Layer Spreading Research Articles

Deep X-Ray Reflectometry of Supermultiperiod A3B5 Structures with Quantum Wells Grown by Molecular-Beam Epitaxy

Elastically strained supermultiperiod (100–1000 periods) AlGaAs/GaAs superlattices with different doping levels and slightly differing period thicknesses have been investigated. The proposed technique of characterization consisting of combined application of deep X-ray reflectometry based on a rigorous calculation method, as well as the well-known method of high-resolution X-ray reflectometry, has made it possible to investigate 100-period structures with 2-nm-wide Al0.3Ga0.7As barriers and 10-nm-wide GaAs wells and determine with a high accuracy the layer thicknesses and spread of interfaces. This achievement can be considered as a first step in further analysis of thick structures using bright synchrotron radiation source. The difference between the expected and obtained by the proposed method layer thicknesses is several percent, including that for samples with a high doping level (up to 1018 cm–3). All supermultiperiod structures are characterized by sharp interfaces with a standard deviation of about 0.1 nm. Based on the obtained data on the thicknesses, one can accurately determine the layer compositions using high-resolution X-ray diffraction.

CFD modeling of multiphase flow in an alkaline water electrolyzer

The hydrodynamic properties of gas–liquid flows in water electrolyzers are of great practical interest since the local distribution of gas influences the amount of electrical energy required to produce hydrogen. We used the Euler-Euler model to simulate the multiphase flow in a water electrolyzer and compared the results to existing experimental data, for a range of current densities. Our study shows that if only the drag force and buoyancy force are incorporated in the model, the spreading of the gas layers formed at the electrodes is not accurately predicted. By adding the turbulence dispersion force to the model, reasonable agreement with the experimental data could be obtained for the higher current densities. The turbulence dispersion had to be implemented via user-defined functions, in order to obtain results that satisfied the momentum balance. In addition the effect of different turbulence models on the turbulent dispersion was investigated.

Experimental study of a lenticular jet.

Non-circular jets, particularly elongated jets and jets with sharp corners, are of interest due to their enhanced large- and small-scale mixing and combustion stability. Non-circular jet geometries such as elliptic, triangular, rectangular, and square have been extensively studied. The non-uniform curvature and elongated design of elliptic and rectangular jets has been shown to facilitate axis switching. Additionally, jets with sharp corners have been shown to facilitate small-scale mixing and further vortex ring deformation, as has been observed in triangular, square, and rectangular jets. There are, however, currently no data on the flow characteristics of a jet that combines these two features, i.e., it enhances both large- and small-scale mixing. The jet is issuing from a nozzle shape defined by two arcs of a circle connecting at sharp corners in the shape of an eye (hereafter termed 'lenticular'). The current study uses experimental techniques to characterize the lenticular jet and compare its behavior to previously studied circular and non-circular jets. Additionally, snapshot POD analysis was used to identify coherent structures and their dynamic features. The lenticular jet was found to have higher entrainment and stronger mixing than a traditional round jet, and comparable mixing and entrainment characteristics as previously studied non-circular jet geometries. It is particularly interesting to compare the jet behavior of the lenticular jet to an elliptic jet. The geometry of these two jets is extremely similar, the difference being sharp corners along the major axis of the lenticular jet. By comparing the lenticular jet and the elliptic jet, the effect of sharp corners, which have been shown to increase small-scale mixing and vortex ring deformation, can be observed. It was found that along the minor axis, where geometry was similar, shear layer turbulence intensity and spreading rate of the jets were also similar. Along the major axis, however, introducing corner features in the lenticular jet lowered the spread rate, resulted in a faster breakdown of turbulence in the shear layer, increased exit turbulence, and resulted in anisotropic centerline turbulence.

