Mass Transfer Conditions Research Articles

Nonequilibrium molecular dynamics study on energy accommodation coefficient on condensing liquid surface—Molecular boundary conditions for heat and mass transfer

Nonequilibrium molecular dynamics (NEMD) studies have been conducted to determine molecular boundary conditions at vapor–liquid interfaces for the kinetic theory of condensation and evaporation. In previous studies, a microscopic formulation of the condensation coefficient was defined as the condensation probability of vapor molecules based on equilibrium molecular dynamics simulations and transition state theory. The condensation coefficient was presented as a function of the translation energy of incoming molecules and surface temperature. Based on this, the velocity distributions of evaporating and reflecting molecules were theoretically expressed under equilibrium conditions. In a practical nonequilibrium situation, the energy transfer by the reflecting molecules is important along with the condensation/evaporation probability. However, it is unclear whether the results obtained under equilibrium conditions can be applied under nonequilibrium conditions. This study, therefore, defines the energy accommodation coefficient of reflecting molecules by comparing the energy transfer due to reflection with that under equilibrium conditions. NEMD simulations are conducted using two surfaces facing each other, an evaporating surface and a condensing surface, for argon molecules under different nonequilibrium conditions. The results show that the velocity distribution of reflecting molecules deviates from those under equilibrium conditions, and the energy accommodation coefficient decreases as nonequilibrium conditions increase. Additionally, an inverted temperature profile is observed. Reflecting molecules play an important role in the sensible heat transfer on the condensing surface, and they are not accommodated on the condensing surface. Thus, they raise the temperature in the vicinity of the condensing surface under nonequilibrium conditions.

Mathematical Modeling of the Process of Convective Drying of Materials Taking into Account their Shrinkage

The authors have discussed the physical regularities of internal mass transfer in drying colloidal capillary-porous materials, which is characterized by the fact that in addition to the phenomenon of mass conductivity (diffusion of moisture) in a fixed coordinate system, the transfer of moisture by the matrix of the material due to its shrinkage is observed. As applied to this case, for a body in the shape of an unbounded plate dried symmetrically from both surfaces, a mathematical model has been written that describes mass conductivity and convective transfer of moisture in the body. The convective transfer of moisture depends on the normal coordinate to the plate’s surface: it is maximum at the surface of the plate and is equal to zero in its central plane. The authors have formulated the problem describing the process of symmetric drying of an unbounded plate on the basis of the convective-diffusion equation with a uniform initial distribution of the concentration and a boundary condition of mass transfer of the third kind. It has been noted that the formulated problem can be solved by numerical methods. For analysis of the degree of influence of the convective component on internal mass transfer, an analytical solution to the problem at constant parameters of the process has been obtained. According to the obtained solution, calculations of the change in the volume mean relative concentration as a function of the Fom number have been performed for the internal problem (Fom = 100) and the mixed diffusion problem (Fom = 10). It has been shown that the maximum effect of the influence of the convective component is observed for the intradiffusion problem.

Metamorphic Differentiation via Enhanced Dissolution along High Permeability Zones

