Presence Of Liquid Film Research Articles

Experimental study of the unconstrained melting of a phase change material for air-conditioning applications

This study analyzed the RT8HC melting in a spherical vessel employing air as a heat transfer fluid. The phase change process was followed both visually and by thermal measurements. Photographs and thermographs allow the following phases' evolution and melting mechanisms. We observed that the melting process was not symmetric around the vertical axis; implications of non-symmetric conditions on the melting mechanisms and heat transfer are discussed and analyzed. This work also includes the effects of outer water vapor condensation on heat transfer in the analyses. We proposed a mathematical model to quantify the heat flux that only requires humidity and temperature measurements and considers both heat convection and condensation effects. Results show that condensation significantly affects heat transfer during the initial stage of the melting process. Beside, these effects extend beyond the condensation time due to the liquid film draining from the spherical vessel surface. Regarding thermography, this technique may be a valuable tool for analyzing and following melting processes inside vessels; however, the presence of liquid film over the sphere affects the temperature measurements obtained from the thermograph analysis. This investigation provides relevant information about using phase change materials in cold thermal energy storage applications, latent thermal loads, and humidity control, which are essential aspects of air-conditioning system designs.

Cellular foam-based trickle-bed DBD reactor for plasma-assisted degradation of tetracycline hydrochloride

Non thermal plasma (NTP) is a promising technology for degrading organic pollutants in water/wastewater, in which the transfer of energetic species at the interface between gas discharge and liquid phase is key to improve degradation efficiency. Herein, this work shows the development of an integrated system of dielectric barrier discharge (DBD) plasma and open-cell ceramic foam (CF) for the degradation of tetracycline hydrochloride (TCH, a model antibiotic) in liquid film. Specifically, a bespoke trickle-bed DBD reactor with liquid distributor was developed to enable the formation of uniform liquid film on the surface of the hydrophilic CF strut. Due to the improved mass transfer across the thin liquid film, the trickle-bed DBD reactor with the hydrophilic CF exhibited the comparatively highest TCH removal efficiency (>80%) and energy efficiency (viz., EETCH removal of ∼0.6 g kWh−1) among the systems under investigation such as the glass bead packed DBD. The findings showed that the presence of liquid film was beneficial to the propagation of homogeneous plasma discharge in the CF bed and promoted the mass transfer of active species from plasma discharge to liquid, and thus improved the TCH degradation efficiency. Results from quenching experiments suggested that the electron induced active species (especially O2−) played the important role in degrading TCH rather than electrons.

Three-dimensional pore-scale observation of drying process of porous media

X-ray micro-tomography was used to comprehensively study the pore-scale drying process and characterize the liquid film region and the invasion front. Our finding reveals that the decrease in saturation is not uniform across the pore size. The capillary rearrangement, which refers to the restructuring of the liquid phase due to the capillary pressure difference, drives the liquid to flow from large pores to small pores. As a result, the decrease in liquid saturation occurs preferentially within the larger pores. A sharp decrease in liquid saturation occurred at the invasion front, where saturation ranged from 1 to around 0.15(0.15<S<1). The obtained invasion width σp that characterized the extent of roughness and disorder of the invasion front obeyed the scaling law of the percolation model. Therefore, this region is significantly limited by the interaction between capillary and gravity. However, with more multi-pore connected clusters observed within the porous medium, the critical exponent τ of the cumulative cluster size distribution CDF(s) obtained from the cluster region (0<S<0.15) deviates from the universal power-law behavior. This result indicates the presence of liquid film within the cluster region, where the viscous force becomes comparable to the capillary force. We also found that the total gas-liquid-specific interfacial area has no effect on the drying rate. This result provides new evidence that the liquid film region is vapor saturated.

Review of Laser Powder Bed Fusion of Gamma-Prime-Strengthened Nickel-Based Superalloys

This paper reviews state of the art laser powder bed fusion (L-PBF) manufacturing of γ′ nickel-based superalloys. L-PBF resembles welding; therefore, weld-cracking mechanisms, such as solidification, liquation, strain age, and ductility-dip cracking, may occur during L-PBF manufacturing. Spherical pores and lack-of-fusion voids are other defects that may occur in γ′-strengthened nickel-based superalloys manufactured with L-PBF. There is a correlation between defect formation and the process parameters used in the L-PBF process. Prerequisites for solidification cracking include nonequilibrium solidification due to segregating elements, the presence of liquid film between cells, a wide critical temperature range, and the presence of thermal or residual stress. These prerequisites are present in L-PBF processes. The phases found in L-PBF-manufactured γ′-strengthened superalloys closely resemble those of the equivalent cast materials, where γ, γ′, and γ/γ′ eutectic and carbides are typically present in the microstructure. Additionally, the sizes of the γ′ particles are small in as-built L-PBF materials because of the high cooling rate. Furthermore, the creep performance of L-PBF-manufactured materials is inferior to that of cast material because of the presence of defects and the small grain size in the L-PBF materials; however, some vertically built L-PBF materials have demonstrated creep properties that are close to those of cast materials.

