Ph.D. Thesis Defense
(Adviser: Prof. Wenting Sun)
“The Effects of Ozone Addition on Flame Propagation and Stabilization”
Tuesday, Oct. 24 2017 @ 1:15 p.m.
Montgomery Knight Room 317
Combustion plays a vital role in transportation and power generation. However, concerns of efficiency, emission, and operations at extreme conditions drive combustion to its limits. The relatively slow combustion process is generally attributed to the slow chemical reactions at low temperature conditions, such as radical production process. If the fuel oxidization pathway can be modified to circumvent these rate limiting processes, 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, which corresponds to the operating condition of most practical combustion devices. Compared to the time scales in most combustors, the lifetime of O3 is long enough to allow it being transported to the desired region from an injection location to modify fuel oxidization and control the combustion process.
This dissertation investigates the effects of O3 addition on some fundamental combustion processes, to serve as a basis for future application on practical combustion systems. These include the propagation of laminar premixed flame and the stabilization of non-premixed jet flames. Previous studies have shown that O3 addition can enhance flame propagation, stability and ignition, but the dependence on pressure and temperature are not clear yet. Such enhancement is generally attributed to the release of reactive O atoms by the thermal decomposition of O3. However, this may not be always true as O3 can react with some fuel directly via ozonolysis reaction before it decomposes. For unsaturated hydrocarbon, such reactions release significant amounts of heat and are rapid even at room temperature. Therefore, one may expect, at proper conditions, ozonolysis reaction can initiate autoignition and fundamentally changes the flame dynamics. However, very limited studies have been conducted to explore such possibility. 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 first, 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 showed that the enhancement in laminar flame speed (SL) increases with pressures. Simulation explains this with the finding that at higher pressure, O3 decomposition becomes a more dominant channel compared to other O3 consumption pathways. This promotes the release of O atoms, which accelerates flame propagation. The positive relationship between enhancement and pressure makes O3 addition an attractive technique for high-pressure applications. Regarding the dependence on initial temperature, simulation results show that adding O3 at higher initial temperature is not as effective as that at lower initial temperature, as another O3 consumption channel is favored at higher temperature. One can boost such enhancement in SL by increasing O3 concentration. 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 to or decreasing the pressure, and enhancement of SL by O3 addition has been 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 designed flow conditions, autoignition events are observed in such burner. Filtered chemiluminescence shows that the ozonolysis product, formaldehyde (CH2O), always accumulates before autoignition happens, which confirms that ozonolysis reactions initiate autoignition. The autoignition timescale is further investigated quantitatively. Overall, the relation between this timescale and inlet velocity is negative. Simulation models are built step by step to interpret this trend. At low Re, this trend is explained by the negative relation between mixing timescale and flow velocity at the present geometry. At high Re, it is attributed to the promoted mixing due to turbulence, which is further confirmed experimentally.
At such autoignitive conditions created by ozonolysis reactions, the stabilization of a lifted non-premixed flame is fundamentally different from the classic ones. It is observed that the ability of a flame to be stabilized at high velocity flows is significantly enhanced. Two mechanisms are proposed to explain this. Firstly, the 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 by several times. Secondly, the conventional propagation process is circumvented as radicals are generated and thermal energy can be released locally, initiated by ozonolysis reactions. So the propagation is not solely controlled by the diffusion process, and autoignition kernels can be generated at the upstream of the existing combustion zone. This effectively moves the flame front at a speed of more than 100 times of SL.
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. As for the non-premixed jet, this dissertation provides data and analysis of ozonolysis-activated autoignition events and their enhancing effects on flame stabilization. This includes modeling the relation between the autoignition timescales and chemistry, mixing, and Reynolds number, and proposing two mechanisms that enhance flame stabilization. That is, ozonolysis i) pre-process the reactants to have higher mass burning velocity, and ii) circumvent the propagation by generating autoignition upstream of the original combustion zone.
A global pathway selection (GPS) algorithm is proposed in this work for the systematic generation of skeletal mechanisms. A 28-species mechanism is generated using this algorithm and applied in the 3D simulation of the non-premixed jet flames with O3 addition. GPS is further extended to a hierarchical framework, Global Pathway Analysis (GPA), to understand the chemical kinetics. A demonstration is given by analyzing the second H2 explosion limit.
Dr. Wenting Sun (AE)
Dr. Jerry Seitzman (AE)
Dr. Timothy Ombrello (AFRL)
Dr. Lakshmi Sankar (AE)
Dr. Jechiel Jagoda (AE)