AbstractMetamorphic differentiation, resulting in segregated mineral bands, is commonly recorded in metamorphic rocks. Despite the ubiquitous nature of compositionally layered metamorphic rocks, the processes that are responsible for metamorphic differentiation receive very little attention. Here, detailed petrography, quantitative mineral chemistry and bulk rock analyses are applied to investigate compositional variations and assemblage microstructure. Furthermore, thermodynamic modelling is applied to provide additional constraints on the P–T–XH2O conditions of assemblage formation and mass transfer. The studied outcrop, located within the Bergen arcs of southwestern Norway, preserves the hydration of anorthositic granulite at amphibolite-facies conditions. The amphibolite-facies hydration is expressed as both a statically hydrated amphibolite and a shear zone lithology, defined by the interlayering of amphibolite with leucocratic domains. Within the granulite, quartz-lined fractures surrounded by amphibolite-facies alteration haloes represent relics of initial fluid infiltration associated with brittle failure. The fracture assemblage (quartz + plagioclase + zoisite + kyanite ± muscovite ± biotite) is identical to that occurring within leucocratic domains of the shear zone. Consequently, the compositional layering of the shear zone lithology is linked to fluid infiltration along localized zones of high permeability that result from fracturing. Mass-balance calculations indicate that quartz-lined fractures and compositional differentiation of the shear zone resulted from mass redistribution internal to the shear zone rather than partial melting or precipitation of minerals from externally derived fluid. The process of internal fractionation within the shear zone is driven by enhanced dissolution along highly permeable fracture planes resulting in the loss of MgO, Fetot and K2O from the leucocratic domains. Elements dissolved in the fluid are then transported and ultimately either precipitated in comparatively impermeable amphibolite domains or removed from the system resulting in an overall mass loss. The mass transfer causing metamorphic differentiation of the shear zone is the result of coupled reaction and diffusion under differential stress. The mechanisms of mass redistribution observed within this shear zone provides further insight into the processes that facilitate mass transfer in the Earth’s crust.

Data-driven in silico prediction of regulation heterogeneity and ATP demands of Escherichia coli in large-scale bioreactors.

Escherichia coli exposed to industrial-scale heterogeneous mixing conditions respond to external stress by initiating short-term metabolic and long-term strategic transcriptional programs. In native habitats, long-term strategies allow survival insevere stress but are of limited use in large bioreactors, where microenvironmental conditions may change right after said programs are started. Related on/off switching of genes causes additional ATP burden that may reduce the cellular capacity for producing the desired product. Here, we present an agent-based data-driven model linked to computational fluid dynamics, finally allowing to predict additional ATP needs of Escherichiacoli K12 W3110 exposed to realistic large-scale bioreactor conditions. The complex model describes transcriptional up- and downregulation dynamics of about 600 genes starting from subminute range covering 28 h. The data-based approach was extracted from comprehensive scale-down experiments. Simulating mixing and mass transfer conditions in a 54 m3 stirred bioreactor, 120,000 E. coli cells were tracked while fluctuating between different zones of glucose availability. It was found that cellular ATP demands rise between 30%and 45% of growth decoupled maintenance needs, which may limit the production of ATP-intensive product formation accordingly. Furthermore, spatial analysis of individual cell transcriptional patterns reveal very heterogeneous gene amplifications with hot spots of 50%-80% messenger RNA upregulation in the upper region of the bioreactor. The phenomenon reflects the time-delayed regulatory response of the cells that propagatethrough the stirred tank. After 4.2 h, cells adapt to environmental changes but still have to bear an additional 6% ATP demand.

Simulation of pellet coating in Wurster coaters.

A combination of computational fluid dynamics (CFD), discrete element method (DEM), and discrete droplet method (DDM), i.e., a CFD-DEM-DDM model, was developed to simulate coating of pellets in a Wurster coater. The model equations were implemented in parallel using an approach that uses the computational resources of both CPU and GPU. Effects of the gas flow pattern, inlet gas temperature, partition gap, and spray characteristics were studied on the process. Decreasing the peripheral gas velocity, increasing the central jet velocity, and reducing the partition gap caused more uniform distributions of the circulation time and draft tube time, while the inlet gas temperature had negligible effect on them. Very high jet velocity caused a wider distribution of the circulation time. The dynamics of the spray and its interaction with pellets had significant effects on the coating mass distribution. Widening the spray angle while maintaining the droplet size constant caused the most uniform coating mass distribution and the highest deposition rate. Heat and mass transfer conditions as well as the deposition pattern changed the distributions of the solvent content and temperature of the pellets.

Corrosion of rebar in concrete. Part I: Calculation of the corrosion potential in the passive state

Knowledge of the corrosion potential (ECP) in the passive condition is important in assessing corrosion in reinforced concrete structures. While ECP is routinely measured, great scatter is reported, because the ECP is a sensitive function of many variables (T, [O2], pH, [Cl−], and the poorly defined mass transfer conditions). In this paper, a Mixed Potential Model (MPM) is used to calculate the ECP. The ECP shifts sharply in the negative direction when the passive current exceeds the limiting current density for oxygen transport to the steel surface. This shift is frequently misinterpreted as indicating passivity breakdown of the steel only.

