Diffusion Velocity Research Articles

Research on gas-liquid interface parameters related to thermal performance of frost-free evaporator of air source heat pump

Direct spray frost-free air source heat pumps (FFASHPs) base on liquid desiccant solutions are a more promising renewable energy technology for developing net-zero emission buildings. In order to improve the thermal performance of the frost-free evaporator, a numerical mode was established based on the penetration theory and the two-film theory to reveal the interfacial heat and mass transfer mechanism of LiCl solution falling film absorption on the air side of the frost-free evaporator. The distribution characteristics of temperature, water vapor concentration, effective diffusion coefficient, morphology, velocity and pressure at the interface under different air Reynolds numbers and temperatures were analyzed. The results show that the distribution uniformity of interface parameters has a greater effect on heat transfer performance than its average value. The critical Reynolds number is 391.0 under the present study, and the distribution uniformity of interface parameters and the intensity of interface fluctuation are improved, and the thermal performance reaches the peak. The COP of FFASHP system can be improved effectively when the frost-free evaporator is operated under the critical condition matching its parameters. The purpose of the study is to provide theoretical support for the performance improvement and efficient operation of frost-free evaporators.

Modeling the transport and mixing of suspended sediment in ecological flows with submerged vegetation: A random displacement model-based analysis

Aquatic vegetation in rivers influences the flow structure, impacting the sediment transport in the river and further changing the ecosystem and geomorphologic evolution of the river. In this paper, an improved random displacement model (RDM) is proposed to investigate the concentration of suspended sediment (CSS) in flows with flexible submerged vegetation. Considering the effect of sediment resuspension on sediment transport, a probability model of sediment resuspension is embedded into the RDM so that the simulation process is more consistent with the natural cases. For the streamwise development of flow with flexible submerged vegetation, the flow is divided into a flow adjustment region and a fully developed region in the longitudinal direction. The vertical distributions of the flow velocity and turbulence diffusion coefficient were analyzed in these two regions, and they were incorporated into the RDM to simulate the along-flow development of the CSS. The simulated concentrations were compared with the experimental data. The mean relative errors (MREs) were below 15%, the root mean square errors (RMSEs) were below 0.20, and the Nash–Sutcliffe efficiencies (NSEs) ranged from 0.71 to 0.99, indicating that the improved RDM is plausible and reliable. In addition, the along-flow distribution of the sediment transport rate (STR) was analyzed. The rate exhibited a decreasing trend during the flow adjustment region and tended to stabilize during the fully developed region. This study provides a new way of thinking for research on sediment transport in rivers with aquatic vegetation, which is of great significance for achieving the sustainable development of river ecosystems and the optimal design of river channels.

Diffusion and mobility measurements for propane gas with a nanodosimetric detector

For the operation of nanodosimetric detectors with propane gas, it is essential to know the gas properties at room temperature, such as drift velocity, ion mobility, longitudinal and transversal diffusion coefficients. For propane gas there is only limited experimental data available for the ion mobility and transverse diffusion coefficients. In this work, the drift velocities and ion mobilities for propane were obtained over the reduced electric field range E/N from 24 to 212 Td (1 Td = 10−17 Vcm2) based on arrival time measurements. The reduced ion mobility K0 for propane was determined to be (0.71±0.09) cm2/Vs. The longitudinal diffusion coefficients were determined from the widths of the experimental arrival time spectra and found to be NDL = (3.7 ± 0.5)⋅1019 1/ms in the low field limit below 30 Td.The results presented in this work extend the range of available experimental data for propane, as well as introduce first experimental measurements of the longitudinal diffusion coefficient of propane.

