Mustapha Karkri

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).

Thermal and mechanical properties of maize fibres–high density polyethylene biocomposites:

In recent years, considerable attention has been given to the development and utilization of natural fibres. This study examines the thermal properties of maize thermomechanical fibre reinforced high density polyethylene composites with competitive mechanical properties. The composites were produced by six different steps, namely: drying, cutting, mixing, compounding, pelletizing and injection moulding. Composite samples with fibre contents in the range 10–40 wt% were chosen to observe their effect on thermal and mechanical properties as the fibre content was increased. Measurements of thermophysical properties were obtained using periodic temperature ramp method. The material characterization was performed on a temperature range that extends from −20℃ to 120℃. It was found that the thermal conductivity and diffusivity of the composites decrease with fibre loading. The results showed that when the temperature is increased, a significant increase of both thermal effusivity and the factor ( ρ c Cp c ) was observed. A high-quality dispersion and adhesion of maize fibre in the high density polyethylene matrix was indicated by scanning electron microscopy. Good mechanical performance of the obtained composites was established considering the stress transfer at fibre–matrix interface.

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.

Improvement of thermal conductivity of paraffin by adding expanded graphite

This paper investigated the use of graphite with different configuration designs to improve the thermal energy storage of phase change material systems. Two types of graphite have been combined with paraffin in order to improve thermal conductivity of phase change material: synthetic graphite (Timrex SFG75) and graphite waste obtained from damaged tubular graphite heat exchangers. Paraffin/graphite phase change material composites have been prepared by the cold uniaxial compression technique. Their morphologies have been observed and analyzed by scanning electron microscope, and their thermophysical properties have been estimated using new experimental tools. Results show that the thermal conductivity and thermal diffusivity can be accurately measured by these new experimental tools. Moreover, results highlight the fact that the phase change material thermal properties are greatly influenced by the graphite addition.

Estimation of radiative and conductive properties of a semitransparent medium using genetic algorithms

The aim of this work is to simultaneously identify the conductive and radiative parameters of a semitransparent sample using a photothermal method associated with an inverse problem. The identification of the conductive and radiative proprieties is performed by the minimization of an objective function that represents the errors between calculated temperature and measured signal. The calculated temperature is obtained from a theoretical model built with the thermal quadrupole formalism. Measurement is obtained in the rear face of the sample whose front face is excited by a crenel of heat flux. For identification procedure, a genetic algorithm is developed and used. The genetic algorithm is a useful tool in the simultaneous estimation of correlated or nearly correlated parameters, which can be a limiting factor for the gradient-based methods. The results of the identification procedure show the efficiency and the stability of the genetic algorithm to simultaneously estimate the conductive and radiative properties of clear glass.

Comparison of experimental and simulated effective thermal conductivity of polymer matrix filled with metallic spheres: Thermal contact resistance and particle size effect

In this paper, we present a numerical and experimental study of a composite material with conducting spheres embedded in a polymer matrix. Our main objective is to study how the particle size and thermal contact affect the overall thermal properties of composites. In the numerical study, finite elements method is used to model heat transfer and to calculate the effective thermal conductivity. A periodical method was used to measure simultaneously thermal conductivity and diffusivity of the composite consisting of an epoxy resin matrix filled with brass spheres of different sizes. A comparison between the numerically calculated, measured and analytical thermal conductivity values for various samples is performed and the significance of the findings is discussed in the paper.

A numerical and experimental study on the effective thermal conductivity of conductive hollow tube composite

In this article, a three-dimensional (3D) finite elements method has been developed for predicting the effective thermal conductivity (ETC) of a conductive hollow tube polymer composite. 3D Representative Volume Element (3D-RVE) was used to represent the composite material. Governing heat transfer equations in both transverse and longitudinal directions for predicting the effective thermal conductivities of composites are used. ETC was numerically calculated using COMSOLTM software. The guarded hot plate method was used to measure the composites conductivities consisting of epoxy resin matrix filled with metallic hollow tube. A comparison between the numerically calculated thermal conductivities, measured and analytical ones for various samples was made. A satisfactory agreement between numerical and experiment takes place.

Smart macroencapsulated resin/wax composite for energy conservation in the built environment Thermophysical and numerical investigations

This study reports the results of experimental and numerical investigations on the thermophysical properties and the process of melting of a phase-change composite material. The proposed phase-change composite material based on epoxy resin with spherical shape paraffin wax (RT27) was used as a new thermal storage system. Thermal characterization was performed using a transient guarded hot plate technique. The results revealed the importance of thermal storage by latent heat. The numerical analysis is realized using numerical COMSOL® Multiphysics 4.3b. The effect of various parameters of the numerical solution on the results is examined: in particular, the term describing the mushy zone in the momentum equation and the influence of temperature melting range. The findings of the experimental investigation compare favorably with the numerical results.

Different Phase Change Material Implementations for Thermal Energy Storage

This paper presents the principal methods available for phase change material (PCM) implementation in different storage applications. The first part is devoted to a non-exhaustive overview of the various chemical processes used to develop stable PCM (such as microencapsulation, emulsion polymerization or suspension polycondensation, polyaddition, etc.) based on the available literature. The second part deals with shape-stabilized PCM, developed from an intimate combination of a polymer matrix and a phase change element. Materials able to include more thermal energy as usual ones are interesting as they increase the thermal inertia of the system that presents by this way advantages. The energy efficiency of buildings may be improved including PCMs that store and provide enthalpy from one hand and without any significant temperature modification during the phase change process on the other hand. If the solid phase of the PCM does not present any problem, it is not the same for the liquid phase which must be maintained mechanically at its assigned location. Furthermore, the PCM in the solid (and furthermore in the liquid phase) does not have mechanical properties which allow to use it as a structural material able to support charge loads. This paper presents different methods to distribute and maintain the PCM in the thermal solid matrix.