Increasing Separation Distance Research Articles

Vapor mediated interaction of two condensing droplets

The present work investigates the internal flow structure of two condensing droplets of aqueous solution during interaction. Velocity of fluid inside the droplets was investigated by confocal micro-PIV technique. Simulation study has also been carried out to supplement the understanding of internal hydrodynamics inside droplets. Condensation on the droplets was carried out inside a closed chamber by generating a difference in vapor pressure between the droplet interface and a reservoir fluid surrounding the droplet in the room temperature without any cooling. Condensation on the droplet leads to spatial variation of solute concentration inside the droplet causing buoyancy driven Rayleigh convection. Fluid flow pattern inside a single condensing droplet is symmetric in nature whereas the fluid flow pattern for two condensing droplets is asymmetric in nature. The neighboring droplet influences the internal convection through vapor mediated interaction between the two condensing droplets. The condensing droplet senses the neighboring droplet from a distance without any physical contact. The asymmetric nature of the flow pattern is attributed to the modification in the distribution of condensation flux of the droplet in the presence of the neighboring droplet. The flow pattern observed for interacting droplets during condensation is opposite to that of evaporating droplets. The effect of the neighboring droplet on the internal convection of the condensing droplet reduces with increase in separation distance between the droplets.

Effect of separation distance on gas dispersion and vapor cloud explosion in a storage tank farm determined using computational fluid dynamics

Storage tank separation distance, which considerably affects forestalling and mitigating accident consequences, is principally determined by thermal radiation modeling and meeting industry safety requirements. However, little is known about the influence of separation distance on gas dispersion or gas explosion, which are the most destructive types of accidents in industrial settings. This study evaluated the effect of separation distance on gas dispersion and vapor cloud explosion in a storage tank farm. Experiments were conducted using Flame Acceleration Simulator, an advanced computational fluid dynamics software program. Codes governing the design of separation distances in China and the United States were compared. A series of geometrical models of storage tanks with various separation distances were established. Overall, increasing separation distance led to a substantial reduction in vapor cloud volume and size in most cases. Notably, a 1.0 storage diameter separation distance appeared to be optimal. In terms of vapor cloud explosion, a greater separation distance had a marked effect on mitigating overpressure in gas explosions. Therefore, separation distance merited consideration in the design of storage tanks to prevent gas dispersion and explosion.

Field emission from AlGaN nanowires with low turn-on field

We fabricate AlGaN nanowires by molecular beam epitaxy and we investigate their field emission properties by means of an experimental setup using nano-manipulated tungsten tips as electrodes, inside a scanning electron microscope. The tip-shaped anode gives access to local properties, and allows collecting electrons emitted from areas as small as 1 µm2. The field emission characteristics are analysed in the framework of Fowler–Nordheim theory and we find a field enhancement factor as high as β = 556 and a minimum turn-on field = 17 V µm−1 for a cathode–anode separation distance = 500 nm. We show that for increasing separation distance, increases up to about 35 V µm−1 and β decreases to ∼100 at = 1600 nm. We also demonstrate the time stability of the field emission current from AlGaN nanowires for several minutes. Finally, we explain the observation of modified slope of the Fowler–Nordheim plots at low fields in terms of non-homogeneous field enhancement factors due to the presence of protruding emitters.

Molecular collapse in graphene: Sublattice symmetry effect

Atomic collapse can be observed in graphene because of its large "effective" fine structure constant, which enables this phenomenon to occur for an impurity charge as low as $Z_c\sim 1-2$. Here, we investigate the effect of the sublattice symmetry on molecular collapse in two spatially separated charge tunable vacancies, that are located on the same (A-A type) or different (A-B type) sublattices. We find that the broken sublattice symmetry: (1) does not affect the location of the main bonding and anti-bonding molecular collapse peaks, (2) but shifts the position of the satellite peaks, because they are a consequence of the breaking of the local sublattice symmetry, and (3) there are vacancy characteristic collapse peaks that only occur for A-B type vacancies, which can be employed to distinguish them experimentally from the A-A type. As the charge, energy, and separation distance increase, the additional collapse features merge with the main molecular collapse peaks. We show that the spatial distribution around the vacancy site of the collapse states allows us to differentiate the molecular from the frustrated collapse.