Experimental study on the spread and burning behaviors of continuously discharge spill fires under different slopes

This paper examines the effects of the slope on the burning and spread process of JP-4 continuous spill fires. Spill fires experiments were conducted on surfaces with different slope angles (0˚–3˚) in a rectangular trench (0.8m×6m). The spread and burning behaviors including the spread process, burning rate and flame height are recorded and analyzed. The results indicate that the whole spread process can be divided, based on the burning area variations with time, into four phases: 1) burning layer spread, 2) shrink process, 3) steady burning, and 4) extinguishment. The results also show that a large slope can increase the spread rate and as a result shorten the duration of the burning layer spread and shrink process phases. In addition, it is found that the slope has a more significant effect on the maximum spread area than the steady burning area. The steady burning rate decreases with increasing slope and the ratio of the steady burning rate of a spill fire and that of the corresponding pool fire is nearly constant. The flame height of continuous spill fires is also well predicted by an empirical model with a dimensionless heat release rate and equivalent pool diameter. The experimental data presented in the work will provide a basis for further studies of liquid fuel spill fire on an inclined surface.

Time-Resolved Dynamics of Struvite Crystallization: Insights from the Macroscopic to Molecular Scale.

The crystallization of magnesium ammonium phosphate hexahydrate (struvite) often occurs under conditions of fluid flow, yet the dynamics of struvite growth under these relevant environments has not been previously reported. In this study, we use a microfluidic device to evaluate the anisotropic growth of struvite crystals at variable flow rates and solution supersaturation. We show that bulk crystallization under quiescent conditions yields irreproducible data owing to the propensity of struvite to adopt defects in its crystal lattice, as well as fluctuations in pH that markedly impact crystal growth rates. Studies in microfluidic channels allow for time-resolved analysis of seeded growth along all three principle crystallographic directions and under highly controlled environments. After having first identified flow rates that differentiate diffusion and reaction limited growth regimes, we operated solely in the latter regime to extract the kinetic rates of struvite growth along the [100], [010], and [001] directions. In situ atomic force microscopy was used to obtain molecular level details of surface growth mechanisms. Our findings reveal a classical pathway of crystallization by monomer addition with the expected transition from growth by screw dislocations at low supersaturation to that of two-dimensional layer generation and spreading at high supersaturation. Collectively, these studies present a platform for assessing struvite crystallization under flow conditions and demonstrate how this approach is superior to measurements under quiescent conditions.

Model-based investigation of membrane biofilm reactors coupling anammox with nitrite/nitrate-dependent anaerobic methane oxidation

An innovative process coupling anaerobic ammonium oxidation (anammox) with nitrite/nitrate-dependent anaerobic methane oxidation (n-DAMO) in membrane biofilm reactors (MBfRs) has been developed to achieve high-level nitrogen removal from both sidestream (i.e., anaerobic digestion liquor) and mainstream (i.e., domestic strength) wastewater. In this study, a 1D biofilm model embedding the n-DAMO and anammox reactions was developed to facilitate further understanding of the process and its optimization. The model was calibrated and validated using comprehensive data sets from two independent MBfRs, treating sidestream- and mainstream-strength wastewater, respectively. Modelling results revealed a unique biofilm stratification. While anammox bacteria dominated throughout the biofilm, n-DAMO archaea (coupling nitrate reduction with anaerobic methane oxidation) only occurred at the inner layer and n-DAMO bacteria (coupling nitrite reduction with anaerobic methane oxidation) spread more evenly with a slightly higher fraction in the outer layer. The established MBfRs were robust against dynamic influent flowrates and nitrite/ammonium ratios. Thicker biofilms were beneficial for not only the total nitrogen (TN) removal but also the system robustness. Additionally, a positive correlation between the nitrogen removal efficiency and the residual methane emission was observed, as a result of higher methane partial pressure required. However, there was a threshold of methane partial pressure, above which the residual methane increased but nitrogen removal efficiency was stable. Meanwhile, thicker biofilms were also favorable to achieve less residual methane emission. Simulation results also suggested the feasibility of methane-based MBfRs to polish mainstream anammox effluent to meet a stringent N discharge standard (e.g., TN < 5 mg/L).

