Mean Film Thickness Research Articles

Experimental investigations of the circumferential liquid film distribution of air-water annular two-phase flow in a horizontal pipe

The circumferential film thickness distribution in an air-water horizontal annular two-phase flow was investigated in 26mm pipe diameter by using the conductance probe. The superficial gas and liquid velocities were 10–40m/s and 0.025–0.4m/s, respectively.In general, the film thickness decreases as the liquid velocity decreases and the gas velocity increases. For low superficial gas velocity, high asymmetry of liquid film thickness distribution is generally found. In addition, the liquid film at the upper half of the pipe mostly consists of surge of waves with frequency lower than that of the bottom. At the lowest superficial liquid velocity of the present experimental range, the mean film thickness at the bottom ranges from 0.24mm to 1.3mm. Meanwhile, at the highest superficial liquid velocity, it ranges from 1.0 to 4.29mm, depending on the superficial gas velocity. At the higher superficial gas velocity, a more uniform of liquid film thickness distribution is commonly found. Under this flow condition, the wave at the upper has a similar frequency to that of the lower half and a coherent liquid wave is normally found.To predict the film thickness at any circumferential position, an empirical correlation is proposed on the basis of the physics of the phenomena. In comparison to the available experimental data, the correlation has a good capability to predict the circumferential liquid film thickness distribution.

Experimental Study of Aqueous Binary Mixture Droplet Vaporization on Nanostructured Surfaces

ABSTRACTIn this study, heat transfer due to vaporization is investigated for low concentration binary mixtures of 2-propanol/water on nanostructured surfaces. The surfaces comprise zinc oxide (ZnO) nanocrystals grown by hydrothermal synthesis on a smooth copper substrate having an average roughness of 0.06 µm. Three nanostructured surfaces used in this study differ only in the duration of the hydrothermal synthesis consisting of 4, 10, and 24 hours of surface growth. Surface geometries were observed to be a function of hydrothermal synthesis time with an increase in area coverage, length, and diameter of nanocrystals with increased synthesis time. ZnO nanocrystals exhibited a mean diameter of 500–700 nm, a mean length of 1.7–3.3 µm, and porosities of 0.04–0.58. Individual droplets between 2.5 and 3.9 mm in diameter consisting of a binary mixture of 2-propanol/water with concentration of either 0.01 M or 0.03 M were deposited at a minimum distance above the surface, equivalent to the droplet radius plus a very small fraction of the radius such that the bottom surface is slightly above the surface without touching it. Droplets detached from the tip of a hypodermic needle from the minimum distance due to their own weight onto a nanostructured surface at temperatures between 110 and 140°C. High speed video was used to record the deposition and vaporization process and through image analysis it was possible to measure heat transfer coefficients based on the wetted area, as well as other parameters. Through the video analysis it was observed that droplets which are approximately spherical, impinged gently on the surface at nearly zero speed and spread into a thin film with mean film thickness between 65 and 400 µm which then evaporated by film evaporation without nucleate boiling. Wettability of each of the surfaces was characterized through contact angle measurements from photographs of the droplet profile when the droplet profile was discernible. When profiles were not discernible due to hydrophilicity of some surfaces, contact angles were calculated by utilizing droplet volume and spread area. Contact angle measurements were performed on the surfaces before and after each experiment in order to document changes in wettability as a result of experimentation. Results from this experiment are compared to water droplet vaporization results from a previous experiment in order to determine whether 2-propanol enhances the heat transfer and found that the heat transfer coefficient was increased by up to 128% in some cases. Heat transfer enhancement was found to be a function of droplet diameter as well as mixture concentration with 3.9 mm 0.01 M 2-propanol/water droplets showing larger enhancement.

