Structural differences of ethanol and DME jet flames in a hot diluted coflow

This study compares the flame structure of ethanol and dimethyl ether (DME) in a hot and diluted oxidiser experimentally and computationally. Experiments were conducted on a Jet in Hot Coflow (JHC) burner, with the fuel jet issuing into a 1250-K coflow at three oxygen levels. Planar measurements using OH-LIF, CH2O-LIF, and Rayleigh scattering images reveal that the overall spatial distribution and evolution of OH, CH2O, and temperature were quite similar for the two fuels. For both the ethanol and the DME flames, a transitional flame structure occurred as the coflow oxygen level increased from 3% to 9%. This indicates that the flames shift away from the MILD combustion regime. Reaction flux analyses of ethanol and DME were performed with the OPPDIF code, and ethane (C2H6) was also included in the analyses for comparison. These analyses reveal that the H2/O2 pathways are very important for both ethanol and DME in the 3% O2 cases. In contrast, the importance of fuel-specific reactions overtakes that of H2/O2 reactions when fuels are burnt in the cold air or in the vitiated oxidant stream with 9% O2. Unsteady laminar flamelet analyses were also performed to investigate the ignition processes and help interpret experimental results. Flamelet equations were solved in time and mixture fraction field, which was provided by non-reactive Large-Eddy Simulation (LES).