Development and validation of a one-dimensional solver in a CFD platform for boiling flows in bubbly regimes

[EN] This paper presents a new one-dimensional solver for two-phase flow simulations where boiling is involved. The solver has been implemented within the OpenFOAM® platform. The basic formulation follows the Eulerian description of the Navier¿Stokes equations. Different closure equations for one-dimensional simulations are also included, as well as a subcooled boiling model in order to perform accurate computations of the mass and heat transfer between phases. In addition to the fluid, a domain is included in order to represent the solid structure, so the solver is able to solve conjugate heat transfer problems. Two different test cases are presented in this work, first a single-phase test case in order to verify the conjugate heat transfer, and then a case based on the Bartolomej international benchmark, which consists of a vertical pipe where the fluid runs upwards while it is heated. Transient calculation were performed, and the results were compared to the TRACE system code, and to the experimental data in the corresponding case. With this calculations, the capability of this new solver to simulate one-dimensional single-phase and two-phase flows including boiling is demonstrated. This work is a first step of a final objective, which consists in allowing a 1D¿3D coupling within the CFD platform, avoiding external links.This work has been partially supported by the Spanish Agencia Estatal de Investigacion [grant number BES-2013-064783], and the Spanish Ministerio de Economia Industria y Competitividad [project NUC-MULTPHYS ENE2012-34585].Gomez-Zarzuela-Quel, C.; Chiva Vicent, S.; Peña-Monferrer, C.; Miró Herrero, R. (2021). Development and validation of a one-dimensional solver in a CFD platform for boiling flows in bubbly regimes. Progress in Nuclear Energy. 134:1-16. https://doi.org/10.1016/j.pnucene.2021.103680S11613

Consolidation of artificial decayed portland cement mortars with an alkoxysilane-based impregnation treatment and its influence on mineralogy and pore structure

Surface treatments, especially hydrophobic agents to prevent water ingress and consolidants able to fill decay- induced cracks, are often proposed as a method for preserving stone cultural heritage, however its use to pro- tect concrete heritage is much less common. New products, specifically designed for concrete, have been developed. These products are based on alkoxysilanes that interact directly with the products of portland cement (OPC) hydration (essentially Ca(OH)2 and C-S-H) to generate additional C-S-H gel. This study assesses the effect of an impregnation treatment, based on alkosysilanes, on artificially decayed cement mortars, in terms of product penetration depth, changes in the porosity of mortars and changes in its mechanical strengths. Reduced porosity and enhanced mechanical strength attested to treatment efficacy. Substrate porosity and pore size distribution were not the only factors found to condition treatment effectiveness, however, mineralogical changes caused by the deterioration processes (such as the presence or absence of portlandite, or the presence of salts) modify the sol gelling time and the substrate surface energy, impacting treatment penetration depth

Studying the bulk hydrophobization of cement mortars by the combination of alkylalkoxysilane admixture and fluoropolymer-functionalized aggregate

In this work, hydrophobic mortars are produced by combining a silane-based admixture with the pre-treatment of the aggregates with an alkoxysilane-ended fluoropolymer. A comparative study is presented to determine the effect of the components, under different curing conditions, on the hydrophobicity, mechanical performance, composition and micro-structure. The combination of both strategies allows obtaining hydrophobic properties at different curing conditions, whereas the silane loses effectiveness at high humidity and the modified aggregate at low humidity. The silane hinders cement hydration and promotes gaps in the aggregate-matrix interfacial transition zone, decreasing mechanical resistance, whereas the modified aggregate changes the interfacial transition zone morphology without significant effects on resistance. The combination of both strategies partially compensates the negative impact of the silane admixture, especially when the mortars are cured at high humidity. Thus, this combination increases versatility of the mortars and poses as a potential route to address the limitations of silane admixtures. © 2022 The Author

A combined data-driven, experimental and modelling approach for assessing the optimal composition of impregnation products for cementitious materials

