Solid Wall Research Articles

Hydrophobic treatments and their application with internal wall insulation

Hydrophobic (or water repellent) treatments have been proposed to mitigate moisture risks associated with internal wall insulation when applied to solid masonry walls. This can reduce risks associated with moisture accumulation within the structure such as mould growth or the deterioration of joist ends and other embedded timber. Where treatments perform well there is a net reduction of moisture content and risk. However, such treatments slow down drying processes, and therefore may result in a net increase in moisture if the treatment is bypassed by, for example, cracks. Some treatments may lead to damage to external masonry surfaces in some situations. Freeze–thaw and salt crystallisation are the two main causes. Hygrothermal simulations may give some indication of risks but techniques to assess the risk of surface damage are either simplistic, impractical outside of the research environment or both. This paper reviews the current field of assessing and predicting the risk of surface damage associated with surface treatments.

Numerical investigation on the heat transfer and pressure loss characteristics of precooler under negative pressure

Precoolers have garnered considerable attention owing to their applications in aerospace and industrial fields. Nonetheless, there is a paucity of research concerning the performance of precoolers under conditions of extremely low pressure. This study conducts a numerical analysis of the heat transfer and pressure loss characteristics of a plate-fin precooler operating under negative pressure, considering the coupled heat transfer interactions among the hot fluid, solid wall, and cooling fluid. The comprehensive performance and flow field on each side of the precooler are investigated. Moreover, the impact of negative pressure on the heat transfer and pressure recovery performance of the hot fluid in the rectangle channel is evaluated. The results show that, even under operating conditions with pressures below 30 kPa and inlet temperatures exceeding 1100 K, the precooler attains a pressure recovery coefficient greater than 95.6 % and achieves a temperature drop exceeding 43.2 % for the hot fluid as the mass flow rate ranges from 0.205 kg/s to 0.564 kg/s. Additionally, the heat transfer coefficient of the hot fluid is less than 1/40 that of the cooling fluid, resulting in increased thermal resistance between the hot fluid and the wall. Consequently, a distinct thermal boundary layer develops between the hot fluid and the wall, which is not present between the cooling fluid and the wall. Furthermore, the performance of heat transfer and pressure recovery for the hot fluid exhibits considerable variability with changes in static pressure at a constant inlet velocity. However, a turning point is observed when these parameters are evaluated in terms of the inlet Reynolds number. At low Reynolds numbers, both the heat transfer coefficient and the friction factor remain relatively stable, notwithstanding fluctuations in static pressure. Conversely, at higher Reynolds numbers, a reduction in static pressure negatively influences these factors. For instance, when the static pressure is 10 kPa, the associated Reynolds number is 500. Nevertheless, the Reynolds number at which this turning point occurs escalates with an increase in static pressure.

Effect of wall contact angle on the conjugate heat transfer for flow boiling processes in rectangular microchannels

Microchannel boiling heat transfer offers a promising solution to address extreme high heat flux in miniaturized and high-density integrated electronic devices. In this work, we performed numerical studies to elucidate the complex flow boiling process in a square microchannel using the open-source platform OpenFOAM. The governing equations of the two-phase flow was solved based on the finite volume framework, and the surface tension together with phase change models were incorporated to study the wettability and conjugate heat transfer in the microchannel. First, the stability of the liquid film shows that smaller contact angles lead to thicker and stable liquid films near the solid wall, while larger contact angles result in thinner and fragile films. Second, the heat transfer, represented by the dimensionless parameter Nu, shows that smaller contact angles result in higher and more uniform distribution of Nu. The velocity gradient causes the Nu to be higher for the sidewalls than for the bottom wall. The contact angle also affects the peak value of Nu. Small contact angles result in large solid wall temperature gradients and more efficient heat transfer. Finally, the simulation results show that smaller thermal conductivity of the solid results in larger temperature gradient in the flow direction, implying that the conjugate heat transfer is less efficient. Overall, the present numerical findings provide useful guidance for efficient thermal management in electronic devices.

