Confinement Boundary Research Articles

Effects of Confinement on Opposed-Flow Flame Spread over Cellulose and Polymeric Solids in Microgravity

Opposed-flow flame spread over solid materials has been investigated in the past few decades owing to its importance in fundamental understanding of fires. These studies provided insights on the behavior of opposed-flow flames in different environmental conditions (e.g., flow speed, oxygen concentration). However, the effect of confinement on opposed-flow flames remains under-explored. It is known that confinement plays a critical role in concurrent-flow flame spread in normal and microgravity conditions. Hence, for a complete understanding it becomes important to understand the effects of confinement for opposed-flow flames. In this study, microgravity experiments are conducted aboard the International Space Station (ISS) to investigate opposed-flow flame spread in different confined conditions. Two materials, cotton-fiberglass blended textile fabric (SIBAL) and 1 mm thick polymethyl methacrylate (PMMA) slab are burned between a pair of parallel flow baffles in a small flow duct. By varying the sample-baffle distance, various levels of confinement are achieved (H = 1–2 cm). Three types of baffles, transparent, black, and reflective, are used to create different radiative boundary conditions. The purely forced flow speed is also varied (between 2.6 and 10.5 cm/s) to investigate its interplay with the confinement level. For both sample materials, it is observed that the flame spread rate decreases when the confinement level increases (i.e., when H decreases). In addition, flame spread rate is shown to have a positive correlation with flow speed, up to an optimal value. The results also indicate that the optimal flow speed for flame spread can decrease in highly confined conditions. Surface radiation on the confinement boundary is shown to play a key role. For SIBAL fabric, stronger flames are observed when using black baffles compared to transparent. For PMMA, reflective baffles yield stronger flames compared to black baffles. When comparing the results to the concurrent-flow case, it is also noticed that opposed-flow flames spread slower and blow off at larger flow speeds but are not as sensitive to the flow speed. This work provides unique long-duration microgravity experimental data that can inform the design of future opposed-flow experiments in microgravity and the development of theory and numerical models.

Strength and stiffness characterization of ultra-high performance concrete (UHPC) cement paste phases through in-situ micro-mechanical testing

Compressive strength and stiffness of concrete materials involve the complex interaction of aggregates and hardened cement, which can involve even more complex phases formed through hydration processes. Typically, the strength and stiffness of concrete materials are determined through material-scale testing, focusing on the bulk behavior while ignoring individual constitutive material interactions. This study aims to identify individual cement phase contributions to bulk concrete properties through the characterization of individual hardened cement-paste phases using micro-scale compressive testing. In this study, in-situ micro-compression measurements are used to generate statistical distributions of compressive strength and stiffness for five hardened cement phases and one phase composite. Results from the in-situ micro-compression testing indicate that AFt/AFm and CH/C–S–H micropillars exhibit higher stiffness and strength when compared to hardened C–S–H and C-A-S-H phases. Comparing measured micropillar strength and stiffness values with traditional nano-indentation methods indicates that nano-indentation approaches may provide a higher estimation of individual phase stiffness due to material confinement boundary effects.

Geometrical control of interface patterning underlies active matter invasion

Interaction between active materials and the boundaries of geometrical confinement is key to many emergent phenomena in active systems. For living active matter consisting of animal cells or motile bacteria, the confinement boundary is often a deformable interface, and it has been unclear how activity-induced interface dynamics might lead to morphogenesis and pattern formation. Here, we studied the evolution of bacterial active matter confined by a deformable boundary. We found that an ordered morphological pattern emerged at the interface characterized by periodically spaced interfacial protrusions; behind the interfacial protrusions, bacterial swimmers self-organized into multicellular clusters displaying +1/2 nematic defects. Subsequently, a hierarchical sequence of transitions from interfacial protrusions to creeping branches allowed the bacterial active drop to rapidly invade surrounding space with a striking self-similar branch pattern. We found that this interface patterning is geometrically controlled by the local curvature of the interface, a phenomenon we denote as collective curvature sensing. Using a continuum active model, we revealed that the collective curvature sensing arises from enhanced active stresses near high-curvature regions, with the active length scale setting the characteristic distance between the interfacial protrusions. Our findings reveal a protrusion-to-branch transition as a unique mode of active matter invasion and suggest a strategy to engineer pattern formation of active materials.

