Packing Ratios Research Articles

Ignition of Forest Fires by Cigarette Butts: Using Pinus massoniana Needles as an Example

As a cigarette butt falls onto the forest surface fuel, it first smolders the fuel, then ignites into flames, and spreads as forest fire under certain conditions. In this study, the needles under a typical stand of P. massoniana were used as the research object. Needle beds with different moisture content and packing ratios were constructed indoors. Cigarette butt-ignition experiments were conducted under different wind velocities, and 30 experiment cycles were conducted under different conditions. There was a total of 5 (packing ratio) × 4 (moisture content) × 6 (wind velocity) = 120 sets of conditions, and a total of 3600 ignition experiments were conducted. The results showed that (1) the total ignition probability of the cigarette butts was 2.36%, which only occurred when the fuelbed moisture content was <10% and the wind velocity was >1 m/s. The ignition time of cigarette butts ranged from 2.73 to 7.25 min. (2) The fuelbed moisture content and wind velocity significantly influenced the ignition probability and time. With an increase in moisture content, the ignition probability of cigarette butts decreased, while the time required for ignition showed an increasing trend. Wind velocity had a dual effect on ignition. The ignition effect was optimal at a wind velocity of 4 m/s. With an increase in wind velocity, the ignition probability first increased and then decreased, and the ignition time first decreased and then increased. (3) The packing ratio had no significant effect on the ignition probability; however, the ignition time significantly decreased as the packing ratio increased. (4) The logistic regression method (LRM), general linear method (GLM), and nonlinear regression method (NLM) were used to establish a prediction model of ignition probability. The prediction effect of GLM was the worst, followed by LRM, and the NLM had the best prediction effect. The GLM was selected to establish the ignition time model, and the error was also within the allowance range. This study elucidated the underlying mechanism of factors affecting cigarette butt-based fuel ignition. In addition, the established prediction model provides a reference for human-caused forest fires and is highly significant for forest fire prevention.

Comparative Assessment of Wildland Fire Rate of Spread Models: Effects of Wind Velocity

Wildland fire rate of spread prediction models are important tools for the effective coordination of resident evacuation and fire suppression efforts. A comparative assessment of ten empirical and semi-empirical rate of spread prediction models is performed, using a selection of 203 laboratory experiments of surface spreading fires; special emphasis is given to the effects of external wind velocity. Five of the evaluated models have been developed using measurements obtained in laboratory-scale tests; these models are combined with two supplementary sub-models that account for the effects of wind. In addition, a selection of five empirical models that have been developed using large-scale field tests are also assessed. The performance of the ten prediction models is evaluated, both qualitatively and quantitatively, by employing a range of statistical error metrics. The laboratory-developed models are found to exhibit high sensitivity on low fuel load values, when no external wind is present, as well as on low packing ratios and high wind velocity values. The field-developed models exhibit significant discrepancies against the experimental data, due to the use of specific parameters regarding fuel type, scale and wind velocity.

Enhanced Performance of Ru-Based Infrared Imaging Sensor Array With Electrospun Thermal Isolation Structure

Predicting and optimizing thermal conductivity of materials used in infrared thermal imaging sensors is an effective method to improve thermal isolation performance, which can substantially reduce the thermal crosstalk among pixels. This paper reports an electrospun P(VDF-TrFE) nanofiber film used as adiabatic support structure in thermal sensor array, and investigates its effect on imaging performance of thermal sensor. Based on solid heat transfer simulation, the thermal conductivities of electrospun P(VDF-TrFE) films with different packing ratios were theoretically calculated, showing a significant decrease compared with the spin-coated counterpart. The temperature increment of thermal sensor using nanofiber film with 43.71% packing ratio is calculated as 1.68 times higher than that using spin-coated film. An average temperature rise of 1.73 times was experimentally obtained, and the thermal response time was also reduced to half, compared with the sensor using spin-coated film. The method of improving thermal isolation performance using electrospun fibrous film could be of great advantage to high sensitivity infrared imaging sensor.

Modeling the drying process of Masson pine needle fuel beds under different packing ratios based on two-phase models in the laboratory.

