Mass Transfer Characteristics Research Articles

Stability Analysis of Mass Transfer Effects on the Unsteady Stagnation-Point Flow Past a Permeable Sheet

This study involves the problem of mass transfer effects on unsteady stagnation-point flow through a permeable stretching/shrinking sheet. Similarity transformations in the form of nonlinear partial differential equations to ordinary differential equations were used to reduce the basic governing equations. The resulting equations were then solved numerically using the bvp4c function and shooting method. The programming codes for bvp4c function were built using Matlab software, whereas the programming codes for shooting method were built using Maple software. The effects of the Schmidt number, stretching/shrinking, suction/injection, unsteadiness and the reaction rate parameter on the flow and mass transfer characteristics were numerically analysed. The boundary layer thickness and velocity gradient are also influenced by these parameters. Dual solutions were figured out for the problems involving stretching/shrinking sheet at the stagnation-point. To determine the stability of the solutions obtained, stability analysis was performed by solving the linear eigenvalue problems using the bvp4c function. The analysis showed that for the first solution, there was an initial decay of disturbance, while the second and third solutions were opposite, where an initial growth of disturbance were detected for both. This result indicated that the first solution was more stable and physically realizable compared to others.

Computational fluid dynamic analysis of an adsorption-based cogeneration osmotic heat engines with stepwise porosity distribution

Adsorption-based osmotic heat engines offer an alternative way for converting ultra-low temperature waste heat into electricity. Here, considering the heat and mass transfer characteristics in the adsorbent bed, a computational fluid dynamic model is developed to describe the adsorption-based osmotic heat engine for power and refrigeration cogeneration. Impacts of the porosity distribution, adsorption time, switching time, fin number and working solution-adsorbent pairs on system performance are comprehensively analyzed under different porosity distribution configurations. Results reveal that Configuration I leads to higher coefficient of performance (COP), exergy efficiency. However, Configuration II renders higher electrical efficiency. Compared with the uniform porosity configuration, COP and exergy efficiency are respectively elevated by 1.96% and 1.19% under the stepwise porosity configuration of 0.3–0.5. The electrical efficiency is increased by 15.5% with porosity configuration of 0.7–0.2. LiCl and AQSOA-Z02 under the stepwise porosity configuration of 0.7–0.2 can lead to the highest exergy efficiency of 7.76%.

STUDY OF MOISTURE RECOVERY PROCESSES OF KNITTED FABRICS FOR THE BASE LAYER OF WINTER CLOTHING SETS

The article considers the problem of predicting the moisture exchange of textile materials in a wet state using the example of knitted fabrics for underwear. Based on the thermodynamic analogy of heat and moisture transfer processes, an experimental scale of the moisture transfer potential of a reference body was constructed. As a result of the study of the mass transfer system “sample – reference body”, the mass transfer characteristics of several knitted fabrics were obtained. Thermodynamic relations are obtained for calculating the intensity of moisture transfer of textile materials in a bag of clothes. The results of calculations are given on the example of the selected samples. The results of the calculations made it possible to establish the relation- ship between the moisture flux density from the sample and the value of the moisture transfer potential. Signifi- cant differences between these flows have been established. The data obtained make it possible to make a fore- cast about the operational efficiency of clothing with various options for the material of the linen layer.

Simulation study on mass transfer characteristics and disk structure optimization of a rotating disk reactor with high viscosity region

AbstractDue to the excellent film‐forming specific surface area of the disk reactor, its application in the polymer devolatilization process can greatly improve the devolatilization performance of the equipment. In view of this, the fluid volume function method in computational fluid dynamics is used to simulate the film forming performance and surface renewal performance in high viscosity zone of disk reactor. In the disk window area, the liquid film thickness becomes thinner and the surface update frequency becomes faster, indicating that the disk window has a positive effect. The devolatilization performance of five new disk structures under different operating conditions is compared and analyzed. It was found that the influence of liquid level height on film area was greater than that of viscosity and rotation speed, the porous sector disk has excellent surface renewal performance and the baffle can achieve better mass transfer efficiency at a distance of 45–60 mm from the disk.

