Executive Summary : | staged gas turbine combustors are widely used in power generation for their capability of achieving low NOx emissions while conserving high thermal efficiency. A particular implementation of the staged combustor is the sequential (reheat) combustion system, which is characterized by two separate combustion chambers with an expansion step in a high-pressure turbine stage in between. In this project, our interest is in modelling the combustion dynamics of the secondary reheat burner, which plays an important role in achieving the desired combustion characteristics. Exhaust gases from the first-stage, which is typically at a temperature of 1000K, are mixed with fuel in a mixing section, which forms the inlet for the second-stage reheat combustor. The high inlet temperatures, as well as the use of highly reactive fuels like hydrogen and hydrogen-blended mixtures, result in tighter flashback margins and significantly shorter ignition delay times at the operating conditions, subjecting the combustor to earlier reactants' auto-ignition and possible flashback. In this project, we will use state-of-the-art direct numerical simulation (DNs) to understand flame stabilization regimes at reheat conditions where autoignition in the premixer can affect flame stabilization by premixed flame propagation. Particularly, we will perform for the first time a 3D DNs in the simplified burner at high pressure to investigate the flame structure and its interactions with turbulence and a parametric study based on 2D simulations to investigate the control mechanism for flame instabilities. We will also investigate the response of the reheat flame to entropy waves that is important in characterising flame instabilities. Further, the DNs data generated from this project will be used to develop a reduced-order model for the mixed-mode combustion phenomena, which can be used for engineering-scale simulations. |