Film Flow Regime Research Articles

Thermo-hydrodynamic characterization and mapping of the two-phase flow in a capillary tube

The study presents an experimental investigation on the behavior of condensate film and liquid–vapor meniscus in the unit cell of a pulsating heat pipe (PHP) at different operating boundary conditions, and their respective roles in determining PHP’s performance. The orientation and the temperatures of evaporator and condenser sections are taken as the variable operating boundary conditions whereas the amplitude of oscillations influences the performance of the PHP. The entire thermo-hydrodynamic phenomenon is captured using high-speed imaging. By tracking the interface of liquid film and vapor, different regimes for film flow in the condenser section are observed and characterized. Also, the oscillation amplitude has been quantified at different operating boundary conditions. Despite a quite small tube diameter, gravity plays a key role in determining the liquid and vapor phase distributions in the tube. The variation of oscillation amplitude is intimately linked with the nuances of film flow regimes and inter-regime transitions. The optimum inclination angle, and evaporator and condenser temperatures are found to exist for the PHP unit cell which maximize the oscillation amplitude. The dependence of optimum inclination angle on condenser temperature has also been understood along with. Further, the associated film flow regime in condenser which facilitates these optima has also been recognized. All variable operating boundary conditions are observed to have a peculiarly mixed influence on the flow regimes and thus oscillation amplitude. The study would help to achieve a better elementary understanding of the role of condensate film flow in determining PHP’s performance and develop a flow regime map particularly for PHPs.

Self-Similar Solution to the Problem of Unsteady Film Condensation on a Vertical Semi-Infinite Plate

Changes in the regimes of condensate film flow along the underlying surface in the presence of a body force are considered within the framework of a simplified physical and mathematical model of hydrodynamic and thermal processes occurring under unsteady film condensation on a vertical semi-infinite plate. The model is used to determine the applicability conditions of the zero-gravity approximation when considering film condensation. It is found that the validity of using the zero-gravity approximation to calculate the film thickness growth rate on any plate section depends not only on the magnitude of the body force, but also on time and distance between this section and the leading edge of the plate.

Experimental study of shear-driven water film with brightness-based laser-induced fluorescence technique

Driven by low-temperature and high-speed airflow, the water droplets impinging the surface of the aircraft will gather into water film, affecting the aerodynamic characteristics and endangering flight safety. In this paper, a high-speed video system with the brightness-based laser-induced fluorescence (BBLIF) technique was used to analyze the transient water film thickness and wavy characteristics on a horizontal metal plate. A series of experiments were performed over a wide range of wind speeds (15.59 m/s ∼ 52.20 m/s) and water volume flow rates (100 ml/min ∼ 900 ml/min). According to the interfacial phenomena observed in the experiment, the water film flow is classified into five regimes: two-dimension-wave, ripple-wave, dual-wave, roll-wave, and entrainment. The water film characteristics such as mean film thickness, film roughness, wave frequency, and rolling wave intermittency are studied under the influences of air velocity and water flow rate. The results show that the complex fluctuation characteristics of water film are the result of the joint action of surface tension, interfacial shear force and inertia term of water film, and are closely related to the water film flow regime. The findings could contribute to a better understanding of the shear-driven water film wavy characteristics and the optimization of the anti-icing or de-icing system.

Stability and Bifurcation Analysis of Two-Immiscible Liquids Film Down an Inclined Slippery Solid Substrate

In this work, the dynamic behavior of linear and nonlinear waves propagating at the separating surface between two thin layers of viscous Newtonian fluids is studied in the presence of the effect of insoluble surface surfactant. The two liquids are confined between two infinite rigid parallel plates and assumed to have different densities and viscosities. The equations of evolution for surface-wave elevation and concentration of surfactant are derived using the lubrication approximation. In the linear stage, by utilizing the normal mode approach, we have derived the dispersion relation that relates the wave angular frequency to the wave number and other parameters that is solved numerically to inspect the influences of some selected parameters on the stability criteria of the fluid flow. Also, analytical expressions for the growth rate as well as its maximum value with corresponding wave number are obtained in the special case of long-wave limiting. It is concluded that the Marangoni number text {Ma} has acquired a significant stabilizing influence on the fluid flow, whereas the inverse of the slippery length of substrate plate beta, resorts to the destabilize the motion of the interfacial waves. Consequently, both of the Marangoni number and the substrate slippy coefficient can be utilized to control the film flow regime, where they preserve the film laminar flow and tend to prevent the film breakdown. These can be useful in many industrial applications such as coating processes, heat exchangers, cooling microelectronic devices, chemical reactors, food processing, thermal protection design of combustion chambers in rocket engines and operation of Laser cutting and heavy casting production processes.

