Ignition delay of oxygenated fuel droplets: development of a 1 second drop tower and initial 1-g test results

United States\u27 dependency on petroleum fuels, much of it imported, has remained at the same level even as alternative fuels become more readily available to the consumer market. One alternative to petroleum based fuel is biodiesel. Biodiesel has been shown to have lower carbon monoxide, hydrocarbon, and particulate matter emissions than standard fuels, but show an increase in the formation of nitrous oxide gases. It has been theorized that several of the physical properties, which are governed by the chemical makeup of the fuel, have an adverse affect on NOx production. But until that time when a thorough chemical kinetic mechanism is developed for biodiesel, a means in which to lower NOx emissions will continue to elude the scientific community. Unfortunately, the chemical makeup of most biodiesel fuels is very complex. Instead, surrogate fuels must be used which have similar chemical structure to biodiesel, but are simple enough to allow for the development of a detailed chemical kinetic mechanism. At this time, there has been relatively little research in the combustion of biodiesel surrogate fuels. One fuel which has been proposed as a surrogate fuel for biodiesel is methyl butyrate. A detailed chemical kinetic mechanism for methyl butyrate has recently been developed, but it has yet to be substantially validated due to the lack of experimental data available on methyl butyrate combustion. In the present study, droplet ignition delay times of methyl butyrate and methanol are investigated. A bench experiment, conducted in normal gravity, was constructed and used to determine the ignition delay times of each fuel. Normal gravity experiments were conducted on both the methanol and methyl butyrate fuels. The methanol ignition delay results showed significant scatter. However, the methyl butyrate data exhibits expected trends. Experiments conducted in microgravity are expected to show higher repeatability. Accordingly, as part of this thesis, a drop tower facility was constructed in order to allow for testing in a microgravity environment. The microgravity environment is necessary to ensure that the experiments are spherically symmetric. A numerical model for methanol was also used to run simulations under prescribed conditions. The numerical model was originally developed at Princeton University and has been used to model methanol, methanol/water, heptane and heptane/hexadecane droplet combustion. The ignition delay times gathered from each set of simulations was compared against the experimental data obtained in the lab. Future work will entail incorporating the methyl butyrate mechanism into the droplet combustion model and comparing the model against the methyl butyrate experiments. By examining the ignition delay times of methyl butyrate under a range of temperatures and initial diameters, the chemical kinetic mechanism can be verified and, if possible, reduced in size. The overall goal is to determine a simplified chemical kinetic model that can be implemented into a CFD program which mimics the conditions present in a standard engine