Critical longitudinal ventilation velocity for smoke control in a tunnel induced by two nearby fires of various distances: Experiments and a revisited model

Fire smoke in a tunnel fire is a key factor causing casualties, thus the critical longitudinal ventilation velocity is a key parameter in tunnel fire safety ventilation design. The correlation of critical longitudinal ventilation velocity for smoke control in a tunnel induced by two nearby fires has rarely been studied experimentally and quantified before, despite the fact that this scenario exists in practice. In this paper, the critical longitudinal ventilation velocities for smoke control in the tunnel, induced by two nearby fires were experimentally quantified, involving various separation distances between the two fires of both axi-symmetrical and linear sources of various heat release rates, with a total of 72 test conditions. Results show that the effect of separation distance between the two fires on the critical longitudinal ventilation velocity, decreases with the increase of fire heat release rates. The critical longitudinal ventilation velocities are found to firstly decrease with increasing separation distance between the two fires, and then remain unchanged when the separation distance between two fires beyond a critical value. A new modified model, involving two fires of various separation distances, is proposed. The model is shown to well represent the experimental results.

Transverse electric surface waves in ferrite medium surrounded by plasma layers

The theoretical analysis of transverse electric surface waves in ferrite medium surrounded by isotropic plasma layers is presented in this manuscript. Maxwell’s equations in differential form are used, and we impose the boundary conditions to acquire the dispersion relation to formulate the proposed structure. The influence of number density, separation distance between the layers of plasma, and dielectric permittivity of ferrite film on the normalized propagation constant is studied. It is concluded from the result obtained that if the number density and values of dielectric permittivity of ferrite film increases the propagation constant Re(β) tends to decreases whereas the increase in separation distance between the layers of plasma tends to increase the propagation constant Re(β). Furthermore, to verify the surface waves, the normalized field distribution for plasma medium as well as ferrite medium are also presented in this manuscript. The present work has potential applications in communication, drug delivery, cancer treatment, and ferrite sensing waveguide structures in the GHz frequency regime.

Investigation of path loss and modeling for digital terrestrial television over Nigeria

Accurate prediction of path losses is a key factor in Digital Terrestrial Television (DTTV) to ensure Quality of Service (QoS). This study investigates the path losses of three Digital Terrestrial Television Base Stations (DTTBS) in Lagos, Kaduna and Katsina cities of Nigeria. The Received Signal Strength (RSS) of the DTTBS was measured at intervals along selected routes around the stations using a digital signal strength meter. So also, the transmitter-receiver distances of data points with their corresponding geographic coordinates and heights were measured using a hand held GPS receiver. Also measured concurrently were some of the location's - based surface meteorological parameters such as temperature, atmospheric pressure and humidity, using a compact wireless weather station. In addition, the corresponding surface radio refractivity values along the routes were computed using the meteorological parameters recorded. Data were collected during dry and wet season months' covering a period of three years. Path losses were calculated using Okumura-Hata model. Results for all the routes and seasons revealed that path loss increases with increase in trans-receiver distances, however path losses were higher during wet compared to dry season's month. Path losses estimated were highest in Kaduna followed by Ikorodu-Lagos and least in Katsina. Average high positive correlation coefficients of 0.76, 0.75 and 0.74 were obtained between path losses and line of sight for Lagos (Coastal zone), Kaduna (Sudan Savannah) and Katsina (Sahel Savannah) respectively. In the same order, negative correlation coefficients of -0.72, -0.79 and -0.63 were obtained between path losses and RSS. In addition, Modified Okumura Hata Model(s) (MOHPL) that incorporates the effect of the specified tropospheric parameters were proposed. Results also revealed that path losses obtained by Okumura-Hata model increase with increase in LOS separation distance from the base station following a consistent exponential rise while the Modified Okumura-Hata Model (MOHPL) followed similar trend with few exceptions of crests and troughs depicting the influence of the incorporated location's-based tropospheric parameters. Another finding is that Okumura Hata model under estimated the path losses associated with digital terrestrial television channel over the study locations. In order to ensure high reliability of power budgets and link's design, the proposed models are recommended for use over the study locations. The overall findings of this work will be useful for the accurate prediction of path losses and the design of power budgets and links over digital terrestrial television and similar wireless channels on the UHF band.

