Minimum Wetting Rate Research Articles

Effect of activated carbon electrode material characteristics on hardness control performance of membrane capacitive deionization.

Capacitive deionization (CDI) is an electrochemical-based water treatment technology that has attracted attention as an effective hardness-control process. However, few systematic studies have reported the criteria for the selection of suitable electrode materials for membrane capacitive deionization (MCDI) to control hardness. In this study, the effect of electrode material characteristics on the MCDI performance for hardness control was quantitatively analyzed. The results showed that the deionization capacity and the deionization rate were affected by the specific capacitance and BET-specific surface area of the activated carbon electrode. In addition, the deionization rate also showed significant relationship with the BET specific surface area. Furthermore, it was observed that the deionization capacity and the deionization rate have a highly significant relationship with the BET specific surface area divided by the wettability performance expressed as the minimum wetting rate (MWR). These findings highlighted that the electrode material should have a large surface area and good wettability to increase the deionization capacity and the deionization rate of MCDI for hardness control. The results of this study are expected to provide effective criteria for selecting MCDI electrode materials aiming hardness control.

Study of the influence of surfactants and surface treatments on the minimum wetting rate of falling films of aqueous LiBr and Carrol solutions

The wettability is one of the critical properties of the falling film type absorber, limiting the wetting area in the absorber, hence the total mass transfer performance. To achieve low values of wetting rate in the absorber is an interesting feature to decrease the pumping power and, therefore, the size of the solution pumps. However, studies about the relationship between surface tension, contact angle, and minimum wetting rate are limited. In this work, working pairs LiBr - H2O and Carrol - H2O (Carrol contains LiBr and EG - Ethylene glycol - with a mass ratio at 4.5:1) with additives 2 - EH (2 - Ethyl - 1 - hexanol) and PCM (1 - Phenylethanol) are studied focusing on the surface tension and minimum wetting rate. The surface tension of the two solutions is measured at various temperatures and concentrations employing the Du Rouy method. The experimental results demonstrate the surface tension of both solutions and the effect of liquid/gaseous surfactants on the surface tension is similar. Also, the minimum wetting rate (MWR) is measured in an annular falling film experimental set-up employing the dewetting method using different materials tubes and fluids. The experimental results of MWR test agree with theoretical predictions and empirical correlations with a discrepancy of less than 10%. When the tube is coated with a hydrophilic surface, a contact angle at 30∘ is achieved, and the minimum mass flow of water to achieve complete wettability on the coated SS tube is reduced from 0.096 to 0.025 kg·m−1·s−1.

Theoretical and Experimental Investigation of the Minimum Wetting Rate of a Falling Water Film on the Vertical Grooved Plate

Theoretical and Experimental Investigation of the Minimum Wetting Rate of a Falling Water Film on the Vertical Grooved Plate

Numerical study of the falling film wettability and heat transfer on the inclined plates with different corrugated structures

AbstractIn this study, 2D and 3D models of corrugated plates using the volume of fluid (VOF) model were set up to simulate the influence of corrugated structure on the wettability and heat transfer. The results show that compared with the triangular corrugation, the rectangular corrugation has a slower growth rate of wetting ratio (WR) as the Weber number (We) increases under the same corrugation length Le and corrugation depth De. The average WR of the triangular corrugation is 2.8% less than that of the smooth plate. For the triangular corrugation, the higher Le contributes to improving wettability, but the average heat transfer coefficient havg first decreases to the minimum at Le = 5 mm and then increases 8% to 9% to a steady value until Le = 8 mm. WR decreases with De at We ≤ 0.76. A WR correlation considering the corrugation steepness St (St = De/Le) and a critical Re correlation corresponding to the minimum wetting rate were fitted for the liquid oxygen. As the inclination angle ϕ increases, the average film thickness decreases and havg increases until ϕ = 75°, while the variation rate slows down afterwards. At Re = 1200, havg has a decreasing trend at St ≤ 0.13, increases at St ≥ 0.14 and reaches a plateau after St = 0.24 as St increases. It increases by 30.6%, 47.4%, and 44.4% for the inclination angle ϕ = 30°, 60°, 90° cases from St = 0.14 to 0.24. The enhancement is caused by the vortices existing at St ≥ 0.14.

