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Energy consumption in the building sector has become a global concern. Among the most energy-intensive equipment: air conditioning units, especially in hot countries. This paper investigates this rise of energy consumption for a case study of a laboratory located at the National Engineering School of Tunis, Tunisia. The key idea of this paper is to look for an optimal solution to reduce the energy consumption of the air conditioner while ensuring occupant’s comfort. For this purpose, firstly a laboratory dynamic thermal model is designed. Then, a Hysteresis control is adopted to ensure thermal comfort and to maintain the ambient temperature within a range predefined by the user. To achieve an outcome compatible with expectations, the thermal transfer through laboratory wall’s envelope constituents, considering loads effects on air conditioner functioning and, meanwhile, the energy balance of the system are analyzed. Then, a comparative study of different solutions to improve the energy consumption of the laboratory was also carried out. The results indicate that the use of double-glazed windows is the most suitable. The window model used guarantees an energy gain of 27% if it is used only for the windows of the east wall and an additive gain of 30% if it is used for all the openings. Otherwise, if an external insulation is adopted, the energy gain is negligible. The proposed model is implemented in Matlab / Simscape and results are validated by the experimental measurements provided by a network of temperature sensors and transmitted to cloud service.

The present work presents a numerical study of the evaporation of a sessile liquid droplet deposited on a substrate and subjected to different heating configurations. The physical formulation accounts for evaporation, the Marangoni effect, conductive transfer in the support, radiative heating, and diffusion–convection in the droplet itself. The moving interface is solved using the Arbitrary Lagrangian–Eulerian (ALE) method. Simulations were performed using COMSOL Multiphysics. Different configurations were performed to investigate the effect of the heating conditions on the shape and intensity of the Marangoni circulations. A droplet can be heated by the substrate (different natures and thicknesses were tested) and/or by a heat flux supplied at the top of the droplet. The results show that the Marangoni flow can be controlled by the heating configuration. An upward Marangoni flow was obtained for a heated substrate and a downward Marangoni flow for a flux imposed at the top of the droplet. Using both heat sources generated two vortices with an upward flow from the bottom and a downward flow from the top. The position of the stagnation zone depended on the respective intensities of the heating fluxes. Controlling the circulation in the droplet might have interesting applications, such as the control of the deposition of microparticles in suspension in the liquid, the deposition of the solved constituent, and the enhancement of the evaporation rate.

IF=1.787; International audience; This work presents a study on the evaporation of a volatile liquid droplet placed on a substrate and surrounded by air. We consider in this work the case of a pure liquid droplet (water, ethanol and butanol), and the case of a binary mixture droplet (water-ethanol and water-butanol). The aim of this work is to study the effect of the volatility of each component within a pure or binary droplet on the evaporation kinetics. The volatility of a liquid is considered as the ability of a substance to evaporate. One liquid is more volatile than another is the one with the lowest boiling temperature. We noticed that for pure liquid droplets, the volatility of the liquid considerably influences its life and cooling effect. After evaporation, the more volatile the liquid, the shorter the life of the droplet and the greater its cooling. The results of the simulation were compared and validated by the literature. Experimental results are used for validation and interpretation of the evaporation of water-butanol binary droplet. The behavior of the butanol in the mixture is different to that when it is pure. Despite its low volatility compared to water, strongly diluted in a water-based droplet, butanol evaporates in a preferential way. In fact, in the diluted mixture, the fraction of butanol evaporates first and is exhausted before that of water. The binary droplet tends to become a pure water droplet. We also show that, the same as in the case of the water-ethanol, the addition of butanol in water accelerates evaporation and decreases the droplet’s life.