A Numerical Investigation on Fluid Dynamics and Heating Characteristics of Co-and-Counterrotating Multiple Swirling Impinging Flames Arranged in 3 × 3 Inline Array

Abstract Swirling impinging flame jets possess the potential of uniformly heating the target surface. Numerical simulations have been carried out for multiple reacting swirling flows arranged in square 3 × 3 array. The turbulence chemistry interactions in the flow field are modeled using RNG based k-ε/ eddy dissipation model (EDM) framework. Co-and-counterrotating configurations have been compared for interactions and spent gas release mechanism at fixed inter-jet spacing and separation distance. Multiple swirling impinging flames undergo strong interactions resulting in distortions of recirculation zones (RCZs) for all the surrounding but central flame. Co-swirling flames result in development of higher turbulence in the interaction regions as compared with counter-swirl case. Tilted heat flux contours have been noticed for co-swirling impinging flames, whereas heat flux impressions are straight for counter-swirl case. Effect of inter-jet spacing (C/Dh = 5, 7, and 9) and separation distance (H/Dh = 3, 5, 7, and 9) studied for co-swirl case revealed that peak heat fluxes decreased with increasing inter-jet spacing and separation distance. With increase in inter-jet spacing, the width of central flame increased due to increased suppression effect induced by the recirculating gases. Increase in separation distance resulted in decrease of width of central flame and thermal dilution taking place led to decrease in heat flux magnitudes.

Soot particle size distributions in premixed flames of RP-3 jet fuel and its distillates

Particle size distributions in premixed burner-stabilized stagnation (BSS) flames of a Chinese RP-3 jet fuel and its three distillate fractions, namely the 0–50%, 50%–90%, 90%–100% distillate fractions, were experimentally measured. The cold gas velocity and C/O ratio in the mixture were kept constant, and the maximum temperatures were similar in the four flames. As the separation distance between the burner and the stagnation plate was above 6 mm, the particle size distribution functions (PSDFs) of the four flames showed bimodal distribution, i.e. nucleation and accumulation modes. With increased separation distance between the burner and the stagnation plate, the accumulation strength increased in all the flames. The 90%–100% distillate fraction showed higher nucleation strength but lower accumulation strength than the other fuels, and thus a higher number of particles and slower particle growth rates. In contrast, the 0–50% distillate fraction exhibited higher accumulation strength than the other fuels. The 50%–90% distillate fraction and the RP-3 jet fuel presented intermediate nucleation and accumulation strengths between those of the 0–50% and 90%–100% distillate fractions. The soot morphology observation from transmission electron microscopy also indicated that the 90%–100% distillate fraction produced smaller primary particle sizes than the other fuels, suggesting their slower particle growth rates.

Capillary forces on wet particles with a liquid bridge transition from convex to concave

The transition of a liquid bridge profile from convex to concave and the associated capillary forces are experimentally studied via particle-particle and particle-plane pairs. The results demonstrate that a convex liquid bridge appears at a relatively large water volume and small separation distance, where the capillary force remains approximately constant. As the separation distance increases, the liquid bridge is stretched from convex to concave, and the capillary force initially doesn't change much but increases to the maximum value and then decreases gradually. By combining analytical analysis and experimental data, a framework is proposed for predicting the evolution of liquid bridge profiles. Additionally, a capillary force model that is applicable to both convex and concave liquid bridges is formulated. The applicability of this force model to multiple-particle systems is demonstrated by comparing DEM predictions of repose angles of sandpile and hopper flow discharge rates with experimental measurements at various water contents.

