Higher Average Velocity Research Articles

Fast lipid vesicles and dielectric particles migration using thermal-gradient-induced forces

Abstract Lipid vesicles are small biological particles that can be used for both targeted drug delivery systems and clinical studies. Their optical manipulation, however, is limited by the small difference in refractive indices with the surrounding medium, as well as the requirement for high laser trapping powers. In this work, we combine gradient force and thermal forces to deliver and trap individual lipid vesicles with low-trapping laser powers. The total optothermal force exerted on liposomes causes them to migrate rapidly toward the laser focus with a high average migration velocity of 1.77 µm s−1 under 7.3% w/v polyethylene glycol (PEG) concentration and low trapping laser power of 1 mW. A high normalized experimental trap stiffness of 0.88 (pN µm) mW−1 was obtained at 7.3% w/v PEG/water solution. This work may open new ways for bioparticle sorting and manipulation with potential applications in cellular studies, drug delivery, biosensing, and medicine.

Experimental and numerical investigation on flame pattern formations of non-premixed CH4 and air in a planar micro-combustor

Non-premixed CH4/air flames in a rectangular micro-combustor of 2.0 mm (width) × 10 mm (height) × 31 mm (length) were experimentally investigated. The results demonstrated that flame cannot occur within the micro-combustor if nominal equivalence ratio ϕ ≤ 1.0 for any average velocity (V). Five flame patterns appeared and were termed as “single internal C-shaped flame”, “dual internal C-shaped flames”, “single ε-shaped flame”, “dual internal and external flames” and “single external flame”. Cold-state numerical simulations unraveled that fuel stratification occurred in the height direction due to the buoyancy effect, which is especially pronounced at ϕ = 1.3–1.5. Consequently, “dual internal C-shaped flames” is formed, consisting of an upper flame cell and a lower flame cell. At ϕ = 1.1–1.3, fuel stratification phenomenon grew weakened, which led to “single ε-shaped flame” at ϕ = 1.2–1.3 and “single internal C-shaped flame” at ϕ = 1.1–1.2. At sufficient high average velocities, one or both flame cells will be pushed out of the combustor. In conclusion, the present work verified the possibilities of formation of stable flames inside the micro-combustor with a width less than the quenching distance of stoichiometric mixture. Moreover, this study revealed the important role of buoyancy effect in flame pattern formations within micro-combustors with a large enough channel height.

An experimental evaluation of drying banana slices using a novel indirect solar dryer under variable conditions

Novel thermal characteristics of drying banana slices in an indirect dryer are presented for four different experimental drying conditions in the forced convection mode. The novel characteristics include measuring the airflow velocity in the drying chamber, measuring the thermal profiles in different trays comprehensively and measuring the relative humidity under different conditions. Two tests are carried for 16 h in two consecutive days (8 h per day for each test). The first test is on acloudy day followed by a sunny day, while the second test is carried out on two consecutive sunny days. Tests 3 and 4 are 24 h tests with high (0.23 m/s) and low (0.11 m/s) average drying chamber airflow velocities under good solar radiation conditions. The maximum temperatures obtained in the collector and the drying chamber are around 80 and 48 °C, respectively, for the 16 h tests. Significantly lower collector and drying chamber temperatures are obtained due to cloudy conditions. Maximum collector temperatures are around 84 and 95 °C for the high and low average airflow chamber velocities for the 24 h tests. The corresponding maximum temperatures in the drying chamber are around 50 °C for the 24 tests. The final moisture ratios are 0.26 (cloudy and sunny days) and 0.20 (two sunny days), respectively, for the 16 h tests. These final moisture ratios are lower than those obtained for the 24 h tests which are 0.32 and 0.28, respectively. Increasing the drying chamber airflow velocity results in faster moisture removal during sunshine hours for the 24 h tests. For tests 1, 2, 3 and 4, the maximum average collector efficiencies during the sunshine period are around 60, 80, 40 and 10 %, respectively. The average drying efficiencies for the total solar drying period for tests 1,2,3 and 4 on day 1 are 6.9, 6.9, 5.5 and 5.7 % respectively. These values are comparable, suggesting that the average collector powers, airflow velocities and efficiencies have a very small effect on the average solar drying efficiency for the whole drying period. The quality of the bananas slices mainly in terms of the colour and shape is also compared with previous studies and commercially available products. A reasonably acceptable quality product is obtained.

