Laurent Ibos

A simultaneous characterization of thermal conductivity and diffusivity of polymer materials by a periodic method

A periodic method is used to determine simultaneously both thermal conductivity and diffusivity of various polymer materials at room temperature. The sample is placed between two metallic plates and temperature modulation is applied on the front side of one of the metallic plates. The temperature at the front and rear sides of both plates is measured and the experimental transfer function is calculated. The theoretical thermal heat transfer function is calculated by the quadrupole method. Thermal conductivity and diffusivity are simultaneously identified from both real and imaginary parts of the experimental transfer function. The thermophysical parameters of several polymer samples (PTFE, PVDF and PA11) with different thicknesses (respectively, 5 mm, 2 mm and 300 µm) were studied and compared with values from the literature. The values identified for the thermal parameters are in good agreement with values from the literature for PTFE and PVDF samples; however, we show that the method reaches its limit for the thinner PA11 sample, owing to inadequacy of the thermal model.

Infrared emissivity measurement device : principle and applications

Infrared emissivity is a necessary parameter for computing models to predict road surface status and pavement temperature irrespective of the weather situation. In this work a new experimental device based on the indirect method was developed for the measurement of surface emissivities. The surface is exposed to modulated isotropic infrared radiation. The intensity reflected by the surface of the sample in a given direction is measured by a detector operating in the spectral range 1–40 µm. This large spectral range allows measurement of the total hemispheric emissivity defined in this case as emissivity. The effect of the temperature modulation frequency, surface composition and surface roughness on the emissivity measurements was investigated. The results show good ability of the device for the determination of emissivities at room temperature.

Complex Interval Arithmetic Using Polar Form

In this paper, the polar representation of complex numbers is extended to complex polar intervals or sectors; detailed algorithms are derived for performing basic arithmetic operations on sectors. While multiplication and division are exactly defined, addition and subtraction are not, and we seek to minimize the pessimism introduced by these operations. Addition is studied as an optimization problem which is analytically solved. The complex interval arithmetic thus defined is illustrated with some numerical examples which show that in many applications, the polar representation is more advisable.

Analysis of uncertainties in thermophysical parameters of materials obtained from a periodic method

In the present work, we are interested in the improvement and the validation of the identification procedure for the simultaneous estimation of thermophysical parameters and their uncertainties using a periodic method. The effect of the statistical uncertainties on the estimation procedure and the reproducibility study of experimental device and measurement method are presented. In addition, the effect of the uncertainties of supposedly known model parameters on the identification procedure was investigated. The results show good reproducibility of the experimental results, and the uncertainty related to reproducibility is negligible. However, conductivity and diffusivity of the copper plate, and heat coefficient exchange, are parameters that strongly affect the result of the identification procedure.

Experimental investigation of a composite phase change material: Thermal-energy storage and release

In recent times, composites made out of polymers and paraffin waxes were thought to be good thermal energy storage materials, in which the heat is stored as latent heat of fusion in the paraffin wax. In this study, phase change composite material with spherical shape calibrated based paraffin wax (RT27) was produced. The properties of the prepared composite phase change material have been characterized. The objective of this article was to study the energy storage and the energy recovery by using a phase change composite material. An experimental set-up consisting of fluxmetric measurement has been constructed to provide the thermal performance of the composite. In addition, a differential scanning calorimetry analysis was carried out. The experimental apparatus allows providing heat storage capacities and “apparent” thermal conductivities of the composite at the solid and liquid states, and also a measurement of the latent heat of fusion. The proposed test provides temperature and heat flux measurements at the material borders. The amount of energy exchanged during the variation of the thermodynamic state samples could be calculated when the boundary temperatures vary. In this article, one shows how it can allow the study of complex composite material with PCM. In particular, heat flux measurements make it possible to highlight very specific behaviors of these products and are thus a very interesting experimental source of data which comes to complete the traditional measurement methods like calorimetric device (differential scanning calorimetry).