Geographical variability in hygrothermal simulation results of historical building envelopes with interior insulation in Belgium

Thermal retrofits of existing buildings offer a great potential to reduce Europe’s energy use and CO2 emissions. The impact of these retrofitting strategies on the risk for degradation in building envelopes is assessed through hygrothermal simulations. Due to the lack of long term climate data, hygrothermal simulations are performed for a certain location, and the results are generalised over a large region. However, this generalisation may not be valid, since the hygrothermal performance of building envelopes is highly influenced by local climate conditions. Therefore, we assessed the geographical variability on degradation risks in building envelopes. This paper presents the results of 34560 hygrothermal simulations of solid masonry walls, with and without interior insulation, and including a range of other parameter variations, for 20 synoptic weather stations across Belgium. Generally, the risk for degradation is higher at locations with a higher elevation above sea level. For Belgium, the elevation correlates well with the mean temperature (invers correlation), and the annual precipitation and wind-driven rain load. For a known wall orientation, the impact of interior insulation on the risk for freeze-thaw damage is most determined by the elevation above sea level. Also the mould index in uninsulated walls correlates best with the elevation. On the contrary, the brick type is more important than the location when considering wood decay of embedded beam heads in retrofitted walls. To conclude, the risk for degradation cannot be generalised, especially over areas with complex variations in orographic and climatological conditions.

Internal heating and distribution in electromagnetic Powell-Eyring [formula omitted]/glycerin hybridized nanomaterial with radiation and wall slip conditions

The synergistic influences of the hybridization of molybdenum disulfide (MoS2) and graphene oxide (GO) nanoparticles on electromagnetic radiation absorption and augmentation of thermal conductivity coupled with the industrial demand for an enhanced non-Newtonian working fluid spurred this study. Therefore, the study examines the radiative properties and thermal behaviour of hybridized MoS2 and GO nanoparticle systems dispersed in a glycerin Powell-Eyring fluid with wall slip conditions. A theoretical mathematical setup is modelled for the nanoparticle thermophysical characteristics with viscous dissipation and heat generation under the influence of the electromagnetic Riga plate. The model is transformed into an invariant dimensionless form and solved utilizing the pseudo-spectral collocation technique. The effect of fluid rheology, nanoparticle concentration, and fluid-wall slip conditions interface on radiative heat transfer and thermal propagation is investigated. The results reveal momentous augments in radiation absorption and thermal conductivity of about 4.13% to 10.22% due to MoS2 and GO hybridized nanoparticles. Also, it was found that thermal transport is enhanced close to the solid boundary wall slip, leading to an improve heat distribution rate. Hence, this study contributes to the interplay complexity between external stimuli, fluid dynamics, and nanoparticle morphology in thermal systems, offering insights into the advanced materials synthesis, thermal management systems, and optimization and design of nanoparticle fluid enhancement applications.

Maxwell boundary condition for discrete velocity methods: Macroscopic physical constraints and Lagrange multiplier-based implementation

In this paper, we propose an algorithm that imposes macroscopic physical constraints with Lagrange multiplier approach in implementing the Maxwell boundary condition within the framework of the discrete velocity method. For the simulation of rarefied gas flows in the presence of solid walls with complex geometry, the distribution function in the reflection region of the wall surface needs to be constructed in the discrete velocity space, to fulfill the specular reflection in the Maxwell boundary condition. The construction process should not consist of interpolation only, but include certain macroscopic physical constraints at the wall surface, so as to correctly account for gas-surface interaction on a macroscopic level. We demonstrate that for the specular reflection, keeping the symmetry of the first three moments of the distribution function between the incident and reflected region is sufficient for maintaining the conservation of mass, momentum, and energy at the wall surface. Furthermore, to strictly satisfy macroscopic physical constraints, a Lagrange multiplier method is introduced into the construction of the distribution function to correct the pure interpolation solution. In addition, the construction process requires the inversion of a large and sparse matrix (of dimension N × N, where N is the number of points in the velocity space). To improve the computational efficiency, the matrix inversion is converted into that of a much smaller matrix, i.e. (D + 2) × (D + 2) in the d-dimensional physical space. A series of numerical experiments are conducted to examine the performance of the proposed algorithm under different flow conditions. We demonstrate that the results obtained by the proposed algorithm are more accurate than the pure interpolation solution, comparing with the benchmark data. Moreover, after the validation of our results with previous studies, we find that the method significantly enhances the conservation of total mass and energy, especially for flows in an enclosed domain.

