Vapor Mass Transfer Research Articles

Sensitivity of the Non-Equilibrium Approach for Mixture Condensation to Heat and Mass Transfer Correlations and Thermophysical Properties

Abstract The prediction of mixture condensation is still complex due to the coupled heat and mass transfer and insufficient data of thermophysical mixture properties. This article analyzes the impact of various heat and mass transfer correlations on the non-equilibrium approach for mixture condensation in a vertical plain tube. Furthermore, the influence of thermophysical properties from different databases is investigated. The results are shown for ethanol-water, but allow conclusions to other fluid mixtures. They indicate that the liquid heat transfer coefficient in the non-equilibrium approach dominates the qualitative behavior of the condensation process, but the vapor mass transfer coefficient can only decrease or increase the quantitative level of the effective heat transfer with minor impact. More importantly, the logarithm in the vapor mass transfer term is central for the prediction of the condensation heat transfer. As this logarithm contains vapor-liquid equilibrium (VLE) data, it proves that there is a strong connection between VLE and overall prediction of mixture condensation. A demonstration of available data for thermophysical mixture properties of ethanol-water shows significant deviations, which affect the calculations as well. Besides, data from our own experiments are presented for mixture viscosity of ethanol-water. It is recommended to focus not only on improved heat and mass transfer correlations but also on thermophysical properties and VLE data for a precise prediction of mixture condensation.

Modeling and Simulation of Distillation of a Binary Non-Reacting Mixture using Rate Based Approach in Packed Column under Total Reflux

Modeling and simulation of a distillation column are very important in many chemical industries where ever it requires separation or purification of a liquid mixture into its pure components. Particularly in the context of improving efficiency, process scale-up, intensification and control it requires a validated model. In this work, both experimental results and simulation results are obtained. A batch distillation equipment that consists of a reboiler, a packed column, and a top condenser is used to distill a mixture of benzene and toluene with various heat inputs to the reboiler. The distillation process in the column is modeled by an evaporation rate-based approach using Raoult’s law for driving force. This particular feature is different from the stage-wise equilibrium model. The evaporation mass transfer coefficients are obtained for benzene and toluene in lab-scale apparatus. The material balance equations for the two components are solved using a finite difference method. The distillate compositions at total reflux obtained from experiments and the simulations are compared for validating of model proportionally. Further insights are obtained in regard to throughput and distillation efficiency.

Scalable High-Efficiency Bi-Facial Solar Evaporator with a Dendritic Copper Oxide Wick.

Solar thermal distillation is a promising way to harvest clean water due to its sustainability. However, the energy density of solar irradiation inevitably demands scalability of the systems. To realize practical applications, it is highly desirable to fabricate meter-scale solar evaporator panels with high capillary performance as well as optical absorptance using scalable and high-throughput fabrication methods. Here, we demonstrate a truly scalable fabrication process for a bi-facial solar evaporator with copper oxide dendrites via the hydrogen bubble templated electrochemical deposition technique. Furthermore, we construct a theoretical model combining capillarity and evaporative mass transfer, which leads to optimal operation conditions and wick characteristics, including superhydrophilicity, extreme capillary performance, and omni-angular high optical absorptance. The fabricated porous surfaces with excellent capillary performance and productivity provide a pathway toward a highly efficient bi-facial solar evaporator panel with meter-level scalability.

Evaluation of frost heave in clay soils

Frost heaving in clayey soils with a low coefficient of permeability raises a lot of questions regarding the cryosuction, surface tension forces, and accompanying phase transfer of water. The freeze-thaw laboratory test results were considered in this work in terms of temperature and volumetric parameters change, dry density, and water mass transfer. The article presents a model for calculating the mass transfer of water (vapour) in the gas state under the influence of cryogenic forces. Findings include the improved understanding of the heat and mass transfer phenomenon during the unidirectional freezing of soils in an open system. Most of the tests for engineering properties registered a slight reduction in relation to strength, cohesion, and angle of internal friction. However, there was a significant increase in the coefficient of permeability after the freeze-thaw cycles with initially dense compacted soil samples, which was due to loosening and moistening of the soil samples during the heave at sub-zero temperatures. The conceptual model for frost heave in soils was developed based on the vapour mass transfer. There was presented algorithm of vapour flow calculation in unsaturated soils using fundamental thermodynamic equations.