Velocity of ultrasound diffusion in heterogeneous media and its applications

This paper investigates the velocity of ultrasonic diffusion in heterogeneous media under the conditions where the diffuse wave approximation is valid (λ≤a). The diffusion velocity is defined as the moving speed of the peak of the energy evolution curve. The peak arrival time as a function of transport distance is calculated for infinite spaces and bounded domains. The results show that the peak arrival time is independent of the domain’s geometry, i.e. the size, shape, and boundary conditions. This is confirmed by comparing the arrival times in an infinite space and a bounded domain with a geometric feature – a surface-breaking crack of varying depth. Therefore, the velocity of ultrasonic diffusion is an intrinsic property of a medium, and the formula for the infinite three-dimensional space is sufficient to calculate the arrival time–transport distance relationship, given the diffuse properties of the medium. These findings eliminate the needs for the finite element numerical simulations in the applications to determine geometric parameters such as the crack depth using diffuse ultrasound. The diffusion velocity is a function of transport distance in general, while its far-field asymptotic value is constant regardless of the dimensionality and geometry of the domain.

Experimental Study on the Diffusion and Burning Characteristics of Continuous Liquid Fuel Spillage on Sloped Ground

When liquid fuel spills on a sloping ground and an ignition source is nearby, it could lead to a continuous spill fire. To investigate the diffusion and burning characteristics of continuous liquid fuel spillage on sloped ground, a series of experiments were conducted, considering different discharge rates (60∼140 ml/min) and ground slopes (2o, 4o, 6o, 8o). Characteristic parameters were measured, including fuel diffusion rate, diffusion shape, flame temperature, and burning rate. A new theoretical model for the diffusion velocity was established and the diffusion shape characteristics of the liquid fuel were revealed for the first time. Results indicate that under non-ignited conditions, the diffusion velocity of the liquid fuel gradually decreases, eventually converging toward an asymptotic velocity. Under ignited conditions, the burning process of the spill fire could be categorized into three distinct stages. Moreover, the mass burning rate per unit area of spill fires is obviously lower than that of pool fires. This is because the fuel thickness in spill fires is so thin that the radiant heat flux emitted by the flame to the fuel cannot be fully absorbed by the fuel. This work could provide a critical reference for predicting the diffusion process of a spill fire.

Buongiorno’s radiative Prandtl-Eyring nanofluid flow through porous medium over a stretching surface with cross-diffusion effects

The main focus of the current analysis is to describe the thermo-magnetic flow, heat and mass transport features of two-dimensional dissipative Prandtl-Eyring nanofluid (PE-NF) flow over a stretching sheet under the influence of porous medium and magnetic Ohmic dissipation numerically. An innovative Buongiorno’s nanofluid model in terms of Brownian motion and thermophoresis is deployed to accurately simulate the nano behavior in Prandtl-Eyring within the boundary layer regime. The thermal radiation and heat source/sink effects are also included to describe the thermal transport process. In addition to this, the thermo-diffusion and diffusion-thermo effect are included to demonstrate the temperature and concentration diffusion mechanism under the presence of chemical reaction process. The emerged coupled nonlinear two-dimensional time-independent partial differential equations are rendered to their dimensionless form through appropriate similarity transformations and solved by deploying Matlab-based BVP4C technique. The graphical visualization showed that, the increasing magnetic number diminished the flow field and enhanced the temperature and concentration profiles in the flow regime. Rising porous parameter decayed the Prandtl-Eyring nanofluid velocity. Increasing radiation and Eckert numbers enhance the temperature field in the flow regime. Magnifying Prandtl-Eyring parameter suppressed the velocity diffusion. Increasing Brownian motion and thermophoresis parameters increases the temperature profile. Amplifying Soret parameter upsurges the concentration profile. Skin-friction coefficient amplified with enhanced magnetic and porous numbers. Heat transfer rate acts as increasing function of thermophoresis and Brownian motion parameters. The main objective of this study is the addition of magnetic Ohmic dissipation, radiation, porous medium, thermal source/sink, Soret and Dufour effects and Buongiorno nanofluid model and this consideration generalizes former studies and gives a new re-defined mathematical formulation of heat and mass transport features of Prandtl-Eyring nanofluid flow over a stretching sheet. Finally, the numerical accuracy of the current similarity results is validated with available solutions and noticed a good agreement.

Genome-wide analysis of the biophysical properties of chromatin and nuclear proteins in living cells with Hi-D.