Hydroelectric energy conversion of waste flows through hydroelectronic drag

Hydraulic energy is a key component of the global energy mix, yet there exists no practical way of harvesting it at small scales, from flows with low Reynolds number. This has triggered a search for alternative hydroelectric conversion methodologies, leading to unconventional proposals based on droplet triboelectricity, water evaporation, osmotic energy, or flow-induced ionic Coulomb drag. Yet, these approaches systematically rely on ions as intermediate charge carriers, limiting the achievable power density. Here, we predict that the kinetic energy of small-scale "waste" flows can be directly and efficiently converted into electricity thanks to the hydroelectronic drag effect, by which an ion-free liquid induces an electronic current in the solid wall along which it flows. This effect originates in the fluctuation-induced coupling between fluid motion and electron transport. We develop a nonequilibrium thermodynamic formalism to assess the efficiency of such hydroelectric energy conversion, dubbed hydronic energy. We find that hydronic energy conversion is analogous to thermoelectricity, with the efficiency being controlled by a dimensionless figure of merit. However, in contrast to its thermoelectric analogue, this figure of merit combines independently tunable parameters of the solid and the liquid, and can thus significantly exceed unity. Our findings suggest strategies for blue energy harvesting without electrochemistry, and for waste flow mitigation in membrane-based filtration processes.

Predicting Leidenfrost Temperature and Effects of Impact Conditions on Droplet Size Distribution in Film-Boiling Regime

Abstract The interaction of a droplet with a solid wall is relevant in various engineering applications. The properties of the resulting secondary droplets are determined by the wall temperature, ambient pressure, impact momentum, and impact angle. This paper presents a comprehensive characterization of drop-wall interactions and the subsequent atomization as a function of the combined effects of such parameters. A drop-wall interaction model is derived for F-24 droplets using smoothed particle hydrodynamics (SPH). The model can predict different impact outcome regimes (deposition, rebound, contact-splash, and film-splash) for different ambient pressures, wall temperatures, and impact parameters. The model also addresses the effect of ambient pressure on the Leidenfrost behavior. Size distributions of secondary droplets are compared for vertical and nonvertical impacts of F-24 droplets on superheated surfaces in the film-boiling regime. The non-dimensional Sauter mean diameter (SMD) of the secondary droplets varies based on the position in the impact plane for all the nonvertical impacts but remains almost unchanged for vertical impacts. The leading arc for nonvertical impact consists of larger secondary droplets, and the size decreases toward the trailing arc. An empirical relation is proposed to represent this trend. This research sheds light on successive droplet impacts by studying the effects of impact frequency on SMD evolution. The results are compared to single droplet impact cases for different fuels and Weber numbers. The size of secondary droplets for successive impacts is observed to be nearly indistinguishable from that of single-droplet vertical impacts.

Examining the relationship between dwelling energy efficiency and fuel poverty for retrofit strategies: A case study of the United Kingdom

In the midst of a global energy crisis, fuel poverty has become increasingly prevalent. This study examines the effectiveness of energy efficiency retrofits (EERs) in alleviating fuel poverty in the United Kingdom. Using the England fuel poverty dataset with XGBoost and interpretive analyses like Individual Conditional Expectation (ICE) and SHapley Additive exPlanations (SHAP), the research investigates the impact of EERs on fuel poverty. Findings reveal that while energy efficiency enhancements hold potential for improving conditions for vulnerable households, their effectiveness diminishes beyond a certain threshold. The study identifies solid wall and cavity wall insulation, condensing boilers, well-insulated lofts, and central heating systems as the most effective retrofit measures. However, the optimal selection of retrofit methods varies depending on household and dwelling conditions due to interactive effects. Additionally, comprehensive cost-efficiency analyses show that installing condensing combination boilers in homes without existing boilers and adding cavity wall insulation to houses with uninsulated cavity walls are more cost-efficient options. These insights shed light on the complex relationship between energy efficiency retrofits and fuel poverty, informing the development of targeted strategies to address this pressing issue in the UK and globally.