Numerical Study of the Forced Convective Condensation on a Short Vertical Plate

ABSTRACTNumerical investigation was conducted on the effects of gravity, surface tension, and wall adhesion upon condensation on a short vertical plate. The volume of fluid method was applied to model the interaction between the liquid and vapor phases and to capture the interface. The surface tension was implemented by employing the method of continuum surface force model. A modified phase-change model, derived from basic equations related to the kinetic gas theory, was proposed and verified based on the cases of Nusselt film condensation of water vapor on a vertical flat plate, the forced convection film condensation on a horizontal flat plate, and the capillary blocking due to condensation in a horizontal miniature circular tube. The predicted results showed that a laminar capillary wavy flow regime exists and the waves enhance the heat transfer of condensation on the plate. The mean film thickness increases and the heat transfer performance becomes worse with decrease of gravity. A high value of surface tension or contact angle, representing a large surface free energy difference, leads to an enhancement of heat transfer on the plate with large-amplitude waves.

A pancake droplet translating in a Hele-Shaw cell: lubrication film and flow field

We adopt a boundary integral method to study the dynamics of a translating droplet confined in a Hele-Shaw cell in the Stokes regime. The droplet is driven by the motion of the ambient fluid with the same viscosity. We characterize the three-dimensional (3D) nature of the droplet interface and of the flow field. The interface develops an arc-shaped ridge near the rear-half rim with a protrusion in the rear and a laterally symmetric pair of higher peaks; this pair of protrusions has been identified by recent experiments (Huerre et al., Phys. Rev. Lett., vol. 115 (6), 2015, 064501) and predicted asymptotically (Burgess & Foster, Phys. Fluids A, vol. 2 (7), 1990, pp. 1105–1117). The mean film thickness is well predicted by the extended Bretherton model (Klaseboer et al., Phys. Fluids, vol. 26 (3), 2014, 032107) with fitting parameters. The flow in the streamwise wall-normal middle plane is featured with recirculating zones, which are partitioned by stagnation points closely resembling those of a two-dimensional droplet in a channel. Recirculation is absent in the wall-parallel, unconfined planes, in sharp contrast to the interior flow inside a moving droplet in free space. The preferred orientation of the recirculation results from the anisotropic confinement of the Hele-Shaw cell. On these planes, we identify a dipolar disturbance flow field induced by the travelling droplet and its $1/r^{2}$ spatial decay is confirmed numerically. We pinpoint counter-rotating streamwise vortex structures near the lateral interface of the droplet, further highlighting the complex 3D flow pattern.

Selection of solitary waves in vertically falling liquid films

Two-dimensional solitary waves at the surface of a film flow down a vertical plane are considered. When the system is subjected to inlet white noise, solitary waves are formed after an inception region and interact with each other. Using open-domain simulations of reduced equation models, we investigate numerically their late time process dynamics. Close to the instability threshold, the waves synchronize themselves into bound states. For higher values of the Reynolds number, the separation distance between the waves increases and the synchronization process at work is weaker. Performing statistics, we show that the mean characteristics of the waves correspond to the minimal value of the mean film thickness along the traveling-wave branch of solutions. In this regime, synchronization occurs through the waves tails which is associated with a change of scaling of the waves features. A similar behavior is observed performing simulations in periodic domains: the selected waves maximize the mean flow rate.

Falling liquid films with blowing and suction

Flow of a thin viscous film down a flat inclined plane becomes unstable to long-wave interfacial fluctuations when the Reynolds number based on the mean film thickness becomes larger than a critical value (this value decreases as the angle of inclination to the horizontal increases, and in particular becomes zero when the plate is vertical). Control of these interfacial instabilities is relevant to a wide range of industrial applications including coating processes and heat or mass transfer systems. This study considers the effect of blowing and suction through the substrate in order to construct from first principles physically realistic models that can be used for detailed passive and active control studies of direct relevance to possible experiments. Two different long-wave, thin-film equations are derived to describe this system; these include the imposed blowing/suction as well as inertia, surface tension, gravity and viscosity. The case of spatially periodic blowing and suction is considered in detail and the bifurcation structure of forced steady states is explored numerically to predict that steady states cease to exist for sufficiently large suction speeds since the film locally thins to zero thickness, giving way to dry patches on the substrate. The linear stability of the resulting non-uniform steady states is investigated for perturbations of arbitrary wavelength, and any instabilities are followed into the fully nonlinear regime using time-dependent computations. The case of small amplitude blowing/suction is studied analytically both for steady states and their stability. Finally, the transition between travelling waves and non-uniform steady states is explored as the amplitude of blowing and suction is increased.

