Combustion and flow characteristics of staged combustors involving confined jets in crossflow
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Staged combustion offers many advantages in high performance aero-propulsion and power generation applications of gas turbine engines. For example, staged combustors can operate at low overall equivalence-ratio and temperature, thereby, pollutant emissions, while maintaining robustness, e.g., ignitability and flame stability over a greater operational range. To be effective, axial staging approaches require rapid mixing and burning of the staged reactants with the vitiated products from the pilot zone. In practice, this is achieved by utilizing a multiple jets-in-cross-flow (JICF) configuration in a highly reactive and confined combustor environment. While most previous work has focused on studying the properties of single, unconfined JICF, there is a paucity of work employing detailed diagnostics to study multiple and confined JICF (CJICF). This thesis examines the mixing, velocity and combustion characteristics of CJICF in air-staged (Rich-Quench-Lean, RQL), and fuel-air-staged (Lean-Quench-Lean, LQL) configurations using natural gas and air at atmospheric pressure and high temperature conditions. The well-characterized facility developed for this study allows for injection from five round jets, each produced by sudden contraction; two jets from the top wall and three interlaced jets from the bottom wall, with independent control of each set. Results are presented for parallel (one-sided injection), and staggered-opposed (two-sided injection) jets in a vitiated crossflow. High speed (10 kHz) stereo particle image velocimetry results are used to elucidate the mixing and flow characteristics, while OH* chemiluminescence imaging is used to study the combustion zone. Chemical reactor modeling is also used to help interpret the combustion results. For multiple confined, high momentum jets, the jet-wall and, to a lesser extent, the jet-jet interactions are found to have a major influence on the flowfield and the mixing characteristics of the jets with the crossflow. For example, the stagnation region where a jet interact with the opposite wall creates an upstream recirculation zone that redirects the crossflow away from the wall. Downstream of the jets, the crossflow can rapidly mix with jet fluid, which is even more noticeable in the regions between the jets due to lateral movement of jet fluid as it is redirected near the wall stagnation zone. The jet-wall impact appears to be more influenced by the total mass injection (or air split ratio in this study) rather than the momentum flux ratio, which is the parameter considered most influential for single, unconfined JICF configurations. In RQL conditions, with the high temperature crossflow containing H2 and CO, chemical times (autoignition delays) are sufficiently fast (~1-100μs), such that flames are stabilized near the jet exits and combustion is found to be mixing limited rather than chemistry limited. For LQL conditions, most of the burning likely occurs due to flame propagation, though a sufficiently high temperature crossflow can lead to enhanced flame stabilization, and burning of the premixed jets before significant mixing with the crossflow. Thus when stabilized in a high temperature crossflow, the LQL jets can burn in the opposed wall stagnation region, while the RQL burning is delayed until mixing with the crossflow occurs.