Capillary rise kinetics on woven fabrics – Experimental and theoretical studies

Kinetics study of vertical water capillary rise on woven fabrics has been studied. Experiments are performed at 20±2°C and 65±2% RH by using a CCD camera and an electronic microbalance. A mathematical model is developed in order to interpret the experimental data in terms of diffusion parameters of water molecules on the porous materials and the height attained at equilibrium. The results show that the theoretical predictions are in reasonable agreement with experimental data with higher correlation coefficients. It is also found that kinetic parameters and the height of water attained at equilibrium are influenced by the composition (type of fibre) and the structure of the woven fabrics. It is concluded that the wicking ability is strongly enhanced with the decrease of the weft count (yarns/cm) and in the presence of higher float length of the weft yarns in the woven structure as in the case of satin 8/5. The sorption ability of the cotton fabrics has also been studied by using measurements of the mass of water absorbed by the textile. The results indicate that the satin structure, which is the less tightened one, has the best sorption ability

Efflorescence chou-fleur, efflorescence croûte et leur impact sur l’évaporation d’un milieu poreux

L’évaporation à partir d’un milieu poreux contenant initialement une solution saline aqueuse est une situation riche en problèmes ouverts étant donné la complexité des couplages entre évaporation, transport des ions et cristallisation. Nous passons en revue quelques avancées récentes qui permettent une meilleure compréhension des phénomènes. Ainsi sont discutés les facteurs affectant la localisation des efflorescences à la surface d’un milieu poreux, l’impact de la nature poreuse de l’efflorescence, et la typologie des efflorescences. Deux types d’efflorescence sont distinguées : chou-fleur et croûte, dont l’impact sur la l‘évaporation est radicalement différent. Les facteurs conduisant à chacun de ces deux types sont discutés

Experimental study of the effect of sodium chloride on drying of porous media: The crusty–patchy efflorescence transition

We present an experimental study of drying in the presence of dissolved sodium chloride. The process is characterized by the formation of a crystallized salt layer, referred to as efflorescence, at the evaporative surface of the porous medium. By varying the average size of the beads forming the porous medium, we show that the formation of the crystal layer does not affect significantly the drying process and can even enhance the drying rate when the beads are sufficiently large. By contrast, the crystal layer can greatly affect the drying process and even blocks or severely limit the evaporation process for sufficiently small beads. We therefore show the existence of two regimes, namely the blocking regime and the enhanced drying rate regime. It is shown that the two regimes correspond to two different types of efflorescence, referred to as crusty and patchy, respectively. Then by varying the initial salt concentration for a given bead size, we show that the interplay between drying and the efflorescence formation leads to a nonmonotonous variation of the drying rate with the initial salt concentration when the efflorescence is patchy but not when the efflorescence is crusty. The crusty–patchy transition is finally discussed from a simple model of capillary rise in the efflorescence

Porous medium coffee ring effect and other factors affecting the first crystallisation time of sodium chloride at the surface of a drying porous medium

We study the distribution of ions in a drying porous medium up to the formation of first crystals at the surface. The study is based on comparisons between numerical simulations and experiments with packings of glass beads. The experimental configuration, which is representative of many previous drying experiments, is characterized by the formation of an efflorescence fairy ring at the surface of the porous medium. The preferential formation of crystals at the periphery is explained by the combined effect of higher evaporation fluxes at the surface periphery, as in the classical coffee ring problem, and variations in the porosity near the wall bordering the packing. It is shown that both effects have a great impact on the time marking the occurrence of first crystals, which is referred to as the first crystallization time. The experiments indicate that the first crystallization time increases with a decreasing bead size for a given initial ion concentration. This is explained by the variation with bead size of the characteristic size of the near wall region where a preferential desaturation of the sample occurs as a result of the porosity increase near the wall. The study also reveals a significant salt supersaturation effect. This represents a noticeable fact in relation with salt weathering issues

Numerical study of turbulent normal diffusion flame CH4-air stabilized by coaxial burner

The practical combustion systems such as combustion furnaces, gas turbine, engines, etc. employ non-premixed combustion due to its better flame stability, safety, and wide operating range as compared to premixed combustion. The present numerical study characterizes the turbulent flame of methane-air in a coaxial burner in order to determine the effect of airflow on the distribution of temperature, on gas consumption and on the emission of NOx. The results in this study are obtained by simulation on FLUENT code. The results demonstrate the influence of different parameters on the flame structure, temperature distribution and gas emissions, such as turbulence, fuel jet velocity, air jet velocity, equivalence ratio and mixture fraction. The lift-off height for a fixed fuel jet velocity is observed to increase monotonically with air jet velocity. Temperature and NOx emission decrease of important values with the equivalence ratio, it is maximum about the unity

