Numerical simulations of hydrogen flames in reheat gas turbine combustor: effect of pressure scaling and fuel blending
With a goal toward a net-zero energy supply, hydrogen, hydrogen-enriched natural gas, and biofuels, which reduce the carbon footprint, are actively being considered for firing stationary gas turbine engines. In this regard, the longitudinally-staged combustion concepts have been demonstrated to achieve low emissions and good fuel flexibility while operating over a wide range of load conditions. A particular implementation of such a concept is the constant pressure sequential combustor in Ansaldo Energia’s GT36 gas turbine comprised of two fuel stages implementing lean premixed combustion with different characteristics. In the first stage, the flame is stabilized aerodynamically and combustion occurs mainly by premixed flame propagation. The product gases from the first stage combustion are then blended with additional air in a dedicated mixer before entering the second stage combustor (also called the reheat burner) where additional fuel is added and combustion takes place at reheat conditions, i.e. it is controlled by spontaneous ignition due to the high temperature of the reactants. The reheat burner operation plays an important role in achieving the overall desired combustion characteristics. Recently, a high fidelity three-dimensional direct numerical simulation of the reheat flame with hydrogen fuel was performed to identify the modes of combustion and quantify their contributions towards fuel consumption at atmospheric conditions. However, the pressure in the practical system varies between 15 and 20 bar. The primary objective of this work is to understand the pressure scaling of flame in a reheat combustor using two-dimensional simulations. The computational domain consists of a mixing duct followed by a sudden expansion into a combustion chamber. A nine species, twenty-one reactions hydrogen-air mechanism is used for the detailed chemistry. Results show that at higher pressures the flame position is very sensitive to small perturbations in pressure/temperature, and can easily transition to an unstable state of combustion. Further, results on the flame structure and the role of auto-ignition will be presented. Chemical explosive mode analysis (CEMA) was used to qualify the fuel consumption rate between the auto-ignition and the flame propagation modes. With the increase in pressure, a significant decrease in fuel consumption due to auto-ignition was observed. To assess the effects of three-dimensional small-scale structures that are absent in two-dimensional simulations, a comparison of results between the two simulations was performed at atmospheric pressure. To understand the performance compromises observed in the hydrogen-rich regime of hydrogen-natural gas blends, further simulations with methane blended hydrogen were performed. Results illustrate a significant change in the flame stability and its structure. A detailed analysis of these results is also presented.