Experimental and numerical characterization of hydrogen combustion in a reverse-flow micro gas turbine combustor

This study conducts experimental and numerical investigations on a stagnation-point, reverse-flow combustor to examine the effect of its design features on the combustion behavior and emission characteristics of non-premixed, turbulent hydrogen/air flames. The study covers a wide range of equivalence ratios, from ultra-lean to stoichiometric, and thermal power inputs varying between 5 kW and 20 kW. The main motivation behind this design is to achieve passive recirculation of flue gas within the combustor to enhance dilution of the incoming charge, thus potentially leading to relatively uniform thermal fields, lower temperatures, and suppressed NOx emissions. This design concept is implemented in a micro gas turbine - a practical energy conversion device tapped to play a major role in decarbonizing CHP systems suitable for small-scale industries. Optical chemiluminescence measurements, in addition to thermocouple-based temperature and Quantum Cascade Laser (QCL) spectroscopy-based exhaust emissions measurements, were conducted for all the test cases. Spatial OH* signal intensity is used as heat release signature to identify distinct features of the test flames, allowing for useful information on the nature and dynamics of the combustion process within the combustor. The results show stable hydrogen/air flames for all the operating conditions; however, a more distributed combustion is observed at higher powers for all the given equivalence ratios. Relatively higher NOx emissions at phi >0.2 suggest the lower achieved mixing between burnt and fresh gases, particularly in the non-preheated case, prevent a thorough dilution of the fresh mixture. The latter is necessary to achieve low-temperature, uniform thermal fields - an essential for low thermal NOx emissions in gas turbine combustors. Further, CFD results show that preheating considerably changes flow field dynamics and recirculation pattern for better mixing; however, this also dramatically increases thermal NOx emission, with the lowest levels observed at phi = 0.2, while the peak occurs at phi = 0.4. Finally, a 'brute-force' sensitivity analysis shows that thermal NO and NNH pathways contribute the most to NO formation in hydrogen/air flames.