Kamilia Abahri

Sensitivity analyses of convective and diffusive driving potentials on combined heat air and mass transfer in hygroscopic materials

ABSTRACT The focus of this paper was to examine the contribution of two key mechanisms—moisture convection and diffusion–on heated air and moisture transfer in porous building envelopes and to define the validity of the sub-models. A numerical simulation was performed and is focused on the one-dimensional problem for drying test boundary conditions. Thereafter, a detailed parametric analysis was carried out in order to investigate the influence of typical nondimensional parameters. Results show that convection is a prominent driving potential with respect to the diffusion process when the hygric state is stable between the environment and the envelope.

Periodic homogenization for heat, air, and moisture transfer of porous building materials

ABSTRACT In this work, a macroscopic model of hygrothermal transfers in porous building materials was developed, using periodic homogenization, where the air infiltration was added to the classical mass and energy balance equations written at the microscopic scale. The corresponding infiltration, hygric, and thermal input parameters were carefully identified. Numerical calculations of thermal and diffusion tensors were performed on a representative concrete elementary cell. Further, the diffusion tensor was compared to the equivalent experimental results available in the literature, and its sensitivity to the water content variations and porosity has been evaluated on the concerned elementary cell.

CONVECTIVE AND CONDUCTIVE THERMAL HOMOGENIZATION FOR NONSTURATED POROUS BUILDING MATERIALS: APPLICATION ON THE THERMAL CONDUCTIVITY TENSOR

Porous materials possess a complex structure on a microscopic scale and present strong heterogeneities, which makes their precise study extremely complex. In fact, the macroscopic behavior of these materials is strongly dependent on mechanisms that act to the scale of their components. The present work focus on the development of a macroscopic conductive; and convective fluid heat transfer model, with a heat source in the unsaturated porous materials. This model is established by periodic homogenization of energy conservation equations written on a microscopic scale in each phase (solid, liquid and gas). The resulting input parameters formulations of the sub model were explicitly identified. Numerical calculations of the homogenized thermal conductivity tensor are performed on a representative three-dimensional elementary cell of the porous medium. Finally, a sensitivity study of this tensor depending of the variation of the water content and porosity of the concerned elementary cell has been performed. This sensitivity is required to be considered in simulations to better understand the behavior of building materials and improve the prediction of energy performance.

Intrinsic properties controlling the sustainability of construction

Abstract The main objective of this chapter is to present the important link between a material’s physical properties and its durability. First, the influence of pore structure and the material composition is considered. The diffusion coefficient evolution of cementitious material is also studied taking into consideration the influence of the W/C ratio. The permeability and the effective thermal conductivity of such material are also evaluated in conjunction with the function of porosity, where different correlations are presented. Finally, the liquid vapor interaction inside porous building material is evaluated through the moisture buffer value parameter.

Total Pressure Gradient Incidence on Hygrothermal Transfer in Highly Porous Building Materials

A new experimental investigation is conducted to extract conclusions about the magnitude of moisture transfer due to the total air pressure difference, generally, caused by wind or pressurization by ventilation, on hygroscopic porous wall. A special constructed setup tolerating the creation of low total pressure gradient at different relative humidity conditions is performed. The resulting moisture flux and the hygrothermal state around and within the material are monitored. Then, a characterization of the intrinsic moisture infiltration coefficient is defined. This coefficient is used as an input parameter in simulation models. The experimental procedure points out diverging conclusions between different testing materials (wood fibrous insulation, OSB and aerated concrete). In fact, the amount of moisture infiltration is completely dependent on both combination of total pressure gradient value and hygrometry of the specimen. It has an important contribution on the moisture transfer, in walls, relatively to that induced by vapor diffusion phenomenon.

Multiscale modelling for better hygrothermal prediction of porous building materials

The aim of this work is to understand the influence of the microstructuralgeometric parameters of porous building materials on the mechanisms of coupled heat, air and moisture transfers, in order to predict behavior of the building to control and improve it in its durability. For this a multi-scale approach is implemented. It consists of mastering the dominant physical phenomena and their interactions on the microscopic scale. Followed by a dual-scale modelling, microscopic -macroscopic, of coupled heat, air and moisture transfers that takes into account the intrinsic properties and microstructural topology of the material using X-ray tomography combined with the correlation of 3D images were undertaken. In fact, the hygromorphicbehavior under hydric solicitations was considered. In this context, a model of coupled heat, air and moisture transfer in porous building materials was developed using the periodic homogenization technique. These informations were subsequently implemented in a dynamic computation simulation that model the hygrothermalbehaviourof material at the scale of the envelopes and indoor air quality of building. Results reveals that is essential to consider the local behaviors of materials, but also to be able to measure and quantify the evolution of its properties on a macroscopic scale from the youngest age of the material. In addition, comparisons between experimental and numerical temperature and relative humidity profilesin multilayers wall and in building envelopes were undertaken. Good agreements were observed.

Numerical analysis of heat, air, and moisture transfers in a wooden building material

The present paper aims to predict the hygrothermal behavior of massive wood panel considered as bio-based building material. In this context, we developed a macroscopic model coupled no linear heat, air, and moisture transfers that incorporates simultaneously the effect of thermal diffusion and infiltration phenomenon on the building material. The model inputs parameters were evaluated experimentally according to the recognized standards of material’s characterization. Therefore, numerous series of hygrothermal calculation were carried out on the 1-D and 2-D configuration in order to assess the dimensionless effect on such wooden material. Two types of boundary conditions were considered and examined. The first are at the material scale of wood drying process. The second type of conditions is at the wall scale, where the conditions of the building ambiance are considered. Moreover, the model sensitivity to the driving potentials coupling and to the parameters variability was considered and examined. It has been found that the coupling in the model had a remarkable impact on both kinetics of temperature and moisture content.

Evaluation of Earth-Air Heat Exchangers Efficiency in Hot and Dry Climates

Energy efficiency of building, promotes strongly the integration of passive strategies, in order to achieve a thermal comfort especially in summer conditions by reducing or preventing the use of air conditioning systems. In this work, building energy performance has been evaluated using an earth-air heat exchanger (EAHE) during summer period. Energy requirements was analysis by the means of dynamic simulation tools called (TRNSYS) for hot and arid climate in the southern Algeria. This analysis was conducted function of different (such as soil typology, tube material, tube length and depth, ventilation airflow rates). Results show that earth-air heat exchanger has the highest efficiency for arid climates. Furthermore, the possibility of coupling of this technology with other passive strategies (nocturne ventilation and thermal mass) has been also examined. High efficiency was observed.