Measure method of effective diffusion in gas oscillating in channels of variable radius or porous medium

The paper discusses a new technique for measuring the diffusion coefficient of vapor of a volatile fluid in air in long straight channels or porous media. The proposed experimental technique is universal and allows a) determining the coefficient of molecular diffusion of vapor of a volatile liquid in air; b) calculating the coefficient of effective diffusion of vapor in oscillating air. The proposed technique was tested in the study of the diffusion of 2-propanol vapor in air at rest and oscillating air in a channel of variable radius and a porous medium consisting of randomly packed hard spheres of equal diameter and was found to be relevant. Initial experiments with a porous medium show that the proposed experimental technique can also be used to estimate the diffusive tortuosity of porous media. The results of studies, finished and planned, are useful for understanding the physical processes taking place in various technical operations including wood and food drying, drug delivery, etc.•The new experimental technique provides accurate measurement of the molecular diffusion coefficient of vapor of a volatile fluid in air.•The experimental setup allows measuring the enhanced mass transfer of vapor of a volatile fluid in oscillating air in a straight channel of constant/variable radius and a porous medium.•The additional advantage of the present technique is that it enables the estimation of the diffusive tortuosity of a porous medium.

Test Data Analysis of the Vented Chill, No-Vent Fill Liquid Nitrogen CRYOTE-2 Experiments

NASA is interested in developing efficient methods with which to transfer cryogenic propellants to enable future in-space cryogenic propellant vehicles, particularly the cryogenic fuel depot. The process of transferring cryogenic propellants between two vessels in a reduced gravity environment is complicated by the low normal boiling point of cryogens, high propensity for boiling during tank chilldown, and the fact that liquid cannot be transferred with the receiver tank vent valve open to space. This paper presents test data analysis of the liquid nitrogen vented chill, no-vent fill (NVF) experiments on the CRYOTE-2 tank. 53 tests were conducted, and while not originally intended, were performed in a somewhat parametric fashion. Performance between three different injectors are compared, as well as the effect of initial receiver tank wall temperature, receiver tank fluid initial condition, supply pressure, mass flow rate, and trigger point on the NVF process. From in-depth test data analysis, the highest performing injector based on maximizing evaporation heat and mass transfer between droplets and ullage as well as condensation heat transfer at the liquid/vapor interface, was the 3-spray injector.

Understanding how non-condensable gases modify cavitation mass transfer through the van der Waals theory of capillarity

Cavitation of a pure fluid continues to be a challenging problem in fluid dynamics. However, most cavitating fluids contain small amounts of non-condensable gases that can change the dynamics of the process significantly. While the effect of non-condensable gases on the surface and bulk properties of the fluid has been studied in detail, its impact on liquid–vapor mass transfer has remained elusive. Here, we provide mechanistic insight into this process using theory and simulations. Our results point to a dual role of non-condensable gases on liquid–vapor mass transfer. While the presence of non-condensable gases always reduces hydrodynamic cavitation, it also leads to a mass transfer mechanism that we call mixing cavitation. We show that mixing cavitation may increase or reduce mass transfer under different physical conditions and can dominate hydrodynamic cavitation. This study opens possibilities to better understand cavitation inception.