Measuring the spreadability of pre-treated and moisturized powders for laser powder bed fusion

For AM processes—specifically Laser Powder Bed Fusion (L-PBF) processes—powder flowability is essential for the product quality, as these processes are based on a thin layer spreading mechanism. However, the available techniques to measure this flowability do not accurately represent the spreading mechanism. Hence, this paper presents two novel applicator tools specifically designed to test the spreadability of l-PBF powders in thin layer application. The results were checked by running standard tests to analyze the powder morphology, moisture content, chemical composition and flowability using the Hall-flowmeter. For this study, four common l-PBF metal powders were selected: Inconel 718, Ti6Al4V, AlSi10Mg and Scalmalloy. From the as-received state, drying (vacuum and air) and moisturizing treatments were applied to compare four humidity states and investigate the feasibility of pre-treating the powders to remove moisture, which is known to cause problems with flowability, porosity formation and enhanced oxidation. The tests reveal that AlSi10Mg is the most susceptible alloy to moisture and oxygen pick-up, considerably decreasing the spreadability and relative density on the build platform. However, the results also reveal how challenging the direct measurement of moisture levels in metal powders is.

Kinetic energy evaluation for the steam explosion in a shallow pool with a spreading melt layer at the bottom

Steam explosion experiments with a melt layer spreading at the bottom of a shallow water pool, namely the PULiMS-E6 and SES-S1 by KTH, Sweden, were simulated by the steam explosion simulation code, JASMINE. The observed impulses in the experiments were successfully reproduced by simulations with assumed premixing conditions. With those simulation results, the adequacy of the kinetic energy evaluation method used for the experiments were examined by comparison of the kinetic energy directly obtained in the simulation, Ek, and the one evaluated based on the impulse and the water mass limited to the center area above the premixing zone, Ekic. It showed that the impulse based kinetic energy evaluation gives about five times overestimation. The impact of the water pool geometry on the validity of the impulse based kinetic energy evaluation method was further examined by a parametric study with variations of the pool geometry in the simulations of PULiMS-E6 and SES-S1 as well as high pressure bubble expansion simulations. The results for the relation of Ekic/Ek and the geometric factors were consistent between the cases for the experiments and the bubble expansion. The results showed that: (1) for the shallow water pool regime, Ekic/Ek shows a trend of convergence to 4–5, (2) for deep water pool regime, the impulse based kinetic energy evaluation with the whole water mass, Eki, rather than Ekic, gives a good estimation. A set of empirical formulas was obtained for Ekic/Ek.

Modeling of the stamping process of box-type forging

A new mathematical model for manufacturing rectangular of box-type forgings is proposed. Along the outer contour of the forging there are walls with thickness comparable to the thickness of the bottom part of the forging. To calculate the power parameters of the technological process under study, the authors use the theory of flow in a relatively thin plastic layer as described by A. A. Ilyushin. A thickness-averaged two-dimensional mathematical model of the plastic layer spreading process is described by nonlinear partial differential equations of the first order with respect to the contact pressure and the flow velocity components. For a metal that is in a flat deformed state, the condition of complete plasticity is assumed, under which the tangent stresses in the layer are small in comparison with normal stresses and they can be ignored, and two normal stresses in the cross section can be considered equal in the first approximation. The third normal stress is equal to the contact pressure of the layer on the tool. In this case, it is sufficient to set a single boundary condition on the border of the regions.

