Measuring the ignition of fuel blends using a design of experiments approach
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A comprehensive set of experiments to determine the autoignition and ignition-delay times of multi-component fuel blends lends itself to a design-of-experiments approach. Possible blends of interest to gas turbine engines include methane combined with ethane, propane, butane, pentane, and hydrogen, with CH4 concentrations lower than 50% in some cases. For the fuel-air autoignition experiments, the only parameters are the fuel components. The proposed measurements will determine the autoignition limits in terms of a maximum residence time for either a given mixture temperature (800 K) or for the maximum mixture temperature for a given residence time. The fuel/ air mixtures in the autoignition experiments will be at elevated pressures (18 atm) and lean equivalence ratios corresponding to typical Lean Premixed Combustor conditions. Orthogonal matrices of 41, 9, and 8 fuel combinations were utilized instead of a full factorial matrix that would require 243 combinations, assuming 3 levels for each factor. A chemical kinetics model was used in a numerical experiment to determine the autoignition times within each matrix, resulting in mathematical correlations. The results of this numerical exercise indicate that neither the 8-or 9-combination matrix can be used in lieu of the larger 41-blend matrix in the autoignition experiments because they did not produce the same results within acceptable error. Several parameters in addition to fuel-blend combinations are required to fully characterize the oxidation kinetics of all possible fuel blends in a combustion environment, including equivalence ratio (0.5 - 2.0), pressure (1 - 25 atm), and diluent mole fraction (75 - 98%) over a range of temperatures from 700 - 2000 K. A 16-test matrix was derived to fully characterize possible pairs of CH 4 and one other fuel (100/0 - 40/60% CH4/other). Additional matrices were formulated to cover oxidation experiments for CO/H 2 blends and a range of possible methane blends with more than one fuel additive.