The characterization of thermophysical properties by thermal impedance measurements taken under random stimuli taking sensor-induced disturbance into account

Abstract

The characterization of thermal systems can be performed by using thermal impedance measurements. Experimentally, this method requires simultaneous measurements of the variations in heat flux and the temperature of the measuring surface. The sensor, placed on the system to be characterized, induces a disturbance. For a slow change in thermal characteristics and for low frequencies, the disturbance due to the sensor is negligible. The low-frequency case has been studied in several works, but we show here that it is possible to extend the method to higher frequencies by taking the sensor disturbance into account. A model accounting for sensitivity to parameters shows that in a zone called 'the middle frequency zone' the disturbance is limited to a resistive effect. This zone is studied in this work and concrete was tested under these conditions. Several tests were carried out on the same sample. The contact resistance was modified for each test and the method used led to a stable value of material effusivity.

Frequency Methods Applied to the Characterization of the Thermophysical Properties of a Granular Material with a Cylindrical Probe

“…The flux density and temperature are measured for a single isothermal surface. If a relationship between the thermal units at the output of the kind θ o = Z o φ o is known, we can then write the relationship [34] in the following way: We can therefore define the input thermal impedance as follows:…”

“…The contact resistance between the probe and the material is integrated in the physical model. Previous studies have shown that it could be identified in every test carried out [34].…”

Determination of Thermal Properties in the Frequency Domain Based on a Non-integer Model: Application to a Sample of Concrete

“…Thermal impedance [5] is defined in the Laplace domain as the ratio of the temperature and heat flux Laplace transforms. Thus, Z ( p) can be written as…”

“…7). PRBS [12] are signals easily generated by way of a series of high and low levels defined by deterministic laws; their statistical properties can be simulated as, and can be shown to be equivalent to, random processes [5]. Their spectrum is very rich as a frequency range, the upper limit depending on the rate of change of state and on the maximum number of bits included in the PRBS.…”

Identification of temperature-dependent thermal properties of solid materials

“…This impedance function is also an analogy with electrical or mechanical systems (Defer et al, 1998). Their values are found by the application of the Fast Fourier transform of the data x(t) and y(t).…”