Characteristics Of Liquid Film Research Articles

Effect of hemispherical ash slag simulants on the thickness distribution and fluctuation characteristics of turbulent liquid film

To explore the influence of ash slag adhering to the scrubbing-cooling tube on the turbulent falling film flow, three hemispheres with different radii (R = 2 mm, 4 mm, and 6 mm) were used to simulate the slag. Ultrasonic Doppler Velocimetry was used to investigate the liquid film’s thickness distribution and fluctuation characteristics under Reynolds number of liquid film Ref ≈8.75 × 103 ––2.63 × 104. The liquid film thickness distribution at low Reynolds numbers exhibits an isosceles triangle when subjected to R = 4 mm and 6 mm hemispheres. At high Reynolds numbers, the average thickness of the liquid film is smaller downstream of the three simulants. The simulants can enhance the liquid film’s perturbations. A mathematical model has been established to predict the mean liquid film thickness on hemispherical simulants, and the relative difference is within 10 %. The probability density distribution curves of the instantaneous liquid film thickness on the simulant exhibit multimodality.

Measuring liquid film thickness during downflow condensation in a minichannel

The availability of experimental liquid film thickness data during in-tube condensation heat transfer is fundamental for the development of reliable correlations for the design of condensers. During annular flow condensation, the heat transfer is directly linked to the liquid film thickness, as the main thermal resistance is due to thermal conduction through the film of condensate. Therefore, understanding the liquid film characteristics is paramount for modelling two-phase annular flows. Although several prediction methods for liquid film thickness are available in the literature, they are mainly related to air-water adiabatic experiments. More liquid film thickness data are required for a better understanding of the condensation phenomena and for the development of more accurate models. In this work, the liquid film thickness characteristics of R245fa, R134a and HFE-7000 have been investigated during condensation inside a vertical channel with 3.38 mm inner diameter. Shadowgraphy and chromatic confocal imaging were used to detect the evolution of the liquid film thickness with high temporal resolution. The experimental data were useful to address the existence of a dependence of the liquid film thickness on dimensionless numbers. The combination of two dimensionless numbers, specifically the vapor Weber number and the Reynolds number of the liquid phase, allows to better catch the liquid film thickness trends with the varying operating conditions.

Study on the effect of wall roughness on the flow characteristics of liquid film in airflow

During the flight, water flows under the action of shear force to form the overflow liquid film and freezes to form overflow ice. Overflow ice will change the aerodynamic shape of the aircraft and affect the safety and stability. In this paper, the three-dimensional structure of the liquid film in airflow was measured transiently with the digital image projection technology. The effects of different flat wall roughnesses, airflow velocities, and liquid film flow rates on the flow characteristics of the liquid film were analyzed. The experimental results show that the lower the wall roughness, the more pronounced the liquid film contraction toward the central axis. When the liquid flow rate drops to 0.228 l/min, the continuous liquid film transforms into rivulets on the surface of a large contact angle or low roughness. The average thickness of the liquid film on the flat is greater than that on the rough flat at low airflow velocity and low liquid flow rate, but smaller than that on the rough flat at high airflow velocity and high liquid flow rate. The wave near the inlet has an obvious acceleration process, and after reaching the peak speed, it gradually decreases and stabilizes near a certain value. The smoother the surface, the faster the acceleration of the wave at the inlet, and the greater the final stable wave speed.

Numerical Simulation of Liquid Film Characteristics during Atomization of Aluminum Alloy Powder