When the moisture content of a fuel bed is higher than the fiber saturation moisture content (0.35 gg-1), the drying process is controlled by evaporation (>0.35 gg-1) and diffusion (>0.35 gg-1). Packing ratio has a significant effect on the drying process. Ignoring the impacts of packing ratio or the separate phases of the drying process is one main reason for inaccurate moisture content predictions. This study simulated the drying process in five Masson pine (Pinus massoniana Lamb.) needle beds with different packing ratios. Using the fiber saturation moisture content as the cut-off point, we divided the drying process into two phases. The drying mechanism of each phase was different and had its own drying equation. Using a model that does not distinguish the two phases of the drying process as a comparison, the prediction effect of the two-phase model was analyzed. The influence of the fuel bed packing ratio on the drying process was also analyzed. We found that, regardless of any changes in packing ratio, the two-phase model could better simulate the drying process, with a mean absolute error (MAE) and mean relative error (MRE) of the two-phase model 18.4% and 25.6% less than the one-phase model, respectively. The time-lag prediction model was established with the packing ratio, and the errors were all within the allowable range, but the prediction effect of the time-lag prediction model based on the two-phase model was larger. It was further demonstrated that considering the packing ratio of the fuel bed and distinguishing the two separate phases of the drying process could both effectively improve the prediction accuracy of the moisture content of fuel beds based on the semi-physical method.

Fire spread along vertical greenery systems from window ejected flame: A study based on a fire dynamic simulator model

Recent studies have shown that, poorly maintained vertical greenery systems (VGS) have a high fire risk and a tendency to cause very high heat release rates (HRR) in a case of fire. High HRR in materials often results in rapid fire spread in upward orientation. This study used numerical fire simulations with computational fluid dynamics (CFD) to investigate the fire spread along a VGS. The fire simulations were conducted in fire dynamic simulator (FDS) using the pyrolysis model that was specifically developed for vegetation. In FDS, the fire was initiated as a room fire of 1 MW followed by a window ejected flame, which ignited a 12 m tall and 9 m wide VGS. Fire spread along a VGS was modelled with four different moisture contents (MC) and four different packing ratios (PR) of the vegetation fuel bed. The different MCs represented regularly and scarcely watered vegetation, while different PRs represented different bulk densities of the vegetation in the VGSs. The fire spread rate was increased when both the MC and PR were decreased. The rate of fire spread in the upward orientation, which is the most hazardous mode of fire spread, was found to increase by 469.8% when the MC was decreased from 80% to 10%, and by 207.1% when the PR was decreased from 0.5 to 0.1. The results are helpful in understanding the design of a VGS with an optimum level of vegetation density and maintaining the vegetation fresh and alive to minimise the fire hazard. • Numerical simulations are conducted with a fire dynamic simulator model. • Moisture content (MC) and packing ratio (PR) of vegetation were varied. • Lower MC and PR of vegetation increase the fire spread rate. • Fire spread is notably faster in the upward direction over green wall. • Keeping vegetation fresh and lush is vital to reduce the fire risk.

Easy snap-folding of hexagonal ring origami by geometric modifications

Hexagonal ring origami is a type of foldable structure that has impressive packing abilities and can be tessellated into two-dimensional or three-dimensional surfaces without any gap or overlap. It can be folded under bending or twisting loads into a peach core-shaped configuration with only 10.6% of its initial area. However, in applications of large-scale foldable structures, folding by bending or twisting is usually technically difficult. Here, we propose strategies to facilitate easy snap-folding of the hexagonal ring by a simple point load or localized twist or squeeze. This is enabled by two geometric modifications made to the hexagonal ring: introducing residual strain and creating pre-twisted edges. By combining theoretical modeling, finite element simulations, and experiments, we systematically investigate the snap-folding behaviors of modified hexagonal rings with residual strain and pre-twisted edges. It is found that the geometric modifications promote easy snap-folding of the hexagonal ring by different mechanisms: introducing residual strain can significantly decrease the energy barrier and thus reduce the required moment to snap-fold the ring, while creating pre-twisted edges allows for easy out-of-plane deformation which is a necessary condition for a ring to fold. Combining the two methods further enables the snap-folding of the hexagonal ring by a point load or localized twist or squeeze. To demonstrate the easy folding of large assemblies of the modified rings, we construct various structures that can be snap-folded from their initial three-dimensional states to significantly lower-volume final states by a simple compression at the corners of the rings. We envision that the proposed geometric modification strategies can provide a new perspective on the rational design of easy-to-fold ring origami-based foldable functional structures with extremely high packing ratios.