Hydrophilic polymers: interactions with lipid layers and ferning patterns

AbstractPurposeTo evaluate (i) the mucomimeticity of pharmaceutically applicable hydrophilic polymers (HP) as well as (ii) their interactions with human meibum films.MethodsFour HP were used‐hyaluronic acid (HA), cross‐linked hyaluronic acid (CHA), carboxymethyl cellulose (CMC) and gellan gum (GG)‐ at the concentrations of 0.0001, 0.001,0.01,0.05, 0.1%. HP mucomimeticity was probed by the formation of tear‐like ferning patterns (TFP) after HP solution drop evaporation. The interactions between HP and meibum at the air/water interface were examined in vitro at blink‐like compression/expansion of the film area by Langmuir surface balance. Surface pressure (π)‐area (A) isocycles and stress relaxations were used to assess the sample’s lateral elasticity and reorganization during area cycling. Films morphology was monitored by Brewster Angle Microscopy (BAM).ResultsSignificant differences were observed in the capability of HP to form ferning patterns: CHA (≥0.001%) &gt; HA (≥0.01%) = CMC (≥0.01%) &gt; GG (≥0.05%). CHA, HA, CMC and GG enhanced the uniform spreading of meibomian layers as well as their elasticity and rapid reorganization at blink like area cycling in concentration dependent manner. BAM micrographs revealed considerable thickening of the meibomian films in the presence of HP.ConclusionsThe combination of tear ferning and Langmuir surface balance studies allows to simultaneously evaluate the mucomimetic and the lipid interaction properties of HP. TFP forming polymers enhanced both (i) the uniform spreading and structure of meibomian layers and (ii) their dynamic surface properties. Cross‐linking HA was particularly efficient both in ferning pattern and meibomian film systems.

Spreading Process Maps for Powder-Bed Additive Manufacturing Derived from Physics Model-Based Machine Learning

The powder bed additive manufacturing (AM) process is comprised of two repetitive steps—spreading of powder and selective fusing or binding the spread layer. The spreading step consists of a rolling and sliding spreader which imposes a shear flow and normal stress on an AM powder between itself and an additively manufactured substrate. Improper spreading can result in parts with a rough exterior and porous interior. Thus it is necessary to develop predictive capabilities for this spreading step. A rheometry-calibrated model based on the polydispersed discrete element method (DEM) and validated for single layer spreading was applied to study the relationship between spreader speeds and spread layer properties of an industrial grade Ti-6Al-4V powder. The spread layer properties used to quantify spreadability of the AM powder, i.e., the ease with which an AM powder spreads under a set of load conditions, include mass of powder retained in the sampling region after spreading, spread throughput, roughness of the spread layer and porosity of the spread layer. Since the physics-based DEM simulations are computationally expensive, physics model-based machine learning, in the form of a feed forward, back propagation neural network, was employed to interpolate between the highly nonlinear results obtained by running modest numbers of DEM simulations. The minimum accuracy of the trained neural network was 96%. A spreading process map was generated to concisely present the relationship between spreader speeds and spreadability parameters.

Pore space characteristics and corresponding effect on tensile properties of Inconel 625 fabricated via laser powder bed fusion

In this work, the tensile behavior of Inconel 625 parts fabricated via laser powder bed fusion (LPBF) at different laser power levels is examined, and correlated to bulk porosity as well as pore characteristics such as pore size, aspect ratio morphology, and polar orientation extracted from X-ray computed tomography (CT). Scanning electron microscopy (SEM) is employed to identify the fracture mode and origin of failure in the pulled samples. Microstructural examination on the as-built samples showed that increasing the laser power resulted in the transition of melting mode, from lack of fusion to keyhole, with an increase in part bulk density from 98.86% to 99.29%, respectively. It was found that the general bulk porosity level does not correlate directly with the Ultimate Tensile Strength (ranging between 780–820 MPa) and strain to fracture (ranging between 0.2–0.39) behavior of the parts. Detailed pore space characteristics obtained from CT datasets before and after the tensile test contributed to establishing a relationship between defects size, morphology, orientation and tensile properties of the samples. In general, it was found that strain to failure is directly influenced by pore space characteristics, while tensile strength is influenced by a combination of pore space and microstructural characteristics. This study also identified that there are systematic bias effects in the LPBF process, likely introduced by the combination of nuisance variables such as powder layer spreading and gas flow.