Percolation transitions of confinement-induced layering and intralayer structural orders in three-dimensional Yukawa liquids.

The disorder-order transitions of layering and intralayer structural orders of three-dimensional Yukawa liquids, under the enhanced confinement effect with decreasing normal distance z to the confinement boundary, is investigated numerically. The liquid between the two flat boundaries is segmented into many slabs parallel to the boundary, with the same slab width as the layer width. In each slab, particle sites are binarized into sites with layering order (LOSs)/ layering disorder (LDSs) and with intralayer structural order (SOSs)/disorder (SDSs). It is found that with decreasing z, a small fraction of LOSs starts to heterogeneously emerge in the form of small clusters in the slab, followed by the emergence of the large percolating LOS clusters spanning over the system. The smooth rapid rise of the fraction of LOSs from small values followed by their gradual saturations, and the scaling behavior of multiscale LOS clustering, are similar to those of the nonequilibrium systems governed by the percolation theory. The disorder-order transition of intraslab structural ordering also exhibits a similar generic behavior as that of layering with the same transition slab number. The spatial fluctuations of local layering order and local intralayer structural order are uncorrelated in the bulk liquid and the outmost layer next to the boundary. Approaching the percolating transition slab, their correlation gradually increases to the maximum.

Confinement improvement during detached phase with RMP application in deuterium plasmas of LHD

In order to explore the compatibility of good core plasma performance with divertor heat load mitigation, the interaction between cold edge plasma and core plasma transport, including the edge transport barrier (ETB), has been analysed in the divertor detachment discharges of deuterium plasmas in LHD with resonant magnetic perturbation (RMP) field application. The RMP application introduces a widened edge stochastic layer and sharp boundary in the magnetic field structure between the confinement region and the edge stochastic layer. The widened edge stochastic layer enhances impurity radiation and provides stable detachment operation as compared with the case without RMP. It is found that ETB is formed at the confinement boundary at the onset of detachment transition. However, as the detachment deepens, the resistive pressure gradient-driven MHD mode is excited, which degrades the ETB. At the same time, however, the core transport decreases to keep global plasma stored energy (W p) unchanged, showing clear core-edge coupling. After a gradual increase of density fluctuation during the MHD activity, a spontaneous increase of W p and the recovery of ETB are observed while the detachment is maintained. Then, the coherent MHD mode ceases and ELM-like bursts appear. In the improved mode, impurity decontamination occurs, and the divertor heat load increases slightly. Key controlling physical processes in the interplay between core and cold edge plasma are discussed. A comparison between deuterium and hydrogen plasmas shows that hydrogen plasmas exhibit similar features to the deuterium ones in terms of density and magnetic fluctuations, impurity decontamination towards higher confinement, etc. But most of the features are modest in the hydrogen plasmas and thus no clear confinement mode transition with clear ETB formation is defined. Better global confinement is obtained in the deuterium plasmas than the hydrogen ones at a higher radiation level.

Transition kinetics of defect patterns in confined two-dimensional smectic liquid crystals.

Topological defects in liquid crystals under confined geometries have attracted extensive research interests. Here, we perform molecular dynamics simulations to investigate the formation and transition of defect patterns in two-dimensional smectic Gay-Berne liquid crystals with a simple rectangular confinement boundary. Two typical types of defect patterns, bridge and diagonal defect patterns, are observed, which can be transformable continuously between each other over time. The transition usually starts from the line or point defect regions, and the competition between neighboring and opposite boundary effects induces the continuous realignments of the smectic layers to connect the neighboring or opposite walls. The relative stability of these two defect patterns can be controlled by changing the confinement conditions. These results deepen our understanding of transition kinetics of defect patterns in confined liquid crystals.

