The effects of ozone addition on flame propagation and stabilization
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Combustion plays a vital role in transportation and power generation. However, concerns of efficiency, emission, and operations at extreme conditions highlight the needs to enhance combustion process. If the rate-limiting chemical pathways can be modified, the ignition and combustion process could be dramatically accelerated. Following this idea, addition of ozone (O3) is proposed as a potential solution. O3 is one of the strongest oxidizers. It can be efficiently and economically produced in situ at high pressures, and transported to the desired region from an injection location to modify fuel oxidization and control the combustion process. To serve as a basis for future application on practical combustion systems, this dissertation investigates the effects of O3 addition on two fundamental combustion processes: the propagation of laminar premixed flames and the stabilization of non- premixed jet flames in autoignitive environment. Previous studies have shown that O3 addition can enhance flame propagation, stability and ignition, but the dependence on pressure and temperature were not clear. Furthermore, few studies have been conducted on the effects of ozonolysis reactions, which are rapid even at room temperature for unsaturated hydrocarbons. The results presented in this dissertation are an attempt to address these questions. The effects of O3 addition on the propagation of laminar premixed flames are investigated with respect to pressure, initial temperature, O3 concentration and fuel kinetics. For alkane/air premixed laminar flames, high-pressure Bunsen flame experiments in the present work show that the enhancement in laminar flame speed (SL) increases with pressures. This is due to the fact that O3 decomposition, which releases reactive oxygen atoms, becomes a more dominant O3 consumption pathway at higher pressure. Simulations show that adding O3 at higher initial temperature is not as effective as lower initial temperatures. A nearly linear relation between the enhancement and O3 concentration is observed at room temperature and atmospheric pressure. If the fuel is changed from alkanes to C2H4, an unsaturated hydrocarbon species, ozonolysis reactions take place in the premixing process. When the heat released from ozonolysis reactions is lost, decrease in SL is observed. In contrast, if ozonolysis reaction are frozen, either by cooling the reactants or decreasing the pressure, enhancement of SL by O3 addition is observed. The study on flame stabilization with O3 addition is conducted with a non-premixed jet burner in a quartz tube using C2H4 as the fuel. At low-dilution conditions, autoignition events are initiated by ozonolysis reactions. The autoignition timescale is further investigated quantitatively. Overall, this timescale decreases as the inlet velocity increases. At such autoignitive conditions created by ozonolysis reactions, the stabilization of a lifted non-premixed flame is fundamentally different from non-autoignitive conditions. Propagation is enhanced due to the “preprocessing” of fuel by ozonolysis reactions, after which the mass burning velocity of the reactants is increased as shown by simulation. This can increase the propagation speed by several times. In summary, for the premixed laminar flame propagation, the present results explain the pressure, initial temperature, and fuel dependence of enhancement of flame propagation by O3 addition. A more comprehensive understanding is thus contributed. Furthermore, this dissertation explores ozonolysis reactions as an alternative to create a platform to conduct fundamental research on flame in autoignitive environment.