2020
Authors
Butte, Carter Vincent, author
Yalin, Azer, advisor
Marchese, Anthony, committee member
Bailey, Ryan, committee member
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Abstract
Progress towards more complex combustion applications has demanded more advanced and versatile ignition techniques. One attempt to address some of the concerns associated with well-established techniques such as spark plugs and igniters is laser plasma ignition. Advantages of laser ignition include flexibility of spark location and timing, reduced NOx formation, leaner engine operation, increased combustion efficiency, and greater system longevity at elevated pressures. Additionally, the non-intrusive nature of laser plasmas results in more unperturbed kernel evolution, as mounting hardware is not required. This is an advantage when compared with spark plug or igniter electrodes which typically act as heat sinks quenching the flame. However, large input energies, complications with beam delivery, and undesirable kernel dynamics have impeded field implementation. Our approach to address these challenges uses a dual-pulse laser plasma where an ultraviolet (UV) beam preionizes a gas mixture and a second near infrared (NIR) beam increases the energy and ionization state of the gas. The use of this technique decouples the processes responsible for ionization, predominantly multiphoton ionization (MPI) and electron avalanche ionization (EAI) through inverse bremsstrahlung absorption, and allows for tailoring of plasma properties through adjustments to beam energies and delay time. Recent work has shown that dual-pulse laser plasmas not only reduce energy requirements but also enhance ignition characteristics such as combustion efficiency, particularly around the lean limit.6 The present thesis serves to fill voids in the existing literature with regards to plasma properties and ignition characteristics in various fuels, as well as present a new resonant preionization scheme targeting molecular oxygen at λ=287.5 nm. Four laser plasmas are investigated in this work: non-resonant single pulse (λ=1064 nm), non-resonant dual-pulse (preionization at λUV=266 nm with energy-addition at λNIR=1064 nm), resonant single-pulse (λREMPI=287.5 nm), and resonant dual-pulse plasma (preionization at λREMPI=287.5 nm and energy addition at λNIR=1064 nm). Each of these plasmas are analyzed for electron density and gas temperature using combined Rayleigh Thomson scattering, and are studied for ignition of propane-air, methane-air, and hydrogen-air mixtures. In the analysis, these experimental results are combined with past results to give a comprehensive picture of the ignition abilities of single pulse and dual-pulse plasmas in propane-air, methane-air, and hydrogen-air mixtures. Together, knowledge of plasma properties and ignition characteristics give us a more complete picture of the capabilities and limitations of each plasma for combustion applications.
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Subject
dual-pulse
Rayleigh scattering
combustion
Thomson scattering
plasma