Study of the stochastic dynamics of particles in the coal screening based on the Markov process

The model of the stochastic dynamics for the screening and screen-penetration processes was developed based on the Markov process to reveal the mechanism of the coal screening effect. As a partial differential equation, the model of the stochastic dynamics describes the relationship between particle position in the y direction, screening time, and particle screen-penetration probability. The obtained model for stochastic dynamics can calculate the screen-penetration probability of a specific particle size at any given time. Moreover, two coefficients, a1 and a2, are defined where a1 represents the average velocity of particles, and a2 represents the combined effect of orderly segregation and mixing of particles in chaotic motion. These coefficients of the stochastic dynamics model were obtained by regression analysis. Additionally, the statistical analysis showed that the fitting errors were all <1. The proposed statistical dynamics model works for all particle sizes. A necessary coefficient, a1, was calculated based on screen-penetration coefficients k, a, and b. A large value of a1 means a high average velocity of particles and a greater screen-penetration probability. The value of a1 for particles smaller than 6 mm declines as the feed mass increases. Moreover, it initially decreases, followed by an increase, in response to a rising proportion of particles <6 mm. In contrast, the a1 value for particles ranging between 6 mm and 13 mm undergoes minimal changes when subjected to variations in feed mass and the proportion of particles <6 mm.

Wettability-modulated behavior of polymers under varying degrees of nano-confinement

Extreme confinement in nanochannels results in unconventional equilibrium and flow behavior of polymers. The underlying flow physics dictating such paradigms remains far from being understood and more so if the confining substrate is composed of two-dimensional materials, such as graphene. In this study, we conducted systematic molecular dynamics simulations to explore the effect of wettability, confinement, and chain length on polymer flow through graphene-like nanochannels. Altering the wetting properties of these membranes that structurally represent graphene results in substantial changes in the behavior of polymers of disparate chain lengths. Longer hydrocarbon chains (n-dodecane) exhibit negligible wettability-dependent structuring in narrower nanochannels compared to shorter chains (n-hexane) culminating in higher average velocities and interfacial slippage of n-dodecane under less wettable conditions. We demonstrate that the wettability compensation comes from chain entanglement attributed to entropic factors. This study reveals a delicate balance between wettability-dependent enthalpy and chain-length-dependent entropy, resulting in a unique nanoscale flow paradigm, thus not only having far-reaching implications in the superior discernment of polymeric flow in sub-micrometer regimes but also potentially revolutionizing various applications in the oil industry, including innovative oil transport, oil extraction, ion transport polymers, and separation membranes.

Elutriation of fine particles in a pulsed fluidized bed

This paper reported on fine particle elutriation for two different static bed heights of 25 cm and 30 cm in a pulsed fluidized bed. The effects of three types of flow including continuous airflow, on-off pulsed airflow, and pulsed airflow imposed on continuous airflow were investigated. Observations showed that the elutriation fraction extremely increased with gas temporal average velocity and static bed height for continuous airflow, while for pulsed and mixed flows at the same condition the increase was not considerable. The effect of pulse frequency from 0.033 Hz to 0.1 Hz as an optimum range led to an extreme reduction loss of particles for pulsed and mixed airflows compared to continuous airflow even at high gas temporal average velocities. The elutriation rate of mixed airflow at 0.033 Hz and 3Umf for 30 cm static bed height on average was 75% and 35.3% less than continuous and pulsed airflows, respectively.

Controlled multiple spectral hole burning via a tripod-type atomic medium

In limit of saturation spectroscopy, we theoretically study the spectral hole burning (SHB) in the absorption spectrum of a probe field through a tripod atomic system. The response function for the probe field is calculated in a Doppler-broadened medium. Burning of spectral holes is observed only for the counter propagation of either one or both the coupling fields in the medium. The SHB is not observed below some critical temperature which is a condition for the electromagnetically induced transparency (EIT) in the medium. The most interesting and significant feature is that the Doppler broadening acts as a decoherence effect in case of EIT, however, the Doppler broadening acts inversely in case of SHB and consequently the burning effect enhances. The SHB is further enhanced and controlled by classes of the average velocity of atoms. The classes of high average atomic velocity in the medium increase the number of spectral hole burns (HBs). The widths of HBs can be controlled by the intensity of the driving fields. A single HB can be switched to multiple HBs in a well-controlled manner using different classes of high average atomic velocity. The various switchable holes can be burned in a desired position of the absorption spectrum which in turn simultaneously slow down multiple probe fields. The phenomenon of SHB may be useful in the construction of multichannel optical switching and storage devices.