The process of atomizing aluminum alloy powder using a rotating disk was studied by numerical simulation and experimental verification. The motion characteristics of the molten metal thin liquid film and the evolution law of atomization into droplets were systematically studied with different disk shapes and speeds. The results showed that the slippage of the liquid film on the surface of the spherical disk was smaller, the liquid film spread more evenly, and the velocity distribution was more uniform. Under the same working condition, the boundary diameter of the continuous liquid film on the spherical disk was 21–29% larger, and the maximum liquid film velocity increased by approximately 19%. In other words, the liquid film obtained more energy at the same rotational speed, the energy utilization rate was higher, and the liquid filaments produced by the splitting region of the disk surface were finer and greater in number. The data showed that the average thickness of the liquid film on the surfaces of different disk shapes was more affected by the speed of the flat disk, and the thickness on the spherical disk was relatively stable and uniform, but the difference in thickness between the two disk shapes decreased from 4.2 μm to 0.3 μm when the speed increased from 10,000 rpm to 60,000 rpm. In particular, the influence of the disk shape on the liquid film thickness became smaller when the speed increased to a certain range. At the same time, the characteristics of the liquid film during the spreading movement of molten metal on the disk and the mechanisms of the primary and secondary breakage of the liquid film were obtained through this simulation study.

Measurement of Three-Dimensional Characteristics of Falling Liquid Film Based on SLIF

Measurement of Three-Dimensional Characteristics of Falling Liquid Film Based on SLIF

Experimental study on the liquid film characteristics of annular flow in the helical rectangular channel

A detailed insight into the liquid film characteristics in the helical channel of single screw expander is of great significance for avoiding the leakage loss of higher pressure gas to lower pressure region. The liquid film thickness and fluctuation characteristics of the annular flow on the outer side of the helical rectangular channel is measured using the digital image processing algorithm, and they are evaluated under various flow rates, dimensionless curvature, and dimensionless pitch at different angle positions. Results show that increasing the Reg will decrease the instantaneous liquid film thickness, and a larger decrease in the liquid film fluctuation is found. A larger average liquid film thickness is located at the angle of 90° than that of the 180° and 270°. The instantaneous and average liquid film thicknesses are decreased with increasing the dimensionless curvature and dimensionless pitch at each angle position. Moreover, increasing the dimensionless curvature can weaken the effect of angle position on the liquid film thickness. The time distribution of the liquid film thickness changes dramatically in both wave height and amplitude as increasing the Rel at the smaller dimensionless pitch.

Heat transfer characteristics of oblique jet impingement liquid film cooling in the forced convection mode

Liquid film cooling by oblique jet impingement is widely used in small liquid rocket engine thermal protection. In this work, heat transfer characteristics of oblique jet impingement liquid film cooling in forced convective mode were experimentally investigated to examine the effect of the impingement velocity and the initial wall temperature. The temperature distribution on the rear surface (opposite to the jet) was recorded by a high-frequency infrared camera, and then the heat flux on the front surface was estimated by solving the inverse heat conduction problem. It was found that the lagging effect by thermal diffusion is nonnegligible and can be well described by the penetration time. In addition, thermal diffusion makes the temperature distribution on the rear surface more uniform than the liquid flow path. In contrast, the heat flux distribution has a clear boundary same as the liquid flow path. The length of the wetting front lwf can be fitted as lwf=atn. The constant a and the wetted area A are affected merely by the impingement velocity. The point with the maximum heat flux is located at the impingement point in the forced convective mode rather than the wetting front in the film boiling mode. Inside the liquid film, there is no clear boundary as the film thickness distribution between the thin layer zone and the raised zone, implying the local film velocity determines the heat transfer rather than the film thickness. The cooling power increases with higher impingement velocity and initial temperature difference. The standardization fitting equation shows the effect of the initial temperature difference on the cooling power is more significant than the impingement velocity.

Modeling and Analysis of Metal Liquid Film Flow Characteristics during Centrifugal Spray Forming

Liquid films are an important part of liquid metal granulation in the process of centrifugal spray forming. The size of the granulated particles has an important influence on the density, grain size and microstructure uniformity of the deposited blanks. The particle size is closely related to the flow characteristics of liquid films. Therefore, enhancing our understanding of the flow characteristics of liquid films can provide guidance for forming blanks. In this study, force analysis of a liquid film on the surface of a high-speed rotating centrifugal disc used in centrifugal spray-forming technology was carried out using D’Alembert’s principle and Newton’s law of viscosity. Then, combined with the principle of mass conservation, a theoretical model of the smooth flow of the liquid metal film was established. The experimental values obtained by Leshev were compared with our values to verify the correctness and accuracy of the model. Through the model, the influencing factors of the liquid film flow were obtained, such as the centrifugal disc speed, centrifugal disc radius, inlet volume flow rate and kinematic viscosity. Taking A390 aluminum alloy as the research object, the influence of the process parameters on the thickness, velocity and trajectory of the liquid film was revealed theoretically, and the relationship between the process parameters and the trajectory length and liquid film thickness was clarified. Modeling and analysis can not only help us to understand the flow of a liquid film, but also help us to predict the relevant parameters, which is convenient for the accurate and rapid regulation of the process to obtain the desired flow parameters. Therefore, the research content of this paper is of great significance for the preparation of billets with a uniform microstructure and excellent mechanical properties.