Neutron diffraction illustrates residual stress behavior of welded alloys used as radioactive confinement boundary

Corrosion-resistant welded alloys are frequently used as a leak-tight boundary in critical applications that require confinement of hazardous and/or radioactive substances, including an increasing population of spent nuclear fuel (SNF) canisters. The behavior of residual stresses generated as a result of irregular elastic–plastic deformation during processes such as welding is one of today's key issues to a full understanding of the aging mechanisms that may compromise the confinement boundary. Whether such processes and any subsequent weld repairs, not subjected to post-weld heat treatment, would negatively affect the initial material by introducing through-thickness tensile stresses remains an open question. Here we report the first residual stress measurements using neutron diffraction on the welded joints of a SNF canister. We found significant tensile residual stresses in the as welded sample, indicating that initiation and through-thickness growth of cracks may be possible. Following repair, we observed a stress redistribution and introduction of beneficial compressive stresses. We anticipate our results will improve understanding of confinement susceptibility to aging and guide improvements in repair techniques.

Investigation of the capsule detection method at the irradiation end of the ITER neutron activation system

Due to the harsh radiation, thermal, and electromagnetic environment of the irradiation end of the ITER neutron activation system, which also forms a primary vacuum boundary and a nuclear safety confinement boundary simultaneously, installing a capsule detection sensor at this location, which is essential for reliable operation of the system is extremely challenging. Alternative capsule detection methods utilizing sensors located in more benign environments are investigated experimentally in this study. The experimental result suggests that a simultaneous measurement of the pressure and the acoustic/vibration wave will provide reliable information on the capsule arrival at the irradiation end.

Role of the phonon confinement effect and boundary scattering in reducing the thermal conductivity of argon nanowire.

The thermal conductivity of model argon nanowires over a wide range of temperatures from 20 K to 70 K has been calculated using the formula obtained by solving the Boltzmann equation and independently by molecular dynamic (MD) simulations. The theoretical predictions for thermal conductivity take into account the effect of phonon confinement and boundary scattering. Two known theoretical approaches were used. The first approach is based on the solution of the Boltzmann equation with given boundary conditions and uses bulk acoustic phonon dispersion and neglects the phonon confinement effect. The second approach includes also the modification of acoustic phonon dispersion due to spatial confinement. In simulations, the square and circular shapes of wire with the transverse size of nanowires from 4.3 nm to 42.9 nm have been considered. It was found that MD simulation results match the theoretical predictions reasonably well. The obtained results showed that the phonon confinement effect influences the thermal conductivity of nanowires, but the dominant factor decreasing the thermal conductivity with the thickness of nanowires is boundary scattering. Moreover, the values of the interface specular parameter indicate that the specular phonon-boundary scattering prevails over diffuse phonon-boundary scattering.

Structural Integrity Assessment of ITER Torus Cryo-Pump Housing

In this article, a structural integrity assessment of ITER Torus Cryo-Pump Housing (TCPH) is presented. TCPH is a penetration structure located on the cryostat cylinder whose main function is to accommodate and support the Torus Cryo-Pump (TCP), connect it to the vacuum vessel (VV), and provide tritium confinement and primary vacuum boundary. The design of the TCPH structure was finalized during the final design review (FDR). After the FDR, based on the manufacturing feasibility study, changes in the design were proposed to facilitate the nondestructive examination of weld joints. To qualify the proposed changes, structural integrity assessment was performed as per Section VIII Div. 2 Part 5 of ASME code by finite-element analysis using ANSYS software. The structural and thermal assessment was done as per TCPH load specification for protection against plastic collapse, local failure, ratcheting, buckling, and fatigue to perform the structural integrity of the TCPH. The results obtained from the integrity assessment qualified the design criteria. Margins available over allowable stress values for plastic collapse, local failure, and ratcheting are 20%, 19%, and 30%, respectively. A buckling factor of 79 was achieved with a minimum allowable value of 13.42. For fatigue, a usage fraction of 0.516 was obtained against allowable maximum value of 1. Thus, the design with proposed changes was qualified and incorporated.