Contact angle measurement for LiBr aqueous solutions on different surface materials used in absorption systems

Wetting surface is a very important issue for the design of absorption applications and heat exchangers. The contact angle is deemed essential in wettability studies; However, LiBr aqueous solution contact angle studies are limited. This work analyses the contact angle of LiBr aqueous solution in the range of 0–55% mass fraction on different material surfaces: copper, aluminum, stainless-steel and polytetrafluoroethylene (PTFE) under atmospheric conditions. A sessile drop technique was used for measuring the contact angles, and a linear relation between solution surface tension and contact angle is observed in the tested materials. The study of three metals show hydrophilic performance (θ < 90°), whereas the PTFE shows hydrophobic performance. Additionally, the effect of the selection of the material, and consequent effect on the contact angle, on the minimum wetting rate and film thicknesses is presented under the working conditions of the absorption technologies. From the wettability point of view, the results show that using stainless-steel and aluminum leads to a slightly better performance than a copper made heat exchanger.

Falling film on a vertical flat plate – Influence of liquid distribution and fluid properties on wetting behavior

Complete and even wetting of the surface with a thin liquid film is essential in falling film units for heat or mass transfer. This work presents the influence of different liquid distribution systems and physical properties of the working fluid on the wetting behavior on a vertical flat plate. The wetting behavior is characterized by the film thickness, wetted coverage and minimum wetting rate. These parameters were experimentally determined for film Reynolds numbers of 0.5≤ReF≤460 with the fluorescence intensity technique. Two distributor hole arrangements with similar cross-sectional areas were compared. Furthermore, the wetting behavior of water and water/glycerol mixtures with Kapitsa numbers of 1.6·10−11≤Ka≤3.9·10−6 was investigated. The minimum wetting rate differs in wetting and dewetting experiments and with physical properties of the working fluid. It is compared with the prediction of different literature theories. A general equation with an uncertainty of ±10% capable to describe the normalized mean film thickness of all measured liquids in dependency of film Reynolds number is presented.

Experimental study of the flow characteristics in a falling film liquid desiccant dehumidifier

The flow characteristics play great role in determining the heat and mass transfer performance of a falling film liquid desiccant dehumidifier. Even though there have been many studies reporting the flow behaviors in different devices and fields, the fundamental researches of the film flow characteristics in a liquid desiccant dehumidifier were still very limited, especially for experiment. Therefore, in the present work, a simple single-channel test rig was established for observing and describing the flow characteristics in the dehumidifier. The impacts of various variables on the flow morphology, coverage ratio, and minimum wetting rate have been analyzed in detail. A correlation was developed to predict the minimum wetting rate of the solution flow. The research results can further verify the correctness of a previous simulation work as well.

Three-dimensional simulation on behavior of water film flow with and without shear stress on water–air interface

In this paper, the simulations of falling film behavior on a flat plate with and without interfacial gas–liquid shear stress were carried out. A three-dimensional numerical model was established based on film flow characteristics. A source term was implanted into the numerical model to take into account the interfacial gas–liquid shear stress. The model was validated by the experimental data. Both continuous film flow and film breakup were simulated. The film thickness, velocity distribution and wall shear stress at different Reynolds numbers were presented to understand the film flow behavior comprehensively. The influence of water–air shear stress on film flow behavior was revealed. A reasonable prediction on both MWR (Minimum Wetting Rate: Δmin) and MLFT (Minimum Liquid Film Thickness: Γmin) were obtained. The model proposed in this study ought to be profitable for studies on mechanism of film breakup.

