Abstract
The use of hydrogen and ammonia in gas turbines, either alone or blended with natural gas, poses various technical challenges for combustion systems, including ignition. Depending on the fuel composition, the laminar flame speed and the ratio of unburned to burned gas density (dilatation ratio) of hydrogen and ammonia flames can be well outside the range seen in natural gas flames. Previous studies in annular combustion chambers have provided evidence of the importance of these properties in determining the ignition dynamics including light-around times. So far, these studies have mostly considered hydrocarbon fuels, have been limited to only a few runs, and have not yet systematically investigated variations in the dilatation ratio and the flame speed but rather have considered them as a lumped parameter. To investigate these effects in more detail, experiments characterizing the light-around times were carried out on an atmospheric annular combustor in which the dilatation ratio and the laminar flame speed was independently varied. This was achieved by varying the equivalence ratio and employing a variety of different hydrocarbon fuels (ethylene, propane, and methane) and fuel blends of methane–ammonia and methane–hydrogen. Light-around times were evaluated from global chemiluminescence measurements obtained using an azimuthal array of photomultipliers placed round the combustor chamber as well as high speed imaging. To improve statistical certainty, more than 3000 ignition and light-around times were measured with 30 repetitions obtained for each operating condition. To provide some insight into the light-around dynamics in specific cases, 900 of the 3000 sets included high-speed OH* chemiluminescence images. Light-around times for premixed pure hydrocarbon flames showed a similar dependence on the laminar flame speed as reported in previous studies. For the range of ammonia fuel blends investigated, an increase in laminar flame speed leads to a predictable increase in the flame propagation speed, as in the case of hydrocarbon fuel. Furthermore, collapse of this dependence for all blends could be achieved when corrected for an effective Lewis number, noting that all Lewis numbers for these blends were above unity. However, for hydrogen fuel blends, a decrease in dilatation ratio was found to decrease the light-around time counter to existing experimental results on the ignition of hydrocarbon fuels for which we currently do not have an explanation.