Heat and mass transfer correlations of supercritical n-decane considering the influence of pyrolysis reaction by simplifying internal heat and mass sources as pseudo-wall fluxes

The convective heat and mass transfer characteristics of the supercritical pressure (SCP) fuel in the regenerative cooling channel of hypersonic vehicles are significantly affected by pyrolysis reactions. In this study, we proposed a novel method to develop the Nusselt number (Nu) and Sherwood number (Sh) correlation of SCP fuel that considers the influence of pyrolysis reaction by simplifying internal heat and mass sources as pseudo-wall fluxes based on the differential global reaction (DGR) model of pyrolysis. The convective heat and mass transfer and pyrolysis characteristics of n-decane were investigated experimentally and numerically in a vertical downward-flow heated tube at pressures of 3-7 MPa. Theoretical analysis of the experimental conditions was conducted sequentially according to the partial differential equations (PDEs) of species continuity and energy, in which the internal source terms of heat and mass due to pyrolysis reactions were simplified and predicted using the DGR model of n-decane pyrolysis. A semi-empirical Sh correlation for predicting the radial distribution of fuel conversion was proposed associated with computational fluid dynamics (CFD) data by equivalently simplifying the internal mass source as pseudo-wall mass flux. A Nu correlation for convective heat transfer was proposed using the same methodology considering the influences of pyrolysis. The correction terms consider pyrolysis endothermicity and thermophysical property variation due to the conversion differences between the pipe wall and the bulk flow. Finally, the Nu correlation was compared to the experimental data, and the influence of heat transfer deterioration (HTD) due to pyrolysis coking was also analyzed quantitatively and empirically.

Experimental and numerical study of tomatoes drying kinetics using solar dryer equipped with PVT air collector

The present work aims to develop a 3D numerical simulation that describes fresh tomatoes' heat- and mass-transfer characteristics using software COMSOL Multiphysics. Besides, a numerical study of the effect of drying parameters (drying temperature, Velocity of air drying and thickness of the slice) on the moisture content elimination from the tomato slice was realized. Also, an experimental study of the effect of the continuity and the discontinuity of the drying process on the water content vaporization is carried out. In contrast, the numerical simulation developed is validated by comparing the experimental data with the numerical results of the temperature distribution and mass transfer inside the tomato slice. The results show that the numerical simulation is agrees with the experimental tests. In addition, the results show that bacteria contamination appears in the tomato slice when the drying temperature decreases under 50 °C. The discontinuity of the drying process harms the quality of the dried product. Furthermore, the results show that the water evaporation rate in the thin slice was faster compared to the thick slice. Consequently, the drying time was reduced by 39 h when the thickness of the product was decreased from 33 mm to 10 mm. Also, the numerical simulation shows that moisture elimination is faster at higher drying air temperatures and increasing the drying air velocity leads to decrease drying time. The results show that the numerical simulation developed in this paper can accurately simulate the drying process characteristics of the agro-food product, especially Tomato, and that it provides a high spatial resolution of the distribution of water content and temperature inside the slice of tomatoes during the drying process.

Global characterization of hydrodynamics and gas-liquid mass transfer in a thin-gap bubble column in presence of Newtonian or non-Newtonian liquid phase

Bubble columns are commonly used as photobioreactors (PBRs) due to their good mixing and mass transfer capabilities. Modification in design and operating parameters are nevertheless needed to intensify volumetric productivity. Thin-gap bubble column can be used in this context to operate with high biomass concentration, up to conditions where the culture becomes non-Newtonian. In the present study, the global hydrodynamics and gas–liquid mass transfer inside a 4 mm thickness thin-gap bubble column are characterized in presence of low viscosity Newtonian fluid (water) and two aqueous solutions of Carboxymethyl Cellulose and Xanthan Gum. These shear-thinning solutions are used to mimic high concentration cultures of Chlorella Vulgaris at 42 g.L−1. A range of superficial gas velocities and different capillary diameters are explored to investigate the effect of sparging on hydrodynamics and gas–liquid mass transfer. Studied parameters are flow regimes, mixing, and gas–liquid mass transfer. Results are compared with literature results in classical bubble columns to put into evidence the original effects of both confinement and liquid rheology. Among others, these results show that compared to Newtonian media, non-Newtonian media result in higher gas holdup and mixing times and a lower gas–liquid mass transfer coefficient. The findings suggest that the presence of smaller bubbles would improve mass transfer due to the increased interfacial area, while the presence of larger bubbles would improve mixing performance due to increased vorticity in their wake and relatively greater species transport.

