Showing papers on "Mass transfer published in 2016"

Aqueous reactive species induced by a surface air discharge: Heterogeneous mass transfer and liquid chemistry pathways.

Abstract: Plasma-liquid interaction is a critical area of plasma science and a knowledge bottleneck for many promising applications. In this paper, the interaction between a surface air discharge and its downstream sample of deionized water is studied with a system-level computational model, which has previously reached good agreement with experimental results. Our computational results reveal that the plasma-induced aqueous species are mainly H+, nitrate, nitrite, H2O2 and O3. In addition, various short-lived aqueous species are also induced, regardless whether they are generated in the gas phase first. The production/loss pathways for aqueous species are quantified for an air gap width ranging from 0.1 to 2 cm, of which heterogeneous mass transfer and liquid chemistry are found to play a dominant role. The short-lived reactive oxygen species (ROS) and reactive nitrogen species (RNS) are strongly coupled in liquid-phase reactions: NO3 is an important precursor for short-lived ROS, and in turn OH, O2− and HO2 play a crucial role for the production of short-lived RNS. Also, heterogeneous mass transfer depends strongly on the air gap width, resulting in two distinct scenarios separated by a critical air gap of 0.5 cm. The liquid chemistry is significantly different in these two scenarios.

Boundary layer flow of Maxwell fluid in rotating frame with binary chemical reaction and activation energy

Abstract: Here we study the heat/mass transfer effects on revolving flow of Maxwell fluid due to unidirectional stretching surface. Mass transfer process is modeled in terms of binary chemical reaction and activation energy. Modified Arrhenius function for activation energy is invoked. Traditional boundary layer approximations are utilized to simplify the governing equations. Using similarity method, self-similar form of boundary layer equations are derived which are solved numerically. The solutions depend on dimensionless numbers such as the rotation parameter λ , the Deborah number β , the Prandtl number Pr , the Schmidt number Sc , activation energy E , fitted rate constant n and temperature difference parameter δ . We found that the solute concentration in binary mixture is proportional to both rotation parameter λ and activation energy E . The reaction rate σ and fitted rate n both provide reduction in the solute concentration. Thermal boundary layer becomes thicker and heat transfer rate diminishes when fluid is subjected to a larger rotation rate.

Influence of hatch spacing on heat and mass transfer, thermodynamics and laser processability during additive manufacturing of Inconel 718 alloy

Abstract: A transient three-dimensional powder-scale model has been established for investigating the thermodynamics, heat and mass transfer and surface quality within the molten pool during selective laser melting (SLM) Inconel 718 alloy by finite volume method (FVM), considering the powder-solid transition, variation of thermo-physical properties, and surface tension. The influences of hatch spacing (H) on the thermodynamics, heat and mass transfer, and resultant surface quality of molten pool have been discussed in detail. The results revealed that the H had a significant influence on determining the terminally solidified surface quality of the SLM-processed components. As a relatively lower H of 40 μm was used, a considerable amount of molten liquid migrated towards the previous as-fabricated tracks with a higher velocity, resulting in a stacking of molten liquid and the attendant formation of a poor surface quality with a large average surface roughness of 12.72 μm. As an appropriate H of 60 μm was settled, a reasonable temperature gradient and the resultant surface tension tended to spread the molten liquid with a steady velocity, favoring the formation of a flat surface of the component and an attendant low average surface roughness of 2.23 μm. Both the surface morphologies and average surface roughness were experimentally obtained, which were in a full accordance with the results calculated by simulation.

Review on hydrodynamics and mass transfer in minichannel wall reactors with gas-liquid Taylor flow

Abstract: Over the past decades, Taylor flow has got an increasing interest due to its potential to intensify reaction processes with fast kinetics which are usually controlled by mass transfer processes. Besides high mass transfer rates, the segmented flow regime offers a sharp residence time distribution of the liquid phase and a low pressure drop. Taylor flow appears in micro and minichannels whereas the terms have not always been used with a clear distinction. Minichannels are channels with characteristic diameters between 400 μm and about 1 mm, which will be in the centre of this work. These channel dimensions appear in monolithic reactors also known as reactors with honeycomb catalyst packings, in concepts using bundles of capillary tubes, as well as in microchannel plate reactors. This article presents a comprehensive overview of hydrodynamics and mass transfer in the minichannels of an open flow structure, i.e. in channels without internals, operated in the Taylor flow regime. The review summarises available correlations to predict Taylor flow characteristics, as well as mass transfer coefficients between all involved phases (gas–liquid, liquid–solid, gas–solid). Within this scope, the impact of operational and design parameters is critically discussed and limits of application for the individual correlations are defined. Special attention is given to the interaction of these mass transfer steps in heterogeneously catalysed chemical reactions.