A numerical analysis of air entrapment during droplet impact on an immiscible liquid film

The air entrapment during droplet impingement is responsible for spontaneous droplet bouncing on an arbitrary solid surface at low Weber numbers. However, for the impact on liquid film surfaces, the outcome would significantly change, making it more favorable for the fabrication of non-wetting lubricant impregnated surfaces (LIS/SLIPS). In this paper, we describe a problem associated with the impact on a liquid surface using a three-phase flow model that captures the details of the gas layer thickness and dynamics of fluid motions. The numerical model was based on the finite volume solution coupled with the volume of fluid method to track the phases. The model was validated with the analytical solution. Consequently, the numerical tool was utilized to investigate the thickness of the entrapped air during the impact process while the behavior of droplet and the immiscible liquid film was quantitatively measured. The morphology of the interfacial gas layer was analyzed for key parameters including impact velocity and film thickness. It was observed that the presence of liquid film can reduce the probability of rupturing the gas layer. The results for the profile of liquid film during droplet impact illustrated that the effect of film thickness can considerably influence the bouncing behavior.

Evaluation method of transit time difference for clamp-on ultrasonic flowmeters in two-phase flows

To achieve efficient energy management in industrial plants, it is crucial to measure the steam flow rates at the various points of consumption. Clamp-on time-of-flight ultrasonic flowmeters are useful devices to measure the steam flow rates in existing pipes. However, it is difficult to accurately measure the steam flow rates because of the large acoustic impedance difference between the pipe material and fluid, strong signal attenuation in the fluid, and high temperature. In addition, the steam wetness increases with heat losses and the presence of liquid film and droplets that creates noise in the detected ultrasonic signals, making it difficult to distinguish the target signals from the noisy ones. Hence, a new signal processing method is proposed to determine the transit time difference, particularly at lower signal-to-noise ratios. Two ultrasonic transducers were used to measure the ultrasonic time-of-flight, which varies depending on the flow rate. The transmitted ultrasonic signals were time-dependent due to the generation of guided waves in the pipe wall. The standard deviations of the target signals increased when the flow regime transitioned from stratified to annular mist flow. The guided waves significantly influence the success ratio of determining the transit time difference between the ultrasonic signals transmitted from the upstream and downstream transducers. Based on the results, the use of the standard deviation of the target signals is proposed for estimating the transit time difference. Further, as the standard deviation varied significantly depending on the flow regime, it can be used to identify the flow regime as well.

Pressure drop of two-phase liquid-liquid slug flow in square microchannels

Networks of microdevices can be used to form, sort, split and recombine droplets by varying the fluidic resistance of the channels. To facilitate design of such systems we propose two models to calculate pressure drop of liquid-liquid slug flow in square microchannels. By approximating a droplet surrounded by liquid film as annular flow, the moving film model includes the velocity profile in the lubricating layer. In the no-film model, the presence of liquid film is neglected. Pressure drop measurements of water-toluene and water-silicone oil slug flow generated in glass/silicon devices of 200 and 400 μm width serve to validate these two models. We found that the droplet viscosity influences the applicability of the models. Good agreement is obtained for water-toluene slug flow, with the average error of 15–20% between the measured and calculated values for both models, proving that the presence of the lubricating film may be neglected for flows with similar viscosities of both phases. On the contrary, both predictions agree poorly with experiments for silicon oil-water flow which we attribute to the lower than expected liquid film thickness around the silicon oil droplets. Therefore we use the moving film model to estimate the thickness of the lubricating layer which we find to lie between 0.5% and 0.75% of the channel width, lower than the assumed 2%. Finally we compare the performance of our models against two correlations adapted from literature. We thus demonstrate that our approach allows reliable pressure drop prediction for liquids with similar viscosities and delivers important information on flow hydrodynamics.