Influence of wind on a viscous liquid film flowing down a thread

A liquid film flowing down a fiber or thread is well known to destabilize into an axisymmetric bead-like pattern in stagnant air. We demonstrate experimentally how side wind affects such a film flow. Our results show that side wind increases the descent speed of the liquid beads, highlighting that the film flow regime can be changed by controlling the magnitude of the wind speed.

Quantifying Coupling Effects Between Soil Matric Potential and Osmotic Potential

AbstractSoil matric potential and osmotic potential are widely accepted as two independent components of total soil water potential. However, laboratory observations repeatedly demonstrated that matric potential can vary with salt concentration, implying a potential coupling between matric potential and osmotic potential. To date, it remains elusive whether matric potential and osmotic potential are independent or not and why so, and a theoretical theory for quantifying the coupling between them is still missing. Herein, a theoretical model is developed to quantitatively explain this problem via a lens provided by a recent concept of soil sorptive potential (SSP). The proposed model substantiates that matric potential and osmotic potential are not independent. The increasing salt concentration can notably depress two variables underpinning SSP, namely relative permittivity and electrical double layer thickness, leading to non‐negligible decreasing (more negative) of matric potential in the high suction range, and increasing (less negative) of it in the low suction range. In turn, the soil‐water interactions redistribute ions in soil water, raising osmotic potential especially for clay with high cation exchange capacity. The proposed model shows excellent performance in capturing experimental data, validating its accuracy. A parametric study implies that the neglection of coupling effects can lead to a significant underestimation of soil hydraulic conductivity in the film flow regime.

Characterization of liquid film distribution and sub-flow regimes of annular flow in helically coiled tubes using ring island array sensor

The distribution of liquid film thickness in helically coiled tubes (HCTs) directly affects the heat transfer performance of the annular flow region. However, due to the limitation of measurement technology, accurate measurement data of liquid film thickness in HCTs is very rare. In this paper, a non-invasive contact liquid film thickness sensor (ring island array sensor) is successfully developed to obtain the refined three-dimensional spatial-temporal distribution data of liquid film thickness in HCTs. According to the results, four typical liquid film flow regimes are defined based on the circumferential position of the thicker liquid film appearing in the tube wall, i.e., the bottom distribution (BD) dominated by liquid phase gravity force, the outside distribution (OD) dominated by liquid phase centrifugal force, the inside distribution (ID) dominated by gas centrifugal force and the inside-outside distribution (IOD) dominated by the secondary flow. These circumferential distributions of liquid film thickness are mainly affected by the HCT structure (e.g., coil diameter and coil pitch) and superficial gas-liquid velocities. Based on the modified Dean number De* and the modified Lockhart-Martinelli parameter X*, which consider the influence of HCT structures, fluid properties and operating parameters, a general sub-flow regime map of annular flow is proposed. The prediction models of transition boundaries of different liquid film flow regimes are also established and verified with present and available data in the literature.

Falling film co-current flow cyclone demister for separating high-boiling droplets: Gas-liquid flow pattern and performance

In many processes, the product gas stream contains liquid droplets that easily condense or solidify, which can deteriorate demister performance and even block equipment during gas–liquid separation. We proposed a falling film co-current flow cyclone demister (F2CFCD) to solve the above problems. The novel idea is that an additional stream of solvent fluid is added to the gas stream containing the droplets. After injecting the demister, the solvent fluid forms a film on the wall of F2CFCD. When the droplets are separated to the wall surface by centrifugal force, they are dissolved by the liquid film, thereby avoiding crust formation. In this work, the gas–liquid flow characteristics and the performance of F2CFCD are investigated numerically and experimentally. A hot-wire anemometer (HWA) is used to measure the gas velocity, and a conductivity method is used to measure the liquid film thickness. A coupled numerical model of the RSM-DPM and Eulerian wall film (EWF) model is applied to discuss the effect of the gas–liquid flow rate on the flow field in the demister. The liquid volume flow rates ranges from 0.5–10.5 m3/h, and the gas volume flow rates range from 640–894 m3/h. The gas flow pattern and the liquid film flow regimes are explored in the F2CFCD. A film thickness model is established to evaluate the average film thickness in the waterfall regime. Experimental observations illustrate that F2CFCD has a high separation efficiency for droplets under all conditions.