Remote handling strategy and prototype tooling of the ITER vacuum vessel pressure suppression system bleed line valve assembly and rupture disk assembly

As part of the development work performed under the ITER Robotic Test Facility (IRTF) program, development and substantiation of the maintenance strategy for the Vacuum Vessel Pressure Suppression System (VVPSS) must be investigated. The VVPSS is a key safety aspect of the vacuum vessel of ITER. It ensures that after a coolant leak, and subsequent pressure event, the vessel is protected from the expanding gasses by controlling and venting them appropriately. After an Ingress of Coolant Event (ICE), or during maintenance, the bleed line valve and rupture disk assemblies will require removal and replacement. During removal the confinement function of vessel must be retained. The remote handling of these components while maintaining the first confinement boundary is challenging due to the access restrictions around the pipe flange and environmental conditions in the area. To understand and develop the remote handling strategy for ITER, prototype tooling a mock-up environment has been created. This allows development and confidence in the proposed maintenance strategy leading to the final solution for use at ITER. In this paper we will discuss the prototype tests, the strategies and challenges which need to be overcome for the ITER application, and design considerations that must be taken to future designs to achieve a feasible solution for ITER.

Numerical Predictions of a Swirl Combustor Using Complex Chemistry Fueled with Ammonia/Hydrogen Blends

Ammonia, a chemical that contains high hydrogen quantities, has been presented as a candidate for the production of clean power generation and aerospace propulsion. Although ammonia can deliver more hydrogen per unit volume than liquid hydrogen itself, the use of ammonia in combustion systems comes with the detrimental production of nitrogen oxides, which are emissions that have up to 300 times the greenhouse potential of carbon dioxide. This factor, combined with the lower energy density of ammonia, makes new studies crucial to enable the use of the molecule through methods that reduce emissions whilst ensuring that enough power is produced to support high-energy intensive applications. Thus, this paper presents a numerical study based on the use of novel reaction models employed to characterize ammonia combustion systems. The models are used to obtain Reynolds Averaged Navier-Stokes (RANS) simulations via Star-CCM+ with complex chemistry of a 70%–30% (mol) ammonia–hydrogen blend that is currently under investigations elsewhere. A fixed equivalence ratio (1.2), medium swirl (0.8), and confined conditions are employed to determine the flame and species propagation at various operating atmospheres and temperature inlet values. The study is then expanded to high inlet temperatures, high pressures, and high flowrates at different confinement boundary conditions. The results denote how the production of NOx emissions remains stable and under 400 ppm, whilst higher concentrations of both hydrogen and unreacted ammonia are found in the flue gases under high power conditions. The reduction of heat losses (thus higher temperature boundary conditions) has a crucial impact on further destruction of ammonia post-flame, with a raise in hydrogen, water, and nitrogen through the system, thus presenting an opportunity of combustion efficiency improvement of this blend by reducing heat losses. Final discussions are presented as a method to raise power whilst employing ammonia for gas turbine systems.

Preparation and formaldehyde sensitive properties of N-GQDs/SnO2 nanocomposite

Graphene quantum dots (GQDs) have both the properties of graphene and semiconductor quantum dots, and exhibit stronger quantum confinement effect and boundary effect than graphene. In addition, the band gap of GQDs will transform to non-zero from 0 eV of graphene by surface functionalization, which can be dispersed in common solvents and compounded with solid materials. In this work, the SnO2 nanosheets were prepared by hydrothermal method. As the sensitizer, nitrogen-doped graphene quantum dots (N-GQDs) were prepared and composited with SnO2 nanosheets. Sensing performance of pristine SnO2 and N-GQDs/SnO2 were investigated with HCHO as the target gas. The response (Ra/Rg) of 0.1% N-GQDs/SnO2 was 256 for 100 ppm HCHO at 60 °C, which was about 2.2 times higher than pristine SnO2 nanosheet. In addition, the material also had excellent selectivity and low operation temperature. The high sensitivity of N-GQDs/SnO2 was attributed to the increase of active sites on materials surface and the electrical regulation of N-GQDs. This research is helpful to develop new HCHO gas sensor and expand the application field of GQDs.