Heat transfer and pressure drop of condensation from superheated vapor to subcooled liquid

The focus of this paper is heat transfer and pressure drop during condensation in a horizontal smooth round tube. R134a in 6.1mm inner diameter is used as an example for heat transfer measurements operated in a range of mass fluxes from 50kgm−2s−1 to 200kgm−2s−1and heat fluxes from 5kWm−2 to 15kWm−2. The paper also presents pressure drop measurements with mass flux from 100kgm−2s−1 to 200kgm−2s−1, showing the effect of mass flux and heat flux on the heat transfer coefficient (HTC) and pressure drop. All the measurements are taken at a constant pressure of 1.319MPa, which corresponds to a saturation temperature of 50°C. By connecting heat transfer results with flow characterizations, the behavior of HTC is explained from the perspectives of both mathematics and physics. The result shows that for fixed heat flux and different mass flux, even though the liquid film for higher mass flux is much thinner in the condensing superheated (CSH) region, the HTC is not affected by mass flux. In the two-phase (TP) region, however, higher mass flux clearly yields higher HTC. Opposite behavior is found when heat flux is varied and mass flux is fixed. In the CSH region, HTC increases with heat flux, while in TP region, HTC does not change with heat flux. For both cases, a peak of HTC is presented at quality one, which seems to imply some counter-acting factors that always put heat transfer largest at quality one. The counter-acting factors, however, should not exist based on flow characterization. After mathematically explaining the behavior of HTC in CSH region, film HTC (HTCf) is proposed and ready to serve as a tool in a unified heat transfer model throughout the entire CSH and TP regions. The result of film HTC shows that the film HTC goes to infinite at the onset of condensation, which is physically correct. Also, the film HTC increases with increasing mass flux. The effect of heat flux, or wall temperature on film HTC is still under discussion. In this study, higher heat flux yields lower film HTC only at high qualities, which is due to the difference in void fraction. In addition, the result of pressure drop measurement also shows that the presence of liquid film tends to increase the pressure drop, which suggests that the wavy structure of vapor–liquid interface increases the friction of the flow. The peak of the pressure drop, however, occurs at lower enthalpy for higher mass flux. The reason could be the later onset of condensation and thinner film, which delays the peak of vapor–liquid interaction at higher mass flux.

Cooling of very hot vertical tubes by falling liquid film in presence of countercurrent flow of rising gases

An experimental investigation of cooling a very hot vertical tube by sudden introduction of a falling liquid film in the presence of a countercurrent flow of rising hot gases air is presented. Experiments were carried out for different rising air flow rates, flow rates of falling liquid film, initial tube temperatures and subcooling of the liquid film. Experiments showed that vapor generated during quenching of the tube can produce a countercurrent vapor velocity which exceeds the onset of flooding limit and any addition of rising air can move the situation to be more closer to zero liquid penetration limit. The results showed that the rewetting velocity (velocity of axial rewetting of the tube hot surface with the falling liquid film) increases with the decrease of initial tube temperature and decreases with the increase of air flow rate until zero quenching rate was obtained. However, the rewetting velocity slightly increased with the increase of the liquid film flow rate and liquid subcooling in case of rewetting without rising air, the presence of rising air finishes the effect of inlet liquid film flow rate and liquid subcooling on rewetting velocity. Air flow rate at which the tube cannot be totally rewetted was determined and compared with that obtained during adiabatic flooding test for the same test section and test conditions. Results were compared with previous ones and good agreement was found.

Capillary torque on a rolling particle in the presence of a liquid film at small capillary numbers

A theoretical analysis was developed for the capillary torque acting on a spherical particle rolling on a flat surface in the presence of a thin liquid film. The capillary number (the ratio of viscous force to surface tension force) is assumed to be sufficiently small that the liquid bridge has a circular cross-section. The theory identifies two mechanisms for capillary torque. The first mechanism results from the rearward shift of the liquid bridge in the presence of particle rolling, which causes the line of action of the pressure force within the liquid bridge to be located behind the particle centroid, inducing a torque that resists particle rolling. The second mechanism results from the contact angle asymmetry on the advancing and receding sides of the rolling particle, which leads to a net torque on the particle arising from the tangential component of the surface tension force. Estimates for these two types of capillary torque are obtained using experimental data, and correlations for both torques are obtained in the form of power-law fits as functions of the capillary number. When combined with a standard expression for viscous torque on a rolling particle, the capillary torque expressions are found to yield predictions for particle terminal velocity that are in good agreement with experimental data for a particle rolling down an inclined surface.