Water Vapor Pathways during Freeze-Drying of Foamed Product Matrices Stabilized by Maltodextrin at Different Concentrations

This study aimed to identify the water-vapor transport mechanisms through an aerated matrix during microwave freeze-drying. Due to the larger surface area and lower water vapor transport resistance of an aerated product compared to the solution, foam structures dry faster. Different foam structures were produced with different maltodextrin (MD) concentrations (10–40%) as a foam-stabilizing agent. Depending on the initial viscosity of the solution prior to foaming, the samples differed in overrun (41–1671%) and pore size (d50 = 58–553 µm). Experiments were partially performed in a freeze-drying chamber of a light microscope to visualize structural changes in-situ. Different mechanisms were identified explaining the accelerated drying of foams, depending on the MD concentration, above or below 30%. At lower MD concentration, high overruns could be produced prior to freezing with big bubbles and thin lamellae with short diffusion pathway length. At 40% MD concentration, the viscosity was too high to integrate much air into the product. Therefore, the foam overrun was low and the bubble size small. Under these conditions, the water vapor generates high pressure, resulting in the formation of channels between bubbles, thus creating the pathways with low resistance for a very fast water vapor mass transfer. In addition, microwave freeze-drying experiments using a pilot plant unit were conducted to validate the findings of the freeze-drying microscope. A reduction of the drying time from 150 min (10% MD) to 78 min (40% MD) was achieved.

Turbulent liquid film evaporation in a partially heated wall along a vertical tube

AbstractIn this numerical study, the evaporative heat and mass transfer of a turbulent falling liquid film in a finite vertical tube are investigated. The liquid film flows in the tube's inner wall, whose outer wall is partially subject to thermal flux. Here, different configurations corresponding to thermal flux imposed on different external surface wall percentages are examined. External face zones where the heat flux is not applied are maintained insulated. The nonlinear set of parabolic mass, momentum, energy, and mass fraction conservation equations combined with boundary and interfacial conditions are treated numerically using implicit finite difference procedure. For falling liquid film analysis, an adapted Van Driest turbulence model is used. For the present work, it is supposed that gas flows in a laminar regime. We examine in this paper the impact of the percentage of heated surface area on flows as well as on heat and mass transfer. Obtained results for a partially heated wall are compared with those produced for an entirely heated wall.

High-temperature effects on the mucus layers in a realistic human upper airway model

Firefighters and workers in some industries often inhale (polluted) air at high temperatures, eg, 50–200°C, and very low relative humidity, say, 5–10%. Such severe environmental conditions may cause health problems or further aggravate existing ones. As in vivo tests are very complex, potentially harmful and costly, and in vitro experiments often lack high resolution and predictive capability, in silico studies become a priority. They may include computer simulations of the air-particle dynamics in human lung-airway models to understand the convection heat transfer as well as pollutant transport, deposition and uptake at high inlet temperatures and low relative humidity. In this study, detailed modeling, simulation and analysis focuses on two-phase flow in a human upper lung-airway model with a realistic 3D mucus lining. Especially modeling of the upper airway mucus layers was of interest in order to simulate the vapor mass transfer to the airflow because of its primary function for airway humidification. Naturally, the humidification causes water loss in the mucus layer, which leads to its reduction or even depletion when exposed to relatively high temperatures. Accounting for the changes in thickness and the rise in temperature in the mucus layer allows for the determination of locations of thermal injury in the human airways due to continuous exposure of such abnormal inhalation conditions. Different temperature profiles and local changes in mucus layer thickness were studied for ranges of severe inlet temperature conditions at a representative flow rate of 20 LPM (liters per minute). For inlet temperature reaching 100°C, mucus-layer thinning was observed in the upper airways. Interestingly, as a confirmation of the Reynolds analogy, the areas of significant wall heat flux and associated wall shear stress coincided with the regions of highest mucus evaporation, resulting in the humidification of the air with low relative humidity. Model development and mucus layer generation were done using C++ programming. All computer simulations were carried out using the open-source computational fluid dynamics toolbox OpenFOAM.