Treatment of model uncertainty from radiation by the Stochastically Perturbed Parametrization Tendencies (SPPT) scheme and associated revisions in the ECMWF ensembles

The Stochastically Perturbed Parametrization Tendencies (SPPT) scheme is used in ensemble forecast systems to represent uncertainties due to the atmospheric physics parametrizations. The scheme works by introducing random perturbations to the tendencies from the physics schemes. The size of the perturbations is modulated by the size of the net tendencies from the full physics suite. Recent work has identified an inconsistency in this approach to attributing model uncertainty: greater uncertainty is attributed to the night‐time sky than to the daytime sky in clear‐sky conditions, which is not deemed physically realistic. This paper documents the details of a revised configuration of SPPT, which became active in the recent operational version of the ensemble forecast system (“CY45R1”) at the European Centre for Medium‐Range Weather Forecasts (ECMWF) — released in June 2018. In this work, a modification to SPPT is outlined such that the clear‐sky contribution from the radiation scheme is subtracted from the physics tendencies that are acted on by SPPT. Inspection of individual vertical profiles of physics tendencies provides a greater understanding of the interactions between processes, and helps demonstrate that the revision gives a more realistic description of the associated uncertainties. Forecast experiments with the revised version of SPPT demonstrate neutral to positive impacts on the skill of the ensemble forecasts in the medium‐range; and in the extended‐range, a significantly improved match between the spread and error of the principal components describing the Madden–Julian Oscillation. In addition, it is shown that including the revisions in the ensemble of data assimilations leads to increases in spread in the boundary layer.

Suppression of epidemic spreading process on multiplex networks via active immunization.

Spatial epidemic spreading, a fundamental dynamical process upon complex networks, attracts huge research interest during the past few decades. To suppress the spreading of epidemic, a couple of effective methods have been proposed, including node vaccination. Under such a scenario, nodes are immunized passively and fail to reveal the mechanisms of active activity. Here, we suggest one novel model of an observer node, which can identify infection through interacting with infected neighbors and inform the other neighbors for vaccination, on multiplex networks, consisting of epidemic spreading layer and information spreading layer. In detail, the epidemic spreading layer supports susceptible-infected-recovered process, while observer nodes will be selected according to several algorithms derived from percolation theory. Numerical simulation results show that the algorithm based on large degree performs better than random placement, while the algorithm based on nodes' degree in the information spreading layer performs the best (i.e., the best suppression efficacy is guaranteed when placing observer nodes based on nodes' degree in the information spreading layer). With the help of state probability transition equation, the above phenomena can be validated accurately. Our work thus may shed new light into understanding control of empirical epidemic control.

Influence spread in two-layer interdependent networks: designed single-layer or random two-layer initial spreaders?

Influence spread in multi-layer interdependent networks (M-IDN) has been studied in the last few years; however, prior works mostly focused on the spread that is initiated in a single layer of an M-IDN. In real world scenarios, influence spread can happen concurrently among many or all components making up the topology of an M-IDN. This paper investigates the effectiveness of different influence spread strategies in M-IDNs by providing a comprehensive analysis of the time evolution of influence propagation given different initial spreader strategies. For this study we consider a two-layer interdependent network and a general probabilistic threshold influence spread model to evaluate the evolution of influence spread over time. For a given coupling scenario, we tested multiple interdependent topologies, composed of layers A and B, against four cases of initial spreader selection: (1) random initial spreaders in A, (2) random initial spreaders in both A and B, (3) targeted initial spreaders using degree centrality in A, and (4) targeted initial spreaders using degree centrality in both A and B. Our results indicate that the effectiveness of influence spread highly depends on network topologies, the way they are coupled, and our knowledge of the network structure — thus an initial spread starting in only A can be as effective as initial spread starting in both A and B concurrently. Similarly, random initial spread in multiple layers of an interdependent system can be more severe than a comparable initial spread in a single layer. Our results can be easily extended to different types of event propagation in multi-layer interdependent networks such as information/misinformation propagation in online social networks, disease propagation in offline social networks, and failure/attack propagation in cyber-physical systems.