Flow patterns and draining films created by horizontal and inclined coherent water jets impinging on vertical walls

The flow patterns created by coherent water jets created by solid stream nozzles impinging on vertical polymethylmethacrylate (Perspex) and glass surfaces were studied for nozzles with diameters 2–4mm at angles up to ±45° from the horizontal. The flow rates studied ranged from 7.1 to 133gs−1 (26–480Lh−1; jet velocities 2.6–10.6ms−1). The width and height of the film jump marking the limit of the radial flow zone were compared with models based on that developed by Wilson et al. (2011), modified to include the effect of gravity and the angle of inclination for non-horizontal jets (incorporating the flow distribution model reported by Kate et al. (2007. Journal of Fluid Mechanics 573, 247–263)). The location of the film jump and the flow pattern around the impingement point were sensitive to the nature of the substrate at low flow rates, but insensitive to substrate nature at higher flow rates. The models predicted the film jump location with reasonable accuracy, and the width of the wetted region at the mid-plane was found to follow a simple relationship to the film jump width there. A first-order model for the width of the rope of liquid draining around the film jump gave a lower bound estimate of this dimension. The falling film generated below the impingement point exhibited three forms of behaviour: a wide film, termed gravity flow; a narrowing film, termed rivulet flow, and a wide film which split into two with the formation of a dry patch. The transition to form a dry patch was found to obey the minimum wetting rate criterion reported by Hartley and Murgatroyd (1964), once loss of liquid due to splashback was accounted for. Dry patch formation within the falling film was only observed with upwardly impinging jets, and the tendency to form dry patches was predicted with some success by a simple two-stream model.

Heat Transfer Characteristics of Falling Film Process on Coated Division Tubes: Effect of the Surface Configurations

In general, mixing of solution could refresh the interface of bulk liquid, and hence lead to the more efficient heat transfer within the liquid film. By using the high speed video camera and thermal imaging technique, the flow patterns and heat transfer characteristics of liquid film on coated division tubes, on which the hydrophobically coated regions and the bare regions presented alternately, were investigated experimentally. Flow visualization revealed that the minimum wetting rate increases with the increase in the effective contact angle of this surface. The cumulative liquid ring was observed at the boundary of bare region and coated region. The fluctuations of liquid film temperature were measured at different liquid film mass flow rates and the initial fluid temperatures. The temperature fluctuation periods on the coated division tube increased with the decrease of flow rate and initial liquid temperature. The liquid film surface temperature on the coated division tubes was higher than that on th...

Falling liquid film stability in microgas/liquid absorption

Increasing mass transfer efficiencies is reached in G/L absorption contactors thanks to stabilization of the liquid flow. Hydrodynamic studies about the stability of a liquid film thickness are carried out to explain variations of mass transfer efficiencies during absorption experiments. These studies are based on the analytical expression developed by Hobler (1964) [1] which enables to predict the minimum wetting rate (MWR) and the minimum liquid film thickness (MLFT). Experiments were carried out to validate the prediction of the hydrodynamic model for a specific chemical system (absorption of tetrachloroethylene in di-ethyl-hexyl adipate). Therefore, two microstructured plates have been investigated in comparison to an unstructured plate. The plates have been embedded in a miniaturized falling film absorber. A strong influence of the geometry on the mass transfer efficiency is found, especially at low liquid flow rates where dry zones decrease the interfacial specific area which differs from the specific surface of the solid support.