To understand the dynamic nature of the genome, the localization and rearrangement of DNA and DNA-binding proteins must be analyzed across the entire nucleus of single living cells. Recently, we developed a computational light microscopy technique, called high-resolution diffusion (Hi-D) mapping, which can accurately detect, classify and map diffusion dynamics and biophysical parameters such as the diffusion constant, the anomalous exponent, drift velocity and model physical diffusion from the data at a high spatial resolution across the genome in living cells. Hi-D combines dense optical flow to detect and track local chromatin and nuclear protein motion genome-wide and Bayesian inference to characterize this local movement at nanoscale resolution. Here we present the Python implementation of Hi-D, with an option for parallelizing the calculations to run on multicore central processing units (CPUs). The functionality of Hi-D is presented to the users via user-friendly documented Python notebooks. Hi-D reduces the analysis time to less than 1 h using a multicore CPU with a single compute node. We also present different applications of Hi-D for live-imaging of DNA, histone H2B and RNA polymerase II sequences acquired with spinning disk confocal and super-resolution structured illumination microscopy.

Compressible fluid limit for smooth solutions to the Landau equation

Although the compressible fluid limit of the Boltzmann equation with cutoff has been extensively investigated in Caflisch [Comm. Pure Appl. Math. 33 (1980), 651–666] and Guo, Jang, and Jiang [Comm. Pure Appl. Math. 63 (2010), 337–361], obtaining analogous results in the case of the angular non-cutoff or even in the grazing limit which gives the Landau equation, still remains largely open, essentially due to the velocity diffusion effect of the collision operator such that L^{\infty} estimates are hard to obtain without using Sobolev embeddings. In this paper, we are concerned with the compressible Euler and acoustic limits of the Landau equation for Coulomb potentials in the whole space. Specifically, over any finite time interval where the full compressible Euler system admits a smooth solution around constant states, we construct a unique solution in a high-order weighted Sobolev space for the Landau equation with suitable initial data and also show the uniform estimates independent of the small Knudsen number \varepsilon>0 , yielding the O(\varepsilon) convergence of the Landau solution to the local Maxwellian whose fluid quantities are the given Euler solution. Moreover, the acoustic limit for smooth solutions to the Landau equation in an optimal scaling is also established. For the proof, by using the macro-micro decomposition around local Maxwellians, together with techniques for viscous compressible fluid and properties of Burnett functions, we design an \varepsilon -dependent energy functional to capture the dissipation in the compressible fluid limit with the feature that only the highest-order derivatives are most singular.

Gas Free Dissipation Characteristics Analysis and Safety Repair Time Determination of Buried Pipeline Leakage Based on CFD

Buried pipelines, as the most common method of natural gas transportation, are prone to pipeline leakage accidents and are difficult to detect due to their harsh and concealed environment. This paper focused on the problem regarding the free dissipation of residual gas in buried gas pipelines and soil after closing the gas supply end valve after a period of leakage by numerical simulation. A multiple non-linear regression model was established based on the least squares method and multiple regression theory, and MATLAB 2016b mathematical calculation software was used to solve the problem. The research results indicated that compared to the gas leakage diffusion stage, the pressure and velocity distribution during the free dissipation stage were significantly reduced. The increase in leakage time, pipeline pressure, leakage size, and pipeline burial depth led to a large accumulation of natural gas, which increased the concentration and distribution range of gas on the same free dissipation stage monitoring line. A prediction model for natural gas concentration in the free dissipation stage was established with an average error of 7.88%. A calculation model for the safety repair time of buried gas pipeline leakage accidents was further derived to determine the safety repair time of leakage accidents.

Microstructural and material property changes in severely deformed Eurofer-97

Severe plastic deformation changes the microstructure and properties of steels, which may be favourable for their use in structural components of nuclear reactors. In this study, high-pressure torsion (HPT) was used to refine the grain structure of Eurofer-97, a ferritic/martensitic steel. Electron microscopy and X-ray diffraction were used to characterise the microstructural changes. Following HPT at room temperature to a maximum shear strain of 230, the average grain size reduced by a factor of ∼30, with a marked increase in high-angle grain boundaries. Dislocation density also increased by more than one order of magnitude. The thermal stability of the deformed material was investigated via in-situ annealing during synchrotron X-ray diffraction. This revealed substantial recovery between 450 K – 800 K. Irradiation with 20 MeV Fe-ions to ∼0.1 dpa caused a 20% reduction in dislocation density compared to the as-deformed material. However, HPT deformation prior to irradiation only had a minor effect in mitigating the irradiation-induced reductions in thermal diffusivity and surface acoustic wave velocity of the material. Microstructural and material property changes are dominated by deformation compared to irradiation. In light of this, the benefits of using HPT to improve the irradiation resistance of Eurofer-97 are limited. These results provide a multi-faceted view of the changes in ferritic/martensitic steels due to severe plastic deformation, and how these changes can be used to alter material properties.