Characterization of ceiling smoke temperature profile and maximum temperature rise induced by double fires in a natural ventilation tunnel

In this paper, the ceiling smoke temperature distribution characteristics maximum temperature rises induced by double fire sources in a natural ventilation tunnel were studied. Longitudinal ceiling smoke temperature distribution induced by double fire sources with different burner separation distances is correlated with an exponential decay law. The variation of longitudinal temperature decay coefficient K for different burner separation distances is found to decrease with the increase of burner separation distance. Then a model of K considering the burner separation distance and dimensionless heat release rate is developed. Comparison between the calculated ceiling smoke temperature profile with the proposed model and experimental data in both this work and those from previous work shows good agreement. Besides, the study also concerns the variation of maximum smoke temperature beneath the tunnel ceiling induced by double fire sources with different heat release rate and different burner separation distances. Under a certain heat release rate, the maximum temperature of the smoke beneath the tunnel ceiling decreases with increasing burner separation distances. A maximum temperature correlation, considering dimensionless heat release rates and the burner separation distances, is proposed. The calculated result show good agreement with the experimental data in this work and those from previous work. These new findings and models can provide a good reference for the design of tunnel fire safety.

Suppression characteristics of double-layer wire mesh on wheat dust flame

Dust explosion accidents frequently occur in dedusting pipes of wheat dust, causing huge casualties and property losses. In this paper, a vertical pipeline of combustion experimental platform was established to obtain the effects of working conditions and double-layer wire mesh parameters on wheat dust flame propagation. The results show that the double-layer wire mesh has an obvious suppression effect on the flame propagation of wheat dust. At the combination of the same mesh number, both the peak flame temperature and the maximum flame propagation velocity decreased with the increasing of the mesh number, and the flame quenching phenomenon occurs at 80 mesh. At the combination of different mesh numbers, the flame temperature at the upper-density and lower-sparseness combination is lower than that at the upper-sparseness and lower-density combination under all working conditions. The flame temperature difference (ΔT) gradually increases with the increasing of mesh number difference, and the maximum ΔT appears at the mesh difference of 40 mesh. As the separation distance increases, the flame temperature rises, and the suppression effect is best at the separation distance of 1cm. The peak combustion pressure is negatively linear with the separation distance.

Interaction of lateral cracks in double scratching of single-crystal silicon carbide

In this paper, a new method is established to analyze the interaction of lateral cracks in double scratching process, which is a basic problem in machining of single-crystal silicon carbide. Applying superposition principle, this interaction problem is reduced to the calculation of stress intensity factors (SIFs) around two scratching-induced collinear cracks under scratching-induced stress field. First, values of SIFs at crack tips under the scratching-induced stress field are obtained applying Fourier transformation, gap functions of displacements and Jacobi polynomials. Then, the scratching-induced stress filed is obtained by the superposition of Boussinesq field, Cerruti field and Blister stress field. Finally, this calculation method is verified by comparing with the results in literature in special case. It is found that values of SIFs decrease dramatically when scratching separation distance increases. The interaction effect can be neglected when the separation distance is larger than 1.6 times of the length of lateral cracks generated in single scratching.

Aggregation of stable colloidal dispersion under short high-shear microfluidic conditions

We have designed and carried out intense shear-driven aggregation of charge-stabilized polystyrene nanoparticles (NPs) to big fractal clusters, in a single and a double Z-shape microchannels (SZ-MC and DZ-MC). Both the MCs have the same cross-section, but the length of DZ-MC is double of SZ-MC. It is found that when the NP dispersion repeatedly passes through each of the two MCs, the conversion of the NPs to big clusters can only reach a plateau value that is far smaller than full conversion. The plateau conversion at a given mean velocity (shear rate) is larger for the DZ-MC than for the SZ-MC, i.e., it increases as the MC length (or the residence time of one pass) increases. This surprising phenomenon can be explained upon considering that, as the conversion increases, the average NP separation distance increases, i.e., the time for the specific shear-driven bi-particle aggregation to occur increases. When the aggregation time becomes larger than the NP residence time in a MC, the NP dispersion passing through the MC does not lead to aggregation. The situation does not change if we let the NP dispersion repeatedly pass through the MC many times (i.e., by increasing the cumulative residence time), because if the residence time of one pass is not enough for the NPs to overcome the interaction energy barrier, when they come out from the MC, the intense shear force disappears and the NPs return to their original interaction state.

