Thermal effusivity measurements for liquids: a self-consistent photoacoustic methodology

A self-consistent photoacoustic methodology for the measurement of the thermal effusivity for liquids is presented. This methodology makes use of the analytical solution for the one-dimensional heat diffusion problem for a single layer, assuming a harmonic heat source in the surface absorption limit. The analytical treatment involves fitting procedures over normalized amplitudes and phases, obtained as the ratio of photoacoustic signals in the front configuration with and without the liquid sample, as functions of the modulation frequency. Two values of thermal effusivity for each liquid sample are obtained, one from the analysis of the normalized amplitudes and the other one from the normalized phases. The comparison between the experimental and theoretical phases allows the description of a simple criterion for deciding on the appropriate modulation frequency range for the analysis in each case. This methodology was applied for measuring the thermal effusivity of some pure liquids; a very good agreement between the thermal effusivity values obtained by this methodology and the corresponding ones reported in the literature was obtained.