Particles with constant speed and random velocity in 3+1 space-time: separation of the space variables

We consider a particle in 3+1 space-time dimensions which constantly moves at speed of light c, randomly changing its velocity which can be represented by a point on the surface of a sphere of radius c. The velocity performs an isotropic Wiener process on this surface so that the velocity trajectories are almost everywhere continuous although not differentiable, while the position trajectories are continuous and differentiable. Together with the process that describes the particle in the ‘rest frame’, where the diffusion of velocity on the surface of the sphere is isotropic, the entire family of anisotropic processes which result from Lorentz boosts is also described. The focus of this article is on stochastic evolution in space. We identify a reduced set of variables whose stochastic evolution is autonomous from the remaining variables, but, nevertheless, carry all the relevant information concerning the spatial properties of the process. The associated stochastic equations as well the Forward Kolmogorov equation are considerably simplified compared to those of the complete set of position and velocity variables.

Numerical simulation on transient electromagnetic response of separation layer water in coal seam roof

Mining stress induces deformation and fracture of the overlaying rock, which will result in water filling the separation layer if the aquifer finds access to abscission space along the fracture channels. Accurate detection is crucial to prevent water hazards induced by water-bearing fractures. The 3-D time-domain finite-difference method with Yee’s grid was adopted to calculate full-space transient electromagnetic response; meanwhile, a typical geologic and geophysical model with a water-bearing block in an separation layer was built according to regional tectonics and stratigraphic developments. By using numerical simulation, the induced voltage and apparent resistivity for both vertical and horizontal components were acquired, and then an approximate inversion was carried out based on the “smoke ring” theory. The results indicate that the diffusion velocity of induced voltage is significantly affected by the water-bearing body in the fracture, and the horizontal velocity of induced voltage is lower than the vertical one. The induced voltage curves indicate that the horizontal response to an anomaly body is stronger than the vertical one, leading to a high apparent resistivity resolution of conductivity contrast and separation layer boundary in the horizontal direction. The results of 3-D simulation making use of a measured data model also demonstrate that the horizontal component of apparent resistivity can reflect the electrical structure in a better way; however, its ability to recognize the concealed and fine conductor is rather weak. Accordingly, the observation method or numerical interpolation method needs to be further improved for data processing and interpretation.

Identifying the in-situ origin of supercluster-induced ductile precipitates in Zr-based bulk metallic glasses

A sandwich architecture including double crystal substrates and a single glass-forming droplet has been successfully designed based on the simplified liquid-bridge methods in multicomponent alloys. The multiscale precipitates mainly consisting of β-phase dendrites with a body-centered cubic (BCC) structure, cubic CuZr2 phase, and icosahedral quasicrystalline phase (I-phase) could be simultaneously identified in dissimilar regions of the glass-forming matrix during heat-induced dissolution or diffusion from crystal substrate to glass-forming droplet. The gradient distributions of crystal-like clusters not only accelerate diffusion velocity due to the collective atomic motion but also can simultaneously modulate the atomic-scale structure, mechanical response, and chemical composition via tracing a high-throughput strategy. The present work provides new insights to discover the formation mechanism of the representative microstructures strongly associated with the atomic-scale structure-property-composition relationships and is also helpful to shed light on the in-situ origin or structural mechanism of supercluster-induced multiscale precipitates in developing bulk metallic glasses (BMGs) and in-situ formed BMG composites (BMGCs).

Miniaturizable Chemiluminescence System for ATP Detection in Water.