Phenomenological model of the suspension droplets evaporation in a plasma flow considering different mass transfer mechanisms

Suspension plasma spraying (SPS) technology allows the production of advanced coatings via deposition of micron and submicron particles, however its effective implementation requires a deep understanding of “droplet – plasma” interaction. Droplets of suspension in a jet drastically change their size due to fragmentation and evaporation of the liquid carrier, a change in the structure of particles occurs, and several processes of mass and heat transfer take place. This paper analyzes the time scales of the droplet breakup, convective (Hill vortex) and diffusion mass transfer of solid phase in a suspension, heating and evaporation of the material, vapor transport due to diffusion and filtration mechanisms. Basing on these analyses, a phenomenological model of the evolution of suspension droplets in the SPS process is proposed. The model includes the stages of a breakup, diffusion drying and the formation of a dry crust, the filtration exit of vapors, and the final heating and melting of the material. A 1-D simulation of the behavior of droplets of a ZrO2 suspension in a uniform air plasma flow was carried out, the influence of the velocity (500–2500 m/s) and temperature (3000–5000 K) of the plasma, the concentration of the suspension (1–60% wt) on the characteristics of the sprayed particles was studied. It was found that after formation of surface crust the vapor pressure inside the particles can reach 1 MPa, and filtration mass transfer of vapor must be considered when analyzing the problem. Equations have been obtained for estimating the size and temperature of particles and duration of each stage of droplet evolution.

On heat and mass transfer using evaporating self-rewetting mixtures in microchannels

Evaporation heat and mass transfer in open microchannels was investigated using pure liquids (ethanol, deionized water and butanol), an ordinary (non-self-rewetting) mixture (5% v/v ethanol/water mixture) and a self-rewetting fluid (5% v/v butanol/water mixture). The applied heating power ranged from 0 mW to 282.76 mW (giving heat fluxes from 0 W/m2 to 4501 W/m2) with wall temperatures ranging from 24 °C to 95 °C. When the microchannel had no external heating, diffusion was found to be the main mechanism of mass transfer. With external heating, the diffusion model underestimated the experiment results, and buoyancy-driven convection may account for this. Evaporation rates remain high for the self-rewetting fluid even when the meniscus recedes significantly (by 6 times the inner diameter) in the microchannel under heating conditions. Additionally, the self-rewetting fluid showed the highest conduction rate in the axial direction in high heating power conditions. Meniscus-wall contact angles and flow visualizations were obtained for various heating rates. The “contact angle shift” phenomenon of the self-rewetting fluid was observed. Marangoni-induced convection in the microchannel accounts for the mechanism of better heat and mass transfer as well as the “contact angle shift” phenomenon for the self-rewetting fluid.

Experimental description of bubble dynamics and heat transfer processes occurring on the pool boiling of water on biphilic surfaces

This study concerns the detailed description of the fluid dynamics and heat transfer mechanisms occurring during single bubble nucleation on biphilic surfaces (superhydrophilic/hydrophilic surfaces surrounding hydrophobic/superhydrophobic isles). A high-speed video camera is synchronized with a high-speed thermographic camera to relate the temporal evolution of bubble dynamics, from generation to detachment. The results allowed identifying several stages of bubble growth in all the biphilic surfaces tested, regardless of the size of the superhydrophilic regions. Bubble dynamics is affected by the size of the superhydrophobic regions. Since bubbles are constrained to the superhydrophobic region boundaries, which alter their base diameter, smaller superhydrophobic regions tend to promote a regular and stable bubble generation, due to the action of stronger surface tension forces acting on the boundary region. As the base diameter increases, the surface tension effects, which only act at the boundary with the hydrophilic region, are lessened and only affect the bubble at larger volumes. Consequently, smaller superhydrophobic regions are associated to higher evaporation mass transfer rates. Temperature gradients are larger at the hydrophilic/superhydrophobic boundaries and despite being small amplitude differences, they clearly promote induced convection of the cold liquid between superhydrophobic regions, as observed in the high-speed thermographic images.