Acoustic radiation of a fluid-saturated microperforated plate

It is known that fluid-saturated microperforated plates (MPP) dissipate energy and are substantially damped through thermoviscous dissipation taking place in the thermoviscous skin on the solid wall of the perforations. The aim of this work is to explore the acoustic radiation of a microperforated periodic cell to investigate the intercellular coupling and the complexity of near-field radiation, and to derive a criterion related to the near-field far-field transition. A numerical modeling using the finite element method is developed to solve the elastic and thermo-visco-acoustic (TVA) multiphysics problem. The TVA calculates explicitly the thermoviscous losses on the acoustic wave in the periodic cell studied. Numerical results show that the distance of the near-far field transition is significant at low frequencies, passing through a minimum for a frequency depending on perforation parameters (diameter and ratio) before increasing again as a frequency function. The radiated acoustic power and radiation efficiency are compared with those derived from the theoretical MPP vibration model. It is found that the microperforations and resulting added damping, around a characteristic frequency, reduce the acoustic radiation efficiency of the MPP compared to the corresponding non-perforated plate.

Vibration behavior of a microperforated plate within an acoustic nonlinear framework

Microperforated plates (MPP) can add substantial damping in the low-frequency range. MPP are known to dissipate energy through thermo-viscous interactions between shearing adjacent fluid layers near the perforation solid walls. Under linear operating conditions, a previous work carried out by the authors showed that the added damping reaches a maximum at a characteristic frequency which solely depends on the perforation parameters. However, MPP is also suitable in environments subject to high levels of mechanical excitation and, consequently, high fluid velocity within the perforations. Two types of nonlinearities should then be considered: (1) an acoustic nonlinearity induced by high fluid velocity, and (2) a nonlinearity induced by large structural displacements. This work only explores the former. The acoustic nonlinearity is modelled by the Forchheimer resistivity correction, a function of the fluid-solid relative velocity in the perforations, introduced into the equations subsequently solved numerically. Experimental measurements using a laser vibrometer on a perforated cantilever beam validate the proposed model. Results show that, under high excitation levels and at the characteristic frequency, the maximum added damping can reach a maximum, depending on the MPP parameters, at a critical value of the relative fluid-solid velocity.

Linear instability of viscous parallel shear flows: revisiting the perturbation no-slip condition

Linear stability analysis currently fails to predict turbulence transition in canonical viscous flows. We show that two alternative models of the boundary condition for incipient perturbations at solid walls produce linear instabilities that could be sufficient to explain turbulence transition. In many cases, the near-wall behaviour of the discovered instabilities is empirically indistinguishable from the classical no-slip condition. The ability of these alternative boundary conditions to predict linear instabilities that are consistent with turbulence transition suggests that the no-slip condition may be an overly simplified model of fluid–solid interface physics, particularly as a description of the flow perturbations that lead to turbulence transition in wall-bounded flows.

Interfacial adhesion of polycarbonate to graphene and silicon oxide: A comparative molecular dynamics analysis

In this study, molecular dynamics was used to evaluate the adsorption-free energy of polycarbonate on a solid wall surface that represented a fiber material. Three solid surfaces were used: clean graphene without functional groups to represent surface-treated carbon materials, graphene with OH groups without surface treatment, and SiO2 to represent glass fiber. The results showed that the adsorption free energy per unit area for clean graphene was greater than the adsorption free energy per unit area for the other two solid wall surfaces, which is different from the experimental results where the surface treated carbon fiber had a smaller adsorption free energy. The structure of benzene rings was analyzed, and it was found that benzene rings are deposited almost parallel to the solid wall on all solid walls and that the density of benzene rings with SiO2 and OH groups is the smallest on Graphene with the smallest angle and closest to the solid wall, resulting in smaller adsorption energy. This suggests that the π-π interaction between the benzene ring and the solid surface may account for a large proportion of the adsorption energy. It also suggests that chemical reactions between the solid surface and the polymer, which were not considered in this study, may have a significant effect on the adsorption energy.