G-1-2 ずり変形下のメニスカスにおける界面すべりとその摩擦力(G-1 マイクロナノ理工学,口頭発表:マイクロナノ理工学)

Recent advances in ultra-fine processing technologies are remarkable. Such as micro machine or magnetic storage device, devices can be smaller and the performance is far advanced. Frictional, capillary and viscous forces are among the most influencing factors in this field of application. In the presence of liquid film, droplet or condensed water from humid air on a surface, a liquid meniscus bridge can be formed between two bodies. The liquid bridge causes a strong interaction heavily influences the operation of micro/nano devices. In this study, we focused on the lateral component of capillarity. The deformation process of the bridge was observed experimentally. And the lateral component of capillary force acting on a body was measured as frictional force. Results shows that an interfacial slip have an important role on the lateral component of capillary force.

Mid-infrared absorption measurements of liquid hydrocarbon fuels near [formula omitted

Quantitative absorption spectra for several hydrocarbon fuels in the liquid phase at 27 ∘ C are presented. Measurements of toluene, n-dodecane, n-decane, and three samples of gasoline were made over the spectral region 2700–3200 cm - 1 to support the development of mid-infrared laser-absorption diagnostics for measurements of fuel vapor in the presence of liquid films and aerosols. A procedure for quantitative Fourier transform infrared (FTIR) absorption measurements of strongly absorbing liquids is described and the resulting absorption spectra are compared with previously measured absorption spectra in the vapor phase. The measured absorption spectra for liquid gasoline are shown to scale with the volume percent of olefin, alkane, and aromatic hydrocarbons in each sample. Finally, the observed frequency shift of ∼ 8 cm - 1 in the spectra of vapor and liquid hydrocarbons is discussed, including the potential for measurements of fuel vapor in the presence of liquid films.

Droplet entrainment correlation in vertical upward co-current annular two-phase flow

Upward annular two-phase flow in a vertical tube is characterized by the presence of liquid film on the tube wall and entrained droplet laden gas phase flowing through the tube core. Entrainment fraction in annular flow is defined as a fraction of the total liquid flow flowing in the form of droplets through the central gas core. Its prediction is important for the estimation of pressure drop and dryout in annular flow. In the following study, measurements of entrainment fraction have been obtained in vertical upward co-current air–water annular flow covering wide ranges of pressure and flow conditions. Comparison of the experimental data with the existing entrainment fraction prediction correlations revealed their inadequacies in simulating the trends observed under high flow and high pressure conditions. Furthermore, several correlations available in the literature are implicit and require iterative calculations. Analysis of the experimental data showed that the non-dimensional numbers, Weber number ( We = ρ g〈 j g〉 2 D/ σ(Δ ρ/ ρ g) 1/4) and liquid phase Reynolds number ( Re f = ρ f〈 j f〉 D/ μ f), successfully collapse the data. In view of this, simple, explicit correlation was developed based on these non-dimensional numbers for the prediction of entrainment fraction. The new correlation successfully predicted the trends under the high flow and high pressure conditions observed in the current experimental data and the data available in open literature. However, in order to use the proposed correlation it is necessary to predict the maximum possible entrainment fraction (or limiting entrainment fraction). In the current analysis, an experimental data based correlation was used for this purpose. However, a better model or correlation is necessary for the maximum possible entrainment fraction. A theoretical discussion on the mechanism and modeling of the maximum possible entrainment fraction condition is presented.

Hot ductility of an Fe‐10%Ni alloy during penetration of copper

Today's available theories for hot crack formation are based on the fact that hot cracks are formed in the presence of liquid films in the interdendritic areas or at the grain boundaries, which are exposed to tensile stress. Copper is well known to cause hot shortness in steels. In order to study how the liquid embrittles the material, high‐temperature tensile tests were performed at two strain rates during the penetration of liquid copper into Fe‐10%Ni. The penetration distance was measured in samples that were exposed to strain without fracturing. The ductility (area reduction), strain and ultimate tensile stress were determined. Microprobe analysis was performed on the fractured samples. The transition temperature of ductility was found at 1400–1450°C without copper penetration whereas it occurred at 1025–1078°C during the penetration of copper, i.e. copper starts to embrittle at a temperature below its melting point. The microprobe measurements showed that the diffusion rate of copper into Fe‐10%Ni was enhanced when the lattice was strained. The results are discussed in terms of a new theory concerning vacancy formation and condensation as the dominating mechanism for hot crack formation during solidification.

Rheological behavior of the mushy zone at small strains

The modeling of the hot-tearing phenomena at the end of solidification requires the development of specific constitutive equations for the mushy zone. The presence of liquid films in the solid-liquid mixture has important consequences on the rheological behavior of the mushy zone. This aspect is particularly treated in order to propose a constitutive equation for the mushy zone in the low strain region. Starting from the behavior of the fully solid material, the mush is modeled as a porous medium saturated with liquid with partial cohesion. The cohesion of the mush is treated as an internal variable for which an evolution equation is proposed. Experimental data on a Sn-Pb model alloy both in the fully solid state and in the mushy state are used in order to study the advantages and the limitations of the proposed constitutive equation.

