Thermal-diffusive instabilities, soot and carbon nanotube formation in unstrained diffusion flames

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Combustion systems are complex machines in which a large number of chemical reaction take place simultaneously and closely coupled with fluid mechanics processes. It is therefore often useful to gather fundamental knowledge on each phenomenon in simple configuration uncoupled from perturbing influences. In this project we are using a unique experimental burner to investigate combustion-related phenomenon in unstrained diffusion flames. In this configuration, the reactants are provided separately to the reaction chamber and the flame has a one-dimensional inner structure which allows for direct quantitative comparison with theoretical models. Although this configuration can seem far from practical applications it is used extensively for the development of theoretical models, for instance on thermal-diffusive instabilities. It is also relevant to the flamelet concept, used extensively for the numerical modelling of the turbulent partially-premixed combustion encountered in gas turbines. The research program presented here targets two main topics where the use of this unique tool can provide novel insight into complex combustion-related phenomena of particular interest for the canadian energy-conversion sector. In the context of the desire to reduce emissions from the combustion of hydrocarbons from fossil or biological origin, several products are the subject of intense research. For instance, carbon dioxide (CO2) is the dominant greenhouse gas, fine particulate such as soot are a serious health hazard and nitrogen oxides (NOx) are responsible for urban pollution and acid rain. However, attempts to reduce one of these emissions is often made at the expense of an increase in another or can result in an unacceptable decrease in performance. The first research topic included in this proposal is the investigation of thermal-diffusive instabilities (TDIs) that form close to the extinction limit. The current trend to operate combustors with lean mixtures, a technique used extensively to to lower emissions in gas turbine applications, makes them more susceptible to the development of TDIs and research is necessary to to identify safe operating conditions in lean mixtures. Our work aims to provide experimental results for the quantitative validation of theoretical models for diffusion flame stability. The second research topic focuses on the formation and aggregation of soot particles in diffusion flame. These phenomena can be advantageously investigated using our experimental configuration where a stable soot layer can be formed and the residence time of the particles can be controlled precisely. Additionally, two more research projects have been initiated following observation made in the burner mentioned above. The third research axis of this proposal focuses on the investigation of carbon nanotube formation in flame environments. More specifically, the unstrained and one dimensional nature of the flow in our reaction chamber offers unique conditions for the growth of a wide variety of carbon nano-structures, including nanotubes without catalysts. The fourth and final research axis of this proposal addresses the need to develop techniques to concentrate and sort aerosols, for instance as a function of the particle size. We are implementing an approach based of the use of acoustic forces to perform advanced concentration and sorting tasks on polydisperse sub-micron aerosols. Although initially motivated by our desire to separate the carbon nanotubes from the soot simultaneously produced by the flame, such acoustic-based techniques are relevant to a wide range of industrial applications in the energy conversion sector.