Spreading Characteristics of Volatile Liquid Film on the Liquid-solid Substrates

The spreading characteristics of volatile liquid film on the liquid-solid substrate have been experimentally investigated. Unlike the liquid film spreading on the liquid substrate, there exists a dip at the top of the solid substrate, as a result, the spreading behaviors are inhibited. It is found that dip characteristics of R’/ R slightly decrease at the beginning then it keeps constant R’/ R=0.83 until the end. Moreover, the comparison of spreading behaviors on two different substrates is investigated. When the volatile film spreads on the liquid-solid substrate, the maximum radii of the outer and inner rings are smaller, but the process lasts longer. Although the evolutions of radii are different, the variation trend of nondimensional ring width is consistent. We further study the spreading rate; results show there exist three stages due to the relative importance of different forces. Our results provide important missing pieces to the rich mechanisms of the multiple-phase flow driven by the Marangoni effect.

Liquid film and heat transfer characteristics during superheated wall cooling via pulsed injection of a liquid jet

Pulse firing is one of the major operation modes of bipropellant thrusters for the attitude control of small-scale spacecrafts. In the pulse-firing mode, the operational range of the thruster depends on the deterioration of liquid film cooling performance. Because cooling performance is determined by the balance of heat removed by the liquid film and transferred to the manifold, the behavior and heat transfer characteristics of liquid films under the intermittent injection of liquid jets must be understood to enable a wider range of operational patterns. We conducted cooling tests on a superheated metal plate via the pulsed injection of a liquid jet for better understanding of the pulsed cooling process. Different injection patterns, including continuous injection, with the same injection quantity were examined on two types of metal plates (aluminum alloy and copper), because the thermal properties of metal plates affect both the heat transfer characteristics of the liquid film and temperature rise of the plate during the inter-pulse duration. The cooling process was evaluated based on the evolution of the liquid film on the metal plate and rear-side temperature of the metal plate. The evolution and temperature were simultaneously visualized using a high-speed camera and infrared camera, respectively. To link the liquid film state to the heat transfer process, the temperature and heat flux on the cooled surface were estimated by solving the inverse problem of three-dimensional transient heat conduction. The results indicate that the wetting front position corresponds to the position of the maximum temperature gradient. Additionally, the residual liquid film is consumed through the evaporation and the droplet dispersion related to the nucleate boiling. The effects of thermal inertia and diffusivity of the metal plate highlight the differences in the amount of heat removed by the liquid film. The heat removed by the liquid film during pulsed cooling exhibited a peak and was higher than that removed by continuous injection on the aluminum alloy plate. Less heat was removed under low-duty-cycle conditions, and the amount of heat removed by the liquid film was lower than that removed by continuous injection on the copper plate.

Quantitative research of the liquid film characteristics in upward vertical gas, oil and water flows

The study of liquid film characteristics in multiphase flow is a very important research topic, however, the characteristics of the liquid film around Taylor bubble structure in gas, oil and water three-phase flow are not clear. In the present study, a novel liquid film sensor is applied to measure the distributed signals of the liquid film in three-phase flow. Based on the liquid film signals, the liquid film characteristics including the structural characteristics and the nonlinear dynamics characteristics in three-phase flows are investigated for the first time. The structural characteristics including the proportion, the appearance frequency and the thickness of the liquid film are obtained and the influences of the liquid and gas superficial velocities and the oil content on them are investigated. To investigate the nonlinear dynamics characteristics of the liquid film with the changing flow conditions, the entropy analysis is introduced to successfully uncover and quantify the dynamic complexity of the liquid film behavior.

