Initial Diameter Effects on Combustion of Unsupported Equi-volume n-Heptane/iso-Octane Mixture Droplets and the Transition to Cool Flame Behavior: Experimental Observations and Detailed Numerical Modeling Initial Diameter Effects on Combustion of Unsupported Equi-volume n-Heptane/iso-Octane Mixture Droplets and the Transition to Cool Flame Behavior: Experimental Observations and Detailed Numerical Modeling