Water Vapor Mass Transfer Research Articles

Heat and mass transfer analysis of desiccant-coated heat exchanger with desiccants and metal-organic framework for bus air conditioning applications

Desiccant-coated heat exchangers (DCHEs) in air conditioning offer advantages like independent temperature and humidity control, reduced energy consumption, and high performance. However, the availability of desiccants with high water adsorption and low thermal energy requirements is limited. The present study proposes that aluminum fumarate (Al-Fum) MOF-coated heat exchangers will outperform conventional desiccants, especially at low regeneration temperatures (around 50°C). In order to evaluate this hypothesis, the performance of DCHEs was evaluated with a validated analytical model for different solid desiccant coatings: silica gel, zeolite, and Al-Fum MOF. Water vapor sorption characteristics, heat and mass transfer, and adsorption and desorption capacities were determined. The results showed that the Al-Fum MOF-coated heat exchanger outperformed silica gel and zeolite at low regeneration temperatures under adiabatic conditions. The MOF exhibited superior water uptake capacity (0.39 g/g) between 20% and 80% RH compared to the other studied conventional desiccants. Moreover, the MOF achieved the lowest outlet air temperature. At 19.5 g/kgda, the MOF demonstrated higher adsorption rates (4 % and 0.3 %) than zeolite and silica gel. These findings highlight the potential of Al-Fum MOFs as energy-efficient solutions for controlling indoor air humidity, particularly for latent-sensible heat separation systems with conventional vapor compression bus air-conditioning systems.

Prediction of heat and mass transfer inside 3D frost microstructure

In the present study, a CFD model based on 3D frost microstructure is developed. The desublimation phase change coupled with mass transfer of water vapor to ice surface and heat conduction through the tortuous ice crystals are calculated to predict accurate heat transfer process inside the frost microstructure. Air velocity and temperature inside the frost structure after 30 min operation are significantly lower than those inside 10 min operation, which are influenced by the structural change of the ice crystals. Vertical variations in ice crystal surface temperature and ice surface mass flux are also calculated, which varied between the frost structures operated for different periods. At 70 μm height of the frost, the mass flux at the ice crystal surface shows significant decrease from 7.48 × 10−5 kg/m2s for the frost structure at 10 min to 1.82 × 10−5 kg/m2s for that at 30 min. This study provides new and fundamental information to understand the mechanism of frost growth which is very important in refrigeration, cryogenics, and aerospace industry, etc.

Beads-on-string structural nanofiber membrane with ultrahigh flux for membrane distillation

Membrane distillation (MD) is a promising technology among various desalination processes. However, the membrane used in MD is still problematic due to its low permeability and tendency to become wet. In this work, we fabricated a novel electrospun nanofiber membrane (ENM) with micro-nano structures resembling beads on a string to enhance water flux and anti-wetting properties in MD desalination. The composite ENM consists of a heat-conducting top layer made of polyvinylidene fluoride co-hexafluoropropylene (PH) nanofibers embedded with carbon sphere (CS) nanoparticles and a heat-insulating support layer made of pH nanofibers. The PH/CS top layer with beads-on-string micro-nano structures can not only provide a rough surface to elevate anti-wetting property of the membrane, but also create a maximum temperature difference inside the membrane to improve mass transfer of water vapor. The PH/CS composite ENMs acquired the highest water flux of ∼ 71.3 L m−2 h−1 at a temperature difference of 40 °C, and its salt rejection rate for a 3.5 wt% NaCl solution was higher than 99.9% in direct contact membrane distillation. Meanwhile, the composite ENM exhibited long-term stability of 5100 min. Our work presents a novel composite ENM with great potential for promoting water flux and anti-wetting properties towards MD desalination.

Water vapor mass transfer in alginate–graphite bio-based hydrogel for atmospheric water harvesting

This study presents experimental and theoretical investigations on water vapor mass transfer of a novel hydrogel compound based on alginate and graphite. This hydrogel enables rapid, reproducible, and thermally driven cycles for the adsorption and desorption of water vapor from ambient air for atmospheric water harvesting applications. We study the impacts of hydrogel composition on sorption capacity and kinetics using sorption/regeneration experiments under various environmental conditions. Theoretical models based on Fick's law of diffusion and Linear Driving Force are developed and validated with experiments to optimize thermal cycling conditions within the temperature range of 20–100 °C. The bio-based hydrogel exhibited remarkable water uptake, ranging from 0.5 to 0.9 g/g, with RH below 30 and 50 %, respectively. This low-humidity setting enables a water production rate of 1.6–2.9 L/kg of sorbent per day with a low-grade thermal regeneration (60–100 °C). Natural graphite microparticles improve water vapor release kinetics during regeneration, with an effective diffusivity coefficient of around 10−11 m²/s.

