Coupled Mixing and Auto-Ignition Dynamics of Turbulent Fuel Jets Issuing into Hot and Vitiated Oxidizing Environments

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1605136 - SuttonA broad range of engineering systems such as transportation and power-generation platforms rely on the injection of a turbulent fuel stream into a high-temperature, oxidizing environment. Under certain mixture and temperature conditions, auto-ignition will occur. Systems such as diesel engines and high-speed scramjets/ramjets rely on auto-ignition for achieving ignition and flame stabilization. Other systems, including gas-turbine and spark-ignition engines are designed to prevent auto-ignition to avoid significant and/or catastrophic damage. For either of these scenarios it is highly desired to understand the physics governing the transient auto-ignition process. Turbulent flows are very complex and when coupled to the chemical reactions governing auto-ignition, a highly dynamic system is formed where turbulent mixing has a direct effect on the reaction chemistry. In this project, time-resolved measurements of fuel/oxidizer mixing, temperature, and species will be made using advanced laser diagnostics, characterizing the flow and chemical conditions necessary for achieving auto-ignition kernel formation under turbulent fuel injection. The impact of the research will be far-reaching, ranging from a new physical understanding of auto-ignition dynamics to assessing numerical simulations and models. This project also will aid in the training of a graduate student and mentoring of a post-doctoral researcher. In addition, a unique aspect of this project is the implementation of a formal direct graduate-to-undergraduate mentoring program, where a graduate student is partnered with and mentors an undergraduate honors student. The PI also will partner with a local elementary school for K-12 outreach, equipping young students with information, inspiration, and initiative in STEM-related topics. The overarching topics, such as combustion, engines, and lasers, provide exciting themes for younger children and can help build the foundation for a life-long interest in science and technology. The proposed research will be transformative in the fact that the dynamic coupling between turbulent mixing, low-temperature chemistry and ?hot? ignition kernel formation will be examined in detail for the first time. High-speed (10 to 100 kHz acquisition rate) laser diagnostics will be used to measure the mixture fraction, temperature, and CH2O/OH concentrations following turbulent fuel injection through auto-ignition. Specific research contributions include quantification of key time-dependent processes which lead to the observed auto-ignition topology including the mechanisms in with turbulent mixing modifies auto-ignition topology and the role of low-temperature chemistry (e.g., CH2O) on ignition kernel formation. Due to the transient and spatially-intermittent nature of the auto-ignition process, multi-dimensional, temporal records are necessary to characterize the flow field scalars at the ignition kernel sites. These measurements will be used to determine space-time correlations between mixture fraction, CH2O (low-temperature chemistry) and OH (hot ignition kernel) as well develop new statistics of the mixture fraction, temperature, and scalar dissipation conditioned on the ignition kernel location for parameterization of the most probable conditions leading to auto-ignition. The proposed measurements will be carried out across a broad range of test conditions, examining the effects of varying Reynolds (Damköhler) number, fuel type, and oxidizer composition and temperature.