NUMERICAL INVESTIGATIONS OF INSTABILITIES IN A NATURAL GAS-AIR FUELED ROTATING DETONATION ENGINE

Recent numerical and experimental studies of Rotating Detonation Engines (RDEs) using air as the oxidizer have primarily focused on the ability to sustain a stable continuous detonation wave when fueled with hydrogen. For RDEs to be a viable technology for land-based power generation it is necessary to explore the ability to detonate natural gas and/or coal-syngas with air in the confines of the annular geometry of an RDE. There are major challenges in obtaining a stable detonation wave for a natural gas air fueled RDE and to a lesser extent for coal-syngas and air. Recently published computational studies have, however, successfully simulated the underlying flow physics of detonative combustion for two-dimensional (2D) unrolled RDE geometries. In the present work, detonation wave characteristics of a hydrogen-natural gas fueled RDE have been numerically investigated and analyzed to understand the stability of natural gas detonations and detonability limits of fuel blends at relatively low operating combustor pressure. A series of detonation sensitivity studies have been conducted by varying the natural gas content in a hydrogen-natural gas fuel mixture, to assess the stability limit of natural gas detonations in an air breathing RDE. The current study explores the maximum percentage of natural gas content in a hydrogen-natural gas fuel blend that produces self-sustained, stable detonation waves. The simulations have been performed in a 2D unwrapped RDE geometry using the open-source CFD library OpenFOAM employing an unsteady pressure-based compressible reactive flow solver with a k - epsilon turbulence model in a structured rectangular grid system. Both reduced and detailed chemical kinetic models have been used to assess the effect of the chemistry on the detonation wave characteristics and underlying flow features. A systematic grid sensitivity study has been conducted with various grid sizes to quantify the weakly stable overdriven detonation on a coarse mesh and oscillating features at fine mesh resolutions. The low and high frequency instabilities have been analyzed from the time dependent pressure and temperature collected at various fixed spatial locations within the detonation height region. The results show that the peak pressure oscillates at low frequencies while for the high frequency instabilities, the peak pressure oscillates irregularly. Furthermore, at higher methane content, the high frequency instability leads to detonation extinction due to decoupling of the flame-front from the shock.front. Wave speeds, peak pressures and temperatures, and dominant frequencies have been computed from the time histories. 2D contour maps of temperature and species concentrations have been used to visualize the flow structures, and calculate detonation height. Global wave speed and detonation height variations for varying methane content indicate the pathway to detonation failure at higher methane content for the current low pressure RDE. Experimental data from an air-breathing RDE fueled by natural gas-hydrogen fuel blends conducted in a detonation research laboratory at NETL, has been incorporated to verify the numerical findings.