Effective Utilization of Limonitic Nickel Laterite via Pressurized Densification Process and Its Relevant Mechanism

Limonitic laterite contains low iron and nickel grades and much high smelting minerals and loss on ignition (LOI), identified as refractory iron ore for sintering. Thus, sinter pot tests of limonitic laterite via pressurized densification sintering and its intensification mechanism were conducted, and the industrial application prospect was explored. The results indicate that the sintering performance of the limonitic laterite of the new process is significantly improved with the tumble index and productivity increased by 19.2% and 18.6%, respectively, and solid fuel rate lowered by 10.3%. The external pressure field promotes the synchronization of heat front velocity and combustion front velocity for better sintering heat and mass transfer conditions, which also greatly improves the mineral compositions and microstructure of the product sinter. The microstructure is converted from large thin-wall pores into small thin-wall or large thick-wall pores with the sinter porosity decreased by 42.4%. Much close interlocking texture between hercynite and silico-ferrite of calcium and alumina (SFCA) is formed with hercynite grains aggregation and growth, and SFCA amount substantially increased. The better sintering performance will bring about a remarkable economic benefit of 282.78 million RMB/a if the industrial application is implemented. The pressurized densification sintering process is considered as one of the effective technologies for improving limonitic laterite sintering.

Influence of Surface Reflection (Albedo) in Simulating the Sun Drying of Paddy Rice

The sun drying of agricultural products is a complicated process involving heat transfer, mass transfer, and variable weather conditions. Surface reflection (albedo), a crop’s radiative property, plays an essential role in energy balance, and understanding its contribution can improve the thermal analysis. In this study, field experiments were conducted in the Philippines to explore the influence of surface albedo on the sun drying of paddy rice. First, we implemented energy and mass balance equations in a transient model with the surroundings using a graphical programming language in Matlab/Simulink®. Second, we identified the influence of albedo on the sun drying model by using a sensitivity analysis. Third, we investigated the relationship of paddy rice albedo and the solar zenith angle. Lastly, we integrated the albedo function into the sun drying model. The simulation outputs were validated with field experiments. A better estimation of the measured exit temperature and instantaneous mass were obtained when a variable albedo was applied. This study makes clear that introducing a variable albedo has a positive impact on model improvement. This information is important for application in solar drying technologies, so that the drying process can be better assessed.

Chemically radiative flow of viscoelatic fluid over stretching cylinder with convective conditions

This articles explores the mixed convection flow of viscoelastic fluid due to astretching cylinder. Heat transfer analysis has been carried out. Energy equation in presence of thermal radiations is taken in to accunt. Convective boundary conditions for heat and mass transfer are employed on the surface of the stretching cylinder. Appropriate transformations are used to reduced the governing nonlinear partial di⁄erential equations into ordinary di⁄erential equations. The transformed equations. The resulting differential equations along with the subjected boundary conditions are solved analytically by using homotopy analysis method (HAM) for obtaining the convergent series solutions. Impacts of physical parameters on the velocityand temperature elds are analyzed. Numerical values of local Nusselt number are computed andanalyzed.

Efficient wettability-controlled electroreduction of CO2 to CO at Au/C interfaces

The electrochemical CO2 reduction reaction (CO2RR) represents a very promising future strategy for synthesizing carbon-containing chemicals in a more sustainable way. In spite of great progress in electrocatalyst design over the last decade, the critical role of wettability-controlled interfacial structures for CO2RR remains largely unexplored. Here, we systematically modify the structure of gas-liquid-solid interfaces over a typical Au/C gas diffusion electrode through wettability modification to reveal its contribution to interfacial CO2 transportation and electroreduction. Based on confocal laser scanning microscopy measurements, the Cassie-Wenzel coexistence state is demonstrated to be the ideal three phase structure for continuous CO2 supply from gas phase to Au active sites at high current densities. The pivotal role of interfacial structure for the stabilization of the interfacial CO2 concentration during CO2RR is quantitatively analysed through a newly-developed in-situ fluorescence electrochemical spectroscopic method, pinpointing the necessary CO2 mass transfer conditions for CO2RR operation at high current densities.