Numerical Investigation Into the Gas–Liquid Two-Phase Flow Regime and Heat Transfer Characteristics in a Gravity Heat Pipe

Abstract In order to reveal the gas–liquid distribution, heat and mass transfer characteristics inside a gravity heat pipe, the bubble behavior and the flow regime transition during the phase-change process were examined by employing a copper-water heat pipe, with the length of 500 mm and Φ22 × 1.5 mm. The results indicate that in the process of phase change, the typical flow regimes of bubble flow, slug flow, and churn flow can be observed in the evaporator, and the presence of bubbles has an obvious disturbance on the flow field. In addition, the discontinuous liquid film plays an important role in the heat transfer mechanism in the condenser, which allows the vapor to contact the wall directly and reduces the heat transfer resistance. The temperature difference between the evaporator and the condenser can be reduced by adjusting the saturation temperature, so as to effectively improve the heat transfer performance of the heat pipe and contribute to the practical engineering design.

Numerical Study on Heat and Mass Transfer of Evaporated Binary Zeotropic Mixtures in Porous Structure

This study investigates the heat and mass transfer characteristics of a binary mixture (R134a/R245fa) evaporated in a porous medium. The Eulerian model coupled with the multiphase VOF model and species transport equations is employed to establish a multi-component evaporation model. The effects of heat flux ranging from 200 kW/m2 to 500 kW/m2, porosity ranging from 0.4 to 0.6, and mass fraction ratios (R134a/R245fa) of 3:7, 5:5, and 7:3 are explored. The results indicate that an increase in heat flux contributes to an increase in the evaporation rate. For the overall evaporation rate, the evaporation rates of R134a and R245fa improve by 11.3%, 6.9%, and 16.3%, respectively, while the maximum improvement in heat transfer coefficient is only 1.4%. The maximum evaporation rate is achieved at intermediate porosity in the porous medium, and the highest heat transfer coefficient is obtained at a porosity of 0.4. With the increase in mass fraction, the evaporation rate of the corresponding species also increases, while the overall evaporation rate and heat transfer coefficient remain almost unchanged.

A general effectiveness-NTU modeling framework for membrane dehumidification systems

Selective membrane dehumidification is a promising technology for energy efficient air dehumidification in buildings and has been ranked as a top alternative technology by the Department of Energy. One critical area that needs to be addressed is system modeling that incorporates device mass transfer characteristics and sizing. This work develops, validates, and applies a simple effectiveness-number of transfer units (ε-NTU) modeling framework for dehumidification technologies that can be used to predict performance and determine required membrane area for a wide range of technologies, including energy recovery ventilators, vacuum membrane dehumidification, and desiccant dehumidifiers. Although the ε-NTU method is a well-established framework for sizing and modeling heat exchangers, past attempts to create εL-NTUL(meaning ε-NTU for latent loads) models for dehumidification (referred to as latent cooling): (1) don’t agree on the definition of NTUL, (2) are not fully analogous to the heat transfer definition of NTU, (3) cannot be easily applied across all technologies, (4) use humidity ratio, not vapor pressure, as the driving force, and (5) are often complex or lack a fundamental basis. In this work, we develop an effectiveness-NTU model for membrane humidity exchangers that is based on vapor pressure differences, employs a novel definition of the latent number of transfer units for dehumidification applications (NTUL) and is fully analogous to the heat transfer effectiveness-NTU models employed for sensible heat exchangers. The resulting εL-NTUL relationships are validated against experimental data with a maximum error of 1.2%. The overall methodology is applied in case studies for vacuum membrane dehumidification system sizing showing that the ratio of required membrane area per building floor area ranges between 0.1 and 3.5 depending on the building type and operating conditions. Finally, the framework is further generalized for applicability to any gas separation application.