Controlling Superhydrophobicity on Complex Substrates Based on a Vapor-Phase Sublimation and Deposition Polymerization.

The superhydrophobic properties of material surfaces have attracted significant research and practical development in a wide range of applications. In the present study, a superhydrophobic coating was fabricated using a vapor-phase sublimation and deposition process. This process offers several advantages, including a controllable and tunable superhydrophobic property, a dry and solvent-free process that uses well-defined water/ice templates during fabrication, and a coating technology that is applicable to various substrates, regardless of their dimensions or complex geometric configurations. The fabrication process exploits time-dependent condensation to produce ice templates with a controlled surface morphology and roughness. The templates are sacrificed via vapor sublimation, which results in mass transfer of water vapor out of the system. A second vapor source of a polymer precursor is then introduced to the system, and deposition occurs upon polymerization on the iced templates, replicating the same topologies from the iced templates. The continuation of the co-current sublimation and deposition processes finally renders permanent hierarchical structures of the polymer coatings that combine the native hydrophobic property of the polymer and the structured property by the sacrificed ice templates, achieving a level of superhydrophobicity that is tunable from 90° to 164°. The experiments demonstrated the use of [2,2]paracyclophanes as the starting materials for forming the superhydrophobic coatings of poly(p-xylylenes) on substrate surfaces. In comparison to conventional vapor deposition of poly(p-xylylenes), which resulted in dense thin-film coatings with only a moderate water contact angle of approximately 90°, the reported superhydrophobic coatings and fabrication process can achieve a high water contact angle of 164°. Demonstrations furthermore revealed that the proposed coatings are durable while maintaining superhydrophobicity on various substrates, including an intraocular lens and a cardiovascular stent, even against harsh treatment conditions and varied solution compositions used on the substrates.

Condensation heat transfer between a vertical superhydrophobic aluminum surface and moist air under natural convection

Condensation heat transfer at superhydrophobic surfaces has raised a lot of interest due to their potential application in atmospheric water harvesting and air-conditioning. In moist air, the condensation heat transfer using a superhydrophobic surface can be negatively affected by the existence of a large amount of non-condensable gas, namely, the dry air. Besides, the air flow pattern would substantially affect the surface condensation rate due to the convective mass transfer of water vapor. In this study, the condensation regime of a superhydrophobic aluminum surface vertically positioned in quiescent moist air was visualized and the condensation rate was experimentally investigated. Condensation experiments were also performed by using hydrophobic, hydrophilic, and superhydrophilic surfaces for comparison. The condensation rate of the superhydrophobic surface was consistent with that of other surfaces, though the condensation regimes of the four surfaces were quite different. The average deviation between the experimental condensation rate and the empirical correlation of natural convection heat and mass transfer was within 12%. The result indicates that vapor transfer induced by diffusion and convection in the boundary layer rather than the heat transfer resistance associated with condensate dominates the condensation heat and mass transfer between the moist air and the superhydrophobic surface.

Numerical Study of a Non-Linear Porous Sublimation Problem With Temperature-Dependent Thermal Conductivity and Concentration-Dependent Mass Diffusivity

Abstract Sublimation heat transfer occurs in a wide range of engineering processes, such as accelerated freeze drying (AFD), energy storage, and food technology. Particularly in the microwave AFD process, preservation of material with the least possible energy consumption is desirable. In connection with this, it is of interest to analyze the effect of temperature/concentration dependent heat/mass transfer properties. Given the limited literature available on sublimation, there is a general lack of physical understanding of this particular problem. The present work analyzes the nonlinear sublimation process driven by convective heat/mass transfer and evaporation of water vapor using the Legendre wavelet collocation method (LWCM). Results from the present work are shown to be in excellent agreement with the exact solution of the special case of a linear problem. Further, the present numerical technique shows good agreement with finite difference method in case of a completely nonlinear model. The model is used for a comprehensive investigation of the impact of the problem parameters, on the rate of sublimation. It is found that the sublimation rate increases with increasing values of β1 and decreasing values of β2. The impact of other dimensionless problem parameters such as Péclet numbers Pe1 and Pem, convection due to mass transfer of water vapor β, latent heat of sublimation l0 and Luikov number Lu on sublimation process is also discussed in detail. These observations offer a comprehensive theoretical and mathematical understanding of sublimation heat/mass transfer for improving the performance and efficiency of freeze-drying and related engineering processes.

