A new diagnostic for hydrocarbon fuels using 3.41-µm diode laser absorption

We report the development of a novel laser absorption diagnostic for accurate, time-resolved and in situ measurement of various hydrocarbon fuels in combustion systems. This diagnostic method utilized a wavelength-tunable interband cascade laser operated near 3.41 µm, providing improved performance in several aspects over the conventional 3.39-μm He-Ne gas laser diagnostic. First, it enabled a simplified and more compact experimental setup that significantly reduced the measurement complexity. Second, it improved the long-term stability over the 3.39-μm diagnostic by at least a factor of 2, leading to substantially reduced measurement uncertainties. Lastly, the new diagnostic also avoided a cluster of CH4 transitions that coincide with the He-Ne wavelength, and hence minimized CH4 interference in other hydrocarbon measurements. Absorption cross-sections of a variety of hydrocarbons at both 3.39 and 3.41 µm were measured in a high-purity shock tube over 531-1659 K, 0.34-3.1 atm, and reported here as functions of temperature. Example applications of this new diagnostic in shock tube pyrolysis studies of methylcyclohexane, n-heptane and iso-octane are also presented. These studies have yielded an improved value of the overall decomposition rate constant of methylcyclohexane as kd = 3.3 × 1015 exp(−38000 K/T) s−1 + 28%/−34%, which is valid over 1260-1400 K and near 1.5 atm.