Numerical investigation of the collision of a vortex pair upon a solid wavy wall

The vortex dynamics produced by the collision of a vortex pair with a solid wavy wall are examined numerically using a proposed high-order vortex particle method. The influences of the Reynolds number (ReΓ) of the vortex pair, the wavelength (λw), and the wave amplitude (Aw) of the wall on the induced vortex structures, interactions among vortices, and the instability and reconnection of secondary vortices are discussed. The boundary layers at the top of ripples separate first to form the secondary vortices moving around the primary tubes. Meanwhile, the boundary layers at the bottom of ripples detach late, move vertically upward, and then form incomplete vortex loops due to an unsuccessful reconnection of the secondary vortices. The ReΓ effects cause the appearance of various vortex structures, such as incomplete primary vortex loops from the bottom of ripples at ReΓ=1000, secondary vortex loops due to interactions of the first loops with the wall at ReΓ=2000, and hairpin vortices at ReΓ=4000. The wavelength of the wall induces instability in the secondary vortex tubes. This instability on the secondary tubes appears at the top of ripples at λw=2r0, while it spreads over the whole secondary vortices at λw=4r0, where r0 is the initial spacing between the primary vortex tubes. This instability results in various wavy tubes that robustly interact with the primary tubes. Due to the effects of Aw, the primary vortex tubes significantly decay with time compared to those in the flat wall case.

Acoustics of wet porous media with evaporation/condensation

This paper investigates acoustic wave propagation in wet rigid-frame porous media accounting for evaporation and condensation. At equilibrium, the solid walls are covered by a thin water film, and water vapor in the air is at its temperature-dependent saturation pressure. Small acoustic perturbations cause water to vaporize or condense, which together with the reversibility of the phase change, lead to a linear problem where the usual local poro-acoustics physics is enriched with the (i) Clapeyron relation linking liquid-wall temperature, vapor pressure, and latent heat of vaporization, (ii) latent heat transfer in the solid frame, (iii) diffusion equation for water vapor in air, and (iv) water vapor's equation of state. The equilibrium temperature highly influences the vapor concentration and the physical parameters of saturated moist air. Using the two-scale asymptotic homogenization method, it is shown that the dynamic permeability is determined similarly to classical porous media, while the effective compressibility is modified by evaporation/condensation and the equilibrium temperature. This modification is influenced by vapor mass and heat flows associated with phase changes through a local fully coupled heat transfer and water vapor diffusion problem, with specific boundary conditions at the gas–water interface. The analysis identifies dimensionless parameters and characteristic frequencies defining the upscaled model's features. Depending on equilibrium temperature, the theory qualitatively and quantitatively determines the characteristics of acoustic waves propagating through the media. The results are illustrated and discussed with analytically developed models.

Examining a Conservative Phase-Field Lattice Boltzmann Model for Two-Phase Flows

In this study, a conservative phase-field lattice Boltzmann model is examined for simulating immiscible two-phase flows with large density and viscosity ratios. This model is built upon the Allen–Cahn equation, incorporating a filtered collision operator and high-order corrections in the equilibrium distribution functions. The numerical model is also integrated with the volumetric boundary scheme and the immersed boundary method to achieve various stationary and moving boundary conditions on arbitrary geometries for different application scenarios. A comprehensive evaluation of this phase-field solver is conducted through a range of academic and industrial cases. First, droplet splashing cases at different Reynolds numbers are studied. The phase-field LB model effectively captures the surface wave motion and splashing of the droplet, although some smearing of small structures is observed due to the diffused interface property of the numerical model. Second, a tuned liquid damper is simulated, accurately capturing sloshing forces on solid walls. In the simulation of a dam-breaking case, predicted gauge pressure and free-surface elevation time histories compare well with experimental data. Lastly, a gearbox case under dip lubrication is simulated, with both the gearbox churning loss and oil distribution being well predicted across a range of rotating speeds. The validation studies demonstrate the effectiveness of the present phase-field lattice Boltzmann model in simulating immiscible two-phase flows with large and realistic density and viscosity ratios. The strengths, limitations, and weaknesses of the conservative phase-field LB model are discussed, and suggestions for the development and application of this numerical model are provided.

Children (and many adults) use perceptual similarity to assess relative impossibility.