Deviceful LiCl salt hydrate confinement into a macroporous silicone foam for low-temperature heat storage application

LiCl is a well-investigated salt hydrate for low-temperature thermal energy storage due to its high energy storage density (∼1250 Wh/kg), which, however, suffers from deliquescence. Its hygroscopicity induces relevant issues during hydration/dehydration cycles due to water vapor mass transfer, swelling, and agglomeration. Finding a proper semipermeable container that retains the salt and allows free water vapor flow would enhance the dehydration/hydration kinetics and conversion. In this study, a new promising macro-porous LiCl filled composite foams was evaluated for storing low-temperature heat below 100 °C. Composite foams at varying salt hydrate content were prepared (0–70% wt.). The optimal formulation was addressed by coupling morphological, absorption, and energy storage density performances. The morphological analysis evidenced a relationship between foam microstructure and salt hydrate content. Hydration/dehydration measurements indicate that the composite foam allows the water vapor diffusion thanks to its interconnected microporous structure, preventing mass diffusion issues. An energy storage density of up to 665 kWh/m 3 was estimated. Furthermore, the relatively homogeneous dispersion of the salt hydrate filler in the matrix facilitates the hydration/dehydration process, leading to a notably less pronounced hysteresis area. Based on sorption, manufacturing, service, and handling feasibility, these results indicate this material as a potentially effective option for this application. • Assessment of a LiCl salt hydrate confinement into a macroporous silicone foam semipermeable container. • Morphological evaluation of salt hydrate composite foams at varying filler content. • Experimental characterization of sorption composite foams for thermal energy storage applications. • Correlation between energy storage capability and macro-micro-structure of the composite was pointed out.

Modelling approach to predict the fire-related heat transfer in porous gypsum based on multi-phase simulations including water vapour transport, phase change and radiative heat transfer

The heat transfer in gypsum exposed to fire is significantly affected by heat conduction, mass transfer and condensation/evaporation effects of water vapour in the porous structure. In the past, numerical models to predict the heat transfer in gypsum were mainly limited to heat conduction and mass transfer of water vapour in thin gypsum boards used in wall assemblies. Thus, in the present study a multi-phase approach is proposed to predict the heat transfer within gypsum under fire exposure including conduction, mass transfer and condensation/evaporation. The consideration of water vapour transport and its condensation in the porous structure was leading to a good prediction of the heating process of gypsum up to approx. 100 °C. Furthermore, the calculated temperatures above 100 °C were adequate up to 2 cm from the fire side. However, at a higher distance from the fire the additional implementation of a thermal radiation model was crucial to improve the heat transfer in gypsum. Including the thermal radiation, the proposed numerical model is able to calculate the temperatures in the gypsum blocks in close accordance to the measurement. • Fire resistance test of gypsum and measurement of the temperature profile. • Lee model to predict evaporation/condensation of water in porous gypsum. • Multi-phase approach for the water vapour transport through gypsum in fire tests. • Thermal radiation modelling in porous gypsum with discrete ordinates model (DOM). • Porosity-corrected DO modelling was successfully used for radiation in gypsum.

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.

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.

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.

Convective drying of a moist porous object under the effects of a rotating cylinder in a channel

Numerical study of evaporation in porous medium during convective drying process was examined, and heat and mass transfer of liquid water and water vapor through porous media was investigated by using the Galerkin weighted residual finite element method. The porous moist object has a rectangular shape and is assumed to represent a food sample. Two-dimensional laminar flow of dry hot air was used in the channel with a rotating circular cylinder. Different locations and angular rotational speed of the rotating cylinder were considered to control the convective heat transfer and mass transportation. The radius of rotating cylinder and velocity of drying air were also varied with five different values. Results showed that rotational angular speed and drying air velocity had significant effect on heat transfer and mass transport phenomenon for the porous moist object.