A comparative analysis of dielectric, rheological and thermophysical behaviour of ethylene vinyl acetate/BaTiO3 composites

Dielectric, rheological and thermal behaviours of ethylene vinyl acetate/barium titanate sphere (EVA/BaTiO3) composite materials were investigated in this work. Several composites were prepared for different volume concentrations (from 5% to 49%) and for two types of spheres sizes d: d = 9 µm and d = 105 µm. We show that the use of larger particles allows an increase of dielectric permittivity and thermal conductivity, particularly at high filler concentrations. A comparison between the thermal, dielectric and rheological behaviours of the composites is also reported in this paper. A linear dependence of the relative variation of thermal conductivity and dielectric permittivity, on the one hand, and thermal conductivity and Newtonian viscosity, on the other, was shown.

Annealing or Storage Influence on Pyroelectricity of Ferroelectric PVDF and P(VDF-TrFE) Copolymer

Abstract The aim of this work is to study the influence of thermal annealing on pyroelectric properties of PVDF and P(VDF-TrFE) 75–25 copolymer. A strong dependence of pyroelectricity decay upon ageing temperature and time is observed. There is no decay for ageing temperatures lower than +50°C for both polymers. A slower decay of P(VDF-TrFE) 75–25 copolymer pyroelectric properties are is observed for ageing temeperatures T a ≤ +110°C, i.e. lower than the Curie transition point (+128°C). The strong decay of the pyroelectric coefficient observed in PVDF is mainly due to shrinkage effects occuring at the α c -transition. The TSC study performed highlighted that changes of crystallinity occur only for high ageing temperatures (> 110°C).

Effective thermal conductivity of random two-phase composites

This work aims to establish a new three-dimensional method for automatically generating three-dimensional FEM models of two-phase composite materials with complex materials arrangement for the purpose of effective thermal conductivity (ETC) evaluation. Tests were carried out in which the shape, spatial distribution, thermal contact resistance and particles volume fraction were taken into account. Thermal conductivity ratio between both component materials was also varied. The aim of the work was to examine how each variable influences the ETC of the composite material. ETC was calculated numerically using COMSOL™ software and a 3D-representative volume element (3D-RVE) was employed to represent the composite material. The results were analysed and compared to various analytical models and experimental data reported in the literature.

Transport properties of polyester composite reinforced with treated sisal fibers

Thermal conductivity, diffusivity, relative permittivity, specific heat, and tensile properties of sisal/polyester composites were investigated as a function of fiber volume fraction and fiber surface modification. The composites were prepared by resin transfer molding (RTM) technique. Sisal fibers were subjected to various chemical and physical modifications such as mercerization, heating at 100°C, permanganate treatment, benzoylation, and silanization to improve the interfacial bonding with matrix. All treatments used in this study improved the relative permittivity, tensile strength, Young’s modulus, and reduced the elongation at break. The use of NaOH and silane treatments decreased the thermal conductivity. Beside these, a comparison between dielectric, mechanical, and thermal behaviors of the composites was reported in this paper. A linear dependence of the relative variation of dielectric permittivity, Young’s modulus, and thermal conductivity was also shown for untreated systems.

Active infrared thermography applied to defect detection and characterization on asphalt pavement samples: comparison between experiments and numerical simulations

This work is devoted to the application of active infrared thermography to defect detection in pavement structures. The challenge is to localize and to determine some properties of defects (e.g. shape and depth) into a highly heterogeneous material. Experimental work was carried out in laboratory conditions using a pavement sample containing two defects (wood and air). Pulsed thermography results were compared with FLUENT numerical simulations. Different preliminary approaches were investigated to analyze data: singular value decomposition of infrared image sequences, contrast image methods and computation of thermal effusivity considering a heat transfer model in a semi-infinite material. This last method is more sensitive to experimental conditions such as the presence of natural convection at a sample surface. However, all methods allow detection of one defect into the pavement sample.