X-Ray Study of Electrochemical Stern Layer: Ordering and Layering

Electrochemical double layers (EDL) form at the interfacial region and have been described by Gouy and Chapman (GC) model[1] or by modified versions of GC model. However, the GC model is suspected to break down at large overpotentials, because it predicts the unlimited rise in differential capacitance. Stern suggested long ago that there should be a layer with a finite ion density, known as ‘Stern layer’[2]. However, direct observation of Stern layer has long been remained elusive to date. In this talk, we present synchrotron x-ray observation[3] of Stern layer on Pt(111) surface at −850 mV (vs. Ag/AgCl) in alkali fluoride and chloride electrolytes. First, by a direct inversion method[4] for normalized crystal truncation rods, the formation of three sharp layers and one diffuse layer was observed. The diffuse layer spreads over one nanometer, consistent with the Cs+ diffuse layer predicted in GC model. The strong sharp layer among three layers is a Cs+ layer near the Helmholtz plane at 3.5 Å from the Pt(111) surface. Second, in-plane diffraction was used to show that the sharp layers form a (2×2) lattice with one Cs+ per unit cell (Fig. 1). The (2×2) nature of unit cell was used in identifying the two weak water layers. Combination of the x-ray results for the structures normal and parallel to the surface yielded a three dimensional model of Stern layer (Fig. 1). The Cs+ layer is close to Pt(111) surface layer leaving no space for hydrating water molecule. However, three water molecules occupy the empty (2×2) sites, therefore, providing a lateral hydration shell. By adding the three more water molecules above each Cs+, they complete the hydration shell with the Cs+-water distance of 3.1 Å that is in good agreement with known hydration shell distance. Additional evidence for the existence of dense Stern layer comes from potential jump experiments. [3] In a series of jump experiments with Li, Na, K, and Cs chloride electrolytes, the mass dependence was observed and explained with a simple model of diffusion. The diffusion of the layers were observed because of the high sensitivity of CTR to layers. By combining all aspects of the x-ray studies, we unambiguously identify the Stern layer and its structure. [1] Bard, A. J.; Faulkner, L. R., Electrochemical methods: fundamentals and applications. 2nd ed.; John Wiley: New York, 2001. [2] Stern, O., The theory of the electrolytic double layer. Zeitschrift Für Elektrochemie 1924, 30, 508. [3] Liu, Y.; Kawaguchi, T.; Pierce, M.S.; Komanicky, V.; You, H., Layering and Ordering in Electrochemical Double Layers, J Phys Chem Lett, 2018, 9, 1265. [4] Kawaguchi, T.; Liu, Y.; Pierce, M. S.; Komanicky, V.; You, H., Direct determination of the one-dimensional interphase structure by crystal truncation rod analysis, J. Appl. Cryst., 2018, 51, 679. Fig. 1. The (2x2) model of Stern layer. The structure is composed of one Cs+ and two water layers with Cs+ and 6H2O. The perspective (a), top (b), and side views (c,d) are shown. Platinum atoms of the working electrode, Cs+ ions, in-plane water molecules, and the top water molecules are shown by grey, blue, red, and pink balls, respectively. Figure 1

Wind(ow) of Change: A Systematic Review of Policy Entrepreneurship Characteristics and Strategies

What lessons can we learn from 40 years of policy entrepreneurship scholarship on policy entrepreneurs’ strategies and defining characteristics? While scholars have offered important insights, many questions remain open. This article systematically reviews 229 articles that were published between 1984 and 2017. Our findings provide (i) an analysis of policy entrepreneurship characteristics by sector, policy domain, individual/group, government layer, and geographical spread, (ii) an empirically based identification and classification of policy entrepreneurship strategies, and (iii) a statistical analysis of the relationship between policy entrepreneurs’ characteristics and strategies. We conclude with an agenda for future studies that will continue to examine new theoretical approaches that advance our understanding of the role that individuals and small groups play in the policy process.