Thermal cycling stage in the optimization of the freezing processes in the technology of drug lyophilized

Introduction . In the lyophilization technology, the freezing stage plays a key role in determining the time of primary sublimation, secondary desorption, and often the appearance of the lyophilisate. The duration of lyophilization is related to the composition and freezing conditions through the parameter resistance to mass transfer, which depends on the size of ice crystals.Objective: to select temperature regimes of thermal cycling (“annealing”) to obtain the most vapor-permeable sublimation layer, calculate the value of resistance to mass transfer of water and select the most optimal lyophilization mode for preparing lyophilisate GK-2 for preparing solutions for injections.Materials and methods . Substance: GK-2 (hexamethyleneamide bis-(N-monosuccinyl-L-glutamyl-L-lysine)) (V.V. Zakusov Research Institute of Pharmacology, Russia); excipients intended for parenteral use: lyoprotector – sucrose (CompriSugar®) (CristalUnion, France), cryoprotector – polyethylene glycol 4000 (PEG, macrogol) (Polyglykol® 4000, Panreac, Spain). Edwards EF-6 Lyophilic Dryer (Italy); microscope Nikon Eclipse E200; digital camera Nikon DS-Ri2; confocal microscope LEXT OLS 4100. Various freezing, lyophilization, direct optical microscopy in a cold chamber, laser microscopy and mathematical formulas were used to determine the values of resistance to mass transfer of water vapor from model lyophilisates.Results . This article discusses the possibilities of optimizing the freezing stage and, accordingly, the entire lyophilization cycle by introducing an intermediate thermal cycling stage, provides calculations of the mass transfer coefficient of the composition under different freezing conditions and ice crystal sizes.Conclusion . A simplified equation for calculating the coefficient of resistance to mass transfer for model solutions of GK-2 based on the correlation dependence was determined. The selection of freezing and “annealing” modes was carried out on the basis of the most important indicators, such as productivity and time of lyophilization.

Simulation of frost growth and densification on horizontal plates with supersaturated interface condition

Numerical simulation in the air subdomain and analytical analysis in the frost subdomain are coupled to simulate the frost growth and densification on horizontal plates. The interface movement, water vapor supersaturation and mass transfer in frost subdomain are discussed. The contribution of interface movement to the water vapor mass flux is evaluated by order of magnitude analysis. An approach to determine the supersaturation degree at the interface is proposed. A new model describing the water vapor diffusion and absorption processes in the frost subdomain is proposed and the empirical mass transfer coefficient is determined by fitting to experimental data. It is found that the supersaturation boundary condition provides reasonable simulate results and the mass transfer coefficient in the frost subdomain increases with air velocity, which indicates that the convective mass transfer in porous frost cannot be neglected.

Impact of hydrocolloid addition and microwave processing condition on drying behavior of foamed raspberry puree

In this study, foamed raspberry puree was dried by microwave-assisted freeze drying (MWFD). The combined use of microwaves and a foamed structure was intended to accelerate the drying process, while creating an innovative product structure with intense aroma impact for consumption as a snack. The influence of potato protein as foaming agent, maltodextrin as foam stabilizer and microwave (MW) power on the drying characteristics of raspberry foams during MWFD was investigated. Conventional freeze drying (FD) experiments were performed as a reference. MD concentration was shown to significantly influence product temperature. As higher MD concentrations yielded smaller bubbles and a more uniform bubble size distribution, lower drying temperatures were needed to reach the same final moisture content.Varying MW power did not significantly influence the drying time. MWFD at 1.0 W g−1 yielded a 3–4-fold decrease in total drying time as compared to FD. The addition of 10% protein led to the most gentle drying at high MW power input due to structural changes enabling a lower resistance against water vapor mass transfer. There was a high correlation between the foam characteristics and the drying behavior. An overrun above 450% led to gentle drying at all tested microwave power input levels. Foam bubble size and bubble size distribution correlated well with drying velocity. Overall, MWFD was shown to be a much faster and more gentle alternative to FD for the production of fruit foams.

Effect of operation parameters on the mass transfer and fouling in submerged vacuum membrane distillation crystallization (VMDC) for inland brine water treatment

Membrane distillation-crystallization process has great potential to recover high quality water and valuable precipitates from inland brine water. Incorporation of submerging membrane in a feed tank with agitation provides opportunity to reduce temperature and concentration polarization and energy consumption. This study evaluated membrane transverse vibration and feed aeration on the mass transfer of water vapor, crystallization and fouling in a submerged vacuum membrane distillation and crystallization for inland brine water. At selected vibration frequency, more than 700h of high desalination performance was achieved. However, at elevated initial feed conductivity, the operation time was significantly reduced due to severe fouling.With accelerated tests with extreme feed concentration, both transverse vibration and aeration were able to increase the initial flux by reducing boundary layer thickness on the membrane surface. However, with transverse vibration, rapid reduction of flux occurred earlier due to the enhanced CaCO3precipitation on the membrane surface. The aeration of feed improved productivity with longer period of high flux with the increased nucleation in the bulk feed solution. Thermal water softening procedure prolonged the operation time by increased calcium precipitation in the feed solution and slower CaCO3 nucleation on the membrane surface.