Graphene Quantum Dots Induce Autophagy and Reveal Protection Against Hydrogen Peroxide-Induced Oxidative Stress Injury.

As zero-dimension nanomaterials, graphene quantum dots (GQDs) have the excellent characteristics of graphene as well as photoluminescence (PL) features that ordinary graphene does not possess for its virtue of quantum confinement effect and boundary effect, which attract lots of researchers interested in the field of biomedical applications. In this work, we found GQDs can be internalized into SGC-7901 cells, and the labeled cells exhibited bright yellow fluorescence in the form of aggregated dots. Meanwhile, GQDs effectively mitigated the injury hydrogen peroxide (H2O2)-induced in SGC-7901 cells including cytotoxicity and apoptosis. While H2O2 exposure increased ROS and decreased mitochondrial membrane potential, it could be arrested by GQDs. Furthermore, GQDs showed protective effects against the reduction of the ratio of mitochondrial apoptosis-related proteins Bcl-2/Bax in SGC-7901 cells. Our work revealed that GQDs had the antioxidant ability against oxidative stress, which meant they had the potential to be an antioxidants. Besides, GQDs increased the level of autophagy-related protein LC3-II and induced autophagic structure observed via TEM, and subsequent the results of LC3 turnover and p62 degradation demonstrated GQDs treatment led to the activation of autophagic flux. Ultimately, GQDs were found to affect autophagy through the mTOR signaling pathway.

EM analyses on the 55.NE.V0 loom system and attached components

The ITER Vacuum Vessel contains transducers for vessel and blanket instrumentation and for the measurement of plasma performance. The electrical and optical signals to and from these transducers are managed through the electrical services infrastructure project 55.NE.V0. This system is made of around 1400 sensors/junction boxes/connectors and more than 1700 MI cables, distributed throughout the vacuum vessel inner surface.This paper concerns EM analyses performed as part of the full set of engineering analyses necessary to assess the design configuration feasibility and to demonstrate a sustainable impact on the VV, which is a confinement boundary and ESPN equipment.Transient EM analyses were carried out for 4 representative looms systems placed at the inboard and outboard region of the VV. The calculated distributed EM loads along the cables and in attached components allow the most critical positions for connecting clamps and junction boxes to be identified, that were subsequently analyzed with detailed component models, thus obtaining an accurate and detailed geometrical distribution of the most critical EM loads inside these components. All these analyses were made possible also thanks to the development of a dedicated “1d wire element” that made the modeling of the looms cables numerically efficient and manageable.

Design evolution of the diamond window unit for the ITER EC H&CD upper launcher

The torus window unit is a very particular component of the ITER EC H&CD upper launcher aiming to provide the primary vacuum and confinement boundary between the vacuum vessel and the transmission lines (TLs). The high power 170 GHz millimeter-wave beams generated by the gyrotrons travel along the TLs and pass through the window units, before being quasi-optically guided into the plasma via the upper launchers. The design of the window unit shall thus meet stringent requirements to guarantee the safety function, the millimeter-wave beam transmission and the structural integrity during normal operation and off-normal events. The unit consists of an ultra-low loss CVD diamond disk brazed to two copper cuffs; this structure is then integrated into a metallic housing by welding. The compliance with the requirements shall be assured by applying the ASME Section III – Subsection NC code and a dedicated experimental qualification program. This paper reports the way in which the design of the unit, already optimized by FEM analyses against the ITER loading conditions, was further improved by the application of the ASME III-NC code, leading to a more feasible and simpler manufacturing and assembling sequence. In addition, the impact of the ITER project decision to change the inner diameter of the waveguide from 63.5 to 50 mm, to improve the beams’ mode purity, was assessed and it is also discussed. Different materials for the metallic housing and in particular for the millimeter-wave inserts of the unit were compared using appropriate engineering criteria to mitigate the significant increase of the millimeter-wave thermal loads on the waveguides when the diameter is decreased.