Dynamics of unsteady compressible cavitating flows associated with the cavity shedding

The main purpose of this work is to shed light on the physics involved in the two distinct cavity cloud shedding mechanisms in cloud cavitating flows, namely re-entrant jet mechanism (RJM) and shock wave propagation mechanism (SWM). A compressible cavitating flow solver, which considers the compressibility effects of both liquid and vapor, is used to account for the wave dynamics in cavitating flows. The compressible Navier-Stokes equations, coupling the mass, momentum energy equations, and phase fraction transport equation, along with the thermodynamic equations of state for both liquid and vapor, Saito cavitation model and the SST-SAS turbulence model, are solved. Numerical results are presented for the conditions of both the re-entrant jet mechanism and shock wave mechanism around a NACA66 (mod) hydrofoil (Leroux et al., 2005), respectively, with emphasis on the process of re-entrant jet development and shock wave formation and propagation. The results show that the re-entrant jet can cause the attached cavity sheet breakup for both the two cavity cloud shedding mechanisms in both high and low angle of attack, while the shock wave formation and propagation process only occurs under shock wave mechanism at low angles of attack. Pressure evolution illustrates that in the re-entrant jet mechanism, cavity cloud collapse will induce high pressure load, while no shock wave and thus the corresponding rebound phenomenon are observed. In shock wave mechanism, during the whole process of the shock wave dynamics, namely generation, propagation and rebound process, pressure fluctuations increase sharply along with the generation of the pressure peaks with large amplitude and short time interval. Further study on the compressible characteristics involved in the two cavity shedding mechanisms illustrates that vapor fraction and mass transfer have a significant effect in sonic speed and Mach number characteristics. The average and standard derivation of maximum Mach number (Mamax) in shock wave mechanism is lower than that in re-entrant jet mechanism. Cavitating flows are characterized by low sonic speed value in the cavitation region and high sonic speed value in the pure liquid region and a sonic speed boundary layer exists between the two regions, the thickness of which is about the size of the local attached cavity sheet on foil surface. In the process of shock wave generation and rebound, cavity volume and cavity volume rate experience large fluctuations, showing strong cavitation instabilities in shock wave mechanism.

Theoretical Analysis of a Sessile Evaporating Droplet on a Curved Substrate with an Interfacial Cooling Effect.

Sessile droplet evaporation is widely encountered in nature, and it has numerous applications in industrial and scientific communities; therefore, the accurate prediction of droplet evaporation has great significance in practical applications. In this paper, for the first time, a comprehensive theoretical model is built up for diffusion-controlled heat and mass transfer for sessile droplet evaporation on a curved substrate in toroidal coordinates. The evaporative mass transfer is coupled with the heat transfer across the gas-liquid droplet interface, as well as the heat transfer across the solid-liquid interface of the curved substrate. The effects of interfacial cooling and thermal conductivity of the droplet and substrate as well as their initial shapes on the droplet evaporation are provided in details. It is found that the evaporative flux usually increases sharply near the droplet edge due to the short distance for heat conduction from the substrate to the droplet; however, it can be reversed from sharp increasing to decreasing at a low thermal conductivity ratio kR < 0.3 of the substrate over droplet or large initial droplet contact angle θ > 30°. The interfacial evaporative cooling effect can always suppress the droplet evaporation. The lifetime of evaporative droplet can be prolonged with the decreasing thermal conductivity ratio, increasing evaporative cooling number, and increasing initial droplet contact angle or tangential angle of a curved substrate. These findings may be of great significance in the applications of droplet evaporation on the curved substrate.

RESULTS OF A STUDY OF THE AMMONIA ABSORPTION EFFICIENCY BY A BRINЕ IN A VORTEX DEVICE

Object of research: vortex mass transfer device for the effective absorption of ammonia by brine in soda production. Investigated problem: intensification of mass transfer processes and ensuring effective absorption of ammonia with small dimensions of the device and low hydraulic resistance Main scientific results: studies of ammonia absorption efficiency in a laboratory device are carried out, which confirm the operability of the proposed device design. The dependences of the efficiency of the vortex device on the ammonia concentration and the brine consumption are revealed. The flow velocity and pressure in the device are simulated at various capacities. Scope of practical use of the research results: the research results can be used in the design of mass transfer equipment, primarily equipment for purifying emissions from ammonia and gas breakers from soda ash columns. Innovative technological product: the design of the vortex device, which allows achieving a high degree of absorption in ammonia with a concentration of up to 130 g/m3 of the order of 99 % at low operational (due to low hydraulic resistance) and capital (due to the compactness and small size of the installation) costs. Scope of the innovative technological product: the vortex device of the proposed design can be used to clean emissions, absorption of ammonia vapor or other mass transfer processes.