Experimental investigation for liquid film characteristics of gas-liquid swirling flow in a horizontal pipe

Experimental investigation for liquid film characteristics of gas-liquid swirling flow in a horizontal pipe

Effect of Nanoparticle Addition on Evaporation of Jet Fuel Liquid Films and Nanoparticle Deposition Patterns during Evaporation.

Jet fuel-based nanofluid fuel has been proposed for improving the energy density and utilization efficiency of jet fuel that is widely applied in aircraft powered by aviation turbine engines. To recognize the evaporation behavior of the formed liquid film as a jet fuel-based nanofluid sprayed onto the engine wall or blades, this paper presents the evaporation and deposition characteristics of the jet fuel-based nanofluid liquid film adhering on the hydrophilic substrate. The changes in contact line, contact angle, volume, and deposition pattern during liquid film evaporation under different substrate temperatures, different nanoparticle concentrations, and different kinds of nanoparticle additions were investigated. The effect of nano-Al addition on the evaporation kinetics and deposition pattern of the nano-Al/jet fuel (nAl/JF) nanofluid fuel liquid film was explored. Repeated pinning and de-pinning of contact lines during evaporation occurred, resulting in the formation of concentric multi-ring deposition patterns. The addition of nano-Al increased the evaporation rate and shortened the evaporation lifetime, demonstrating a promotion effect on jet fuel liquid film evaporation. The existence of an energy barrier shows that the movement of three-phase contact lines on the hydrophilic solid surface presented not a continuous sliding behavior but a "stick-slip" behavior, and there were multiple jumps in contact lines and contact angles. Finally, a comparison was made with the deposition pattern of jet fuel liquid films with different graphite and Fe nanoparticle additions during evaporation. The mechanism of deposition phenomena was deeply revealed by the analysis of capillary flow and Marangoni recirculation.

Heat transfer enhancement in wavy falling films studied by laser-induced fluorescence

The characteristics of thin liquid films flowing down a uniformly heated and inclined plane are investigated, with heat transfer across the wavy films quantified using up-to-date optical measurement techniques based on laser-induced fluorescence (LIF). A planar two-colour LIF technique provides the temperature distribution inside the films, but requires a high degree of wave regularity for the spatial reconstruction. A pointwise adaptation of the aforementioned technique, with much finer temporal sampling, provides simultaneous measurements of the average temperature over the film height and of the film thickness. Despite the loss of spatial resolution, the latter technique can be applied to diverse situations, especially when the waves lose their regularity and have large amplitudes. With these two approaches, the enhancement of heat transfer due to surface waves is traced along the film flow. A growing thermal boundary layer is found close to the inlet of the flow (i.e., first few cm), but its thickness remains small relative to the film thickness. Therefore, the heat transfer coefficient (HTC) is observed to be insensitive to the shape and amplitude of the waves at the free surface. A critical distance is necessary for the thermal boundary layer to be thick enough to interact with the flow structures associated with the waves, and the critical length scales with the Peclet number of the flow based on the specific flow rate. Several experiments are conducted to quantify the influence of the main flow parameters that control the HTC, such as the Reynolds number, the inclination angle and the wave frequency. For moderate wave amplitudes, the internal structure of the film is insensitive to the wave dynamics, and the temperature distribution is essentially dominated by thermal diffusion in the direction normal to the heated wall. Classical Nusselt theory is found to be applicable to the unperturbed (flat) film flows with some limited adjustments to predict the heat transfer rate. However, for the waves that have a larger amplitude, the classical Nusselt theory diverges from the experimental results. A sharp increase of the HTC by several tens of percent is observed over just a few cm, compared to an equivalent undisturbed liquid film with the same Reynolds number. Refined images of the temperature field are then used to better understand the mechanisms by which heat transfer is enhanced. Mixing appears in regions close to the wave front, then progressively extends to other film regions, tending to make the temperature more homogeneous. This has a strong effect on the local HTC in the troughs of the waves, with deviations of up to 40% relative to the flat film theory. Finally, a loss of wave regularity, observed after a few tens of cm from the inlet, accelerates the mixing by further altering the distribution of the temperature field over the entire liquid domain.