Дослідження оптимального розміщення витяжного вентиляційного обладнання в пташнику за допомогою чисельного моделювання

Keeping poultry in damp and cold rooms with poor ventilation system reduces the weight gain of the bird, reduces its egg production and increases the incidence of young animals, as well as excessive feed consumption and exceeding the growing period established by technical regulations. The aim of the study was to determine the effective placement of exhaust ventilation equipment at the height of the end wall of the poultry house to improve the ventilation system, reduce stagnant air zones and improve the microclimate. The numerical modeling of hydrodynamics, heat and mass transfer processes during air ventilation in poultry buildings is carried out. The analysis of the conditions of heat and mass transfer in the poultry house, depending on the placement of fans along the height of the house, and the efficiency of the location of such equipment was determined. The system for maintaining the microclimate in poultry houses was considered in the presence of a system for cooling the outside air with water from an underground well. The ventilation system uses exhaust ventilation equipment with a fan wheel diameter of 1.25 m. In the simulation, the fans were installed at a height of 1.125, 1.5 and 1.875 m from the floor to the center of the fan axis. Simulation was performed for 2D CFD models using ANSYS Fluent software. The results of CFD analysis of the air flow pattern and the thermal state inside the house are presented. As a result of numerous studies, the geometry of the location of the ventilation equipment has been found. It is shown that it is advisable to install ventilation equipment at a height of 1.5 m. At the same time, the size of stagnant zones and the uneven distribution of air velocity near the bird are reduced. Numerical modeling was carried out in order to minimize the size of stagnant zones, equalize the air flow and improve the temperature indicators in the poultry house.

Determination of dynamic interfacial tension in a pulsed column under mass transfer condition

AbstractInterfacial tension is an essential physical property in two‐phase flow, and it changes due to the mass transfer. The measurement of dynamic interfacial tension (DIFT) in such a condition is a difficult problem. In the previous study (Zhou et al., Chem Eng Sci. 2019; 197:172–183), we presented the quantitative relation between the droplet breakup frequency function (DBFF) and interfacial tension. It is found that the DBFF is highly depended on interfacial tension. Therefore, the DBFF is a suitable parameter to quantitatively characterize the interfacial tension. Based on this concept, the DIFT in the column is determined by regression method after the DBFF under mass transfer condition is measured. It is found that the DIFT is smaller than the static interfacial tension. This result indicates that interphase mass transfer leads to decreasing of the interfacial tension. The decreasing extent of the DIFT has a positive correlation with the mass transfer flux.

A Hypersonic Flow Effect on the Melting Rate of a Heat-Protective Surface under Degradation Conditions

The paper presents the results obtained for the distribution of heat flux incident upon a refractory plate of a hypersonic flight vehicle moving at space velocities at various distances from the Earth’s surface. The paper also provides the results of studying phase transitions in the near-wall boundary layer occurring during the hypersonic flow over the ablating surface. The effect of the catalytic wall on heat flux is analyzed. The emphasis is on the analysis of mass loss from the surface of high-velocity flight vehicles based on detailed consideration of the mechanism of heterogeneous catalytic reactions taking place under the surface mass transfer conditions. A temperature distribution is provided across the thickness of the boundary layer at the stagnation point of a blunt-nosed body with a refractory coating for a specific section of the flight path. The mass loss from the surface of crystalline refractory bodies is determined.