The effectiveness of sol-gel based treatments for the protection of concrete depends on their capacity to penetrate into the material pores. Optimization of sol formulation to achieve maximum penetration depth is not a straightforward process, as the influence of different physical properties of the sol varies with the pore size distribution of each concrete. Thus, a comprehensive experimental programme to evaluate this large number of materials would require a significant number of experiments. This manuscript describes an approach, using combined computational and experimental approach to design tailor-made impregnation products with optimized penetration depth on concrete or cementitious materials with different pore size distributions. First, a process-based numerical model, calibrated experimentally for one sol composition and several cementitious material samples with different pore structures is developed. The model calculates the penetration depth for a specific pore structure. The optimization process utilizes the probabilistic and non-parametric Gaussian Processes regression method Gaussian Processes at two steps; first to make the choice of the optimal experimental design, and second to make predictions of physical properties based on the obtained training points. In the final step, the penetration depth is calculated for each mix combination in defined parameter range. The effectiveness of this approach is demonstrated on three cases. In the first instance, we optimized the impregnation product for the maximum penetration depth without any restrictions. With another two cases, we impose the restrictions on the gelation time, i.e. the time in which the sol reacts to gel. The validation of the procedure has been made by the use of a blind validation and shows promising results. The impregnation product penetrated significantly deeper with a product selected by using the described procedure compared to the considered best product before this optimization. The proposed procedure can be applied to a wide range of cementitious materials based on their pore size distribution data. This offers significant advantage compared to purely experimental approaches, where a set of experiments is required for each considered material

Chemistry of the interaction between an alkoxysilane-based impregnation treatment and cementitious phases

Chemical compatibility with a wide range of materials is among the features that has driven the use of alkoxysilanes as consolidants in built structures. Such compatibility is particularly important in cementitious materials where the reaction with portlandite may generate C-S-H gel, one of the main hydration phases of OPC. The cementitious matrix is a complex system, however, and the reaction of its many phases with alkoxysilanes, while poorly understood, may determine treatment efficacy. This article describes a detailed study of the individual interactions between an oligomeric alkoxysilane-based impregnation treatment previously shown to interact with the portlandite present in cement paste and the cementitious phases generated in ordinary portland cement hydration. The findings show that both portlandite and C-S-H gel interact with the silicon oligomers in the hydrolysed impregnation treatment to generate a C-S-H gel (in the case of portlandite) and a rise in C-S-H gel mean chain length (MCL). Ettringite is also altered in the presence of alkoxysilanes, transforming to gypsum and AH(3). Its transformation generates a tetrahedral aluminium that is taken up into a high silicon gel sourced from the treatment to form an amorphous aluminosilicate gel. Monocarboaluminate and katoite also partially decompose in the interaction with the product, whereas gibbsite remains unaffected

Achieving superhydrophobic surfaces with tunable roughness on building materials via nanosecond laser texturing of silane/ siloxane coatings

In this work, we employ a versatile laser-based top-down approach that allows to create superhydrophobic and hydrophobic surfaces, with controlled roughness and wetting properties, on marble and potentially other building materials. The process involves two stages: (1) application of an organically modified silica coating to reduce surface energy. (2) Controlled texturing of the coating by ablation using a nanosecond-pulse laser. In general terms, at higher laser fluence (energy per unity of area), the contact angles increased from 110 degrees of the non-textured surface to values around 155 degrees, following a nearly linear correlation with the measured roughness values. Starting from fluence values of 154 J cm-2, the surfaces displayed water repellence (hysteresis <= 10 degrees) and the micrographs showed the formation of sub-micrometric structures on top of the micro-roughness by melting and re-deposition of the coating material, suggesting the formation of a Cassie-Baxter wetting regime. Ablation at lower fluences created a random micro-roughness, leading to static contact angles of 135-145 degrees but no water repellence, which is indicative of a Wenzel wetting regime. At the highest fluence values tested, the increasing trends respect roughness and hydrophobic/water repellent properties are inverted due to the damages suffered by the coating. In terms of durability, the coating demonstrated a good adhesion to the stone surface, maintaining its superhydrophobic properties after repeated cycles of an "adhesive tape test". The sand falling test showed that water repellence is relatively sensitive to abrasion, although the hydrophobic character of the coating is maintained

On the One-Dimensional Modeling of Vertical Upward Bubbly Flow

[EN] The one-dimensional two-fluid model approach has been traditionally used in thermal-hydraulics codes for the analysis of transients and accidents in water¿cooled nuclear power plants. This paper investigates the performance of RELAP5/MOD3 predicting vertical upward bubbly flow at low velocity conditions. For bubbly flow and vertical pipes, this code applies the drift- velocity approach, showing important discrepancies with the experiments compared. Then, we use a classical formulation of the drag coefficient approach to evaluate the performance of both approaches. This is based on the critical Weber criteria and includes several assumptions for the calculation of the interfacial area and bubble size that are evaluated in this work. A more accurate drag coefficient approach is proposed and implemented in RELAP5/MOD3. Instead of using the Weber criteria, the bubble size distribution is directly considered. This allows the calculation of the interfacial area directly from the definition of Sauter mean diameter of a distribution. The results show that only the proposed approach was able to predict all the flow characteristics, in particular the bubble size and interfacial area concentration. Finally, the computational results are analyzed and validated with cross-section area average measurements of void fraction, dispersed phase velocity, bubble size, and interfacial area concentration.The authors sincerely thank the Plan Nacional de I+D+i for funding the Projects MODEXFLAT ENE2013-48565-C2-1- P, ENE2013-48565-C2-2-P, and NUC-MULTPHYS ENE2012- 34585.Peña-Monferrer, C.; Gómez-Zarzuela, C.; Chiva, S.; Miró Herrero, R.; Verdú Martín, GJ.; Muñoz-Cobo, JL. (2018). On the One-Dimensional Modeling of Vertical Upward Bubbly Flow. Science and Technology of Nuclear Installations. 2018:1-10. https://doi.org/10.1155/2018/2153019S110201

