A thickness measurement technique based on low-coherence interferometry applied to a liquid film with thermal gradient

Certain combustion engine technologies can imply the formation of a liquid film on the walls of the combustion chamber, which produces significant pollutant emission. In this context, experimental study of the flame/film interaction is necessary, which in turn necessitates the measurement of the liquid film thickness. For this purpose, a low-coherence interferometry technique has been used in a specific configuration involving reactive flow and, consequently, temperature gradients, which contribute to measurement errors because of the thermo-optic coefficient. In the present work, in order to estimate these errors, the group refractive indexes of hexane, heptane and octane were measured by low-coherence interferometry according to the temperature at a wavelength of 1310 nm. The comparison of the thermo-optic coefficients indicated a good concordance between experimental data and the application of models derived from electromagnetic theory. The knowledge of these coefficients permits the estimation of a fairly reasonable film thickness uncertainty.