Acetophenone preparation via continuous bubbling of ozone for catalyst-free ethylbenzene oxidation

Acetophenone is essential to chemical synthesis. Hence, completely using coal tar resources via liquid-phase oxidization to obtain acetophenone is crucial. This study demonstrates a novel, efficient, and environment-friendly technique for converting ethylbenzene to acetophenone based on ozone oxidation and continuous bubbling. Analysis of the mass transfer characteristics of the continuously bubbling ozone in various polar organic solvents showed that alcohol solvents had a higher equilibrium concentration than ethyl acetate and acetone. Equilibrium ozone concentrations were identical in solvents such as chlorinated methane. The solubility of ozone in different solvents may be affected by hydrogen bonding or dipole–dipole interactions between ozone and the solvent molecules. Ethyl acetate was found to be the best solvent after the ozone oxidation conditions of ethylbenzene were refined. The highest amount of acetophenone produced was 74%, and the reaction duration was 120 min; and the corresponding molar ratio of ozone to ethylbenzene was 1.6. Finally, the oxidation of ethylbenzene using ozone was proposed. Hydrogen peroxide accelerated this process, while ethanol slowed it down. The hydroxyl, alkyl, and superoxide free radicals in the reaction system were confirmed via electron paramagnetic resonance studies. Results showed that the key chemical process is indirect ozone oxidation.

MHD Free Convection Flows for Maxwell Fluids over a Porous Plate via Novel Approach of Caputo Fractional Model

The ultimate goal of the article is the analysis of free convective flow of an MHD Maxwell fluid over a porous plate. The study focuses on understanding the dynamics of fluid flow over a moving plate in the presence of a magnetic field, where the magnetic lines of force can either be stationary or in motion along the plate. Further, we will investigate the heat and mass transfer characteristics of the system under specific conditions: constant species and thermal conductivity as functions of time. The study involves a symmetric temperature distribution that provides heat on both sides of the plane. Our analysis includes the study of the model for different instances of plate motion and variations in temperature. The fluid dynamics of the system are mathematically described using a system of fractional-order partial differential equations. To make the model independent of geometric units, dimensionless variables are introduced. Moreover, we employ the concept of fractional-order derivative operators in the sense of Caputo, which introduces a fractional dimension to the equations. Additionally, the integral Laplace transform and numerical algorithms are utilized to solve the problem. Finally, by using graphical analysis the contribution of physical parameters on the fluid dynamics is discussed and valuable findings are documented.

Refinement and predicting formaldehyde concentrations of indoor fabric: Effects of temperature and humidity

Indoor air pollution resulting from volatile organic compounds (VOCs) is a significant health concern, especially formaldehyde. Therefore, predicting indoor formaldehyde concentration is essential for environmental control. In this research, the authors develop a thermal and wet coupling calculation model of porous fabric that considers the influence of different phases of wet components and the coupling effect of heat and humidity on formaldehyde migration. We propose a modified calculation method of the formaldehyde mass transfer characteristic parameters of fabric to obtain the diffusion coefficient D and partition coefficient K. The heat and humidity coupling model and formaldehyde mass transfer model of fabric are simultaneously solved, and the authors analyze the influence mechanism of fabric loading rate, fabric type, temperature, and humidity on indoor formaldehyde mass transfer characteristics. We study the variation trend of fabric formaldehyde mass transfer characteristics coefficient and the temporal and spatial distribution of indoor formaldehyde concentration. The theoretical model is applied to practical problems by pre-evaluating the indoor formaldehyde concentration of decorated residential buildings in typical climate areas of China before occupancy. The authors obtain the variation rule of indoor formaldehyde concentration of residential buildings under typical hot and humid climate conditions, building materials, furniture, and fabrics. To provide theoretical support for indoor environmental control and human health protection.