Numerical Study of Liquid Film Characteristics at Varying Operating Conditions in Prefilmer Airblast Atomizer

AbstractThe atomization of liquid fuel in gas turbines is one of the key factors that affect the combustion performance. After the adaptation of the k‐ϵ‐turbulence model for two‐phase flows and the definition of the film thickness, this work investigates and reports the influence of different parameters on the behavior of the liquid film on the prefilmer of airblast atomizers. A correlation that can be used to predict the mean film thickness has been developed and is proposed in the present study. This correlation can be used in order to optimize the design of airblast atomizers.

Friction Reduction in Lubricated Rough Contacts: Numerical and Experimental Studies

Combining the contact model of elastic-layered solid with the concept of asperity contact in elastohydrodynamic lubrication (EHL), a mixed-lubrication model is presented to predict friction coefficient over rough surfaces with/without an elastic-layered medium under entire lubrication regimes. Solution of contact problems for elastic-layered solids is presented based upon the classical model of Greenwood and Williamson (GW) in conjunction with Chen and Engel's analysis. The effects of the Young's modulus ratio of the layer to substrate and the thickness of the layer on the elastic real area of contact and contact load for a fixed dimensionless separation are studied using the proposed method, which is used for the asperities having contact with an elastic coating. Coefficient of friction with elastic-layered solids in boundary lubrication is calculated in terms of Rabinowicz's findings and elastic-layered solutions of Gupta and Walowit. The effect of rough contacts with an elastic layer on friction coefficient in lubrication regimes has been analyzed. Variations in plasticity index ψ significantly affect friction coefficients in boundary and mixed lubrications. For a large value of ψ, the degree of plastic contact exhibits a stronger dependence of the mean separation or film thickness than the roughness, and for a small value of ψ, the opposite result is true. The effect of governing parameters, such as inlet oil viscosity at ambient pressure, pressure–viscosity coefficient, combined surface roughness, and El/E2 on friction coefficient, has been investigated. Simulations are shown to be in good agreement with the experimental friction data.

Structures in gas–liquid churn flow in a large diameter vertical pipe

Gas–Liquid two phase co-current flow in a vertical riser with an internal diameter of 127 mm was investigated in the churn flow pattern. This paper presents detailed experimental data obtained using a Wire Mesh Sensor. It shows that the most obvious features of the flow are huge waves travelling on the liquid film. Wisps, large tendrils of liquid and the product of incomplete atomisation, which had previously detected in smaller diameter pipes, have also been found in the larger diameter pipe employed here. The output of the Wire Mesh Sensor has been used to determine the overall void fraction. When examined within a drift flux framework, it shows a distribution coefficient of ∼1, in contrast to data for lower gas flow rates. Film thickness time series extracted from the Wire Mesh Sensor output have been examined and the trends of mean film thickness, that of the base film and the wave peaks are presented and discussed. The occurrence of wisps and their frequencies have been quantified.

Gas/liquid flow behaviours in a downward section of large diameter vertical serpentine pipes

An experimental study on air/water flow behaviours in a 101.6 mm i.d. vertical pipe with a serpentine configuration is presented. The experiments are conducted for superficial gas and liquid velocities ranging from 0.15 to 30 m/s and 0.07 to 1.5 m/s, respectively. The bend effects on the flow behaviours are significantly reduced when the flow reaches an axial distance of 30 pipe diameters or more from the upstream bend. The mean film thickness data from this study has been used to compare with the predicted data using several falling film correlations and theoretical models. It was observed that the large pipe data exhibits different tendencies and this manifests in the difference in slope when the dimensionless film thickness is plotted as a power law function of the liquid film Reynolds number.