We present the design, fabrication, and testing of a low-cost, miniaturized detection system that utilizes chemiluminescence to measure the presence of adenosine triphosphate (ATP), the energy unit in biological systems, in water samples. The ATP-luciferin chemiluminescent solution was faced to a silicon photomultiplier (SiPM) for highly sensitive real-time detection. This system can detect ATP concentrations as low as 0.2 nM, with a sensitivity of 79.5 A/M. Additionally, it offers rapid response times and can measure the characteristic time required for reactant diffusion and mixing within the reaction volume, determined to be 0.3 ± 0.1 s. This corresponds to a diffusion velocity of approximately 44 ± 14 mm2/s.

Nitrogen diffusion along the layer boundaries after nitriding of multilayer materials

Diffusion processes play a key role in formation of the structures of new materials and technological processes of strengthening heat treatments, since diffusion is the reason for redistribution of substances in solids. An urgent task is to develop technologically advanced and effective methods for strengthening materials in order to improve their performance properties. There is an increasing need to improve chemical heat treatment methods, which directly affects the wear resistance of working surfaces, and, consequently, the product service life. Near-surface volumes experience increased loads, so the formation of high-strength layers becomes an important task. Quite a few methods of surface hardening are known, among which carburization, nitriding, nitrocarburization and others are widely used. The most interesting is nitriding, since it increases hardness, strength, fatigue limit, and heat resistance. However, despite the proper advantages, nitriding has a number of disadvantages, including the holding duration and small thickness of diffusion layers. The solution is related to intensification of the technological process by increasing the nitriding temperature, activating the nitriding media or directly the parts surface. All these solutions are aimed at accelerating diffusion processes, both in grain volume and along grain boundaries, the velocity along which is many times higher than the velocity of volumetric diffusion. It may be effective to use a new type of structural metal materials with a multilayer structure of hundreds of layers, with thicknesses in the micron and submicron ranges separated by large angular boundaries. The results of metallographic studies showed the effect of the steel layers interchange in the multilayer metal material on diffusion depth after chemical heat treatment. The authors proposed an accelerate diffusion model of diffusible element along the layer boundaries.

Aspects of double slip on multiphase inclined channel flow of Casson and Cu-nanofluid

The key aspect of this study is to inspect the entropy generation (EG) of heat and momentum transport within two immiscible Casson and nanofluid (NF) with mass diffusion of copper nanomaterials in an inclined channel (IC) with thermal and velocity slips. The IC is divided into two regions. Region-I contains the Casson fluid (CF) whereas Region II is occupied with copper-water NF. Moreover, the effects of Lorentz force are also taken in region II having copper-water NF. The mathematical model is solved by employing the perturbation method. The behavior of different aspects, i.e., mass diffusion of copper nanomaterials, Lorentz force, velocity and thermal slips on EG, and transport of mass and heat are presented in graphs and tabular forms. Furthermore, the numerical outcomes of friction at the walls and the rate of heat transport are presented in tabular form. Our results show that the EG and the rate of EG decline when the dispersion of nanomaterials is increased. The velocity and temperature decline when Lorentz force is increased in Region II. Furthermore, temperature augments with the rise in CF parameter and thermal slip parameters whereas velocity also increases with the rise in velocity slip parameters. So, the thermal performance can be enhanced in an IC by using blood fluid in one phase and thermal slips at the walls.

Influence of Sn grain orientation on mean-time-to-failure equation for microbumps in 3D IC technology

The mean-time-to-failure (MTTF) associated with microbumps is a pivotal factor influencing the reliability of cutting-edge consumer electronic devices. Within Sn-based solder joints, the orientation of Sn grains significantly impacts the diffusion velocity of Cu and Ni atoms, thereby impacting MTTF. While a large solder joint encompassing numerous Sn grains can be treated as isotropic, the scenario shifts in a small microbump with just one Sn grain, rendering it anisotropic. In such cases, the Sn grain orientation emerges as a dominant factor dictating the microbump's longevity. Herein, we investigated the influence of Sn grain orientation on MTTF of electromigration, thermomigration, and thermal aging. A new parameter, α, was introduced, which signifies the angle between the c-axis of the Sn grain and the direction of external stressing.