Numerical study of MHD heat and mass transfer of a Jeffrey fluid over a stretching sheet with chemical reaction and thermal radiation

Abstract: A numerical model is developed to study the effects of chemical reaction and heat source on MHD heat and mass transfer of an electrically conducting Jeffrey fluid over a stretching sheet in the presence of power law form of temperature and concentration. Similarity transformations are used to convert the governing partial differential equations to a set of coupled non-linear ordinary differential equations. The resulting equations are then solved numerically by shooting method with Runge–Kutta fourth order scheme. The influence of various dimensionless parameters on the velocity, temperature and concentration distributions are analyzed and discussed through graphs and tables. It is observed that the Deborah number ( β ) and ratio of relaxation and retardation times parameter ( λ ) have opposite effects on the skin friction coefficient. However, the effects of β and Pr on the Nusselt number profiles are similar. Subsequently the present results are in very good agreement with the results obtained for a viscous fluid.

Unsteady three-dimensional flow of Casson–Carreau fluids past a stretching surface

Abstract: In this study, we investigated the effects of nonlinear thermal radiation and non-uniform heat source/sink in unsteady three-dimensional flow of Carreau and Casson fluids past a stretching surface in the presence of homogeneous–heterogeneous reactions. The transformed governing equations are solved numerically using Runge–Kutta based shooting technique. We obtained good accuracy of the present results by comparing with the already published literature. The influence of dimensionless governing parameters on velocity, temperature and concentration profiles along with the friction factors, local Nusselt and Sherwood numbers is discussed and presented graphically. We presented dual solutions for flow, heat and mass transfer in Carreau and Casson fluids. It is found that the heat and mass transfer rate in Casson fluid is significantly high while compared with the Carreau fluid.

Mathematical model for intermittent microwave convective drying of food materials

Abstract: Intermittent microwave convective drying (IMCD) is an advanced technology that improves both energy efficiency and food quality in drying. Modelling of IMCD is essential to understand the physics of this advanced drying process and to optimize the microwave power level and intermittency during drying. However, there is still a lack of modelling studies dedicated to IMCD. In this study, a mathematical model for IMCD was developed and validated with experimental data. The model showed that the interior temperature of the material was higher than the surface in IMCD, and that the temperatures fluctuated and redistributed due to the intermittency of the microwave power. This redistribution of temperature could significantly contribute to the improvement of product quality during IMCD. Limitations when using Lambert's Law for microwave heat generation were identified and discussed.

Drying characteristics and modeling of yam slices under different relative humidity conditions

Abstract: The drying characteristics of yam slices under different constant relative humidity (RH) and step-down RH levels were studied. A mass transfer model was developed based on Bi-Di correlations containing a drying coefficient and a lag factor to describe the drying process. It was validated using experimental data. Results showed that the drying air with constant RH levels of 20, 30, and 40%, temperature of 60°C, and air velocity of 1.5 m/s had an insignificant effect on drying time. This phenomenon was likely attributed to the fact that higher RH led to a rapid increase in sample’s temperature. The higher sample temperature could provide an additional driving force to water diffusion and thereby promote the moisture movement, which could minimize the negative effect of lower the drying rate in the initial drying stage. Applying air with 40% RH for 15 min in the initial stage achieved the desired color and reduced the drying time by 25% compared to the drying time under continuous dehumidification fr...

Heat and mass transfer effects on the mixed convective flow of chemically reacting nanofluid past a moving/stationary vertical plate

Abstract: The problem of conjugate effects of heat and mass transfer over a moving/stationary vertical plate has been studied under the influence of applied magnetic field, thermal radiation, internal heat generation/absorption and first order chemical reaction. The fluid is assumed to be electrically conducting water based Cu-nanofluid. The Tiwari and Das model is used to model the nanofluid, whereas Rosseland approximation is used for thermal radiation effect. Unified closed form solutions are obtained for the governing equations using Laplace transform method. The velocity, temperature and concentration profiles are expressed graphically for different flow pertinent parameters. The physical quantities of engineering interest such as skin friction, Nusselt number and Sherwood number are also computed. The obtained analytical solutions satisfy all imposed initial and boundary conditions and they can be reduced to known previous results in some limiting cases. It is found that, by varying nanoparticle volume fraction, the flow and heat transfer characteristics could be controlled.