Visualization experiment and numerical simulation on behaviors of gas-liquid falling film flow on the air-side of two-row plain finned-tube heat exchanger

The frost-free air-source heat pump (ASHP) with integrated liquid desiccant dehumidification is a more promising energy-saving alternative to the conventional ASHP unit. The performance of spray-type finned-tube heat exchanger (FTHE), as one of the key equipment of this frost-free ASHP system, is affected by a combination of various factors including wall wettability, liquid film flow pattern, liquid film thickness, velocity distribution in a liquid film, interface wave, interface velocity and so on. To provide a theoretical basis for the operation, design and optimization of high-performance spray-type FTHE, a visualization experiment system and a three-dimensional (3D) numerical model are employed to study the behaviors of counter-current gas-liquid falling film flow on the air-side of two-row plain FTHE under different parameters in the present paper. According to the experimental results, the flow pattern of liquid film on the finned-tube surface is a mixture flow composed of the rivulet flow and the thin falling film flow, demonstrating that the assumption of the uniform distribution for liquid film thickness proposed in previous studies is not completely consistent with the actual situation. Meanwhile, the numerical results indicate that the transient-average film thickness on the surface of two-row plain finned-tube is significantly 18.3% – 32.5% higher than that on the vertical plate calculated by Nusselt's model. After that, the critical gas and liquid Reynolds numbers of the flow pattern transition for the liquid film are obtained, and it is pointed out that the wall contact angle of 10° is more conducive to the formation of complete film flow on the air-side of the two-row plain FTHE within the fin pitch range of 4.8 – 6.0 mm. The changes of the falling liquid film and gas-liquid interface characteristics are fundamentally attributed to the drag effect of the interfacial shear force. Furthermore, there is a cubic curve relationship between average interface shear force and gas Reg. A quadratic curve relation exists between average wall shear stress and gas Reg. At the same time, the velocity of falling film fluid at the bottom region of laminar flow is reduced, resulting from the inhibiting effect of the interfacial shear force.

Influence of Liquid Film Characteristics on Hot Cracking Initiation in Al–Cu Alloys at the End of Solidification

Influence of Liquid Film Characteristics on Hot Cracking Initiation in Al–Cu Alloys at the End of Solidification

Numerical Analysis of Flow Characteristics of Upper Swirling Liquid Film Based on the Eulerian Wall Film Model

The Upper Swirling Liquid Film (USLF) phenomenon that occurs in the upper cylinder of the Gas–Liquid Cylindrical Cyclone (GLCC) separator is the direct cause of the low separation efficiency of the liquid phase. In this study, first, the USLF formation and development were simulated by an improved Eulerian-EWF coupled simulated method. By introducing a profile-defined inlet boundary and considering entrainment droplet size distributions, the Eulerian-EWF method got reasonable results which agreed well with the experimental. Then, the flow characteristics and changing laws of the USLF including film thickness, film axial velocity, and film tangential velocity were analyzed by this method under different gas–liquid flow rates. It suggested that the liquid film thickness often reaches a maximum at the aspect ratio (z-z0)/D=(1.2–3.9) above the tangential inlet, and the film thickness appears to be more sensitive to the gas flow than to the liquid flow. For the film axial velocity, the direction of film velocity on the front and back sides seems to be generally opposite. Finally, typical distributions of the aforementioned USLF variables were presented and corresponded accordingly, and two obvious rules were found. One is that the position where the thickest liquid film is located always corresponds to the position where the axial film velocity turns from positive to negative for the first time. The other is that the tangential film velocity has a strong synchronous relationship with the film thickness. This research might provide somewhat valid information for the future LCO-prevented measurement in GLCC separators.