This study is an experimental investigation of the oceanic environment effect on a plate heat exchanger performance. Indeed, an experiment was carried out on a single plate of the exchanger to generate a turbulent airflow in which fine water droplets were injected into a horizontal vein where a heated plate was placed. The experimental tests were conducted for different air velocities and various water concentrations of freshwater and saltwater. In fact, two plate forms were considered: The first one is flat while the second is corrugated. Three main facts were observed in this work: (i) the correlations linking the heat transfer rate to the operating conditions, (ii) the effect of fog addition and the plate form on convective heat transfers, and (iii) the impact of the formation of a salt layer on the surface of the plate in the case of salt water.

International audience; As part of the research in the field of thermal control of electronic components, we confined a phase change material (plastic paraffin) in a liquid and we heated it on one vertical side by a hot plate. The presence of the liquid around the phase change material (PCM) prevents the direct contact between the hot plate and the PCM (increases the lifetime of the PCM by reducing overheating zones). It improves heat transfer by increasing the thermal conductivity around the PCM (increases the thermal exchange surface) and by accelerating the convective transfer (it starts very quickly without waiting for the beginning of melting). In this work, we examine experimentally and numerically the effect of the choice of liquid on the PCM and on the heating plate. In this paper, water and Siloil M40.165/200.10 are employed around the phase change materials and a comparative study of some important results examining the melt front, total melting time, flow direction and thermal behavior is conducted to investigate which liquid is the most optimal to use. Finally, it has been found that the thermal properties of water (especially heat capacity) seem to be the most interesting for a thermal protection role.

International audience; In this work, we present an experimental study of the evaporation of oil drops deposited on both textured and smooth silicon substrates at two different temperatures (20 °C and 270 °C). We show that the sessile drops take a hexagonal form and are linked by an oil film (a droplet sitting on a mixture of solid and liquid). This wetting regime represents the Wenzel-like regime. We are particularly interested in the propagation of the oil film on the textured surface over time. The effect of the surface fraction of the micro-textures ∅ and the temperature of the substrate Th on the film propagation were also studied. We reveal that the spreading length of the oil drops increases as ∅ decreases and Th increases. We also demonstrate that the textured surface favors oil drop evaporation.

International audience; A numerical simulation using the commercial software “COMSOL Multiphysics” was carried out to study the flow around two fixed spheres filled with a phase-change material (PCM). This work focuses essentially on the process of fusion of the two interactive spheres located at different distances and on the heat exchange with the air flowing around two capsules. Two dispositions were examined: the first for two aligned capsules, while the second for two nonaligned ones. The separation distance d between the two capsules ranged from 2R to 8R. The influence of the inter-capsule distance on the thermal behavior of the system was demonstrated. The results show that the interaction rate between the capsules decreases as a function of the separation distance and that there is an optimum distance d from which each capsule behaves independently of the other. This distance is equal to d=6R for the first case and d=3R for the second case. It was noted that the local exchange coefficients’ variation on the surfaces was influenced by the effect of the distance between the two capsules.

International audience; In this work we focus on the study of the acoustic and mechanical behavior of compressed earth blocks (CEBs). The aim was to study the influence of compaction pressure on the compressive strength and intrinsic acoustic parameters influencing sound absorption of these materials (porosity, tortuosity, airflow resistivity, viscous characteristic length). Specimens made by varying the applied compaction pressure and therefore having various bulk densities were studied. Low bulk density CEBs where stabilized by adding 15% cement. The acoustic absorption coefficients of the different specimens were determined experimentally employing data obtained using the Kundt tube. The intrinsic acoustic parameters were identified by minimizing the discrepancies between the experimentally measured absorption coefficient (α) and the theoretical one modeling the CEBs using the equivalent fluid model. The results showed that the acoustic and mechanical behavior of CEBs were strongly influenced by the applied compaction pressure including, inter alia, the bulk density of the specimen and the added cement used as stabilizer.

