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Convective heat transfer is, by nature, generally nonuniform and unsteady, a fact reflected by three-dimensional flow near a wall. However, most experimental studies concerning convective heat transfer have been performed in a time-averaged manner or using one-point measurements. This frequently results in poor understanding of the heat transfer mechanisms. Measurement techniques for the temporal and spatial characteristics of heat transfer have been developed using liquid crystals (Iritani, et al., 1983; among others) or using infrared thermography (Hetsroni & Rozenblit, 1994; among others), by employing a thin test surface having a low heat capacity. However, the major problem with these measurements is attenuation and phase delay of the temperature fluctuation due to thermal inertia of the test surface. This becomes serious for higher fluctuating frequencies, for which the fluctuation amplitude weakens and ultimately becomes indistinguishable from noise. In addition, lateral conduction through the test surface attenuates the amplitude of the spatial temperature distribution. This becomes serious for smaller wavelength (higher wavenumber). These attenuations are considerably large, especially for the heat transfer to gaseous fluid such as air for which the heat transfer coefficient is low. The recent improvement of infrared thermograph with respect to temporal, spatial and temperature resolutions enable us to investigate detailed behavior of the heat transfer caused by flow turbulence. Analytical study indicated that the spatio-temporal distribution of the heat transfer to air caused by flow turbulence can be observed by employing modern infrared thermograph (for example, NETD less than 0.025 K and frame rate more than several hundred Hz) that records temperature fluctuations on a heated thin foil of sufficiently low heat capacity. In the former part of this chapter (sections 2 – 4), an analytical investigation was described on the frequency response and the spatial resolution of a thin foil for the heat transfer measurements. In order to derive general relationships, nondimensional variables of fluctuating frequency and spatial wavenumber were introduced to formulate the amplitude of temperature fluctuation and/or distribution on the test surface. Based on these relationships, the upper limits on the detectable fluctuating frequency fmax and spatial wavenumber kmax were formulated using governing parameters of the measurement system, i.e., thermophysical properties of the thin foil and noise-equivalent temperature difference (NETD) of infrared thermograph for a blackbody. In the latter part of this chapter, this technique was applied to measure the spatio-temporal distribution of turbulent heat transfer to air by employing a high-speed infrared

Spatio-Temporal Measurement of Convective Heat Transfer Using Infrared Thermography