Application results of a prototype ultrasonic liquid film sensor to a 7 MPa steam–water two-phase flow experiment

A prototype ultrasonic liquid film sensor was applied to a high-temperature steam–water two-phase flow experiment. The liquid film sensor was vertically installed in a loop which was connected to HUSTLE, a multi-purpose steam source test facility. The hydraulic diameter of the measurement section was 9.4 mm. The output waveforms of the sensor were acquired with a digital oscilloscope. The fluid temperature and system pressure were kept at 288 °C and 7.2 MPa, respectively, during the experiment. The pulse-echo method was used to calculate the liquid film thickness. The cross-correlation calculation was utilized to determine the time difference between the pulse reflected at the sensor surface and the pulse reflected at the liquid film surface. The time-averaged liquid film thicknesses were less than 0.055 mm in the annular flow condition. The increase of the time-averaged thickness was small with the change of the gas momentum flux. The film thicknesses measured with the sensor were compared with the past experimental results; the former were smaller than one-fourth of the thickness estimated as the mean film thickness. The comparison results suggested that the continuous liquid sublayer thickness was measured with the liquid film sensor.

Measurement of liquid film thickness in micro tube annular flow

Heat transfer in micro scale two-phase flow attracts large attention since it can achieve large heat transfer area per unit volume. At large flow rate and high quality, annular flow becomes one of the major flow regimes in micro two-phase flow. Heat is transferred by evaporation or condensation of the liquid film, which are the dominant mechanisms of micro scale heat transfer. Therefore, liquid film thickness is one of the most important parameters in modeling the heat transfer phenomena. In the present study, time averaged annular liquid film thickness is measured by laser confocal displacement meter (LCDM), and the gas–liquid interface profile is observed by a high-speed camera. Glass tubes with inner diameters of D=0.3, 0.5 and 1.0mm are used. Degassed water and air are used for working fluids, and the total mass flux is varied from G=100 to 500kg/m2s. Flow patterns are observed and flow pattern map based on Reynolds numbers of gas and liquid flows is suggested. Pressure drop is measured and compared with the prediction using Lockhart and Martinelli parameter. Pressure drop is well predicted with Lockhart–Martinelli correlation. Dimensionless mean film thickness is then plotted against quality, and compared with the annular film model assuming flat gas–liquid interface. Flat interface model overestimated the experimental data. It is considered that the shear stress on the gas–liquid interface in the real annular flow is larger than that estimated in the ideal flat interface model. Prediction using new empirical correlation considering the effect of the non-flat gas–liquid interface showed good agreement with the experiment data.

Effects of Liquid Viscosity on the Wave Velocity and Wave Frequency in Horizontal Annular Flow

An investigation on the liquid holdup, wave velocity, and wave frequency in horizontal annular flow has been experimentally conducted through the measurement of liquid holdup using constant electric current method (CECM) sensors. To investigate the effect of viscosity, water and glycerin were used as working liquid, using superficial liquid velocity and superficial gas velocity of 0.05 to 0.2 m/s and 12 to 40 m/s, respectively. Liquids with higher viscosity give the higher liquid holdup, lower wave velocity, and lower wave frequency. Correlations for liquid holdup and mean film thickness, wave velocity, and wave frequency have been developed with mean average errors (MAE) of 13.5%, 9.2%, and 8.6%, respectively.

Experimental study of the film thickness in the dehumidifier of a liquid desiccant air conditioning system

A liquid desiccant air conditioning system is one possible substitute for the high energy consumption of the conventional system. The dehumidifier performance is of great importance as it is the key component of such systems. Film thickness plays a vital role in determining the heat and mass transfer performance of a falling film dehumidifier. However, no experimental studies on the liquid film thickness of the dehumidifier within liquid desiccant air conditioning systems have been conducted. Therefore, the film thickness of LiCl solution flow in a single channel dehumidifier is chosen to be investigated in this study. It was found that the mean film thickness without air flow followed the same trend as that predicted by the Nusselt relation, but was about 20% greater. The opposite air flow exerted a shear force on the liquid film to increase the mean film thickness non-linearly. The mean and changing film thickness over time are studied and comparisons made between the lower and upper points of the film. When no air flow existed, the mean film thickness at the lower point was found to be about 10.7%–16.3% thicker than at the upper point. An increase of the air velocity enhanced the wave at both the upper and lower points. The falling film statistical characteristics are also analyzed, and the probability density of the film thickness was found to effectively describe the flow condition. It was also shown that an increase in both the solution flow rate and air velocity enhanced the surface wave velocity.