Migration and cooling characteristics of low-temperature gaseous CO2 in high-temperature loose coal

To investigate the extinguishing effect of low-temperature gaseous CO2 on coal spontaneous combustion, a similar simulation experiment was designed and conducted to test its migration and cooling characteristics in high-temperature loose coal. During the injection of low-temperature gaseous CO2, the temperature along the central axis of the release port decreases rapidly. The size of the cooling range and the diffusion rate of the cooling area are inversely proportional to the distance from the center axis and directly proportional to the injection time. The injection time plays a dominant role in the cooling effect of the high temperature coal, followed by the injection position. Gaseous CO2 diffuses inside the loose coal body rapidly, filling the vicinity of the release port within 5s and the container within 30s. The average diffusion velocity of CO2 within the loose coal is 0.051 m/s. The results of this study provide a theoretical basis for the application of low-temperature carbon dioxide to combat coal fire disasters.

Effects of the nozzle structure and fluidized gas composition on the gas-particle two-phase jet characteristic in a powder fuel scramjet

The interaction between nozzle design and fluidization gas composition significantly influences the dynamics within a powder fuel scramjet's combustion chamber. To investigate this relationship, an experimental study utilized high-speed shadow imaging technology to explore the macroscopic aspects of powder fuel injection. The investigation examined various convergence angles, nozzle throat lengths, and fluidized gas compositions. Key findings include: During jet development, powder fuel initially concentrates near the axis, with non-convergence angle nozzles exhibiting longer concentrated distribution periods than convergence angle conditions. Decreasing nozzle convergence angles lead to increased penetration distance, frontal velocity, and radial diffusion distance during the initial stages of jet development. Additionally, stable jet shapes show larger divergence angles as nozzle convergence angle decreases, with the largest divergence angle observed at α=60°. In the initial 0–7 ms of jet development, the powder fuel jet demonstrates greater penetration distance and frontal velocity under certain conditions. Moreover, penetration distance and frontal velocity increase with throat length from 7 to 20 ms, accompanied by changes in divergence angles. Specifically, at a throat length (l) of 2 mm, the near-field divergence angle measures 46.50°, and the far-field divergence angle is 22.25°. Conversely, at l=8mm, the near-field divergence angle is 33.49°, and the far-field divergence angle is 23.21°. The fluidization gas composition minimally affects jet penetration distance and frontal velocity during the initial 0–3 ms. However, due to hydrogen's low density, hydrogen/powder fuel jets exhibit shorter distances and velocities compared to nitrogen/powder fuel jets. Hydrogen fluidization also results in larger divergence angles, particularly in the near field. These findings underscore the importance of nozzle design and fluidization gas composition in optimizing scramjet performance and efficiency.

Hierarchical pore engineering of MOF-5 derived electrocatalysts with high mass transfer for zinc-air flow batteries

From micro to macro level, the performance of zinc-air flow battery (ZAFB) hinges on two crucial factors: the effective oxygen reduction reaction (ORR) electrocatalysts, and the durable triple-phase electrodes. However, sluggish kinetic process always occurs due to insufficient active sites and hysteretic mass transfer. In this work, a well-confined iron phthalocyanine (FePc) structure onto carbonized metal-organic framework (C-MOF5) electrocatalyst is constructed through a pyrolysis-free strategy. The macrocyclic skeleton of MOF-5 has been confirmed to offer a stronger and more abundant environment for hosting FePc molecules. Attributing to the hierarchical pore engineering of MOF-5, the FePc@C-MOF5 electrocatalyst reveals a high content of Fe (5.68 wt%) with multiple exposed active sites. Notably, the FePc@C-MOF5 exhibits a remarkably high half-wave potential of 0.915 V for ORR in 0.1 M KOH, surpassing that of commercial Pt/C by nearly 60 mV. From a higher-level device perspective, the highest oxygen diffusion velocity and local current density of the FePc@C-MOF5 electrode is verified by computational fluid dynamics (CFD) simulations. As a result, the as-assembled ZAFB demonstrates long-term stability with 200 h’ cycling. This work not only presents the hierarchical pore tailoring strategy for designing ORR electrocatalysts but also offers a deeper understanding of the essence of interfacial mass transfer.