Investigation of Ozone Initiated Ethylene Oxidation at Room Temperature: Chemistry and Flame Dynamics
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Ozone (O3) addition has been proved to be efficient and effective in combustion enhancement and control. For saturated fuels, it is recognized that the O3 decomposition at elevated temperatures dominantly contributes to the improvement. However, for unsaturated fuels, the knowledge is limited, due to the much more complicated kinetic pathways induced by direct reaction between fuel and O3, i.e., the ozonolysis reaction. In this dissertation, the O3 initiated ethylene (C2H4) oxidation is experimentally investigated at room temperature using multiple diagnostic methods. To accommodate the rapid reaction between C2H4 and O3, a novel flow reactor system with online fast-mixing feature is designed, manufactured, and deployed. By coupling the flow reactor system to 255 nm LED absorption technique, the global reaction rate constant of C2H4+O3 is measured at ambient conditions. Being supplementary to the results in previous studies, many new products and intermediates are rigorously identified in this chemical system of room-temperature C2H4 oxidation, using both gas chromatography and tunable photoionization mass spectrometry. Based on determined molecular structures by quantum chemistry calculations, the detected species can be mainly categorized into alcohol, aldehyde, and peroxide, while many of them have been widely considered as key intermediates in low-temperature oxidation chemistry. Additionally, the effect of ozonolysis reaction on laminar flame dynamics is studied. Stable C2H4 lifted flames are established with oxygen/nitrogen co-flow at reduced oxygen content conditions. By adding certain amounts of O3 into the oxidizer co-flow, non-monotonic flame dynamic behaviors are recorded. Depending on the initial value of the flame liftoff height before O3 is added, it is observed that the flame liftoff height could either increase or decrease. Formaldehyde (CH2O) planar laser-induced fluorescence (PLIF) measurement shows that prompt ozonolysis reaction between C2H4 and O3 produces large amounts of CH2O upstream of the flame. In contrast to previous studies of O3 addition on lifted flames—with saturated fuels in which O3 decomposition dominates—the ozonolysis reaction between C2H4 and O3 considerably changes the chemical composition of fuel jet even at room temperature. Such chemical reaction causes the simultaneous increase of both triple flame propagation speed of lifted flame and axial jet velocity along the stoichiometric contour, which also therefore changes the dynamic balance between these two values to stabilize the flame.