International audience; Water spraying is widely being used in many industrial applications because of the benefits it has shown over cooling. This paper presents a numerical investigation to simulate evaporating water spray along a plate of an exchanger. Indeed, we were interested in a horizontal channel whose lower wall is the plate of the exchanger maintained at a heating flow. This plate is exposed to a turbulent airflow in which water droplets were injected. The equations governing the continuous phases (air) and the dispersed phase (water) were developed. These equations were solved using Comsol Multiphysics. A comparison of the simulation results and those of the experiment reveals an acceptable concordance. Therefore, the numerical results present the thermal behavior by studying transversal and longitudinal evolution of temperature. Moreover, the maximum evaporated water flow (mmaxew) and the plate temperature are investigated for several physical parameters of the continuous phase, such as velocity, relative humidity and inlet temperature. The results show that these parameters have significant impact on the cooling of the plate. In addition, mmaxew witnesses a remarkable decrease by increasing relative humidity. However, increasing air velocity and inlet air temperature could improve mmaxew.

Water spraying in exchanger systems is widely used to allow cooling and improving their performance. However, transfers within the spray mixture are difficult to express because the combined mass and heat are transferred between phases, which complicates the design of the spray systems. This article presents a numerical study of the influence of water volume fraction on the distribution of the temperature in a canal. A 2D numerical model of a horizontal channel was generated and the equations governing the continuous phases (air) and the dispersed phase (water) were developed. These equations were solved using Comsol multiphysics. A comparison of the simulation results and those of the experiment reveals an acceptable concordance.

A numerical study of a solar desalination system based on the humidification-dehumidification thermal process was carried out. Air humidification is carried out in the evaporator by evaporating saline water. Air dehumidification takes place in a condenser where the distilled water is recovered by cooling the moist air coming from the humidifier. To achieve proper dehumidification the countercurrent configuration is considered. Energy needed to evaporate water is provided by solar energy. This study was conducted using a one-dimensional mathematical model to simulate heat and mass transfers that occur in the evaporator and the condenser. The proposed model is based on energy and mass balances of moist air, cooling water and saline water film. The resulting differential equations of ordinary derivatives are solved using the finite difference method in an iterative scheme. The mathematical model is validated against experimental and numerical results. The effect of feed saline water and air temperature at the solar evaporator entrances as well as its insulation thickness on the distillated water production of the desalination unit is studied.

Compressed earth blocks (CEB) were used for the construction, always and until now systematically with a bulk density of about 1800 and 2100 kg m −3 . These earth construction materials have a relatively high thermal conductivity of about 1.1 W m −1 K −1 . This study aims to obtain lightweight CEB by varying their bulk density in order to modify their porosity and consequently to act on their thermal conductivity. The approach adopted here involves an experimental study of the influence of compaction pressure on the bulk density of the compressed earth block (CEB) and its effects on their thermal performance (thermal conductivity and thermal effusivity) and mechanical properties (compressive strength and modulus of elasticity). Results show that bulk density has a strong influence on thermal and mechanical behavior of CEB. The decrease in the bulk density of CEB was accompanied by a significant reduction in their thermal conductivity and their thermal effusivity. This variation in thermal properties as a function of bulk density is performed linearly. However, the bulk density should not decrease under certain minimum values below which the CEB lose their cohesion and compressive strength.

International audience; This work addresses the recovery of a liquid effluent by thermal evaporation. It is a digestate of pig manure, rich in N, P, and K (Granier 1993; Latimier 1996; Levasseur, Composition et volume de lisier produit par le porc: Données bibliographiques. TECHNI 21(4):16–19, 1998). The effluent was subjected to anaerobic digestion and phase separation by centrifugation. Several recovery scenarios for the liquid phase of a waste with 2.3% DM (Dry Matter) are analyzed.Because of the high consumption of fossil energy in the world, we analyze in this work the evaporation of this effluent using solar energy, in order to concentrate N, P, and K elements. The device consists of a stainless steel plate with a tilt angle of 30° and is not covered with glass. The liquid circulates as a film on the steel plate and is exposed to a 6000-W solar simulator.This work is divided into two primary parts. The first part is the thermophysical characterization of the effluent comparing to water. In the second part, evaporation tests with liquid effluent were performed. So, experimental tests will be valid by determining the equations of heat and mass balances on the plate and on the film (Bouchekima 2001; Chen 1986; Brau, Support de cours de convection pour 3 GCU. Insa de Lyon, département de Génie Civil et Urbanisme, 2006).Before starting the tests, the plate is exposed to solar radiation until stabilization of its temperature. A validation of the temperature of the plate, depending on solar flux and temperature of air, in laminar regime, was performed (Huetz-Aubert, Revue Phys Appl 17:251–260, 1981). We find that when the solar flux increases, the temperature of the plate increases. However, if solar flux is constant, the temperature of the plate decreases when the air temperature increases.The liquid circulates as a film flowing from the top of the plate. Local heat flux measurements, temperatures, and evaporated flow for input flow rate from 1 to 5 g/s are determined and compared with experimental results.

