An active thermographic technique for highly resolved heat transport measurements in paper drying

A novel measurement technique, based on infrared thermography, has been applied for characterizing the thermal properties of paper with high spatial resolution. The technique is referred to as 'active thermography' since the temperature response of a paper sheet is observed and analysed with respect to an external periodic heating. Through the analysis of the temperature response of the paper surface in the Fourier domain for different modulation frequencies of incident heat flux, heat transfer velocities across the solid-gas interface can be estimated. This in turn results in estimation of the heat capacity of paper by solving the heat balance equations of the system. The thermal heating is applied spatially homogeneously. Therefore all these calculations are performed for all pixels in the image sequence. The spatial distribution and temporal development of all these parameters were visualized. The technique shows high potential for non-invasive dynamic measurements in paper drying.