Date of Award

June 2017

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Benjamin Akih-Kumgeh

Subject Categories

Engineering

Abstract

Cleaner and more efficient combustion systems are expected to operate at conditions where successful spark ignition is difficult to achieve. Laser ignition is a proposed alternative ignition system capable of stable engine performance under these conditions. Fundamental studies are needed to fully characterize the complex, multi-physics nature of the laser ignition process. This thesis is a contribution in that direction, also characterizing the ignition and flame behavior of some engine-relevant fuels.

This work investigates experimentally the early stages of the laser ignition process, characterizing breakdown and laser-induced shock waves. It then explores self-sustained flame behavior from early flame emergence to complete propagation or quenching.

Regarding the early stages of laser ignition, the influence of focusing optics, thermodynamic conditions, and chemical structure of fuels on optical breakdown threshold is examined. These results are presented in a universal representation of the breakdown threshold, facilitating their comparison. The results agree with previous studies and new data sets are generated.

Thermomechanical differences between breakdown in non-reactive and reactive mixtures are quantified, isolating the effect of exothermicity on plasma and shock wave propagation. The thermodynamic conditions of the gas near the focal volume are investigated and quantified using two-color interferometry. This information is applied toward developing accurate initial conditions for simulations based on absorbed laser energy and early kernel geometry.

With respect to flame propagation, schlieren and interferometric imaging techniques are used to examine early flame behavior, especially near flammability limits. This provides insight into the mechanisms controlling quenching of fuel-lean laser ignited flames as well as the time-scales involved. Four fuels (methane, biogas, iso-octane, and E85) are characterized, highlighting thermochemical effects which control their flame kernel development, the dynamics, and fate of initially sustained flames.

Laser ignition is further put into context by contrasting with the better established spark ignition process. The duration of energy deposition and heat transfer to the spark plug electrodes are found to be the main reasons for differences between laser and spark ignited flames.

By examining these different physical aspects of laser ignition, this thesis advances understanding of forced ignition, consolidating this by contrasting with spark-ignition behavior. The results are useful for the design of fuel-flexible and lean combustion technologies. The data set is also useful for CFD simulations and simplified modeling of the ignition process.

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Open Access

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Engineering Commons

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