The comparison between two methods of membrane cleaning to control membrane fouling in a hybrid membrane photobioreactor (HMPBR)

In this study, the effect of two membrane cleaning methods, such as aeration and addition of granular particles in a hybrid membrane photobioreactor (HMPBR) including Spirulina sp. with the concentration of 1.5 g·L−1 was investigated. Three different spargers [i.e., different diameters of orifice (do ): 0.5, 1, and 1.5 mm] and granular particles (i.e., three packing ratios of 0.5, 1, and 1.5%) were used. The results showed that with the increase in granular packing ratio, Rc /Rt decreased significantly. The best result was achieved by combining aeration with an orifice diameter of 1.5 mm and a granular packing ratio of 1.5%, which led to the lowest Rc /Rt value (0.38), while Rc /Rt value with a do of 1.5 mm was 0.68 without particles. In contrast, the ratio of pore blocking resistance to total resistance (Rp /Rt ) increased by 4.2% under the combined application of 1.5 mm of orifice diameter and 1.5% of granular packing ratio. The results from protein concentrations in the cake layer showed that as the do became larger, cake protein concentration decreased by 40%, whereas increasing the granular packing ratio from 0.5 to 1.5% increased protein concentration by 60% in the cake layer.

Investigation of compact packing strategy for large rigid-panel deployable antennas

Aiming at the problem of low packing ratio of the rigid-panel deployable antenna, this paper presents a strategy consisting of three concrete schemes. The first one is put forward by the immediate geometric relationship of the folded antenna. On this base, the second scheme is obtained by a rotation of the panel and then an enlargement of the aperture, which leads to an increase in packing ratio. Inspired by the aforementioned operation and the handling of branch interference in parallel mechanisms, the minimum-variable kinematic model describing the gap between adjacent panels with only two generalized coordinates is established. From this model and in combination with Bisection method, the aperture-scaling algorithm for the maximum packing ratio of the third scheme is developed. Extensive simulations reveal the algorithm converges and the resultant folded configurations are reasonable, in addition, the packing ratios of the three schemes increase significantly compared with those of the existing antennas. The parameter analysis is also made to further increase the packing ratio. It is concluded that the packing strategy displays distinct advantages in packing large-aperture rigid-panel deployable antennas.

Phase diagram and mechanics of snap-folding of ring origami by twisting

Ring origami is a new strategy to design bistable foldable/deployable structures by assembling multiple rings with the same geometry. The successful folding of ring origami assembly leverages the snap-folding capability of single rings. It is thus important to model the snap-folding of ring-shaped structures for the rational design of ring origami with good foldability, stability and high areal packing ratio. This paper studies the folding mechanics of differently shaped rings (circular, elliptical, rounded rectangular and rounded triangular rings) under twisting loads by developing theoretical and three-dimensional finite element analysis (FEA) models. The rod model is re-formulated for different ring geometries as two-point boundary value problem systems, which are then solved through numerical continuation, allowing one to capture complex equilibrium paths with multiple fold points. The rod model, verified by FEA simulations and experiments, can accurately, quantitatively capture the snap-folding behaviors for rings of different geometries. Using the rod model, we analyze the folding modes in terms of foldability, stability and snapping type. Phase diagrams of folding modes are then constructed with respect to geometric parameters for the ring profile and cross-section for differently shaped rings. Diagrams of areal packing ratios are obtained using the reduced planar rod model. This work provides comprehensive mechanics insights into the ring folding through twisting and can guide future ring origami design.

Effects of Particle Size Distribution with Efficient Packing on Powder Flowability and Selective Laser Melting Process.