The importance of physical parameters for the penetration depth of impregnation products into cementitious materials: Modelling and experimental study

The performance of impregnation treatments used for protection and remediation of porous building materials relies on sufficient penetration depth. The penetration of sol-gel impregnation products into partially saturated porous material is driven by capillary suction and depends on material properties, such as pore size distribution on one hand, and on the other hand on sol physical properties, viscosity, density, surface tension and contact angle, along with the time in which the sol gels. In this work we analyse, by the way of modelling and experiments, the penetration depth of a sol-gel impregnation product as the function of pore size distribution and sol properties. The main goal is to determine the importance of sol's physical properties for the penetration depth for a specific pore size, which will serve as a basis of the optimization of impregnation products to maximize their penetration depth. The model is first calibrated in terms of penetration depth and sol uptake by the experimental data obtained from mortar samples each with a specific pore-size distribution. The correlation between penetration depth and physical parameters is then established by the use of Monte-Carlo method. The results show that the most important parameters for the optimization are surface tension, whose influence increases for larger pores, and gelation time, which with decreasing importance for larger pores. (C) 2020 The Author(s). Published by Elsevier Ltd

A patchy particle model for C-S-H formation

The composition and structure of Calcium-Silicate-Hydrate (C-S-H) phases depends on various reaction parameters leading to its formation. Molecular Dynamic simulation studies probing the formation and structure of C-S-H are generally computationally expensive and can reach only very short time scales. Herein, we propose a coarse graining approach to model the formation of C-S-H, using patchy particles and a modified Patchy Brownian Cluster Dynamics algorithm. The simulations show that patchy particle systems can recover the qualitative kinetic evolution of C-S-H formation, and the obtained final structures were comparable to previously reported molecular dynamics studies and experiments. The model was extended to study the effect of water in the polymerization of tetraethoxysilane oligomers, the principal component of an impregnation treatment for deteriorated concrete surfaces. The intermediate system properties predicted by the simulations, such as viscosity and gel time, and structure were found to be well in accordance with the tailored experiments.The work described in this manuscript has been performed under InnovaConcrete EC project, supported by funding from the European Union's Horizon 2020 Research and Innovation Programme under Grant Agreement N◦760858. AP and JSD also acknowledge the support received from the BASKRETE initiative and the Joint Transborder Lab-oratory (LTC) “Aquitaine-Euskadi Network in Green Concrete and Cement-based Material

Multifunctional silane-based superhydrophobic/impregnation treatments for concrete producing C-S-H gel: Validation on mockup specimens from European heritage structures

Many of the concrete structures that conform our modern cultural heritage are in need of repair and protective interventions. Silane-based impregnation treatments can be used to repair onset cracks and reinforce the surface due to their ability to produce silica and C-S-H gels, and can be modified by incorporating hydrophobic precursors to create multifunctional treatments that also protect from water ingress. Since the effectiveness of impregnation treatments is dependent on substrate properties and chemical-physical changes it may have experienced over time, validation using standard materials may not always be representative of on-site application. In this work, the effectiveness of three innovative silane-based impregnation treatments developed by our group (two of them combining superhydrophobic properties) was evaluated on mockup specimens, which simulate the properties of the cementitious materials from six different heritage structures across Europe, artificially aged to simulate weathering by three methods: carbonation, chloride ingress and physical damages (freeze–thaw and thermal cycles). The characterization of the treatments showed they are compatible in terms of chemical interaction, applicability and minimal aesthetical alterations. Surface resistance and ultrasound pulse measurements have been used to assess the improvement in mechanical properties. The incorporation of hydrophobic components and fumed silica has a relatively low impact over the mechanical properties while it significantly reduces water absorption and grants water repellent properties to the surface, giving rise to a superhydrophobic performance. © 2022 The Author