Analysis of the cladding melt relocation along the surface of the fuel pin with help of the SAFR module

The presented work is dedicated to the development of approaches to simulate cladding melt relocation along the surface of the fuel pin. Development of the approaches is based on the results of the experiments carried out at the NSI RAS and IT SB RAS. Features of the melt relocation are studied in the experiments. It is demonstrated that the laminar film flow regime in the heated part of the fuel simulator is the main flow regime. Model of the melt relocation is constructed. This model is the part of the SAFR module of the EUCLID/V2 coupled code. It is shown that the proposed approaches allow simulating the melt relocation with good accuracy.

Effects of cross-sectional geometry on flow characteristics in spiral separators

ABSTRACT As flowing film gravity concentrators, spiral separators are extensively used in the processing of numerous minerals. Appropriate design of their cross-sectional geometries has been known as an effective method to improve the performance of spiral separators. However, limited work has been published on the impact of cross-sectional geometry on flow field characteristics in spiral separators. In this study, the effects of cross-sectional geometries with various parabolic profiles (|x|=m|y| n ) of a spiral separator on the flow film thickness, flow pattern and flow regime were investigated by numerical simulation. The validity of the simulation approach was confirmed by the reasonably good agreement between the predicted and measured flow film thickness in the own laboratory spiral separator. Results have shown that the flow film thickness and the mean primary velocity in the inner and middle troughs are reduced by decreasing downward bevel angle or increasing parabolic index. The dominant region of the secondary flow can be expended inward by increasing the downward bevel angle and decreasing the parabolic index. As the radial position extends inward, a larger downward bevel angle is more conducive to improving the intensity of secondary flow. A wider laminar flow region can be obtained with a smaller downward bevel angle or a larger parabolic index. The flow film thickness has a significant positive correlation with the trough slope in the inner and middle troughs. It is recommended that the matching relationship between material properties and flow field characteristics should be fully considered when selecting the cross-sectional geometry. Thus, this study aids the structural design of the cross-sectional geometries of spiral separators.

Application of the PTV with the use of a light-field camera to study three-dimensional wave regimes of liquid film flow

We present the results of studying the influence of surfactants on three-dimensional waves, in particular, on the flow structures in the wave. Volumetric fluid velocity measurements in waves, as well as the determination of their shape, were carried out using a light field camera. An aqueous solution of the nonionic surfactant Triton-X was used as a working fluid. The presence of surfactant leads to a significant change in the structure of the transverse flow regions under the main hump of the wave. Besides, capillary precursor and backflow in it are suppressed.

Сomputational Modeling of Liquid Film Flow Regimes in Heat and Mass Transfer Devices at Low Reynolds Num-bers

Сomputational Modeling of Liquid Film Flow Regimes in Heat and Mass Transfer Devices at Low Reynolds Num-bers

Experimental and theoretical study of two-phase flow in wide microchannels

Experimental and theoretical studies of the two-phase flow regimes in a wide microchannel with the height of 164 μm were performed. The ranges of parameters of the formation of the main flow regimes are determined: jet, bubble, churn, stratified, and annular. The pressure drop in single-phase and two-phase flows was measured. Using a flat flow model, we performed a theoretical study of the pressure drop and compared it with experimental data. Regular waves were experimentally detected in film flow regimes. The formation of waves on a liquid film was investigated, and characteristic wavelengths were measured. As for the theory, the study of linear stability of the stratified two-phase flow regime in a wide microchannel was performed. The comparison of numerical and experimental results indicated the validity of the proposed approach for modeling waves in film regime in wide microchannels.

Identification of condensation flow regime at different orientations using temperature and pressure measurements

While many prior works in the field relied upon direct optical access to determine condensation flow regimes, the present work outlines a new methodology utilizing temperature and pressure measurements to identify condensation flow regimes. For vertical upflow condensation, amplitude of dynamic temperature and pressure oscillations are shown to clearly indicate transition from counter-current flow regimes (i.e., falling film, oscillating film, flooding) to annular, co-current flow (climbing film flow regime). In horizontal flow condensation, standard deviation between multiple thermocouple measurements distributed around the tube circumference was calculated at all axial (stream-wise) measurement locations. High values of standard deviation are present for stratified flow (stratified flow, wavy-stratified, plug flow), while axisymmetric flow regimes (i.e., slug flow, annular flow) yield significantly lower values. Successful development of this technique represents a valuable contribution to literature as it allows condensation flow regime to be identified without the often-costly restriction of designing a test section to allow optical access. Identified flow regimes in both vertical upflow and horizontal flow orientations are compared to regime maps commonly found in the literature in pursuit of optimum performing maps.

