Interband cascade laser absorption of hydrogen chloride for high-temperature thermochemical analysis of...

A mid-infrared absorption spectroscopy technique has been developed to quantitatively and spatially resolve gas temperature and molecular abundance of $ ^1{{\rm H}^{35}}{\rm Cl} $¹H³⁵Cl in the high-temperature pyrolysis and oxidation layers of chlorinated polymers. Two transitions in the R-branch of the fundamental vibrational band of HCl near 3.34 µm are selected due to their relative strength and spectral isolation from other combustion products at elevated temperatures, and they are probed using a distributed feedback interband cascade laser. A scanned-wavelength direct absorption method is employed with a Voigt line-fitting routine to recover projected absorbance areas across the exit plane of a cylindrical polymeric slab, wherein the gaseous core comprises the axisymmetric reaction layer. Tikhonov-regularized Abel inversion of the projected absorbance areas yields line-integrated radial absorption coefficients, from which a two-line ratio is used to infer a radially resolved temperature between the gas core and solid polymer surface. The method is demonstrated to provide insights into the gas-phase combustion physics that lead to the formation of toxicants when a fire-resistant polymer, polyvinyl chloride (PVC), is burned in the presence of oxygen. Two-dimensional images were generated by assembling multiple tomographic reconstructions for several cylinder lengths. The quantitative images highlight the technique's ability to characterize the thermochemical evolution and toxicant formation during the burning of a halogenated polymer fuel.