Combustion Performance of Hydrogen Partially Premixed and Non-Premixed Flames via Elliptical Cross-Section Radial Swirl Burners

The combustion of hydrogen in the partially premixed and the non-premixed flame modes via a radial swirl burner of an elliptic cross-section was investigated. The swirler cross-section aspect ratio was varied in the range from 1.00 to 2.00 for the vane angles of 30, 45 and 60 degrees, while the equivalence ratio changed in the range of Φ > 1.0 for the partially premixed flames in comparison to having pure fuel jets in the non-premixed flames. It was found that increasing the aspect ratio displaced the central recirculation zone upstream and increased the turbulent kinetic energy to a maximum of 11.3 m2/s2 and the OH concentration peak magnitude by a maximum relative magnitude of 37.8% at Φ = 1.1. Inasmuch as this was associated with higher air entrainment rates, the peak flame temperature decreased and the corresponding peak NOx emission index decreased to 14.8 mg/kWhr at the aspect ratio of 2.0 in combination with a staging degree of 60%. Because of the flame shortening which took place to a relative extent of 25% as a result of the higher strain rates due to either the larger swirl angle or the greater aspect ratio, there was a reduction in the flow residence time across the non-premixed flame envelope. Such reduction was found to have a dominant role in the reduction of the peak NOx exhaust emission index from the non-premixed flame to 37.2 mg/kWhr at a combustion power density of 225 MW/m3. Employing the secondary air supply for the staged combustion in the form of opposing jets at 90° normal to the combustor centreline provided large turbulent kinetic energies that were satisfactory for complete hydrogen oxidation. Increasing the combustor/swirler confinement ratio to 2.5 and the swirler inlet/outlet diameter ratio to 2.25 increased the turbulent kinetic energy to values as high as 16.5 m2/s2.