Optimization and application of the concentric conductance probes in liquid film thickness measurement in a helically coiled tube

The liquid film distribution in the helically coiled tube (HCT) is affected by curvature-induced centrifugal force and gravity force, showing more complex characteristics than straight tubes. However, due to the limitation of the measurement technology, the relevant data on liquid film in HCT are still rare. In this paper, based on the time-multiplexed excitation-probing scheme, a type of non-intrusive concentric conductance probes (CCPs) is developed at first. After reporting the operation principle of CCPs, the numerical simulation of potential field distribution in CCPs is conducted, and the effects of electrode structure parameters and liquid film thickness on the performance of CCPs are then analyzed in detail. It is found that in a certain range, the larger the inner electrode and outer electrode diameter, the greater the signal penetration depth (i.e. the larger the measurement range of liquid film thickness). Finally, the spatio-temporal distribution of liquid film on the wall of the HCT is experimentally studied using the optimized CCPs. The results indicate that this probe can realize the high-frequency accurate capture of liquid film flow details such as the occurrence and evolution process of liquid film inversion and the fluctuation characteristics of liquid film in HCT. The wave frequency of the liquid film is demonstrated to be not consistent along the circumferential direction. Using Strouhal number and Lockhart–Martinelli parameter, the wave frequency correlation at the maximum film thickness in the state of film inversion in HCT is obtained.

Towards an Understanding of Multiphase Fluid Dynamics of a Microfluid Jet Polishing Process: A Numerical Analysis

The microfluid jet polishing (MFJP) process is a manufacturing technology in which small abrasive particles (such as diamond, alumina, and ceria) are premixed with a carrier fluid (typically water) to form a liquid suspension that is pressurized and expelled through a nozzle for material removal. The resulting microjet beam—with a typical nozzle exit diameter in the range from 0.1 to 1.0 mm—impinges the workpiece surface for material removal by erosion and/or abrasion and produces an ultraprecision surface. This work applies a computational fluid dynamics (CFD) model to analyze the key phenomena in the interaction of the liquid suspension and the workpiece surface. The liquid film characteristics (film height, minimum film height, positions of the minimum film height, and hydraulic jump) obtained from the CFD simulations are compared with the results derived from empirical formulations found in the literature. Subsequently, the numerical results are utilized to investigate the impact velocity, pressure distribution, and shear stress caused by the suspension on the workpiece surface. It is observed that the shear stress strongly depends on the injection pressure of the liquid suspension and is weakly dependent on the abrasive suspension concentration (the liquid suspension with different densities, viscosities, and surface tensions). Additionally, the particle behavior is investigated in order to estimate the impact velocity and to identify the impact and erosion zones of the liquid suspension on the workpiece surface. Numerical results indicate that ~50% of total particles are impinging the workpiece surface almost perpendicularly (with a mean impact angle of ~86 degrees) for the first time in the stagnation region, where they are strongly decelerated by the carrier fluid before they reach the workpiece surface. These particles, however, rebound on the surface and are reaccelerated by the carrier fluid, impinging the workpiece surface further in the radial direction.

Effects of a flow obstacle on liquid film thickness in relation with CHF enhancement

Abstract There is evidence in literature that the insertion of flow obstacles in the coolant flow of a boiler flowing in an annular regime postpones the film dryout emergence on the heated surfaces. Specifically, the obstacle is believed to force the deposition of droplets to the wall, hence to increase the film flow rate, and therefore to delay dryout. The two objectives of the present research are (i) to confirm the increase in the film flow rate due to the obstacle, by (ii) building an appropriate flow-loop equipped with suited techniques for film thickness and film velocity measurements. A solid cylinder is inserted in the center of a vertical water-air annular tube flow. Variations in liquid film characteristics, e. g. film thickness and interfacial parameters (disturbance waves velocity and frequency), are measured through a local conductance probes technique. This work shows that the presence of an obstacle in an annular flow leads to an increase in the film thickness, specifically the substrate thickness, and does not affect the interfacial behavior of the film. An increase in the droplet deposition constant is observed and agrees reasonably well with predictions from the Windecker et al. (1999) correlation. The flow-loop and the measurements techniques are suited for extensive droplet deposition measurements.