Long term air ion monitoring in search of pre-earthquake signals

Air ion concentrations at ground level have been measured both indoors and outdoors to address a wide variety of questions, such as estimating air quality or assessing the impact of ions on human health. While most studies rely on discrete measurements, other applications require continuous monitoring over extended periods of time. In this paper, we discuss the challenges posed by long term air ion monitoring within the frame of earthquake forecasting. We first provide a brief state of the art, outlining the main hypotheses advanced to explain anomalies in air ion concentrations that were reported prior to earthquakes, namely the p-hole theory and the radon exhalation theory. We then describe the array of Gerdien capacitors deployed by QuakeFinder in the US and abroad with the goal to identify pre-earthquake variations in air ion data: we discuss the characteristics of these instruments and derive the criticial mobility associated with their parallel-plate geometry. We then present the results of a Parallel Sensor Test (PST) during which positive and negative air ion counters were deployed side by side at increasing separation distances. This test provides insight about the sampling rate at which air ion concentrations should be measured with the QuakeFinder air ion counters.The main processing steps applied to raw air ion data are then described. Particular emphasis is set on the unipolarity coefficient and how it may be used to identify faulty sensors. One of the key issues encountered while monitoring air ion concentrations for extended periods of time is the condensation forming on the electrodes due to increased relative humidity levels. It appears that the relative humidity working range provided by instruments manufacturers is not accurate for long term outdoors measurement and that sensors saturate at relative humidity values lower than the threshold advertised in the instrument specifications. We detail some of the strategies implemented to automatically reject data acquired during episodes of high relative humidity. Finally, we discuss the instrumental improvements that can be made to prevent moisture from forming on the plates of the Gerdien capacitors and that QuakeFinder plans to implement in the near future.

Molecular dynamics simulations of Lennard-Jones systems confined between suspended nanoscale graphene sheets.

The distribution of particles interacting with Lennard-Jones potentials and confined between parallel graphene sheets is investigated by molecular dynamics simulations. For small separation distances, the particles are densely localized in the central region between the graphene sheets. However, two high-density layers appear as the separation distance increases. The particle distribution also depends on the temperature, tensile force of the graphene sheets, and the initial configuration, and various configurations are observed for large separation. For example, an argon cluster initially located between the graphene sheets changes shape, and a bridge between the parallel walls is formed at low temperature. In contrast to the Lennard-Jones system sandwiched between rigid walls, the flexibility of the graphene sheets strongly affects the distribution of particles in the direction perpendicular to the graphene sheets.

Airborne transmission during short-term events under stratum ventilation

Past studies on airborne spread of expiratory droplet nuclei between occupants were focused on long-term exposure under steady-state conditions. However, exposure during short-term events can be widely found in practice, e.g. medical examination or short meeting. Airborne transmission during short-term events under stratum ventilation was examined experimentally in this study. Two breathing thermal manikins were employed to simulate a standing infected person and a standing exposed person. The manikins were placed face-to-face and face-to-back to reproduce the exposure conditions with the highest and the lowest risk, respectively. Tracer gas was dosed into the air exhaled by the “infected” manikin to simulate the droplet nuclei. A newly developed average exposure index was used to evaluate the exposure risk. The time-averaged exposure index increases over time, but the increasing rate depends strongly on the duration of exposure time, e.g., the exposure index increases much faster during the first 5 minutes than during the period after 5 minutes. The exposure index during short-term events does not always decrease with the increase of separation distance. These findings imply that the control measures formulated based on steady-state conditions are not necessarily effective to short-term events.