The powder bed-based additive manufacturing (AM) process contains uncertainties in the powder spreading process and powder bed quality, leading to problems in repeatability and quality of the additively manufactured parts. This work focuses on identifying the uncertainty induced by particle size distribution (PSD) on powder flowability and the laser melting process, using Ti6Al4V as a model material. The flowability test results show that the effect of PSDs on flowability is not linear, rather the PSDs near dense packing ratios cause significant reductions in flowability (indicated by the increase in the avalanche angle and break energy of the powders measured by a revolution powder analyzer). The effects of PSDs on the selective laser melting (SLM) process are identified by using in-situ high-speed X-ray imaging to observe the melt pool dynamics during the melting process. The results show that the powder beds made of powders with dense packing ratios exhibit larger build height during laser melting. The effects of PSD with efficient packing on powder flowability and selective laser melting process revealed in this work are important for understanding process uncertainties induced by feedstock powders and for designing mitigation approaches.

Experimental study on fire spread over discrete fuel bed-Part II: Combined effects of wind and packing ratio

This work experimentally investigates the fire spread of discrete fuels by using fuel beds of laser-cut cardboards in a wind tunnel. Two distinct particle ignition modes are identified: under lower wind speed and packing ratio, a part of the tilted flame front inside the fuel bed touched and ignited the fuel elements; under a higher wind speed or packing ratio, the flame trough intermittently impinged on and ignited the fuels. When the wind speed is lower than a critical value (1.5 m s−1 in this work), the rate of spread (ROS), varying with the packing ratio, has a weak dependence on the wind speed. When the wind speed exceeds the critical value, ROS decreases with the packing ratio, and the dimensionless ROS almost linearly increases with the wind speed. The optimal packing ratio decreases with the wind speed. The characteristic length (the distance the flame front spreads in the shallow fuel layer) increases with the wind speed for lower packing ratios (0.018–0.032 in this work), while it first fluctuates and then increases with the wind speed under higher packing ratios (0.054 in this work). The temporal surface and internal flow velocities strongly depend on whether a stable peak-and-valley structure is sustained, affected by the wind speed and packing ratio. Nevertheless, the internal flow velocity near the flame front increases under all conditions, enhancing the internal convective heating on the unburned fuel.

Finite element analysis for prediction of thermal conductivity of composites with high packing ratios of particles using representative volume element models

Polymer composites with high thermal conductivity offer new possibilities for thermal management in electric systems. The objective this paper is to develop the prediction method of the thermal conductivity of polymer composites with high packing ratios of fillers. Fillers were spherical alumina and boron nitride with platelet shape. Measurements of thermal conductivity of polymer composites using steady state method and finite element analysis for thermal conductivity using representative volume element (RVE) models of polymer composites were carried out. The analytical thermal conductivity of multi-scale RVE models increased with increasing homogenization numbers and volume fraction alumina. When contacts between fillers were considered in RVE models, the finite element analysis results and experimental results were in good agreement.

Gravity-driven membrane filtration of primary wastewater effluent for edible plant cultivations: Membrane performance and health risk assessment

In this study, gravity-driven membrane (GDM) systems packed with Icelandic lava stones (0%, 26% and 53%) were employed to treat primary wastewater for reuse. The three GDM systems could achieve > 75% removal of organics. While, the lava stone biocarriers facilitated higher nitrogen removals (>51% vs. ~37% without biocarriers), which were related to the presence of manganese oxide (MnOx) in the lava stones. A combined mechanism of pore constriction and cake fouling was predominant during flux stabilization, regardless of lava stone packing ratios. Compared with non-cleaning or geothermal water-/persulfate-based cleaning, sodium hypochlorite-based cleaning (0.5% in geothermal water at 50 °C, 10 min per 3–4 days) was more effective in improving flux recovery (~46–79%) and water permeability (~23–79%). Furthermore, GDM permeate was used to irrigate two vegetables (tomato and basil), with tap water and commercial fertilizer as comparison baselines. Statistically insignificant (p > 0.05) results were observed for vegetable growth profiles and non-essential heavy metal contents in soils. Even though the GDM permeate-irrigated vegetables had a slightly higher uptake of non-essential metal cadmium (Cd) compared to those with tap water, the hazard quotient index was still less than 1, indicating negligible human health risk.