Anisotropic fluid flows in black phosphorus nanochannels.

With the development of advanced micro/nanoscale technologies, two-dimensional materials have emerged from laboratories and have been applied in practice. To investigate the mechanisms of solid-liquid interactions in potential applications, molecular dynamics simulations are employed to study the flow behavior of n-dodecane (C12) molecules confined in black phosphorus (BP) nanochannels. Under the same external conditions, a significant difference in the velocity profiles of fluid molecules is observed when flowing along the armchair and zigzag directions of the BP walls. The average velocity of C12 molecules flowing along the zigzag direction is 9-fold higher than that along the armchair direction. The friction factor at the interface between C12 molecules and BP nanochannels and the orientations of C12 molecules near the BP walls are analyzed to explain the differences in velocity profiles under various flow directions, external driving forces, and nanochannel widths. The result shows that most C12 molecules are oriented parallel to the flow direction along the zigzag direction, leading to a relatively smaller friction factor hence a higher average velocity. In contrast, along the armchair direction, most C12 molecules are oriented perpendicular to the flow direction, leading to a relatively larger friction factor and thus a lower average velocity. This work provides important insights into understanding the anisotropic liquid flows in nanochannels.

A numerical study on the mixing characteristics and mechanisms of (H2+CH4)/air in a miniature combustor

In the present study, the influences of three operating parameters on the mixing performance of H2/CH4 blends and air in a miniature combustor with a rectangular cross section were numerically investigated. The results show that: (1) The mixing process of H2 and air is faster than CH4 and air. (2) The mixing process of H2 and air is diffusion-dominated under low velocities, while convection-dominated under medium and high velocities; However, the mixing of CH4 and air is diffusion-dominated under both low and high average velocities, but dominated by convection under medium velocities. (3) An increase in the nominal equivalence ratio creates a larger fuel concentration gradient perpendicular to the mainstream, which leads to an increase in the diffusion rate. (4) A larger hydrogen blending ratio intensifies the fuel stratification at the combustor entrance, which deteriorates the mixing performance. (5) An empirical correlation is established between the mixing performance evaluation criterion and the three parameters. The nominal equivalence ratio is the most significant factor to the mixing performance.

Estimation of glacier dynamics of the major glaciers of Bhutan using geospatial techniques

Abstract The purpose of this study was to determine the glacier displacement, velocity, and thickness of seven major glaciers of Bhutan and predicted potential glacier lake formation site with its depth. We named the glacier identification (ID) number 1–7 for seven glaciers. From the study, the glacier velocity between the central trunk and snout saw rapid fluctuations in 1976–1978 with an average uncertainty velocity of ± 27.10 m/year and a decreasing velocity trend. The year 2013–2014 has the lowest uncertainty in glacier velocity, with a value of ±1.24 m/year. The glacier velocity progressively increases from the snout to the main central trunk for all the glaciers with a value of 0 to 98.63 m/year. The glacier with the highest average velocity is glacier ID 5, which has a velocity of 25.58 m/year. From 2000 and 2022, all of the glaciers’ thicknesses significantly decreased from 0 to 468.2 m. The thickness of glacier ID 6 was lowered by −192.3 ± 1.99 m, making it the highest among the seven glaciers. In the future, a glacier lake is predicted to form at the base of each glacier. Glacier ID 6 is predicted to form the largest lake with a surface area of 2.572 km2 and a depth of 208.5 m.

Molecular Alignment-Mediated Stick-Slip Poiseuille Flow of Oil in Graphene Nanochannels.