Pillow Plate Heat Exchangers – Investigation of Flow Characteristics and Wetting Behavior at Single‐Flow Conditions

AbstractExperimental and theoretical results on the flow pattern and film thickness of liquid falling films on the outside of a vertical pillow‐type plate heat exchanger are presented. With the experimental setup, the wetting of a pillow plate and the local film thickness can be determined. The calculated mean film thickness is compared to literature data with an experimental investigation of a flat plate to assess the presented evaluation technique. Correlations for the mean film thickness on a flat plate and a pillow plate are given. The minimum flow rate per wetted length for a pillow plate is investigated.

Measurements of liquid film and droplets of annular two-phase flow on a rod-bundle geometry with spacer

Measurements have been conducted to simultaneously consider both liquid films and droplets of the annular flow on a 3×3 simulating BWR fuel rod-bundle test-section with spacers. The optical system of a high speed camera and a tele-microscope was used to record the backlight images at the gap between a corner rod and a side rod of the bundle at high time and space resolutions. The data obtained from the liquid film showed that the mean film thickness, wave height, power spectral density, and wave velocity at the corner rod are larger than those at the side rod, and that the influences of the spacer are different in the cases of low and high gas superficial velocities. Simultaneously, the data containing size and spatial distributions as well as the axial velocity distribution of liquid droplets were obtained. In the case of lower gas flow rates, the spacer generates not only a large number of small droplets but also big droplets whose size exceeds the maximum droplet diameter at upstream. At further downstream, the spatial distribution of the droplets indicates an asymmetry characteristic, which emphasizes the contribution of the droplet impingement mechanism to the entrainment phenomena. Moreover, a close-up observation at right up- and downstreams of the spacer was conducted to describe the interactions between the two-phase flow and this structure. By using these new experimental arrangements, the interaction mechanisms among the wavy liquid film, droplets and spacer were discussed.

A double-helix neutron detector using micron-size 10B powder

A double-helix electrode configuration is combined with a 10B powder coating technique to build large-area (9in.×36 in., or about 23cm by 91cm) neutron detectors. The neutron detection efficiency for each of the four prototypes is comparable to 3.7×103cm3 of 3He inside a cylindrical tube 91cm long. One unit has been operational continuously for 18 months and the change of efficiency is less than 1%. An analytic model for pulse height spectra is described and the predicted mean film thicknesses agree with the experiment to within 30%. Further detector optimization is possible through film texture, powder size, moderator box and gas. The estimated production cost per unit is less than 3k US$ and the technology is thus suitable for deployment in large numbers.

Pattern formation in soft elastic films cast on periodically corrugated surfaces—a linear stability and finite element analysis

Length scales of instabilities exhibited by soft, thin elastic films cast on smooth and sharp corrugated surfaces and in adhesive contact with an external contactor are investigated by means of linear stability analysis (LSA) and non-linear finite element analysis. The instability length scales are found to decrease with either an increase in the amplitude β or a decrease in the wavelength λp of the substrate pattern. For same substrate parameters, a step-patterned surface with higher RMS roughness is generally found to engender smaller length scales. The linear stability analysis with sinusoidally patterned substrates shows a reduced scaling of critical wavelength with substrate amplitude: λc = 2.96(1 − β)h, where h is the mean film thickness. The largest substrate amplitudes explored with finite element analysis are limited to β = 0.7, which is found to beget instability length scales of 0.89h, that are much smaller than the 3h length scale that is obtained with flat substrates. This suggests that an increase in substrate roughness via patterning can prove to be an attractive route for the production of miniaturized patterns.

