Characterization and Chemical Kinetic Analysis of the Ignition of Representative Conventional and Bio-Derived Fuels
Date of Award
Doctor of Philosophy (PhD)
Mechanical and Aerospace Engineering
Biofuel Ignition, Chemical kinetic modeling, Fuel blends, Furans, Model reduction, Shock tube
Fossil fuels are the main energy source in the world. However, they are responsible for negative environmental impacts, such as global climate change and rising sea levels. Biofuels are an environmentally friendly alternative which can substitute fossil fuels without major engine modications, especially in the transportation sector. Furans, a class of biofuels, are considered as possible alternative fuels for SI engines. They can be produced from sugars, derived from non-food biomass sources. This thesis is a contribution to fundamental characterization of their combustion properties.
Reactivity trends in furan combustion are established through ignition delay measurements of selected furans; 2,5-dimethyl furan (DMF), 2-methyl furan (2-MF), and furan. The isomer effect on the ignition of alkylated furans is also investigated to understand the general trends between dimethyl and ethyl isomers of cyclic fuel components. Since near term use of biofuels involves blends with fossil fuels, the relative ignition behavior of the least reactive furan, DMF, the gasoline surrogate, iso-octane, and their blends, is investigated. Experiments are carried out in a shock tube, a reactor that can generate instantaneous high temperature and pressure conditions by means of reflected shock wave, leading to chemical reactions and subsequent ignition of a test mixture of fuel and oxidizer.
Experimental results are compared with chemical kinetic model simulations and the models are analyzed to gain insight on leading chemical pathways. The experimental results for furans and iso-octane are compared to the most recent chemical kinetic models of each fuel and a combined DMF/iso-octane model is developed for the analysis of fuel blend combustion. The new blend model is used to clarify the chemical interactions during ignition of fuel blends.
The thesis also considers the ignition of saturated furans. In this respect the ignition behavior of tetrahydrofuran (THF) and methyl tetrahydrofuran (MTHF) is investigated to establish relative reactivity trends. The results are put into context by comparing with the unsaturated furan, 2-MF.
Cyclic fuel components of non-biofuel nature are considered. The high-temperature autoignition delay times of dimethyl and ethyl isomers of cyclohexane are carried out behind reflected shock waves to establish reactivity dierences between these dimethyl and ethyl isomers, which could further be explored in chemical kinetic modeling. The study is designed to test whether the observed trend is indicative of general reactivity differences between dimethyl and ethyl isomers of cyclic hydrocarbons, oxygenated or non-oxygenated. The ignition delay times of ECH are compared to model predictions to test the model performance. The pronounced dierences in the high-temperature ignition delay times of these isomers are clearly established using the shock tube technique and motivate further mechanistic explorations of distinguishing reaction pathways, without necessarily invoking the more
complex low-temperature chemistry.
With regards to model reduction, the existing Alternate Species Elimination (ASE) model reduction method is employed for the reduction of recently reported iso-octane and n-heptane models. The ASE approach is expanded into a stochastic species sampling approach, referred to as the Stochastic Species Elimination (SSE) method. The SSE method allows for a linear reduction process, and involves new features leading to reduced computational resource requirements, compared to the standard ASE method. Larger systems, such as the recent literature model of n-octanol, are approached with the SSE method with multiple species sampling, which allows for a less time consuming model reduction process. Resulting skeletal models are shown to adequately predict ignition delay times as well as flame propagation, compared to the predictions of the detailed models.
The work advances understanding of biofuel combustion. The established reactivity trends between the various fuels investigated in this work is of great importance to transportation fuel technology. The resulting experimental data sets are expected to fill the gap in the understanding of furans and gasoline combustion. The combined DMF/iso-octane model is a main contribution that allows for better insight into the combustion chemistry of furans, iso-octane, and their blends. Further, the proposed SSE model reduction method contributes to the use of combustion chemistry in practical combustion analysis in the form of cost-effective reduced models.
Eldeeb, Mazen, "Characterization and Chemical Kinetic Analysis of the Ignition of Representative Conventional and Bio-Derived Fuels" (2015). Dissertations - ALL. 385.