Computational fluid dynamics assessment of effect of different openings configurations on the thermal environment of a facility for coffee wet processing

This study aimed to analyze the effect of the area size and location of openings for natural ventilation on the temperature and relative humidity inside a typological facility for coffee wet processing that have been using in Colombia and some South America Countries as well, with mechanical drying inside, using modeling with computational fluid dynamics modeling, in order to find the best suitable condition for preserve the quality of the coffee parchment. A significant effect was found regarding the area and location of the openings for natural ventilation on the internal hygrothermal environment, but no significant effect was found on the temperature. It was also found that the chimney effect plays a decisive role in the mass transfer of water vapor and heat to the outside of the building, and helping to maintain a suitable internal environment for the preservation of coffee.

TransPoly: A theoretical model to quantify the dynamics of water transfer through nanostructured polymer films

Mass transfer of penetrants is among the most important physical properties of polymer films to properly ensure function during service. In the case of interaction between penetrants and polymer, non-Fickian diffusion can be observed. This phenomenon is described by numerous reports and a few models have been proposed. Nevertheless, few of them quantify Fickian and non-Fickian behaviour using parameters with clear physical meaning. The present work develops a new non-Fickian mass transport model: TransPoly (Transfer through Polymers). Based on coupled heat and mass transfer, it accounts for simultaneous Fickian transport and molecular relaxation. This model is applied to data of water vapour mass transfer in nanocomposites of polylactide and cellulose nanocrystals. The contribution of the non-Fickian mass transfer on water vapour sorption is highlighted. TransPoly can be applied to several kinds of polymers and used for prediction purposes once the parameters are determined on a learning database.

Development of mass-transfer evaporation model for Lake Nasser, Egypt

Abstract Evaporation from free water surface is considered a very important constituent in both the energy and hydrologic cycles. Precise measurement of evaporation from the free water surface is almost impossible. This is why we need a calculation model for free water evaporation. In this study, a simple mass-transfer evaporation model was developed to be applicable over Lake Nasser in the hyper-arid region located in the south of Egypt. Measured meteorological data (2011–2014) at two stations, Aswan and Abu-Simbel, were used to calculate free water surface evaporation using Priestly–Taylor equation. Priestly–Taylor equation was used because it is the most appropriate equation for Lake Nasser evaporation according to the literature. Results from this model were used to develop a simple mass-transfer evaporation model. The statistical analysis for both calibration and validation periods were very good. The slope of the regression line is about 0.9, with a coefficient of determination of 0.98. The t value is 0.6, at p value of 0.544, which is much greater than 0.05. The developed model could be used with confidence at Aswan meteorological station or on the average of the two meteorological stations, while it should be used carefully on Abu-Simbel meteorological station.

A multi-scale approach to optimize vapor-liquid mass transfer layer in structured catalytic packing

In order to implement the multi-scale model to optimize the structure of vapor-liquid mass transfer layer, vapor and liquid mass transfer coefficients that embrace the effects of structure parameters of mass transfer layer should be obtained in advance. In the study, the influences of four structure parameters on mass transfer performance were studied by the tracer method. Moreover, the corresponding mass transfer coefficients were correlated. By comparing with the experimental data of different types of structured catalytic packings, it has been found that the correlations exhibit good universality. Furthermore, the correlations were applied to the non-equilibrium model for the hydrolysis of methyl acetate (MA), so as to investigate the influences of four structure parameters on catalytic distillation process. The results indicated that wavy height and the area ratio of mass transfer layer to catalyst layer have more significant influences on the conversion of MA than inclined angle and base length.