An Experimental Characterization of a Multi-Element Lean Premixed Pre-Vaporized Combustor for Supersonic Transport Applications

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Passarelli, Mitchell Louis
Steinberg, Adam M.
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Recent trends have driven a re-emergence of research and development in aircraft engines for commercial supersonic transport (CST). Despite the vast body of literature that exists for gas turbine combustors operating at conditions relevant to conventional subsonic flight, there is little to validate extensions of this knowledge to the conditions encountered by CST engines. Further complications arise from advanced combustor designs that involve multiple different flames or flow devices. The interactions of such combustor elements can lead to individual behaviours that differ from that of single elements. The existing literature on flame and flow interactions is focused on conditions relevant to the operation of conventional, subsonic aircraft engines. While such works provide a baseline understanding of the physical phenomena involved in such interactions, they do not necessarily predict the behaviours exhibited by different combustor configurations and/or at different conditions. Some recent studies have employed numerical simulations to determine the characteristics of various combustor schemes, including lean direct injection and lean premixed pre-vaporized (LPP) designs. These studies are limited by the lack of empirical data for validation and model development. The work presented herein aims to characterize experimentally the flow field, flame dynamics and operating limits of a multi-element LPP combustor operating at CST-relevant conditions. Simultaneous laser and probe-based diagnostics were employed to obtain measurements of pollutant emissions, flow velocities, heat release rate, fuel-air mixing and thermoacoustic dynamics. The effects of combustor inlet pressure, temperature and fuel-air ratio are studied via corresponding parameter sweeps. Numerical chemistry simulations provide estimates of relevant flame properties, complementary to the experimental results. A second set of experiments investigated the forced response of the combustor. Overall, the results presented in this thesis demonstrate the importance of flame and flow interactions. In particular, the interactions of the pilot flame with neighbouring main flames are found to be critical in determining the stable operating range of the combustor. Furthermore, the pilot is found to dominate the dynamics of the combustor at forced and unforced conditions. Empirically-computed flame transfer functions at different forcing frequencies show that the pilot is most sensitive to acoustic perturbations and that this sensitivity is enhanced by interactions of the pilot with the main flames. This work also demonstrates the viability of LPP combustors for CST applications in three aspects. First, the pollutant emissions characteristics of the combustor studied are in line with future emissions targets. Second, the mean flow field, flame and dynamical characteristics do not vary strongly with operating conditions or undergo sudden or unexpected bifurcations, except when exceeding blowoff limits. A Damköhler number (Da)-based blowoff analysis shows that this combustor design exhibits enhanced stability compared with previously reported bluff-body stabilized flames. The analysis itself also demonstrates the robustness of a simple Da correlation for blowoff prediction, which works for a complex geometry such as the one studied in this work.
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