Numerical study on hydrogen heterogeneous reaction characteristic in a micro catalytic combustor with blunt body

A numerical investigation has been conducted to explore the effect of an inserted blunt body on the heterogeneous reaction (HTR) characteristics in a micro-catalytic combustor operating with premixed hydrogen/air. The two-dimensional numerical model incorporates a comprehensive heterogeneous reaction mechanism of hydrogen employed by experimental verification. The influences of blockage ratio at various inlet velocities have been investigated to obtain optimized design parameters of the combustor. The findings of this research highlight analysis of the positive influence of an inserted blunt body within the combustor, including enhancement of chemical reaction rate and hydrogen conversion ratio. Numerical results shows that the gaseous mixture between the blunt body and catalytic surface noticed a considerable velocity increase, leading to a greater adsorption–desorption of bulk species on the catalytic surface. At the same inlet velocity, the hydrogen conversion ratio gradually increases with higher blockage ratios. When the blockage ratio is less than or equal to 0.6, the average inner wall temperature is positively affected by the increased blockage ratio. However, the effect becomes negative at higher blockage ratios (0.8). The Pr (Prandtl number) number near the gas–solid interface adjacent to the blunt body initially decreases and then increases, while Nu/Nu0 (Nusselt number) and Pe (Peclet number) exhibit an initial increase followed by a decrease. The presence of the blunt body leads to a reduction in the cross-sectional area of the channel, causing an acceleration in fluid velocity near the blunt body and exerting an impact on the boundary layer near the gas–solid interface. This indicates that the addition of the blunt body reduces the boundary layer thickness and enhances the mass and heat transfer characteristics in the region. The study reveal that as the blockage ratio increases, the overall field synergy of the combustor initially increases and then decreases. The peak field synergy is attained at an optimal blocking ratio of 0.6. The findings of this study have important implications for the design of micro-scale combustors.

Influence of Dofour and Soret on Eyrig-Powell Nanofluid Flow from A Circular Cylinder with Viscous Dissipation

The impact of Viscous dissipation on the heat and mass transfer characteristics of an Eyring Powell nanofluid flow past a horizontal circular cylinder is intensively investigated in the presence of Dufour and Soret effects. The free laminar flow is subject to a uniform transverse magnetic field. The continuity, momentum, energy, and concentrations equations are transformed into a nonlinear system of partial differential equations using appropriate non-similarity variables. The transformed system was solved numerically using the fourth order Runge Kutta method. The effect of parameters including Prandtl number, Dufour effect, Soret effect and Schmidt number were studied and presented graphically. Nusselt and Sherwood numbers have also been derived and discussed numerically.

The role of ion‐membrane interactions in fast and selective mono/multivalent ion separation with hierarchical nanochannels

AbstractPrecise control over the nanofluid behavior of polyelectrolyte‐based membranes is a primary step toward understanding the structure‐morphology‐property relationships to ultimately determine the mass transfer characteristics. In this study, a high‐performance multistacked polyelectrolyte‐based cation exchange membrane (CEM) with a heterogeneous structure and versatile surface chemistry was developed to achieve selective ion conductance. The self‐assembled CEM can facilitate ion permeation with fluxes of 2.9 mol m−2 h−1 for K+ and 0.22 mol m−2 h−1 for Mg2+, reaching a mono/multivalent ionic selectivity of up to 13, outperforming mono/divalent fractionation when compared with state‐of‐the‐art membranes. Molecular dynamic (MD) simulations illustrated the ionic transport trajectory in hierarchical channels with angstrom‐scale cavities using multilayered CEMs. Both the experimental measurements and theoretical simulations indicated that ionic fractionation was associated with a large disparity in the energy barrier between mono/multivalent cations, which was the primary origin of the differences in the ion dehydration‐rehydration processes in the angstrom‐confinement membrane ion channels.

Performance evaluation and improvement of prototype rice husk fueled mixed flow rough rice dryer using CFD model

The majority of the rural areas in developing countries are not connected to the electric grid. Rice production produces a significant amount of rice husk as a byproduct/waste but this waste can be used as a source of energy for drying of the rough rice. The aim of this research was to evaluate the performance and improve the design and operation of a rice husk fueled mixed flow grain dryer for small scale rural farmers using a validated 3-D computational fluid dynamics (CFD) model. The model was applied to predict the airflow and heat and mass transfer characteristics of the designed prototype rough rice dryer. The model took into account the geometrical detail of the dryer assembly, the relevant boundary and initial conditions, and model input parameters. The results identified relatively stagnant regions at the top and the side walls of the dryer. The observed relatively high variation in drying air velocity distribution caused a relatively high variation in drying characteristics of the rice grain. The model was applied to modify the design and operation procedure of the dryer to improve the performance in terms of drying uniformity and drying time. After 2 h drying time, the difference between the maximum and minimum moisture content values for the original and modified design was 12.9% and 8.5%, respectively. There was an exponential relationship between drying time and drying air temperature and specific flowrate. Mixing and recirculation of the grain at 30 min interval significantly improved the uniformity of drying, and the difference between maximum and minimum moisture content after 2 h of drying was clearly reduced to 1.9%. The study indicated the capability of 3-D CFD model to improve the design and operation of mixed flow rice husk fueled dryer to attain the required performance.

