Didier Defer

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.

Non-destructive testing of a building wall by studying natural thermal signals

Abstract The behaviour of civil engineering works (structures, buildings, dams, etc.) in time is a current problem which is the subject of deep consideration and numerous research projects. These studies — which are aimed at adopting a better approach to repair, maintenance and reinforcement operations — have revealed a significant need for the development of means to diagnose and monitor structures. Many non-destructive testing techniques already exist but a major difficulty in applying them arises from the fact that they are not universal. It is therefore necessary to analyse their limits and define fields of application. Choosing a suitable technique is always a delicate process. In addition, the results obtained are generally affected by a considerable degree of uncertainty; cross tests using different techniques make it possible to improve the quality of the diagnosis. Thermal approaches are currently emerging and being developed quickly. They are typically based on infrared thermography measurements. These techniques involve a contact-free analysis and provide overall information on the structure. They are adapted to a qualitative type of research in which the prime objective is to highlight anomalies. However, it is generally complicated and difficult to make a quantitative interpretation of the results [1] . This article presents a new thermal method based on the concept of thermal impedance, which can be measured at the surface of a structure. It is adapted to a local quantitative analysis and should be used as a complement to the overall measurements taken by infrared thermography to quantify and refine the analysis.

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.

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.

Laser Scanning Vibrometry and Holographic Interferometry Applied to Vibration Study

The applications of high performance materials in the aerospace and in the automotive technology in the next years need to develop new vibration study, nondestructive testing, predictive maintenance and industrial control methods.The Laser Scanning Vibrometry and Holographic Interferometry methods of vibration study and nondestructive testing by modal analysis are described. The Laser Scanning Vibrometer PSV 400 is made by Polytec GmbH and the PSV software reconstructs the 3D model of the measured micro-deformation of the object. The holographic laser system HLS-3 from Lumonics Inc. has 100 MW ruby laser peak power and 30 ns pulse width.Using mechanical excitation to induce a measurable vibration, the Laser Scanning Vibrometry and Holographic Interferometry modal analysis measurements show up the vibrational signatures and the damaged areas of the objects made by high performance materials - polymers, composites.

Optimization of District Heating Consumption Using Random Heating Scenario Generator

This paper proposes a new method to optimize the control of the heating system of a district in a way that fulfills the thermal regulations and comfort in the whole district from one side and guarantees the minimization of the energy consumption (and bill) from the other side. This new method depends on coupling buildings with different functionalities and needs. A heating plan for the coupled entities will be generated afterwards. This plan makes advantage of the difference in occupation timing of the buildings and has as constraints the thermal comfort of the occupants and the maximum available energy. This heating plan takes into consideration the building inertia to design the supply plan and maintain the constraints.