Caractérisation d'un jet synthétique en vue de son utilisation pour le contrôle

Dans le présent travail, on a mis au point un actionneur modulable fonctionnant à l'aide d'un haut-parleur. Trois inclinaisons de la fente ont été choisies (30°, 45° et 90°). Pour chacune, plusieurs fréquences et amplitudes de la vitesse à la sortie ont été étudiées. On a fait un balayage à 2 mm de la fente. Pour chaque cas, on a obtenu les caractéristiques de l'écoulement moyen et fluctuant complété par une analyse spectrale à l'aide de l'anémométrie à fil chaud. Un effet de ralentissement a été observé pour une inclinaison de 30° par rapport à l'horizontal. Des variations importantes ont été notées quand on fait varier la fréquence. La fréquence d'excitation pilote la dynamique du jet et va sûrement avoir un effet moteur en présence de l'écoulement. L'augmentation de l'amplitude permet aux tourbillons générés de s'éloigner de la fente et d'influencer l'écoulement extérieur loin de la fente. Des mesures par Vélocimétrie par Images de Particules (PIV) viennent confirmer ces constations

The impact of salt crusts and salt cauliflowers on evaporation from a porous medium

Evaporation in a presence of a dissolved salt is important in relation with various applications such as the conservation of our cultural heritage, the injection of CO2 in saline aquifer or the soil salinization problem, to name only a few. The problem is made particularly interesting by the crystallization of the salt resulting from the evaporation process. Here we are interested in situations where the salt crystallizes at the surface of the porous medium and forms crystallized salt structures referred to as efflorescence. Although advances have been made in recent years on the understanding of the factors controlling the formation and growth of efflorescence, many questions are still open. Here we are interested in the impact of efflorescence on the evaporation kinetics of the porous medium. The study is based on drying experiments in the presence of dissolved sodium chloride. By varying the average size of the beads forming the porous medium, we show that the formation of the efflorescence does not affect significantly the drying process and can even enhance the drying rate when the beads are sufficiently large. By contrast the efflorescence can greatly affect the drying process and even blocks the evaporation process for sufficiently small beads. We therefore show the existence of a transition between the two regimes, namely the blocking regime and the enhanced drying rate regime. It is shown that the two regimes correspond to two different types of efflorescence, referred to as crusty and cauliflower (or patchy) respectively. Then by varying the initial salt concentration for a given bead size, we show that the interplay between drying and the efflorescence formation leads to a non-monotonous variation of the drying rate with the initial salt concentration when the efflorescence is patchy but not when the efflorescence is crusty. This has to do with a porous medium “coffee ring” effect due to evaporation fluxes higher at the periphery of the sample

Evaporation-driven growth of large crystallized salt structures in a porous medium

Subflorescence refers to crystallized salt structures that form inside a porous medium. We report a drying experiment revealing major development of subflorescence in the dry region of the porous medium away from the liquid zone. Using a combination of image analyses and numerical computations, we show that the growth is directly correlated to the evaporation flux distribution along the boundary of the growing salt structure. This indicates that the salt is transported into the domain occupied by the salt structure in the porous medium up to the structure periphery, where salt deposition takes place. This is confirmed when a growing salt structure encounters dry subflorescence formed earlier during the drying process. The dry subflorescence is reimbibed and resumes its growth. The analysis also suggests that the solution within the growing subflorescence is in equilibrium with the crystallized salt wall. These results shed light on the growth mechanisms of subflorescence, a phenomenon that can play a fundamental role in several important issues such as carbon dioxide sequestration or salt weathering

Etude expérimentale et numérique d'un écoulement diphasique gaz-particules

Dans ce travail, une étude numérique et expérimentale est réalisée dans un jet diphasique turbulent (gaz-particules) issu d'un brûleur coaxial. Le modèle numérique repose sur une approche eulérienne-lagrangienne permettant une description fine des mécanismes d'interactions entre les deux phases. La technique d'Anémométrie à Phase Doppler (PDA) est utilisée pour mesurer la taille et la vitesse simultanément de particules solide sphériques de diamètre du 102 µm jusqu'à 212 µm

Gas injection in a liquid saturated porous medium. Influence of pressurization effects and liquid films

We study numerically and experimentally the displacement of a liquid by a gas in a two-dimensional model porous medium. In contrast with previous pore-network studies on drainage in porous media, the gas compressibility is fully taken account. The influence of the gas injection rate on the displacement pattern, breakthrough time and the evolution of the pressure in the gas phase due in part to gas compressibility are investigated. A good agreement is found between the simulations and the experiments as regards the invasion patterns. The agreement is also good on the drainage kinetics when the dynamic liquid films are taken into account