International audience; This work presents a numerical study on the thermal behavior of a PCM heat sink for the purpose of electronic cooling. Introducing copper fins inside a PCM heat sink was examined. A parametric study was performed to maximize the critical time elapsed before reaching the allowable temperature limit for different fin lengths, numbers, and thicknesses for the same copper amount. This amount was also evaluated as a perimeter envelope of the PCM. The enthalpy method was carried out. Natural convection in melted PCM was taken into account. The governing equations were solved by Comsol Multiphysics. This model was validated by comparing results with numerical data by Huang et al. (Int J Heat Mass Transf 47:2715–2733, 2004). The results indicated that the inclusion of fins can enhance the thermal performance of heat sink by increasing the exchange surface and ensuring better heat repartition inside the PCM. The fin geometry presented an important role in thermal control improvement. Although a significant difference was showed in temperature between the copper envelope and the copper fins, they present the same efficience for low heat flux.

International audience; This paper presents a numerical and experimental study of a turbulent flow of air in a T-bifurcation. This configuration corresponds to a stator containing radial vents oriented vertically to the rotor–stator air gap in electrical machines. Our analysis focuses on the local convective heat transfer over the internal surface of the vents under a turbulent mass flow rate. To model the cooling installation in this region, computational fluid dynamics simulations and an experiment using particle image velocimetry (PIV) are performed. The resulting flow generally produces recirculation zones in various channels. The effect of the flow ratio and diameter of the bifurcation on the dynamic and thermal behavior of the flow is also examined. In this study, we apply a numerical approach based on the k–ω shear stress transport (SST) turbulence model (using the commercial software, “comsolmultiphysics”) to numerically solve the Navier–Stokes equations and energy equation of the system under consideration. We describe the different hypotheses necessary to formulate the equations governing the problem, initial conditions, and boundary condition. The velocity in the bifurcation calculated using the simulation is compared with that obtained by the experiment and it reveals a good agreement. The effect of the branch diameter of the bifurcation and flow ratio on the heat transfer is specifically analyzed in this research work.

This study describes the development of an experimental setup that tracks the evolution of the melting and freezing fronts of a Phase Change Material (PCM), in this case paraffin. The results obtained enable the examination of the shape and movement of the melting front of the PCM. Two modes of heat transfer were identified during the melting process: conduction when melting began and natural convection, which becomes dominant in the remainder of the cycle. Monitoring of the melt over time shows that the melt fraction, expressed as the ratio of the molten volume and solid volume, is proportional to the difference between the imposed temperature and the melting temperature. Experimental results confirm the linearity proposed by other researchers.

This work addresses the recovery of a liquid effluent by thermal evaporation. It is a digestate of pig manure, rich in N, P, and K (Granier et al. 1995; Latimier et al. 1996; Levasseur, TECHNI 21(4):16–19, 1998). The effluent was subjected to anaerobic digestion and phase separation by centrifugation. Several recovery scenarios for the liquid phase of a waste with 2.3% DM (dry matter) are analyzed.