Minimum Wetting and Distribution Rates in Falling Film Evaporators

Falling film evaporators are used extensively in the food industry for their ability to process heat sensitive liquids. A coherent liquid film is required to maintain heat transfer efficiency and minimize fouling. It is likely that most evaporator fouling occurs after film breakdown as the substance within the evaporator dries out. The minimum flow rate required to maintain a film is known as the minimum wetting rate which is defined as the minimum mass flow rate per unit circumference. In this work, minimum wetting rates were determined in a 1 m long, 48 mm internal diameter, vertical, stainless steel tube. Water and aqueous solutions of glycerol, alcohol and calcium chloride were used. These substances were chosen so as to give a wide range of properties such as viscosity (0.5-39 mPa s), density (950-1410 kg m–3), surface tension (35-90 mNm–1) and contact angle (64-98°). In a separate set of experiments, the minimum flow rate required to distribute liquid and completely wet the top of industrial evaporator tubes was measured using a range of sucrose solutions. The tube wetting results obtained fitted a dimensionless power law relationship well. Surface tension and contact angle had a strong influence on the wetting rate but viscosity and density were found to have very little effect. The minimum flow rates for distribution were found to nearly always exceed the minimum wetting rates showing that more attention needs to be given to distributor design.

Minimum Wetting Rates for Falling Films on Stainless Steel

AbstractThe minimum wetting rate is defined as the minimum mass flowrate per unit circumference that is required to maintain of complete falling film of liquid on a surface. In this work minimum wetting rates in a 48 mm i.d. stainless steel tube were determined jor water, 50% sucrose solution, and reconstituted skim milk under isothermal and heat transfer conditions. It was found that initial distribution of the liquid at the top of the tube was critically important. The minimum wetting rates under isothermal and heat transfer conditions ranged from 0.22 kg m−1 s−1 for 50% sucrose at 20% to 0.12 kg m−1 s−1 for water az 70°C. Nearly all rates decreased with temperature. Equations in the literature did not accurately predict the results obtained.

Minimum thickness of a flowing down liquid film on a vertical surface

The minimum total energy (MTE) criteria for determining the minimum wetting rate and the minimum thickness of an isothermal, thin liquid film flowing down a vertical adiabatic surface are examined. An analytical expression of the profile of a stable liquid rivulet and of the two-dimensional velocity distribution in the rivulet developed using the Ritz method are incorporated into the MTE criterion to improve its predictions of the minimum wetting rate (MWR) and the corresponding minimum liquid film thickness (MLFT). The present predictions of the MWR and MLFT are in good agreement, to within ±10–20%, with experimental data for water and glycerol–water mixtures and with the values calculated from the MWR measurements, respectively. The accuracy of the present predictions is demonstrated for three conditions, namely: (a) incipient breakup of liquid film, (b) formation of a stable dry patch, and (c) rewetting of a dry surface. For the latter the advancing contact angle is substituted for the equilibrium contact angle in the developed analytical expressions for the MLFT and corresponding MWR.

Flow characterization in a gas-liquid column: application to a bioscrubber for the deodorization of waste gases

The flow pattern of a countercurrent gas-liquid packed column is characterized for gas and liquid phases. Three packing materials are used (10 mm ceramic Raschig rings, 16 mm polypropylene Pall rings and 45 mm×18 mm polypropylene Spiralpac) and various operating conditions are applied. A first approach consists in the study of pressure drops. On the one hand, this study enables to determine, using the Ergun relation, the representative parameters of the three packing materials towards the flow. On the other hand, a study in a three-phase system is performed to establish the limit conditions using the Eckert model (flooding point, conditions of minimum wetting rate). An investigation of residence time distribution allows the hydrodynamic parameters to be obtained for each phase. A dispersion model represents the gas flow. The tanks-in-series with mass exchange model is retained for the washing solution flow representation. The influence of operating conditions on hydrodynamic parameters is evaluated. The determination of these parameters is of great interest to scale up and optimize the biological system of deodorization.

2436 R-113 垂直管外等温流下液膜 : 最小濡れ膜流量と液膜上波の特性について

The minimum wetting rate (MWR) of the isothermal R-113 falling film was examined. The amplitude, frequency, velocity and wave length on the film were also investigated. The MWR was not obtained even at extremely small flow rate of 0.38cc/min (Re_f=0.70). That flow rate was much smaller than the value expected for the smooth surface film although the surface observed was flat. In Re_f<&acd;300,waves were symmetrical between the front and the back side and behaved like theoretical small-perturbation waves which was postulated on the smooth-laminar film. When Re_f>&acd;300,the waves grew as proceeding to downstream, began to behave like roll waves and the characteristics deviated from those of the small perturbation waves.