Elasticity-based-exfoliability measure for high-throughput computational exfoliation of two-dimensional materials

Two-dimensional (2D) materials are promising candidates for uses in next-generation electronic and optoelectronic devices. However, only a few high-quality 2D materials have been mechanically exfoliated to date. One of the critical issues is that the exfoliability of 2D materials from their bulk precursors is unknown. To assess the exfoliability of potential 2D materials from their bulk counterparts, we derived an elasticity-based-exfoliability measure based on an exfoliation mechanics model. The proposed measure has a clear physical meaning and is universally applicable to all material systems. We used this measure to calculate the exfoliability of 10,812 crystals having a first-principles calculated elastic tensor. By setting the threshold values for easy and potential exfoliation based on already-exfoliated materials, we predicted 58 easily exfoliable bulk crystals and 90 potentially exfoliable bulk crystals for 2D materials. As evidence, a topology-based algorithm indicates that there is no interlayer bonding topology for 93% predicted exfoliable bulk crystals, and the analysis on packing ratios shows that 99% predicted exfoliable bulk crystals exhibit a relatively low packing ratio value. Moreover, literature survey shows that 34 predicted exfoliable bulk crystals have been experimentally exfoliated into 2D materials. In addition, the characteristics of these predicted 2D materials were discussed for practical use of such materials.

Experimental study on fire spread over discrete fuel bed-Part I: Effects of packing ratio

The fuel packing ratio (β) significantly influences the fire spread in discrete fuels; however, the underlying mechanism remains unclear. This study performed experiments using laser-cut cardboards with different packing ratios to explore the heat transfer in fire spread. We identify two distinct spread behaviors under varying packing ratios. Heat flux data indicate that radiation controls the surface heat transfer (denoting the heat transfer received by the fuel bed surface) far from the flame, while convective heating plays a considerable part surrounding the flame. The surface heat transfer is enhanced under lower packing ratios (Stage 1) and is responsible for the increase of the rate of spread (ROS) in this stage. Under higher packing ratios (Stage 2), the surface heat transfer does not vary significantly, and the surface radiation transfers more energy to fuels than surface convection. ROS reduction in Stage 2 is attributed mainly to the internal heat transfer (denoting the heat transfer received by the sides of the fuel particles) dominated by radiation. The dense fuel bed impedes the response of in-bed fuel to internal heat transfer, which, however, does not significantly influence the time integral of the internal total and radiant heat fluxes. Besides, the flame residence time almost linearly increases with the packing ratio when the flame can spread.

Contribution of dynamic capillary pressure to rainfall infiltration in thin homogeneous growth substrates

The use of green roofs to help mitigate storm water contributions to urban flooding has been gaining popularity but is hindered by the limited data on the performance of such roofs with regard to storm water runoff mitigation. The underlying issue stems from the inherent complexity of modeling subsurface multiphase flow. Modeling of this phenomena requires calculating the contributions of substrate microstructure characteristics, the influence of the wetting and non-wetting phases upon each other, and the effect of the microstructure on the wetting phase. Previously we have observed how the microstructure can affect detention, however the quantification of this relationship is still missing. In the present paper we present numerical simulations of wetting phase infiltration of a thin monodisperse packed bed in order to understand and quantify the impact of microstructure geometry on storm water infiltration of a green roof substrate. For a slightly hydrophilic case, (θ=82°), we find that a dominant mechanism underlying this relationship is the microstructure-induced dynamic behavior of the capillary pressure. We determine that at larger packing ratios (ratio of packed bed depth to particle size), the influence of hydraulic head diminishes and behaves conversely for thinner layers, particularly when larger pores are present. Indeed, thin beds composed of large particles can exhibit high flow velocities that in turn affect the capillary pressure within the substrate. We observe that the capillary pressure can shift from negative values denoting capillary suction to positive ones which cause valve-like blocking effects on the flow; dependent upon the flow velocity as determined by the microstructure. In particular, we find that the capillary pressure depends on the value of the pore-scale gravity-induced flow velocity, quantified through a characteristic Capillary number. The provided quantification of this relationship can be invaluable from a design perspective to understand the behavior of capillary pressure of different substrates under a variety of flow rates prior to testing substrate candidates. In addition, a comparison of the behavior of the dynamic component of capillary pressure to other works is undertaken. Flow homogeneity is also found to be linked to the flow velocity, and consequently to the microstructure.

