Numerical and experimental investigation of a geometrically simplified, two dimensional flamesheet^tm hydrogen burner

Hydrogen is presently emerging as a convenient, chemically simple and carbon-free chemical for large-scale energy transport and storage with good balancing potential in future energy systems dominated by unsteady, non-dispatchable renewable power generation from solar and wind resources. Therefore, the capability to operate with hydrogen-enriched fuels reliably, cleanly and efficiently is an increasingly important requirement for gas turbines combustion systems. In this context, the innovative FlameSheet™ combustion system platform, developed by PSM with continued technology refinements by Thomassen Energy, both sister Hanwha companies, represents a competitive Dry Low Emission (DLE) device that has already proven able to handle gaseous fuel blends with high hydrogen fractions at 1350 C gas turbine firing conditions and above. This is mainly due, among a number of crucially important characteristics, to a carefully designed fuel-injection system and to an aerodynamic flame-stabilization strategy characterized by a unique flow pattern (U-bend) of the premixed reactants, ultimately resulting in increased resistance to premixed flame flashback. In the present work, we report a joint research effort consisting of a comprehensive numerical modelling study and of a experimental measurements campaign conducted on a geometrically simplified “FlameSheet^TM-like” burner fired with hydrogen-air mixtures at varying equivalence ratios. A two-dimensional, planar version of FlameSheet^TM (originally a cylindrical burner) is developed at TU Delft in collaboration with Thomassen Energy to enable better optical access and improved diagnostics of the turbulent reactive flow. Massively parallel Large Eddy Simulation (LES) of several geometrically simplified FlameSheet^TM configurations are performed at SINTEF in conjunction with detailed chemical kinetics and a Partially Stirred Reactor (PaSR) model for the turbulence-chemistry interaction. The LES results are validated against the experimental measurements and used, jointly with the latter, to provide new insights about the physical mechanisms that lead to stable flames or, alternatively, to the occurrence of flashback. It is found that, depending on the shape of the tip of the inner combustor-liner wall, flashback takes place along an inner route, around the blunt-shaped tip, or follows an outer route along the outer wall of the U-bend, for a sharp-shaped tip. Furthermore, as the critical equivalence ratio is approached, the amplitude of acoustic pressure fluctuations, excited by the interaction of the flame with the vortex-shedding immediately downstream of the U-bend, significantly increases ultimately leading to abrupt upstream flame displacement and to the occurrence of flashback. Finally, the LES model predictions confirm that the ratio of the channel thickness confining the flow upstream and downstream of the U-bend represents one of the main tuning parameters in flashback control.