Evaporative cooling of the liquid film in slot channels with capillary-porous walls under natural convection

Implementation of resource-saving technologies can be based on improving the qualitative characteristics of heat and mass transfer equipment, which is widely used in power engineering, chemical, petrochemical, gas and food industries.One of the most effective ways of interaction between the coolant and the working medium is the direct contact of a gravitationally flowing liquid film and a gas. A promising method for passive intensification of heat and mass transfer between the liquid film and gas is the use of contact surfaces with regular roughness, with capillary-porous or mesh coating of channel walls.The launch and emergency stop of the heat and mass transfer device may generate a transition in the cooling mode of liquid from forced to natural convection. This leads to a change in the conditions of heat and mass transfer, and, consequently, to a change in the parameters of the coolant at the output of the device. In this case, using the dependences obtained for forced convection in the channel leads to a significant error in determining the parameters of the heat carrier. Therefore, in order to design and manufacture heat-mass-transfer units of contact type, one would need the information on the patterns of heat and mass transfer under different operating conditions.An important condition for ensuring the efficiency of contact-type devices is to know the features of the interaction between the liquid and gas mediums. The development of boundary layers in the film and gas leads to the need to allocate the initial thermal region and the stabilized heat and mass transfer region. These factors are not taken into account in the engineering calculation methods that exist today.

Computational fluid dynamics modelling of stirred tank photobioreactor for Haematococcus pluvialis production: Hydrodynamics and mixing conditions

Agitated photobioreactors (PBR) have been widespread for the cultivation of microalgae because of their advantages in mixing and mass transfer conditions. However, it is difficult to investigate the behaviors of cultivation broth flow with experimental procedures in these PBRs. Computational Fluid Dynamics (CFD) is a common method that has been used to simulate the performance of fluid containing systems in order to investigate the fluid flow, reduce the design cost and improve the efficiency. The objective of this study was to evaluate hydrodynamics and mixing conditions of stirred tank PBR for Haematococcus pluvialis production experimentally and numerically. The cells were first cultivated in a stirred tank PBR containing Rushton turbine impeller and the cell growth was examined during 10 days. At the end of cultivation, the cell concentration was reached the value of 3.1 ± 0.1 × 105 cells mL−1 with the total carotenoid content of 2.47 ± 0.01 mg L−1. Then, the hydrodynamic analyses were conducted to evaluate the average velocity magnitude, turbulence properties and dead zone inside the PBR. The simulation results obtained from realizable k-ε turbulence model showed that the uniformity index of average velocity was found as 0.86. This result showed that the fluid flow showed similar behavior in the most parts of PBR except around the impellers. The velocity contours supported that the mixing conditions were supplied efficiently, however the vortex formation was observed around the impellers in spite of the baffles. In addition, the turbulence kinetic energy having a vital influence on mixing characteristics was found between the values of 1.0 × 10−2 and 3.0 × 10−2 m2 s−2 and the lowest values were observed between two impellers due to extent of mixing. Obtained results could be used to re-design PBR configurations considering the impeller type, the distance between impellers and cultivation conditions.

Mathematical Modeling of UF6 Desublimation in a Tank with Horizontal Ribbing

Unsteady-state UF6 desublimation into a vertical immersed tank 6.0 × 10–2 m3 in volume with horizontal internal ribs has been numerically modeled using an in-house developed software-implemented two-dimensional mathematical model. The dependences of the characteristics of UF6 desublimation on the pressure and temperature of gaseous UF6 in the collector; the coolant temperature; and the number, arrangement, and geometry of the ribs in the tank are described. It is shown that increasing the number of ribs, changing the heat- and mass-transfer conditions, and increasing the diameter of the central openings in the horizontal ribs significantly increase the average productivity of the tank and decrease its filling time.

Unsteady Mass Transfer of Nutrients in Gels with Channels of Different Spatial Structures

The dynamics of the development of immobilized cells in a gel under conditions of limited mass transfer of nutrients across the interface was considered. The goal of this study was to find methods for effective cultivation of living microorganisms in the gel due to the correct choice of the structure of internal channels, when nutrients are delivered to the gel by the convective-diffusion mechanism. The variation of the relative intensity of transmitted light along the height of the agarose gel sample with living yeast cells during nutrient solution diffusion from the surface with time was determined by spectroscopy. The depth of the gel layer at which the vital activity of microorganisms is provided was evaluated. The possibility of forming time-stable linear and branching channels in the gel was established experimentally. The possibility of their stable use for convective supply of nutrient in the volume was verified. The tendencies of diffusion of nutrients from the channels in the gel with immobilized cells were determined. The 3D structure of channels that provides an increase in the efficiency of cell nutrition in the gel volume was discussed.