Effect of airborne particle on SO 2–calcite reaction

In modern urban atmosphere, sulphur dioxide (SO 2) attacks calcite (CaCO 3) in calcareous stone-producing gypsum (CaSO 4·2H 2O) which forms crust at rain sheltered surfaces and accelerates erosion at areas exposed to rain. The airborne particles collected on stone surfaces have always been considered to enhance the gypsum crust formation and thus it is believed that they should be removed from the surface to decrease the effects of SO 2. In this study, our aim was to investigate this event by carrying out a series of experiments in laboratory using pure calcium carbonate powder to represent calcareous stone. Sodium montmorillonite, activated carbon, ferric oxide, vanadium pentoxide and cupric chloride were mixed in the pure calcium carbonate powder as substitutes of the airborne particles in the polluted atmosphere. The samples have been exposed at nearly 10 ppmv SO 2 concentrations at 90% relative humidity conditions in a reaction chamber for several days. The mineralogical composition of the exposed samples were determined by X-ray diffraction (XRD) analysis and infrared spectrometer (IR). Sulphation reaction products, calcium sulphite hemihydrate, gypsum and unreacted calcite, were determined quantitatively using IR. Exposed samples have also been investigated morphologically using a scanning electron microscope (SEM). Experimental results reveal that calcium sulphite hemihydrate is the main reaction product of the SO 2–calcite reaction. It turns out that airborne particles play an important catalytic role in the oxidation of calcium sulphite hemihydrate into gypsum, although their presence does not very significantly affect the extent of sulphation reaction. This behaviour of airborne particles is explained by the presence of liquid film on the calcium carbonate surface where a series of reactions in the gas–liquid–solid interfaces takes place.

Lubricant Effect on Noncontact-Mode Atomic Force Microscopy Images of Hard-Disk Surfaces

In this paper we present our results of imaging ultrathin liquid films of perfluoropolyether (PFPE) lubricants on a hard-disk surface using the noncontact mode of atomic force microscopy (AFM). The presence of liquid films on the disk surface is reflected by features resulting from capillary interactions occurring when the AFM tip comes into contact with the lube layer. With the continually increasing tip−sample separation, the features of the capillary forces disappeared gradually and finally the images show a similar topography of the disk surfaces. The results demonstrate that the lubricants spread over the whole surface and coated surface grains of the carbon overcoats smoothly. More lubricants pool in the grain boundaries. The thickness is estimated in the range of 20−30 Å.

MODELING OF DRYING IN CAPILLARY-POROUS MEDIA: A DISCRETE APPROACH

ABSTRACT Drying experiments are carried out in two-dimensional etched networks under quasi-isothermal conditions. The evolution of phase distributions within the network are visualized and compared to numerical discrete simulations. The phase evolution determined by numerical simulations agrees very well with experimental results. However, the comparison between numerical and experimental drying kinetics does not show good agreement. The differences are explained by the presence of liquid films on the micromodel walls and edges during drying. These films are not taken into account in the numerical model.

A Meniscus Model for Optimization of Texturing and Liquid Lubrication of Magnetic Thin-Film Rigid Disks

A meniscus model to predict the effect of disk texture and liquid films on stiction at a head-disk interface is developed using a newly derived equation for meniscus force in the presence of a liquid film. Meniscus contribution to the stiction in the presence of liquid films is calculated for multiple asperity contacts. The meniscus force and the resulting stiction force are calculated as a function of mean peak radius (RP), standard deviation of peak heights (σP), total number of peaks (NO), and liquid film thickness (h). In addition, critical ranges for σP, RP, NO and h, outside which severe stiction and/or plastic deformation may result, are obtained. A limiting case of N identical asperities with a constant spherical radius R and a constant height is also considered. Based on the analysis, the design curves are presented which can be used in selecting the optimum parameters, such as σP,RP, and NO as a function of h, to prevent severe stiction and plastic deformation.

Transition from Boundary Lubrication to Hydrodynamic Lubrication of Slider Bearings

The transition from boundary lubrication to fully hydrodynamic lubrication is investigated for air-lubricated slider bearings using the electrical resistance method. Intermittent contacts are shown to exist even under conditions for which the numerical solution of the Reynolds equation or white light interferometry predicts steady state spacings in the spacing region from 0.125 to 0.25 µm. The transition is similar to the one found in the presence of liquid films, being influenced for a given surface roughness of disk and slider by load, speed, and hydrodynamic design.