Characterization of Non-volatile Particulate Matter in Pressurized Premixed Laminar Jet-A Flames Via Thermophoretic Sampling

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Manikandan, Sundar Ram
Steinberg, Adam M.
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Production and subsequent emissions of non-volatile particulate matter (nvPM) pose a challenge for both optical diagnostics and physical probing, especially at conditions relevant to practical combustors. Key to enabling nvPM mitigation is in-situ optical measurements, particularly laser induced incandescence (LII). However, interpreting the LII signals is challenging. To quantitatively use LII in gas turbines, their measurements must be calibrated and validated against physical nvPM samples. The preferred approach for extracting these physical samples is in-situ thermophoretic soot sampling followed by transmission electron microscope (TEM) imaging. This thesis work deals with the design of a multi-probe thermophoretic soot sampling system capable of extracting nvPM samples in laminar, rich flames of prevaporized jet-A/air premixtures at elevated pressures. The flames under investigation were observed to exhibit thermal-diffusive instabilities, that are responsible for the flame to form corrugated structures. Moreover, these instabilities cause the corrugated flame to exhibit spatio-temporal variations, which exacerbate the challenges in implementing diagnostics. For the soot sampler, a significantly larger sampling time of 125 ms was required to obtain sufficient soot deposition on the TEM grids, which can enhance the extent of restructuring in the deposited soot particles. Visualization of the data through the TEM revealed (i) a wide range of soot particle size varying between 10 – 250 nm; (ii) presence of non-soot organic matters that include (1) fibers, (2) sharp contrasted mineral-like structures, and (3) uniform and porous spherical structures with varying contrast; and (iii) the dominant morphological characteristics of the flame generated soot particles that are indicative of its chemically reactive nature and restructuring. Furthermore, the quantitative results show (i) increasing soot particle size with pressure, and (ii) an increasing-decreasing trend for the mean soot particle size with height above the burner. While the effect of pressure is explained by the enhanced extent of graphitization and maturity in the nanostructures of soot particles at elevated pressures; the dependence on height can be explained through particle agglomeration for the initial increase in size with height, followed by oxidation of the particles respectively. However, considering the range of tested HAB when compared to the flame length, the possibility for inconclusive variation stems from preheat temperature variations and restructuring effects
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