The flow behavior of oil in nanochannels has attracted extensive attention for oil transport applications. In most, if not all, of the prior theoretical simulations, oil molecules were observed to flow steadily in nanochannels under pressure gradients. In this study, non-equilibrium molecular dynamics simulations are conducted to simulate the Poiseuille flow of oil with three different hydrocarbon chain lengths in graphene nanochannels. Contrary to the conventional perception of steady flows of oil in nanochannels, we find that oil molecules with the longest hydrocarbon chain (i.e., n-dodecane) exhibit notable stick-slip flow behavior. An alternation between the high average velocity of n-dodecane in the slip motion and the low average velocity in the stick motion is observed, with a drastic, abrupt velocity jolt of up to 40 times occurring at the transition in a stick-slip motion. Further statistical analyses show that the stick-slip flow behavior of n-dodecane molecules originates from the molecular alignment change of oil near the graphene wall. The molecular alignment of n-dodecane shows different statistical distributions under stick and slip motion states, leading to significant changes of friction forces and thus notable velocity fluctuations. This work provides new insights into the Poiseuille flow behavior of oil in graphene nanochannels and may offer useful guidelines for other mass transport applications.

Numerical and experimental investigation of flow and heat transfer in a fixed bed of non‐spherical grains using the DEM‐CFD method

AbstractFixed‐bed drying of grains is a widely used method for determining temperature and humidity changes and pressure drop characteristics during aeration. In this study, an accurate particle‐resolved computational fluid dynamics (CFD) numerical model was developed to investigate the flow and heat transfer in fixed beds of soybean, wheat, and maize grains. A randomly filled non‐spherical grain fixed bed structure was generated using the discrete element method (DEM), and the contact areas between the packed particles are automated. The distribution of grain particles near the wall surface was approximately circular, and the circle became more regular with increase grain sphericity. Compared with the fixed beds of soybean and maize, there was no significant stratification in the fixed beds of wheat, and the difference between the peaks and troughs of porosity oscillations was smaller. Detailed particle‐resolved CFD simulations of the flow and heat transfer in fixed beds with different grains were performed. The results showed that the pressure drop results of the DEM–CFD simulation of different grain shapes were in good agreement with the Nemec–Levec equation and experimental results. The velocity and temperature distribution in the fixed bed have obvious non‐uniform distribution characteristics, which are more visible in the maize pile. Regions with high porosity have a higher average velocity, and the temperature near the wall is higher than that at the center when the heat transfer is unbalanced. The DEM‐CFD model includes the heat conduction process between the fluid and solid phases, which is consistent with the experimental temperature results.Practical ApplicationsIn this work, an accurate particle‐resolved computational fluid dynamics numerical model was developed to investigate the flow and heat transfer in fixed beds of soybean, wheat, and maize grains. The randomly filled non‐spherical grain fixed bed structure is generated by the discrete element method (DEM), and the contact areas between the packed particles are automated treated. The pressure drop and heat transfer processes of different grains were measured using the fixed bed drying experimental platform. The results show that the pressure drop results of DEM–CFD simulation of different grain shapes are in good agreement with Nemec–Levec equation results and experimental results. The DEM‐CFD model includes the heat conduction process between fluid and solid phases, which is in close agreement with the experimental temperature results.

Gravel arrester beds as an important motorway safety element

Motorways are usually perceived as the safest part of the road infrastructure. Nevertheless, the junctions and exit ramps on them can still present a safety challenge due to high average velocities and obstacles installed to contain the out-of-control vehicles within the roadway. Recently, the gravel arrester-beds have been constructed on many locations on motorway junctions and exit ramps. They present a viable alternative to the rigid barriers and deformable crash-cushions normally used on these locations. They help decelerate the vehicles by dissipating the kinetic energy in the gravel bed and thus stop the vehicles more gradually on a longer distance, causing lower average decelerations and thus reducing passenger injuries. Furthermore, by proper design, they can reduce the damage sustained by the vehicles and the infrastructure and thus shorten the times required to re-open roads after incidents. The paper provides an insight into the subject and presents some recent findings obtained from measurements on a test site, and from simulations with different types of vehicles. The parameters and their required values for optimizing the efficiency of gravel arrester beds for different requirements are discussed in the final part.