Development of Primary Vacuum Windows for ITER Diagnostics

Most of ITER's diagnostics will be provided with viewing lines (optical, microwave, and spectroscopic) for the monitoring of key characteristics of the plasma or for the achievement of physical measurements inside the vacuum vessel. The nature of the physical signal transmitted through the viewing lines requires the implementation of window assemblies incorporating nonmetallic window. Placed at the vacuum boundary, the window assembly shall also ensure the vacuum integrity required for the plasma. Moreover, the diagnostic window assemblies form part of the ITER primary confinement boundary, and thus, directly related to the nuclear safety. This paper gives an overview of their design and related on-going engineering assessments by analysis or tests in the final design phase.

Influence of electron quantum confinement on the strength of carbon nanotube bundles

Radial confinement and specific boundary conditions lead to inhomogeneous spatial distributions for conduction electrons in metallic carbon nanotubes.Such uneven spread of negative charge on the surface of a carbon nanotube can induce Coulomb-type interactions between parallel tubes, contributing to the relative friction. Thus, the axial conduction electron density on a single idealized carbon nanotube closed at both ends was obtained in the framework of the two-dimensional quasi-free electron approximation. A Coulomb potential energy between two parallel nanotubes was calculated as a function of the longitudinal offset between them. The study provides a simple method for estimating the friction force related to Coulomb inter-tube interactions appearing during a parallel sliding motion, with implications in the stretching resistance of various carbon nanotube bundles and in designing various nano-electromechanical devices.

Mechanical behaviour of segmental lining of a sub-rectangular shield tunnel under self-weight

In this paper, a novel loading setup was developed for performing full-scale loading tests on ‘standing’ segmental lining of a sub-rectangular shield tunnel, which for the first time allows the mechanical behaviour of segmental lining subject to self-weight to be assessed in a full-scale loading test. Granular rubber bearings with an equivalent coefficient of subgrade reaction close to hard plastic clay were used to simulate subgrade soils. The experimental results showed that the internal force distribution and deformation pattern of lining ring subject to self-weight loading differed significantly from that observed in a ‘lying’ loading test considering soil-and-water pressures due to the absence of lateral confinement and different boundary conditions. Specifically, maximum positive bending moments were observed at the crown and invert regions, and the largest negative bending moments occurred at the two waists. Stress concentration was not observed at the corners. The axial force was not universally compressive with tensile axial forces present at the crown region. Furthermore, the convergence deformation under self-weight loading was non-negligible. The overall lateral expansion deformation was close to the vertical convergence deformation, but the deformation was smaller at the invert than that at the crown. It is suggested that the influence of self-weight be considered in future full-scale loading tests, especially for shallowly-buried tunnels.

Confined beryllium atom electronic structure and physicochemical properties

Confined beryllium atom ground and first excited states electronic structures are calculated by the direct variational method, taking into account the system asymmetric nature of the trial wave function, adding a cutoff function to ensure confinement boundary conditions. The trial wave function is built up from hydrogenic functions, which constitute an adequate basis for energies calculation. Physicochemical properties such as kinetic energy, pressure, and polarizability are also calculated from energy results previously obtained to different confined radii. Using different variational parameters in each hydrogenic function, the energy approximation obtained is improved. Electronic configuration changes as we move toward the strong confinement region (small cavity radii) in function of its atomic number using impenetrable walls, this region was obtained for Z = 4. This is a conclusion of this work. Another important result is that this method is computationally simpler and gives values inside the experimental precision. Aforementioned results are compared with other theoretical publications.