People see some impossible events as more impossible than others. For example, walking through a solid wall seems more impossible if it is made of stone rather than wood. Across four experiments, we investigated how children and adults assess the relative impossibility of events, contrasting two kinds of information they may use: perceptual information and causal knowledge. In each experiment, participants were told about a wizard who could magically transform target objects into other things. Participants then assessed which of the two transformation spells would be easier or harder, a spell transforming a target object into a perceptual match (i.e., a similar-looking thing) or one transforming it into a causal match (e.g., an item made of similar materials). In Experiments 1-3, children aged 4-7 mainly thought that transformations into the perceptual match would be easier, though this tendency varied with age. Adults were overall split when choosing which spell would be easier. In Experiment 1, this was because of variations in their judgments across different pairs of spells; in Experiments 2 and 4, the split resulted because different subsets of adults preferred either the perceptual or causal match. Overall, these findings show that children, and many adults, use perceptual reasoning to assess relative impossibility. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Anchors to solid clay brick masonry in tension: Behavior under monotonic and repeated loading for varied embedment depths

The paper presents the results of tension tests on anchors installed in solid brick masonry walls. Anchors were installed in various locations (either in bricks or in mortar joints), in walls either uncracked or with cracks passing through mortar joints. All tests, either monotonic or repeated, were executed under displacement-controlled conditions. The purpose of the experimental campaign was to investigate the behavior of anchors embedded to masonry under seismic conditions. To this aim, several parameters were investigated, namely, the depth at which anchors are embedded in masonry, their location within the wall, the presence of cracks, either crossing the anchor or being located at its vicinity. The measured tension resistances, depending on the examined parameters, are compared against values calculated based on current design guidelines. This comparison shows that the calculated values for the expected failure mode are sufficiently conservative.

Physically consistent immersed boundary method: A framework for predicting hydrodynamic forces on particles with coarse meshes

In the present paper, a fluid-particle coupling method is directly derived from the Navier-Stokes equations (NSE) by applying the concept of volume-filtering, yielding a physically consistent methodology to incorporate solid wall boundary conditions in the volume-filtered flow solution, thereby allowing to solve the governing flow equations on non-body conforming meshes. The resulting methodology possesses similarities with a continuous forcing immersed boundary method (IBM) and is, therefore, termed physically consistent IBM (PC-IBM). Based on the recent findings of Hausmann et al. [1], the closures arising in the volume-filtered NSE are closed by suitable models or even expressed analytically. The PC-IBM is fully compatible with the large eddy simulation framework, as the volume-filtered NSE converge to the filtered NSE away from solid boundaries. The potential of the PC-IBM is demonstrated by means of different particle-laden flow applications, including a dense packing of fixed particles and periodic settling of 500 particles. The new methodology turns out to be capable of accurately predicting the fluid forces on particles with relatively coarse mesh resolutions. For the flow around isolated spheres, a spatial resolution of six fluid mesh cells per diameter is sufficient to predict the drag force with less than 10% deviation from highly resolved simulation for the investigated particle Reynolds numbers ranging from 10 to 250.

Green and sustainable bamboo based composites with high self-bonding strength

Self-bonding technology is a forming method utilized for the production of bio-composites with notable advantages in terms of high strength and water resistance, while also being free from formaldehyde. The successful achievement of high-strength self-bonding biomaterials has been demonstrated, however, most of these achievements have relied on various surface chemical modifications with limited attention given to exploring non-additive approaches. This study aimed to investigate the feasibility of self-bonding formation through examining the effects of particle dispersion, lignin melting, and chemical bonding, in distinct unit morphologies without the use of any chemical additives. Also, the self-bonding mechanism of bamboo based composites (BBCs) was revealed, by examination of its microstructure, chemical composition and thermal stability. The findings indicated that raw component morphologies based on power, fiber, and bundle all effectively achieved high-strength self-bonding structures at a temperature of 155 °C, a pressure of 55 MPa, and a duration time of 60 min. The density of the BBCs approached that of solid cell wall in bamboo, reaching a maximum of 1.44 g/cm3. The morphology of raw components had significant effect on the self-bonding performance of BBCs, with the powder exhibiting the highest performance, followed by the bundles, and the fibers showing the lowest. The powder-based BBC demonstrated a remarkable flexural strength of 61 MPa, a notable surface hardness of 32.5 kgf/mm and low 24 h thickness swelling of 6.8 %, all much more excellent than those observed in ordinary panels. The BBCs demonstrated the benefits of being environmentally friendly (free from formaldehyde), processing excellent water resistance and strong mechanical strengths. They were suitable for use in high-humidity environments and can meet the requirements for strong load-bearing conditions, even replacing some metal sliding bearing materials.