Numerical simulation of condensate removal from gas channels of PEM fuel cells using corrugated walls

The removal of condensate water droplets from gas channels is necessary for proper operation of proton exchange membrane fuel cells. In the current work, it is shown that corrugated wall gas channels can help in the removal of condensate water droplets formed on the channel walls. Removal of sessile droplets from channels having semicircle, rectangular dent, and saw-tooth corrugation and at different gas velocities is modeled numerically. It is shown that the time of condensate removal is much shorter in a corrugated channel as compared with that in an uncorrugated channel. Three different droplet removal regimes are identified: droplet, film, and misty flow regimes. The transition from one to another regime is mapped based on the inlet flow velocity and the type of the channel corrugate.

Investigation of the effect of a gap between the cylindrical substrate and curvilinear ring on the regimes of liquid film flow

Experimental studies have been carried out to reveal the peculiarities of the influence of the arrangement of the curved ring on the flow regimes of a liquid film along a cylindrical surface. Regimes under which the estimated thickness of the gap between the flowing film and the cylinder may not be sufficient. Recommendations are given to determine the minimum value of the distance between the ring and the surface along which the film flows, so that at any flow of liquid there is no contact between the liquid and the ring.

Characteristics of film flow during transition to three-dimensional wave regimes

Results of experimental study of transition processes from two-dimensional to threedimensional wave regimes for the case of isothermal falling liquid film are presented. Experiments were carried out using liquids with various physical properties in the range of Reynolds numbers 5 < Re < 100. It is shown that the process of transition to the threedimensional wave motion is accompanied by formation of rivulets and that the downflow evolution of the rivulets usually has non-monotonic character. The statistical analysis of the 3D wave's characteristics allowed us to characterize in detail different scenarios of downflow evolution of 3D waves and to determine the regimes of fully developed steady-state 3D wave flow. Noticeable differences between natural and forced wave evolution have been observed for the relatively small Reynolds numbers whereas for the higher Reynolds numbers fast evolution of all investigated statistical characteristics with achievement of the steady-state 3D wave regimes of film flow occurs.

The transition from two-dimensional to three-dimensional waves in falling liquid films: Wave patterns and transverse redistribution of local flow rates

This article presents the results of experimental investigations of the process of transition from two-dimensional (2D) to three-dimensional (3D) waves in liquid films falling down a vertical plate. The method of laser induced fluorescence was used to obtain instant shapes of three dimensional waves and to investigate the regularities of formation of 3D wave patterns arising due to transverse instability of 2D waves. The obtained results were compared to the results from the published literature on the modeling of 3D wave regimes of film flow. Although many details of 3D wave patterns correspond well, there are a few significant distinctions between our experiments and modeling. In particular, during 2D-3D wave transition, we observed a strong transverse redistribution of liquid leading to the formation of rivulets on the surface of isothermal liquid film, which is a phenomenon not described previously. Possible discrepancies between modeling and experiments, including applicability of boundary layer models and downstream periodic boundary conditions, are discussed. The authors hope that the results presented in the article are of interest not only for modeling of film flows but also for practical applications because at large distances from the film inlet due to 2D-3D wave transition the local flow rates can differ several times at the transverse distances of about 1 cm, which is an effect that cannot be neglected.

Absorption of poorly soluble gases during an upward cocurrent

The processes of the absorption of three different gases (helium, oxygen, and Freon-12) by water have been studied experimentally under laboratory conditions. Short glass pipes had a diameter of 12.4 mm. In order to exclude diffusion resistance in the gas phase single-component, gases were used, but not a mixture of gases. The density of the irrigation of the perimeter of the pipes (0.1–0.8 cm2/s) corresponded to the film flow regime in which entrainment is absent. During the treatment of the results, dimensionless average concentration was represented as a function of the perimeter of longitudinal coordinate \(\varepsilon = \frac{{Dx}} {{qh}} \). The estimate in terms of two-film model of mass transfer showed that the coefficient of mass transfer does not remain constant in the liquid phase, but decreases along the pipe. A calculation method has been proposed that is based on the results of the theoretical solution of the problem about diffusion in the laminar film during an ascending cocurrent.

Diagrams for Analysis of Film Units

Diagrams are proposed for determination of film-flow regimes of water along pipe walls, and for calculation of film thickness and pressure losses during ascent of a parallel-current flow. The question concerning the driving force for absorption in parallel- and counter-current flow regimes is examined.