Comparison of barrel-shaped fixed bed drying for different ventilation methods by mathematic simulation

Unsteady one-dimensional partial differential equations for heat and mass transfer in peanut drying are used in mathematical simulations to compare the drying performance of a barrel-shaped bed in three ventilation modes: bottom up, inside out and outside in. Results obtained using the finite difference scheme show that the mathematical simulation reliably predicts the drying process of peanuts, and that the drying delay of bottom-up ventilation is the most significant at the beginning of drying. This is because the drying enhancement afforded by the high average velocity of air along the ventilation direction is insufficient to offset the drying delay caused by the large ventilation thickness compared with that afforded by the inside-out and outside-in ventilation. Furthermore, as the air volume flow increases, the time consumption and moisture content difference decrease, whereas the productivity, energy consumption, and uniformity increase for all three ventilation modes, consistent with the classical ventilation drying law. Under the same air volume flow, the time and energy consumptions for the bottom-up ventilation are the lowest, followed by those for the outside-in and inside-out ventilation. The productivity afforded by the bottom-up ventilation is the highest, followed by that by the outside-in and inside-out ventilation. The outside-in ventilation indicates the least moisture content difference and the best drying uniformity. Meanwhile, the inside-out ventilation indicates the most significant moisture content difference and the worst drying uniformity.

Light control of droplets on photo-induced charged surfaces.

The manipulation of droplets plays a vital role in fundamental research and practical applications, from chemical reactions to bioanalysis. As an intriguing and active format, light control of droplets, typically induced by photochemistry, photomechanics, light-induced Marangoni effects or light-induced electric fields, enables remote and contactless control with remarkable spatial and temporal accuracy. However, current light control of droplets suffers from poor performance and limited reliability. Here we develop a new superamphiphobic material that integrates the dual merits of light and electric field by rationally preparing liquid metal particles/poly(vinylidene fluoride-trifluoroethylene) polymer composites with photo-induced charge generation capability in real time, enabling light control of droplets on the basis of photo-induced dielectrophoretic force. We demonstrate that this photo-induced charged surface (PICS) imparts a new paradigm for controllable droplet motion, including high average velocity (∼35.9mm s-1), unlimited distance, multimode motions (e.g. forward, backward and rotation) and single-to-multiple droplet manipulation, which are otherwise unachievable in conventional strategies. We further extend light control of droplets to robotic and bio-applications, including transporting a solid cargo in a closed tube, crossing a tiny tunnel, avoiding obstacles, sensing the changing environment via naked-eye color shift, preparing hydrogel beads, transporting living cells and reliable biosensing. Our PICS not only provides insight into the development of new smart interface materials and microfluidics, but also brings new possibilities for chemical and biomedical applications.

CFD investigation of air-oil two-phase flow in oil jet nozzle

In aero-engines applications, nozzle geometric parameters have major effects on the lubricating oil to accurately reach target gears or bearings. To investigate the deviation phenomenon of oil jet flow in the lubrication system, the computational fluid dynamics (CFD) technique integrating with the volume of fluid (VOF) method and SST k- ω turbulence model is adopted to deeply understand the flow characteristics of the nozzle. By leveraging this method, the relationship between nozzle geometry, inlet pressure, jet velocity, and jet deviation are firstly analyzed. The results reveal that a higher average velocity and smaller jet deviation can be determined via adjusting the nozzle geometry. Furthermore, a specific test rig, composed of an oil supply system, target system and data collection system, is set up for oil injection test; experimental outlet velocity and mass flow rate of oil passing through the hole on the target plate are monitored and recorded. The experimental findings compare well with numerical results obtained by the CFD method.

Discharge dynamics of primary and secondary streamers in a repetitively pulsed surface dielectric barrier discharge