Oil Slick Characterization Using a Statistical Region-Based Classifier Applied to UAVSAR Data

During emergency responses to oil spills on the sea surface, quick detection and characterization of an oil slick is essential. The use of Synthetic Aperture Radar (SAR) in general and polarimetric SAR (PolSAR) in particular to detect and discriminate mineral oils from look-alikes is known. However, research exploring its potential to detect oil slick characteristics, e.g., thickness variations, is relatively new. Here a Multi-Source Image Processing System capable of processing optical, SAR and PolSAR data with proper statistical models was tested for the first time for oil slick characterization. An oil seep detected by NASA`s Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) in the Gulf of Mexico was used as a study case. This classifier uses a supervised approach to compare stochastic distances between different statistical distributions (fx) and hypothesis tests to associate confidence levels to the classification results. The classifier was able to detect zoning regions within the slick with high global accuracies and low uncertainties. Two different classes, likely associated with the thicker and thinner oil layers, were recognized. The best results, statistically equivalent, were obtained using different data formats: polarimetric, intensity pair and intensity single-channel. The presence of oceanic features in the form of oceanic fronts and internal waves created convergence zones that defined the shape, spreading and concentration of the thickest layers of oil. The statistical classifier was able to detect the thicker oil layers accumulated along these features. Identification of the relative thickness of spilled oils can increase the oil recovery efficiency, allowing better positioning of barriers and skimmers over the thickest layers. Decision makers can use this information to guide aerial surveillance, in situ oil samples collection and clean-up operations in order to minimize environmental impacts.

Experimental study on the liquid layer spread and burning behaviors of continuous heptane spill fires

Spill fires usually occur with liquid fuel storage and transportation, which may pose a threat to process safety and environmental protection. Analysis on the liquid layer spread and burning behavior is vital for preventing and mitigating spill fire accidents. In the study, a number of heptane continuous spill fire experiments on a fireproof glass sheet were conducted to better understand the spreading and burning of continuous spill fires. In the experiments, the liquid surface area, burning rate and heat transfer between liquid layer and the glass sheet were analyzed. The results showed that the heptane burning rate of spill fires was lower than that for pool fires of the same burning size. The experimental data showed that the radiative penetration through the liquid layer was mainly responsible for the lower burning rate at the quasi-steady burning phase. Based on the heat transfer process, a modified burning rate model of spill fires by considering the heat loss of liquid layer was developed and validated by the experimental data.

A Comprehensive Approach to Powder Feedstock Characterization for Powder Bed Fusion Additive Manufacturing: A Case Study on AlSi7Mg.

In powder bed fusion additive manufacturing, the powder feedstock quality is of paramount importance; as the process relies on thin layers of powder being spread and selectively melted to manufacture 3D metallic components. Conventional powder quality assessments for additive manufacturing are limited to particle morphology, particle size distribution, apparent density and flowability. However, recent studies are highlighting that these techniques may not be the most appropriate. The problem is exacerbated when studying aluminium powders as their complex cohesive behaviors dictate their flowability. The current study compares the properties of three different AlSi7Mg powders, and aims to obtain insights about the minimum required properties for acceptable powder feedstock. In addition to conventional powder characterization assessments, the powder spread density, moisture sorption, surface energy, work of cohesion, and powder rheology, were studied. This work has shown that the presence of fine particles intensifies the pick-up of moisture increasing the total particle surface energy as well as the inter-particle cohesion. This effect hinders powder flow and hence, the spreading of uniform layers needed for optimum printing. When spherical particles larger than 48 µm with a narrow particle distribution are present, the moisture sorption as well as the surface energy and cohesion characteristics are decreased enhancing powder spreadability. This result suggest that by manipulating particle distribution, size and morphology, challenging powder feedstock such as Al, can be optimized for powder bed fusion additive manufacturing.