Ring Origami: Snap‐Folding of Rings with Different Geometries

Origami folding and thin structure buckling are intensively studied for structural transformations with large packing ratio for various biomedical, robotic, and aerospace applications. The folding of circular rings has shown bistable snap‐through deformation under simple twisting motion and demonstrates a large area change to 11% of its undeformed configuration. Motivated by the large area change and the self‐guided deformation through snap‐folding, it is intended to design ring origami assemblies with unprecedented packing ratios. Herein, through finite‐element analysis, snap‐folding behaviors of single ring with different geometries (circular, elliptical, rounded rectangular, and rounded triangular shapes) are studied for ring origami assemblies for functional foldable structures. Geometric parameters' effects on the foldability, stability, and the packing ratio are investigated and are validated experimentally. With different rings as basic building blocks, the folding of ring origami assemblies including linear‐patterned rounded rectangular rings, radial‐patterned elliptical rings, and 3D crossing circular rings is further experimentally demonstrated, which show significant packing ratios of 7% and 2.5% of the initial areas, and 0.3% of the initial volume, respectively. It is envisioned that the reported snap‐folding of origami rings will provide alternative strategies to design foldable/deployable structures and devices with reliable self‐guided deformation and large area change.

Spotting ignition of larch (Larix gmelinii) fuel bed by different firebrands

Spot fire increase the difficulty of fire-fighting and threaten public safety, and therefore it is important to study ignition probabilities of fuel bed by different firebrands, in order to understand ignition mechanisms and analyze the formation of spot fires. This will provide an important basis for further study to improve the fire-fighting efficiency and reduce casualties. In this study, the ignition probabilities of larch (Larix gmelinii) fuel beds with different moisture levels and packing ratios by diffreent firebrands, including cones and twigs of different sizes, was investigated. Ignition experiments were conducted at different wind speeds generated by fans. The results show that, regardless of moisture content and packing ratio, ignition probability is zero when there is no wind. Both moisture content and wind speed significantly influence ignition probability, while packing ratio has almost no effect. The maximum moisture content at which firebrand ignition occurred was 50%, and ignition probability increased with wind speed and decreased with moisture content. Cones have the highest ignition probability, followed by large twigs and by small twigs. Ignition probability is also affected by firebrand shapes and sizes that determine their potential heat and contact area to the fuel bed. Two empirical models were established to link ignition probability with fuel properties and wind speed. This study will help clarify the mechanism of spot ignition and reduce corresponding losses.

DNA Organization along Pachytene Chromosome Axes and Its Relationship with Crossover Frequencies.

During meiosis, the number of crossovers vary in correlation to the length of prophase chromosome axes at the synaptonemal complex stage. It has been proposed that the regular spacing of the DNA loops, along with the close relationship of the recombination complexes and the meiotic axes are at the basis of this covariation. Here, we use a cytogenomic approach to investigate the relationship between the synaptonemal complex length and the DNA content in chicken oocytes during the pachytene stage of the first meiotic prophase. The synaptonemal complex to DNA ratios of specific chromosomes and chromosome segments were compared against the recombination rates obtained by MLH1 focus mapping. The present results show variations in the DNA packing ratios of macro- and microbivalents and also between regions within the same bivalent. Chromosome or chromosome regions with higher crossover rates form comparatively longer synaptonemal complexes than expected based on their DNA content. These observations are compatible with the formation of higher number of shorter DNA loops along meiotic axes in regions with higher recombination levels.

A 2D hysteretic DEM model for arbitrarily shaped polygonal particles

A 2D hysteretic Discrete Element Method (DEM) model is developed for simulating the flow of food particles, specifically with multi-material 3D food printing processes in mind. Particles are modeled as arbitrarily shaped polygons due to the diverse nature of food powders, which can be highly irregular in shape. The developed hysteretic force model is applicable to both convex and concave polygonal particles. It is adjusted to use a proportional weighted maximum intersection area upon splitting of contact areas. This results in a continuous force trajectory, which would otherwise not be guaranteed. The model is validated with in literature reported packing ratios for ellipses. Simulated deposition of sugar-shaped particles shows that for dense packings the fraction of splitting interactions occurring can be up to 7%. Furthermore, the influence of particle shape on the coordination number and packing density is shown with a simulation of the deposition of sugar-like material.