(Adviser: Prof. Wenting Sun)
2 p.m. Thursday, Feb 23 -- Weber 200
Abstract
Combustion plays a vital role in transportation and power generation. However, concerns on efficiency, emission, and operations at extreme conditions drive combustion into its limits. One example is in high-speed air-breathing propulsion systems, such as supersonic combustion ramjet (scramjet) engines. The short flow residence times in the engine highlights the need to enhance and control ignition and flame stabilization. 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, ozone (O3) injection is considered as a promising technique to enhance and control combustion. O3 is one of the strongest oxidizer and can be easily produced by electric discharge. It decomposes at moderate temperature and releases reactive O atoms. Furthermore, explosive exothermic ozonolysis reactions (spontaneous reactions between O3 and unsaturated hydrocarbons) can occur even at room temperature which may activate autoignition. However, its effect on combustion has never been investigated before.
In this work, the effects of O3 addition on different combustion phenomena are systematically investigated, toserve as the basis for the proposed active combustion control technique using O3. Firstly, the effect of O3 addition on laminar flame speeds (SL) is investigated for premixed fuel/oxidizer mixture. Increase of SL due to O3 addition is consistently observed for alkanes, CH4 and C3H8, and this enhancement is more significant at elevated pressure. In contrast, both detrimental and beneficial effects due to O3 addition are observed for the unsaturated hydrocarbon fuel, C2H4, depending on the experimental conditions. The effects of rapid exothermic ozonolysis reactions of unsaturated hydrocarbons on flame dynamics are further investigated using a non-premixed jet burner for C2H4. As O3 is added in the oxidizer, autoignition occurs and several different flame stabilization mechanisms are observed subsequently. These include the conventional flame propagation, autoignition-assisted flame propagation and coexistence of multiple autoignition kernels. The autoignition-assisted propagation features a high propagation speed (approximately 30 times of laminar flame speed at corresponding condition). The coexistence of multiple autoignition kernels significantly accelerates the “propagation” of flames to a speed of approximately 100 times of SL. In the next stage, more detailed diagnostics will be conducted and large-molecule fuels will be investigated as well.
Committee
Dr. Wenting Sun, AE, Dr. Jerry Seitzman, AE, Dr. Timothy Ombrello, AFRL