Theoretical and Experimental Studies of Processes of Oily Wastewater Treatment with a Synergetic Mixture

Theoretical studies of processes of oily wastewater treatment for removing various impurities determined the main mechanism of the formation of coagulation structures and aggregates with their subsequent settling and phase separation, which disturb the aggregation and kinetic stability of the medium. It was found that a more advanced coagulation leads to the destruction of interlayers of the medium between particles, which brings the particles into direct contact with each other. This results in the formation of rigid aggregates with fractal structure, which consist both of hard particles, and of asphaltenes and resins, sometimes shaped as flakes. If the particles coagulate in a turbulent flow, their number decreases, although their sizes increase up to the sizes of aggregates. It was noted that the formation of coagulation structures is beneficial for separation of the solid phase from the liquid phase in wastewater treatment. An investigation was made of the potential of a synergetic mixture of slaked lime and ferric chloride for accelerating the formation of coagulation structures in oily wastewater treatment. Experimental studies showed the efficiency of using such a mixture for removing petroleum products and solid particles in wastewater treatment. In intense stirring of this mixture in a mixer at 1500–2000 rpm, isotropic turbulence is reached, which favors the coagulation and aggregation of particles. It was found that the best composition of the mixture is 1.0 g of slaked lime and 0.06 cm3 of ferric chloride, which best promote the high-quality coagulation and settling of particles. Based on the boundary conditions for mass transfer, kinetic models for calculating the consumption of the mixture were proposed, and a kinetic parameter was estimated. The results of the performed experiments showed that, with increasing concentration of the mixture, the impurity content of wastewater progressively decreases, and wastewater becomes almost transparent. Using the synergetic mixture in wastewater treatment creates conditions for the formation of stronger structures of aggregates prone to natural settling.

On the influence of hypersonic flow at the speed of a reflow heat-proof surface in terms of destruction

The results on the distribution of heat flows coming to the refractory plate of a hypersonic aircraft moving at different distances from the Earth's surface with cosmic velocities are obtained. The results of studies related to the study of phase transitions in the wall boundary layer occurring during the flow of hypersonic flow ablating surface are presented. The influence of the catalytic wall on the heat flow is considered. The main attention is paid to the analysis of the surface entrainment of high-speed aircraft, based on a detailed account of the mechanism of heterogeneous catalytic reactions in the conditions of surface mass transfer. The temperature distribution over the thickness of the boundary layer at the critical point of a blunted body with a refractory coating for a particular section of the flight path is given. Mass entrainment from the surface of crystalline refractory bodies is determined. Ill. 7. Ref. 16.

Characterization of mass transfer in a shallow fluidized bed for adsorption processes: Modeling and supporting experiments

This study characterizes mass transfer processes in a multi-stage fluidized bed (MSFB) using shallow fluidized beds. Multi-stage fluidized beds are of interest for sour gas removal using adsorption, examples are H2S/CO2 removal from natural gas and CO2 capture by amine sorbents. A phenomenological model based on the classical two-phase fluidization model is used to study the mass transfer process. Independent measurements and literature data are used to describe mixing, hold-ups, intraparticle mass transfer and adsorption kinetics. The bubble hold-up was experimentally measured. Mixing was investigated with a temperature response measurement. The mixing in the emulsion gas phase varies between plug flow and ideal mixing with increasing superficial gas velocity, while the solid phase is always ideally mixed. The presented fluidized bed model is able to describe the effects of superficial gas velocity, solid flux, bed height and inlet concentration. Mass transfer was further studied using a sensitivity analysis, which showed that the used shallow fluidized bed is already fairly optimized with respect to mass transfer. Shallow fluidized beds benefit from fast gas interchange because bubbles do not have the chance to grow. Optimal mass transfer and gas interchange conditions can be achieved in shallow fluidized beds for application in MSFBs.