Discharge dynamics of primary and secondary streamers in a repetitively pulsed surface dielectric barrier discharge (SDBD) are investigated based on experimental and numerical simulations. Plasma propagation and coupled energy of the primary streamer are restricted in subsequent pulses, but the deposited energy of the secondary streamer increases. When the pulse repetition frequency reduces, a longer plasma length and higher average velocity of the primary streamer can be observed, but the influences on propagation length and velocity of the secondary streamer are very limited. These phenomena indicate that the residual surface charges left by the previous pulse should have a critical effect on the discharge dynamics of subsequent discharges. In order to have a deeper insight into the influence of residual surface charges in a repetitively pulsed SDBD, a numerical model characterized with a pre-charging of homogeneous charge accumulated on the dielectric surface is built. Pre-charging of positive charges deposited on the dielectric surface can inhibit the electric field of applied voltage, resulting in a decrease in the expansion of the primary streamer and the positive peak of current, which is in qualitative agreement with the experimental measurements. However, there is an opposite evolution rule when the negative charges are deposited on the dielectric surface. Although the electric field strength of the secondary streamer is enhanced for a high pre-charging value, there is no great impact on the negative peak of current during the secondary streamer due to the remaining heavy mass ions.

COVID-19 실내 확산 방지를 위한 급배기 특성 CFD 분석

The outbreak of COVID-19 still seriously affects our daily lives despite the actions with governmental level to prevent the situation from getting worse. Among various infection routes of COVID-19, airborne spread needs to be carefully handled in the perspective of HVAC. In particular, cluster infection in multi-use facilities which unspecified number of people use is now the big issue. One of the most simple solution is to dilute virus pollution through the proper ventilation. Therefore, this study analyzes the effect of the location of diffusers and the ventilation rate on the indoor air distribution in the large space through a numerical method. The results show that the zigzag arrangement was the most suitable for ventilation because of its high average and minimum air velocity.

Numerical analysis of bladeless ceiling fan: An effective alternative to conventional ceiling fan

A ceiling fan is one of the cost-effective alternatives for poverty-ridden societies and tropical countries short on natural resources. However, due to the inherent working principle, a lot of losses occur during electrical to mechanical energy conversion. Apart from losses, other problems associated with conventional ceiling fans are buffeting airflow, internal heating, and dirt accumulation on fan blades. Recently, the idea of a bladeless ceiling fan has been patented which has the potential to resolve most of the issues associated with the conventional fans, however, no details have been shared regarding flow physics and their effectiveness in constrained spaces. In this study, an annular type bladeless ceiling fan is computationally analyzed in a standard empty room (4m × 4m x 4m) environment. Parametric analysis of different design features such as fan radius, height from the ceiling and floor, fan jet width, mass flow rate, and orientations was studied and their effect on the perceived comfort level in terms of velocity spread and average velocities were captured computationally. The results show that the fan height in the close vicinity of ceiling does not affect the flow field in the occupied zones, however, as the fan gets closer to the floor the velocity field in the occupied zone changes due to creation of a strong vortex at the center of floor. Thus, fan installation closer to the ceiling (less than 0.5m from the ceiling) is a preferred choice. With an increase in fan radius from 0.3m to 0.5m, an increment of 33% was observed in velocity spread thus increasing the zone of influence. A finer jet width under the same volume flow rate produces higher velocities across the room e.g. 2 mm jet produces 36% higher average velocities as compared to 4 mm jet, which effectively increases zone of influence. Effect of jet orientation was also studied and a 90° (straight down) orientation produced better overall velocity spread in comparison with 110° and 130° orientation. In comparison with conventional fans, a minimum of 57% increment in velocity spread was recorded for all bladeless fan configurations under consideration in the present work. The flow physics of bladeless fan has prospects for further increment in the zone of influence by optimizing the geometric features in light of present work.

Numerical investigation of additional gas supply angle influence on flow in jet mill ejector

The results of numerical simulation of the flow in the jet mill ejector which is equipped with an additional gas supply channel have been presented. The influence of the supply system parameters on the nature of the flow in the ejector accelerating tube has been investigated. A comparative analysis of the change in the flow structure in the ejector for different values of the additional gas flow angle has been carried out. The dependences of the flow velocity at the outlet of the accelerating tube, the ejection coefficient and the thickness of the protective layer on the angle of the additional energy carrier supply have been scrutinized. The maximum values of the average output velocities for the given ranges of the additional flow parameters have been determined. Recommendations for the choice of geometric and gas-dynamic parameters of the additional gas flow to ensure the highest efficiency of the acceleration process in the jet mill ejector have been developed. The choice of the angle of the additional gas flow supply, which provides the highest average velocity at the outlet from the accelerating tube with the highest ejection coefficient, has been substantiated.