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43689-G9
Combustion of Methane in Countercurrent Shear Layers
Countercurrent shear flows have found application in flow control studies where it is advantageous to induce enhanced turbulent energy production. The current study is focused on exploring the entrainment and turbulent flow characteristics of reacting countercurrent shear layers, where the presence of countercurrent shear and heat release tend to have opposite effects on the turbulence in the shear layer. The results of the study will provide guidance and insight into the potential benefits of countercurrent shear flow control for combustion systems.
A novel shear layer facility was developed that allows for the establishment of a spatially-developing countercurrent shear layer. Initial studies were focused on establishing the flow quality and validation of the non-reacting shear flow through comparison to existing data for similar flow configurations. The proximity of the confining wall on the counterflow side of the shear layer was found to have an influence on the shear layer characteristics, including an effect on the entrainment process and the turbulent stress components, notably the cross stream normal Reynolds stress. The effects of confinement have been shown to be related to the local distance from the shear layer to the wall and the local shear layer thickness. Under confined conditions, the turbulent flow interacts with the wall, causing a highly disturbed secondary stream.
The experiment has been modified to minimize the effects of confinement for the non-reacting flow. The effects of confinement on the reacting flow may be considered once the more canonical unconfined configuration is fully documented. Preliminary combustion studies have been conducted to determine qualitative effects of the use of counterflow on the flame dynamics. Under combustion scenarios, a rich fuel and air mixture is used in the primary stream, while the secondary stream is air. This configuration results in a shear layer mixture composition with an equivalence ratio that is dictated by the equivalence ratio in the main flow and the relative entrainment ratio of the shear layer. Without counterflow, flames have been stabilized in the shear layer for a range of primary flow equivalence ratios from approximately 1.4 to 2.3. With a velocity of 10 m/s for the primary flow, a single-stream shear layer configuration will result in a flame that detaches from the trailing edge and forms a leading-edge flame in the downstream region. The application of modest levels of counterflow resulted in flame reattachment at the trailing edge. Hence the counterflow has been shown to be an effective suppressor of flame lift-off, which is advantageous for maximizing combustion efficiency through the avoidance of fuel leakage around the flame.
The project is currently being optimized to make detailed flow field measurements using particle image velocimetry under combustion conditions. Emphasis will be placed on the effects of combustion on the entrainment and turbulence within the countercurrent shear layer.
The impact of the project on the development and growth of the involved graduate students and the principal investigator has been extensive. One graduate student graduated with a Master of Science in Mechanical Engineering during the summer of 2007. The student gained significant experience and understanding of shear layer flows, advanced instrumentation including particle image velocimetry and hot-wire anemometry, and developed data analysis codes in Matlab® to study the experimental data. The student also gained experience in validation and optimization of the experimental configuration. A second graduate student has taken over where the previous left off, and the new student has prepared the experiment for combustion conditions while optimizing the particle seeding system including the development of a solid particle seeder to be used for PIV measurements under burning conditions. This second master student has become well trained in experimental methods and will produce data that will result in a number of publications in archival journals.
The principal investigator has benefited tremendously from the project. The project allowed for the development and validation of an independent experimental facility that will be the focus of additional research efforts for years to come. The work that is currently underway on the quantitative study of the countercurrent shear layer will result in several publications that explore the effects of heat release and countercurrent shear on the turbulence characteristics of the shear layer. The results of the ongoing work will be used as preliminary data in upcoming proposal to the National Science Foundation and potentially other agencies that focus on the continued exploration of ways to enhance combustion systems in terms of efficiency, size, and emissions. The reacting shear layer facility may also be employed for other studies including the flame and ignition dynamics of alternative fuels and other flow control strategies such as synthetic jet actuators that alter the reacting shear flow.