The present work represents a numerical study of a solar desalination system that operates on the humidification-dehumidification (HDH) principle. This system consists essentially of a humidifier integrated into a solar collector and a tubular condenser. A mathematical model governing evaporation and condensation in the desalination unit has been elaborated. This model is based on energy and mass balances at the humidifier and the condenser. The work consists essentially of a parametric study to improve the desalination unit production installed at the faculty of sciences of Bizerte, Tunisia.

International audience; This paper presents an experimental investigation of phase change material (PCM, Plastic paraffin) behavior in a rectangular enclosure with three discrete heat sources flush-mounted on the right vertical wall. The remaining walls of the cavity are adiabatic. Maximizing the critical time (time required by one of the electronic components before reaching the critical temperature) is the global objective of this study. Conserving constant the total flux, the thermal performance of system is examined by different heat flux repartition between sources. Also, the intermittent regime is studied for different heat flux cycle. The results show that the thermal performance depends strongly on the heat flux repartition and the maximum heat transfer is seen for the lower source. Placing the greater part of heat flux at middle or lower source seems the best manner to extend the critical time. For intermittent regime, it is concluded that fractionating cycle length into 4 or 8 cycles can keep plate temperature under critical condition for respectively 3 and 5 times comparing to one cycle.

International audience; The objective of this work was to analyze the influence of compaction pressure on the intrinsic acoustic parameters (porosity, tortuosity, airflow resistivity, viscous and thermal characteristic lengths) of compressed earth blocks through their identification by solving an inverse acoustic wave transmission problem. A low frequency acoustic pipe (60-6000 Hz of length 22 m, internal diameter 3.4 cm) was used for the experimental characterization of the samples. The parameters were identified by the minimization of the difference between the transmission coefficients data obtained in the pipe with that from an analytical interaction model in which the compressed earth blocks were considered as having rigid frames. The viscous and thermal effects in the pores were accounted for by employing the Johnson-Champoux-Allard-Lafarge model. The results obtained by inversion for high-density compressed earth blocks showed some discordance between the model and experiment especially for the high frequency limit of the acoustic characteristics studied. This was as a consequence of applying high compaction pressure rendering them very highly resistive therefore degrading the signal to noise ratios of the transmitted waves. The results showed that the airflow resistivity was very sensitive to the degree of the applied compaction pressure used to form the blocks.

IF=2.624; International audience; The thermal control of electronic components is aimed at ensuring their use in a temperature range compatible with their performances. This paper presents an experimental study of the behavior of phase change materials (PCMs) as the cooling system for electronic devices. Four configurations are used to control the increase in the system temperature: pure PCM, PCM in a silicone matrix, PCM in a graphite matrix and pure PCM in a system of fins. Thermo-physical properties of different PCMs are determined and found to be desirable for application in this study. Solid liquid interface visualization and temperature evolution are employed to understand the mechanism of heat transfer during the different stages. Results indicated that the inclusion of PCM can lower component increase temperature and extends twice the critical time of the heat sink. The use of Graphite matrix filled by PCM showed more improvement on system thermal performance than silicon matrix. Also, for the same fraction of copper, it was found that incorporating long copper fins with suitable spacing into PCM, can enhance heat distribution into PCM leading to longer remain component temperature below the critical limit. This work therefore shows that the combination of PCM and long, well-spaced fins presents an effective means for thermal control of electronic devices.

One of the challenges to reduce energy consumption in the construction sector around the world is the development of appropriate construction materials. Due to its thermal properties, gypsum plaster is one of these materials. Tunisia ranks second worldwide for the production of gypsum. It used almost exclusively for decorative purposes although it has interesting thermal properties. This study deals with the thermal characterization of Tunisian gypsum plaster from Meknassi region. Particular attention is paid to the effect of temperature and water content on the thermal conductivity. The plaster thermal conductivity vs. temperature exhibits an interesting behavior for buildings walls. Thermal analysis by DSC was conducted to explain this behavior.