A Study on the Minimum Wetting Rate of Isothermal Films Flowing down on Outer Surface of Vertical Pipes.

The minimum wetting rate (MWR) was investigated experimentally with an water film flowing down isothermally on the outer surface of test pipes arranged vertically. A dry patch was generated by blowing a small air jet onto the film temporary, and then observation was made to discriminate whether the dry patch was rewetted or not. The contact angle of the film at the top edge of the dry patch and the amplitude, length and velocity of large waves on the film were measured. The MWR decreased rapidly as the film flowed down and reached a nearly constant value at a position around 0.6 m down from the film inlet. The contact angle at the top edge of the dry patch varied periodically in a range synchronizing with the arrival of the waves. When the contact angle exceeded the maximum advancing contact angle, the rewetting of the dry patch was initiated. The existing correlations where the smooth surface film was assumed considerably over-predicted the MWR. The MWR was properly given by supposing that the dry patch is rewetted when the maximum of the fluctuating dynamic pressure of the film exceeds the upward component of the surface tension corresponding to the maximum advancing contact angle at the top of the dry patch.

Capillary wetting rates in nylon fibrous assemblies

The capillary wetting rates in nylon fibrous assemblies were evaluated from the change in electrical conductivity due to wetting. The wetting liquids used were aqueous potassium chloride solutions of various pHs, in both the absence and the presence of ionic surfactants. Sodium dodecyl sulfate and dodecyltrimethylammonium chloride were chosen as surfactants. The contact angles of the wetting liquids on the nylon fiber were measured by the Wilhelmy method and the isoelectric point of the fiber was determined from electrokinetic data. The minimum wetting rate and the maximum contact angle were observed near pH 4, which coincided with the isoelectric point of the nylon fiber. In the presence of surfactant ions, the minimum wetting rate was also obtained at the concentration where the surface charge of nylon was neutralized. These experimental findings indicate that the capillary wetting rates in nylon fibrous assemblies are influenced by the electrical phenomena at the nylon/ solution interfaces via the contact angle and the structure of the adsorbed surfactant layers.

A semi-theoretical model of stability of vertical falling liquid films

A semi-theoretical model of minimum wetting rates for gravity-driven liquid films under isothermal conditions is presented. It is based on the experimental observations that disturbed liquid layers break up more easily than smooth ones and that the contact angle is a variable quantity. The theory comprises the main features of the three classical models used so far: flow distortion, force balance considerations, minimum-energy requirement. Some experimental evidence supports the main ideas of the model. There is a reasonably good agreement with respect to the boundaries of the transition region.

Dehumidification of Air by Aqueous Lithium Chloride in a Packed Column

Abstract A packed bed absorber-stripper system has been designed to dehumidify moist air by contact with aqueous solutions of lithium chloride. The packing material used in the study is 1.6 cm (5/8 inch) polypropylene Flexi rings, which have a surface to volume ratio of 342 m2/m3 (104 ft2/ft3). The absorber is capable of handling air face velocities from 3362.4 to 6746.4 kg/m2h and liquid flow rates from 2534.1 to 54648 kg/m2h. Solutions of 30% and 40% lithium chloride in water were employed as the dehumidifying agent. The minimum liquid flow rate calculated from the equilibrium data would be too low to wet the packing surface completely, and could not be used in the actual operating system. Therefore, liquid flow rates greater than the minimum wetting rate for the packing were used. Measured flooding conditions corresponded closely with exiting empirical correlations. Mass transfer coefficients ranged from 0.062 kmol/m3s at 40% flooding to 0.166 kmol/m3s at 80% flooding for the 40% lithium chloride solution. The height of a transfer unit calculated from the experimental data ranged from 0.340 m at a column efficiency of 71.6% to 0.617 m at a column efficiency of 50%.