Frequency response and spatial resolution of a thin foil for heat transfer measurements using infrared thermography

A technique for measuring the spatio-temporal distribution of convective heat transfer has been developed using a test surface fabricated from a thin foil heated electrically. If the heat capacity of the test surface is sufficiently low, the fluctuating temperature on the foil can be measured using high-frame-rate infrared thermography. This method, however, has an inherent problem in that the temperature on the test surface attenuates both in time and space due to thermal inertia and conduction. In the present study, the frequency response and the spatial resolution of a thin foil were examined analytically considering heat losses. In order to derive general relationships, non-dimensional variables of fluctuating frequency and spatial wavenumber were introduced to formulate the temporal and spatial amplitudes of the temperature on the test surface. Based on these relationships, the upper limits on the detectable fluctuating frequency and spatial wavenumber were successfully formulated using governing parameters of the measurement system. This enables us to evaluate quantitatively the reliability of the heat transfer measurement by infrared thermography. The values, evaluated here for the practical conditions, indicated that this measurement technique is promising for investigating the spatio-temporal behavior of heat transfer caused by flow turbulence.