Interactions between nanoparticles and fractal surfaces

This study evaluated attachment of a 30-nm nanoparticle to and detachment from fractal surfaces by calculating Derjaguin-Landau-Verwey-Overbeek (DLVO) interaction energies in three-dimensional space using the surface element integration technique. The fractal surfaces were generated using the Weierstass-Mandelbrot function with varying values of fractal dimension D (2.3 ≤ D ≤ 2.7) and fractal roughness G (0.000136 ≤ G ≤ 0.136). Results show that maximum energy barrier is reduced at peak areas of a fractal surface, and hence attachment in primary minima is favored. Some nanoparticles attached in primary minima at the peak areas can be detached by decreasing ionic strength (IS) due to monotonic decrease of interaction energy with increasing separation distance at low ISs. While the attachment in primary minima at valley areas is irreversible to IS reduction, the attachment is inhibited due to enhanced maximum energy barrier at these areas. A nonmonotonic variation of attachment efficiency in primary minimum (AEPM) with IS is present at high fractal dimension (D ≥ 2.4) or low fractal roughness (G < 0.00136), whereas the AEPM decreases monotonically with decreasing IS at low fractal dimension (D < 2.4) or high fractal roughness (G ≥ 0.00136). The AEPM decreases monotonically with increasing D or decreasing G at ISs from 1 mM to 200 mM. The decrease of AEPM with D or G is much slower at 10 mM compared to other ISs. These theoretical findings can explain various experimental observations in the literature, and can have important utility to development of water filtration techniques in engineered systems and to assessment of environmental risks of nanoparticles.

On the influence of distance between two jets on flickering diffusion flames

The interactions of multiple flames are often encountered in real-world fire and industrial burners. The distance between two jets has a strong influence on the instability mode, oscillation frequency, and mean height of flickering diffusion flames, which is experimentally studied and analyzed in this paper. Five different types of instability modes are identified as the separation distance between two nozzles is increased. When the nozzle separation distance is smaller, the flame mode is similar to that of a single-nozzle flame. In this case, the flame can switch between the merged sinuous mode and the merged varicose mode due to external disturbances on the flame. As the nozzle separation distance increases, the probability of mode switching from merged varicose to merged sinuous decreases. As the nozzle separation distance increases further, the flame mode translates into symmetric sinuous mode, alternated sinuous or independent mode. In addition, the flame height and oscillation frequency of a dual-nozzle flame have their own characteristics, which are different from that of the single-nozzle flame. The increase of the nozzle separation distance, leads to a decrease and then an increase of the flame mean height. It is interesting that the flame mean height of the alternated sinuous flame is the lowest, even lower than that of the single-nozzle flame. The oscillation frequency of the symmetrical sinuous flame is lower than that of the single-nozzle flame, and the frequency of alternated sinuous flame is higher than that of the single-nozzle flame.

Aeroacoustic Analysis of a Lift-Offset Coaxial Rotor Using High-Fidelity CFD/CSD Loose Coupling Simulation

This paper presents the aeroacoustic analysis of a lift-offset coaxial rotor in high-speed forward flight using the high-fidelity computational fluid dynamics/computational structural dynamics (CFD/CSD) loose coupling software Helios. Acoustic simulations are performed using the software PSU-WOPWOP at eight microphones positioned below the coaxial rotor. The total power of the three speed cases—100, 150, and 200 kt—is validated against flight-test data and shows good agreement. A series of parametric studies is also conducted to investigate the effect of lift offset, flight speed, and rotor-to-rotor separation distance on acoustics of the coaxial rotor. Strong blade-crossover and self-blade–vortex interaction events of the coaxial rotor, which are major sources of loading noise, are captured via high-fidelity CFD simulations in all speed cases. Highly impulsive acoustic pressure signals are identified in all simulation cases, and the magnitude of mid-frequency sound pressure level (SPL) increases significantly with increasing flight speed and lift offset. The strength of mid-frequency SPL, on the other hand, is reduced significantly as the rotor-to-rotor separation distance increases at 100 kt. However, the higher speed cases do not show a significant reduction in mid-frequency SPL with increasing separation distance.