Date of Award

December 2019

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Benjamin Akih-Kumgeh

Keywords

CFD, Laser-Ignition, Shock Waves

Subject Categories

Engineering

Abstract

This thesis investigates various aspects of laser-ignition. Laser ignition is a form of combustion initiation by means of a focused laser pulse in a combustible mixture. The ignition process consists of a chain of processes with varying degrees of importance to the prediction of a successful flame propagation. Some of these processes include plasma formation, induced shock wave, emergence of a flame kernel, and successful transition to a self-sustained flame or flame quenching. This thesis will explore various aspects of this process using computational fluid dynamics and model analysis with the aim of identifying the controlling processes and simplified ways of capturing successful or failed ignition based on the injected laser energy, focusing optics and combustible gas compositions.

The problem is motivated by practical considerations. Combustion systems are still the main energy conversion technologies and it appears that they will continue to be dominant in the near future. To address environmental pollution and sustainability concerns, clean and efficient systems are being explored. One of the key challenges encountered is the problem of assuring dependable ignition in these emerging technologies. Laser ignition is considered to be a promising technology which would guarantee smooth functioning of advanced clean and efficient engines. Benefits include its non-intrusive nature and the easy control of the spark location, timing, and energy deposition.

For laser ignition systems to be useful, a good understanding of the process is needed. Understanding the degree to which each of the associated processes contributes to the development of a flame can lead to cost-effective models of ignition. This would align with current trends in computer aided engineering where simulations with physics-based models drastically reduce product development cycles. A perceived weakness in the laser ignition literature is the lack of simulations that compare models of different complexity in predicting the ensuing chemically reacting flows.

The proposed research will focus on the laser ignition of methane and biogas from the perspective of numerical simulations. Experimental results will be used as validation targets for these simulations. The flow field and thermochemical features controlling the emergence of flame kernels will be determined. Explanations of possible quenching of the flame kernel will be sought.

The problems addressed include numerical simulations of the laser-induced shock wave propagation, the transition of the laser-spark to a self-sustained flame with the help of chemical reactions, and the quenching of lean biogas flames. The shock wave study is found to accord with the blast wave theory, wherein the outward propagation can be predicted based on absorbed energy. Plasma kinetics is found to be unnecessary for the shock wave propagation. Using a compact or more detailed chemical scheme enables the prediction or the emergence of the flame. For prediction of the observed flame quenching behavior, however, the detailed scheme is necessary since the compact chemical scheme fails to capture the quenching event. Characteristic flow features are observed and explained in a manner that accords with experimental observations of global ignition features.

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Open Access

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