Local and global gas–liquid mass transfer in a micro‐packed bed reactor utilizing a noninvasive colorimetric technique

AbstractMicro‐packed bed reactor (μPBR) presents great potential in the field of multiphase reactions due to the features of safety and high efficiency. However, the deeper cognition of mass transfer needs to be taken into consideration that is the foundation of reactor design. In this work, local and global gas–liquid mass transfer in the μPBR were studied utilizing a noninvasive colorimetric technique. In reactor level, the qualitative and quantitative comparisons were conducted; in particle level, liquid flow and mass transfer textures were assessed for the first time. The diversities of local mass transfer characteristics from temporal and spatial dimensions were obtained, and the heterogeneity of local and global mass transfer was revealed. The predicted correlations of in μPBR with churn flow and pseudo‐static flow were established with deviations generally within ±18%. This study contributes to improve the understanding of mass transfer and points out the process intensification direction of μPBR.

Mass transfer characteristics at cathode/electrolyte interface during electrodeposition of nickel microcolumns with various aspect ratios

The filling behavior of electrodeposited microcolumns is strongly influenced by the mass transfer characteristics at the cathode/electrolyte interface. This study aims to elucidate the influence of the mass transfer characteristics (ion supplementation via diffusion and ion consumption via deposition) on the electrodeposition of microcolumns, thus providing feasible solutions for improving void defects with different feature sizes. The results indicate that ion consumption plays an important role in the mass transfer within large-width microcavities (100 μm). For large-width microcolumns, longer electroforming times lead to higher ion consumption, resulting in nonuniform ion concentration distribution, and consequently uneven deposition rates along the microcavity wall. In microcavities with high aspect ratio (5:1), ion supplementation plays a major role. The low ion supplementation rate does not support a uniform deposition, resulting in a large void defect and a low filling ratio in the deposited microcolumns. Therefore, reducing the ion consumption rate by decreasing the current density from 1 A dm−2 to 0.25 A dm−2 can effectively increase the filling ratio in large-width microcolumns with no significant effect on high aspect ratio microcolumns. On the contrary, the pulse reverse current (forward pulse current density 1 A dm−2, reverse pulse current density 2 A dm−2, frequency 1 Hz, forward pulse duty cycle 0.9) can increase the filling ratio in the high aspect ratio microcolumns by accelerating ion supplementation through dissolution of the deposited layer. By further increasing the reverse pulse current density from 2 A dm−2 to 6 A dm−2, void defects can be completely eliminated and even void-free deposition of high aspect ratio microcolumns (5:1) can be achieved.

Performance of high temperature steam injection in horizontal wells of heavy oil reservoirs

Steam huff and puff and steam flooding are important development methods for heavy oil reservoirs. At present, there are certain deficiencies in the research on the timing and methods of steam stimulation, steam flooding, and subsequent steam flooding. In addition, the research on the mass and heat transfer characteristics of high temperature steam injection in heavy oil reservoirs is still in the initiation stage. This article first studies the high temperature steam flow model in the horizontal wellbore of the SAGD process. On this basis, a horizontal well pattern model was established through numerical simulation software to analyze the heat and mass transfer characteristics of high temperature steam huff and puff and high temperature steam flooding. Results show that: (a) for heavy oil reservoirs with high temperature steam injection huff and puff, the first cycle of steam injection ends (day 22): the temperature near the well pattern of the horizontal well pattern is higher. In the area between the horizontal well patterns, the temperature rise is not obvious; (b) when the high temperature steam injection huff and puff phase of the heavy oil reservoir ends (900 days), most of the heat energy is recovered, and the formation residual heat is less; (c) After the huff and puff transfer, the thermal connectivity between the horizontal well patterns is better.