Emmanuel Antczak

Characterisation of the thermal effusivity of a partially saturated soil by the inverse method in the frequency domain

The purpose of this article is to present a method for identifying the apparent thermal effusivity of a wet soil. This thermal magnitude is indeed fundamentally important, as it fully characterises the behaviour of the system provided it is possible to make the assumption of a semi-infinite medium. The condition is very frequently encountered while studying heat transfers in soils. The apparent thermal effusivity of a soil is also particularly sensitive to the water content of the material. Because of this property, it is possible to evaluate the instantaneous water content of the medium from a correlation law linking thermal effusivity with water content.

Measurement of low-thermal effusivity of building materials using thermal impedance method

Thermal impedance is a way of defining the characteristics of thermal systems. It is a function that represents the relation between the frequency components of temperature and the flux density in a plane for each frequency. Up to now, its use has been restricted to one-directional conductive systems. From the experimental point of view, it is determined simply by measuring the flux density and temperature simultaneously in a measurement plane. In practice, a fluxmeter in which a thermocouple has been placed is put in contact with the sample. The changes in flux density and temperature measured in this way are different from those in the material access plane. The reasons for this perturbation are the presence of the sensor and the sensor/material contact resistance. In the case of slow changes, due, for example, to micro-climatic variations or day/night stresses of the order of 10-5 or 10-4 Hz, this perturbation is negligible. Studies in these frequency ranges have been exploited in several works. In the present study, we show that it is possible to use thermal impedance as a way of characterizing thermal systems for higher frequencies, taking into account the perturbation created by the measuring instruments. By means of a sensitivity study, we demonstrate several cases linked with the nature of the test material. A frequency range is determined where the perturbation due to the measuring instruments is not too great, allowing the materials to be characterized. Several common construction materials are studied. Particular emphasis was laid in this work on characterizing insulating materials, which are hard to study in variable conditions. The tests discussed in this article were performed in the laboratory in ambient temperature conditions close to 20 °C. The pseudo-random stresses were generated artificially. Series of 100 tests were run for each material. They led to the determination of thermal effusivity with less than 5% error. The method gives results that are reproducible and can be validated by simulation.

Formalism of thermal waves applied to the characterization of materials thermal effusivity.

Thermal characterization of materials, especially civil engineering materials, in the way of non-destructive methods, are more and more widespread. In this article, we show an original point of view to describe the used method, the thermal waves, to obtain the thermal impedance of the studied system, using a specific sensor--a fluxmeter. The identification technique, based on a frequential approach, is optimized by applying a random input to the system. This kind of random heating is shown to provide a frequency range where the thermal effusivity is able to be identified and not correlated to another parameter. The strength of the method is also the determination of the contact resistance of the system, that allows to validate the identification process. Experimental results obtained from a sample with well-known thermal properties (polyvinyl chloride) are used to validate the proposed method.

In situ characterization of thermophysical soil properties—Measurements and monitoring of soil water content with a thermal probe

In a period of surging energy prices, resource depletion, and concerns over the use of nuclear power, energy savings are paramount and a major component of ongoing sustainable development. Geothermal energy is the energy stored in the form of heat beneath the surface of the Earth. Related to this, the thermal properties of soils are of great importance, particularly with regard to the modern trends of utilizing the subsurface for transmission of either heated fluids or high power currents. For example, in geothermal hydrology or geotechnical engineering applications, the thermal conductivity must be determined to assess the energy potential of the soil. The presence of water (groundwater, rainfall, natural moisture) improves both the thermal conductivity and thermal capacity fields. We present an original method—based on a thermal study and the use of non-integer order models—to determine the thermophysical parameters of different soils in near-surface layers, and link them to the water content variations...

Thermal characteristics in situ monitoring of detached house wall constituted by raw clay

ABSTRACT The recent evolutions of the thermal regulation and the conclusions brought in the Grenelle environment forum encourage a part of the housing builders to be inspired by the traditional housing environment. The ecological impact of materials, for manufacturing, use and destruction, is more and more taken into account. We find in this way the brick of raw clay, the brick of straw clay or of wooden shavings to build walls of detached houses. The conception is also reanalyzed to favour thermal inertia, phase displacement and energy storage in the structure. The difficulty remains the sensibility of this type of material opposite humidity, even if their role of hydrous regulator can be also noted.

Understanding the Dynamic and Static Thermal Transfer in Brick Walls

The aim of this paper is to study the thermal heat transfer through a 33 cm brick wall, typical of old houses in Lille, a northern French town. First, the wall was studied in a steady state case in order to determine its equivalent resistance using the electrical analogy. Then, the wall is replaced by an equivalent homogeneous wall in order to compare the 1D and the 3D thermal transfer. The results show a perfect consistency between the two models, representing a big advantage when other layers are added to the model like thermal insulation and facing.

Numerical and experimental investigation of heat and mass transfer within bio-based material

In this paper, the coupled heat and mass transfer within porous media has been studies. First, the studied materials have been characterized experimentally and than evaluated their thermal properties, namely thermal conductivity and specific heat in different states (dry-wet). The hygroscopic properties, namely water vapour permeability, water vapour sorption. At second time, we present and validate the mathematical model describing heat and mass transfer within bio-based materials, by the confrontation with the experimental results. The materials properties obtained from the characterisation part are used as model’s input parameters. Moreover, a test facility is mounted in the laboratory in order to compare the numerical and experimental data. The founded results show a good concordance between the simulated and measured data. According to this results the mathematical model of Philip and de Vries gives a good prediction of hygrothermal behaviour of biobased material. This model will allow us to save money and time of the experimental part in the future.

Development of a Cylindrical Probe Designed for Thermal Characterization of Granular Materials

This article describes a new thermophysical characterization method for granular materials. It is based on the creation of a cylindrical probe, fitted with peripheral flux and temperature sensors. These sensors record changes in thermal variables in the case of cylindrically symmetrical heat diffusion. A numerical exchange model integrated into an inversion algorithm identifies the thermal effusivity and diffusivity of the material being tested and the sensor/material contact resistance. In this first study, the method was applied to dry sand.