The Question of Thermal Waves in Heterogeneous and Biological Materials

There have been two recent experimental studies that have sparked a renewed interest in thermal wave behavior at the macro-scale level. Both reported thermal relaxation times of 10 seconds or higher. However, no further experimental evidence of this behavior has been reported. Due to the extreme significance of these findings, the objectives of this study were to try to reproduce these earlier studies and offer an explanation for the outcome. These two previous studies were repeated following the experimental protocol provided in the studies as closely as practically possible. First, studies on heterogeneous materials were repeated using sand, an ion exchanger, and NaHC0₃. A second set of experiments was conducted on bologna. In both cases, the temperature response to a specified boundary condition was recorded. The results from the first set of experiments suggested that the thermal relaxation times presented in the previous study were actually the thermal lag expected from applying Fourier’s Law taking into account the uncertainty of the temperature sensor. In the second set of experiments, unlike the distinct jumps in temperature found previously, no indication of wave behavior was found. Here the explanation for the previous results was more difficult to ascertain. Possible explanations include problems with either the data acquisition system or the temperature sensors used.