Lock-in thermography as a measurement technique in heat transfer

Abstract

An experimental temperature oscillation technique is described for determining local distributions of the heat transfer coefficient or local distributions of the thermal diffusivity of heat transferring walls. By heating uniformly one surface of the wall with sinusoidally modulated energy a temperature oscillation is generated which results in a wavelike propagation behavior of heat flow and temperature within the wall. The characteristic of the temperature oscillations at both faces of the wall depends directly on the local heat transfer conditions and the thermal diffusivity of the wall material. So the local values of the heat transfer coefficient or the thermal diffusivity can be calculated from the measured amplitudes or from the phases of the temperature waves at the surfaces. To demonstrate the applicability of the method first experiments were performed. The measured results agree reasonably well with data obtained from literature. 1. Introduction Because of their simplicity and accuracy temperature oscillation techniques obtained an increasing importance for measuring purposes in heat transfer. The basic idea of these techniques is to supply modulated energy to the testing object which results in a wavelike propagation behavior of heat flow and temperature within the object material. By measuring and analyzing the temporal and spatial propagation behavior of the thermal waves, numerous thermophysical parameters of the testing object can be determined. With an earlier developed method [1] for measuring the local heat transfer coefficient or the local thermal diffusivity periodic temperature oscillations are optically generated at one spot of the object surface by periodic heating with a focused laser beam. This method was then refined by Wandelt [2] to achieve a much higher accuracy. Measurements which cover the whole object surface have to be performed point by point in a rasterlike fashion. At each pOint one has to wait a minimum time, until the local stationary state is achieved. So measuring times can be long which might be not acceptable for practical applications. The technique presented overcomes the restrictions of the method described above. Local distributions of the heat transfer coefficient as well as local distributions of the thermal diffusivity can be determined for the whole testing object within a short time. 2. Measurement principle The measurement principle is illustrated in figure 1. Sinusoidally modulated thermal energy is supplied uniformly to the whole surface of the object under consideration. From the surface a nearly plane thermal wave propagates into the material after the initial transient behavior. The local amplitudes and phases of the temperature waves at both sides of the wall depend on the properties of the wall, the frequency of the oscillation and the heat flux to the surrounding. By measuring the local temperature oscillations at each surface point and by evaluating the local phases and amplitudes of these oscillations, a phase angle image and an amplitude image can be extracted for the whole surface. In principle, a map of the local heat transfer coefficient or a map of the locally varying thermal diffusivity can now be calculated from the data of both the images. For monitoring and recording thermal waves a rapid infrared scanning device (AGEMA THV900LW/ST) is used, which allows a non-contacting measurement of the small wall temperature changes with high thermal, spatial and time resolution. The thermal information from all scanned pOints are analyzed with a PC.