Phase change mass transfer model for frost growth and densification

A frosting mechanism was used to develop a phase change mass transfer model to predict the frost layer growth and densification. The model with a criterion can describe the mass transfer of water vapor from the humid air to increase the frost thickness and the frost density. Frost formation on a cold surface with local cooling was simulated by using the model as a source term in FLUENT to predict the frost morphology, temperature distribution and frost weight which are all in good agreement with the experimental results. The results show that the frost first forms around the cooling block and then extends gradually to other directions, with the frost above the cooling block having the greatest density. The average frost thickness increases gradually with time with the growth rate slowing down, while the average frost density increases with the increases rate getting faster. The temperature distribution in the computational domain is related to the frost layer morphology and the frost surface temperature is usually lower than the freezing point temperature. As the frost layer grows, the average air velocity increases with the maximum local velocity located above the thickest part of the frost layer.

蒸发式冷凝器传热传质数值模拟研究

采用VOF法，建立二维水蒸气传热传质顺流和逆流的计算模型，计算所得的平均液膜厚度模拟值比实验测量值大3.03%~6.90%，比Nusselt理论的预测值大8.33%~10.34%，在合理的误差范围，说明该模型切实可行。运用该模型计算并分析水与空气顺流和逆流过程中，液态水的流动分布、空气与水流速度分布以及空气中水蒸气的质量含量分布情况，结果表明：在气–液两相逆流过程的气–液界面的总传热量中，潜热传热量所占比值在90%以上，比气–液两相顺流时高，在气–液相界面处是以水蒸发传质引起的潜热换热为主、温差引起的显热传热为辅的换热形式，逆流比顺流更有利于传热。 The VOF (volume of fluid) method is used to establish a two-dimensional water vapor heat and mass transfer parallel flow and counter flow model. The deviation of calculated film thickness, which is changed with Re values, is greater than the experimental measurement’s value by 3.03% - 6.90%, and the predicted value of the Nusselt theory by 8.33% - 10.34%; it indicates that the model is feasible. The model is used to calculate and analyze liquid water flow distribution, air and water quality and velocity distribution, water vapor quality content distribution in the air, in water and air’s parallel flow and counter flow processes. The results show that: in gas-liquid two- phase counter flow process, the ratio of latent heat transfer rate in total heat transfer rate is above 90% at gas-liquid interface, higher than that of gas-liquid two-phase parallel flow; at air-liquid interface, the main heat transfer is latent heat transfer caused by water’s evaporation and mass transfer, and the supplemented heat transfer is sensible heat transfer caused by temperature difference; counter flow is more conducive to heat transfer than parallel flow.

Flux enhancement in membrane distillation by incorporating AC particles into PVDF polymer matrix

In this paper, a novel method of preparing porous hydrophobic membrane was proposed and further experiments had been carried out for verification. A kind of hydrophobic porous inorganic particles, macroporous structure activated carbon (AC), was introduced into polyvinylidene fluoride (PVDF) polymer matrix to fabricate porous membrane for vacuum membrane distillation (VMD) application. The incorporation of AC particles in the polymer matrix can efficiently enhance the membrane permeability because of the intrinsic porous structure of AC particles could directly provide water vapor mass transfer channels in the VMD process. Results showed that: the VMD flux of PVDF hydrophobic membranes was obviously improved by the addition of AC particles. When 0.09wt% AC particles were added in membrane casting dope, a VMD flux of 54.9kg/m2h was obtained. The flux is 30% higher than that of pure PVDF membrane (42.0kg/m2h). Furthermore, the conductivity of the M-3 membrane was kept lower than that of pure PVDF membrane in the 16h continuous desalination experiments. All of the results demonstrated that the prepared PVDF/AC hollow fiber membrane not only has higher permeability but also exhibited satisfying performance stability. It performed well in the VMD program and is expected to be applied in the field of MD to improve the MD membrane performance.