Results for high heat-flux flow realizations in innovative operations of milli-meter scale condensers and boilers

This paper defines the nature of high heat-flux innovative boiler and condenser operations that keeps these devices functional for many critically needed thermal management applications where gravitational force is negligible (naturally or made to be so) relative to other forces (shear, pressure, etc.). These applications include heat removal from narrow spaces to address electronic cooling needs (i.e. mm-scale boilers and condensers with μm-scale flows), gravity insensitive aircraft operations, and zero-gravity applications. Further, the paper presents fundamental condensing and boiling flow results that describe the physics that needs to be utilized to achieve significantly enhanced heat-flux values within the context of these applications.Reported experimental results demonstrate: (i) the ability to replace ineffective and problematic liquid–vapor configurations (plug-slug, etc.) with annular flows where a flowing thin liquid film covers the entire heat-exchange surface, (ii) the ability to use pulsation-induced interfacial wave phenomena in conjunction with sub-micron thin film phenomena (such as contact line physics) to achieve very high heat-flux during boiling and condensation, and (iii) the ability to desensitize these flows that would otherwise be highly sensitive to inadvertent but ever present flow fluctuations (due to sensitive coupling between vapor and liquid motions, as well as sensitivity of these micro-meter scale thick liquid films to minuscule transverse vibrations of the heat-exchange surface) when flow channel hydraulic diameters reduce to milli-meter or sub milli-meter scales.Innovative realizations of shear-driven boiling and condensing flows have been investigated within horizontal channels (with condensation or boiling on the bottom, horizontal surface). Results from boiling and condensing flows of FC-72 fluid in horizontal rectangular cross-section (2 or 6mm gap height and 15 or 24mm width, respectively) ducts of 1m length are presented. Utilizing a controlled presence of pulsatile mass flow rates, significant enhancements in heat-transfer rates are obtained for these innovative devices at a location within the device length – these enhancements (relative to non-pulsatile conditions) are sometimes >860% for condensing flows and >190% for boiling flows. The paper reports representative time-varying heat-flux values at this location in support of the understanding that this phenomena can potentially be used (in future experiments) to significantly enhance average heat-flux value over the entire length of an innovative boiler/condenser. The reported phenomena arises from superposing relatively fast time-scale (2.5–30Hz) flow rate pulsations on otherwise steady-in-the-mean flows to beneficially change certain flow variable averages – over the longer time-scale (<0.01Hz) of practical interest – relative to their values obtained in the absence of externally imposed pulsations. Such externally imposed pulsations may cause standing waves to form on the thin film interface. The observed asymmetric reduction in the mean film thickness (associated with high heat-flux values) is likely due to “stickiness” of wave-troughs resulting from interaction of phenomena associated with several length scales (namely nano-scale, micro-scale, and continuum length scales) and two different time scales – short (less than 0.5s) and long (greater than 1min). That is, associated micro-scale flows at wave-troughs interact with and, possibly, de-stabilize the local adsorbed layer (whose thickness may be <200nm and is exposed to phenomena such as disjoining pressures) on the wetting heat-exchange surface.

Cobalt sulfide thin film as an efficient counter electrode for dye-sensitized solar cells

The power conversion efficiency of dye sensitized solar cells (DSSCs) was improved using a nano rose petal like structured cobalt sulfide (CoS2) counter electrode (CE) with an iodine triiodide (I−/I3−) electrolyte. The CoS2 nano rose petals like structure with a mean film thickness of 280nm prepared using a solution containing 0.1M cobalt chloride, 0.8M urea and 0.8M thioacetamide by inexpensive chemical bath deposition at 80°C for 3h resulted in an improved fill factor, short circuit current and open circuit voltage compared to the Pt electrodes. Tafel polarization and electrochemical impedance spectroscopy showed that CoS2 electrode has smaller charge transfer resistance than Pt electrodes. DSSC fabricated with the CoS2 CE achieved a power conversion efficiency of 5.32% under 1 sunlight illumination (100 mW cm−2, AM 1.5 G), which surpassed that of the device based on Pt (5.02%).