Liquid Film Flow Rate Research Articles

Liquid film flow rate from measurements of disturbance wave characteristics for applications in thin film flow

Liquid film flow rate from measurements of disturbance wave characteristics for applications in thin film flow

Modeling and simulation of air-water upward annular flow characteristics in a vertical tube using CFD

Annular flow refers to a special type of two-phase flow pattern in which liquid flows as a thin film at the periphery of a pipe, tube, or conduit, and gas with relatively high velocity flows at the center of the flow section. This gas also includes dispersed liquid droplets. The liquid film flow rate continuously changes inside the tube due to two processes-entrainment and deposition. To determine the liquid holdup, pressure drop, the onset of dryout, and heat transfer characteristics in annular flow, it is important to have proper knowledge of flow characteristics. Especially a better understanding of entrainment fraction is important for the heat transfer and safe operation of two-phase flow systems operating in an annular two-phase flow regime. Therefore, the objective of this work is to develop a computational model for the simulation of the annular two-phase flow regime and assess the various existing models for the entrainment rate. In this work, Computational Fluid Dynamics (CFD) in ANSYS FLUENT has been applied to determine annular flow characteristics such as liquid film thickness, film velocity, entrainment rate, deposition rate, and entrainment fraction for various gas-liquid flow conditions in a vertical upward tube. The gas core with droplets was simulated using the Discrete Phase Model (DPM) which is based on the Eulerian-Lagrangian approach. The Eulerian Wall Film (EWF) model was utilized to simulate liquid film on the tube wall. Three different models of Entrainment rate were implemented and assessed through user-defined functions (UDF) in ANSYS. Finally, entrainment for fully developed flow was determined and compared with the experimental data available in the literature. From the simulations, it was obtained that the Bertodano correlation performed best in predicting entrainment fraction and the results were within the ±30 % limit when compared to experimental data.

Dynamic Simulation of the Stability of Annular Flows in Gas Wells

This paper provides a holistic view of the development and progression of multiphase gas-liquid annular flow in gas wells. With the help of a momentum balance, the stability of a fully developed annular flow is defined. The Euler-Euler model was used to model the multiphase flow system, while the Reynolds-Averaged Navier–Stokes (RANS) model was used to depict a time-dependent solution for a vertical fluid flow system, considering acceleration due to gravity and fluid compressibility. The resultant flow regime was tested for stability by gradually reducing the field velocity of flow and observing the effects using a computational fluid dynamics (CFD) software. The principal parameters considered are the superficial gas velocity and liquid film flow rate. The properties of the region where the flow becomes unstable are documented and related to the incidence of liquid loading in a typical gas well. The effect are critically observed and compared to flow regimes at lower superficial gas velocity. The resultant flow regime configurations are analyzed and compared to the annular flow system. This study will help researchers understand the behaviour of gas wells that have potential in developing liquid loading problems and the movement of liquid film on the wall of the tubing when in annular flow regime.

Experiments on the onset of boiling entrainment from a falling liquid film with gas sheared flow

The process of droplet entrainment caused by nucleate boiling within a falling liquid film was visualized using a high-speed camera to investigate the mechanism and the onset conditions of the boiling entrainment. In the experiments, air was flowed along the liquid film to explore the potential effects of vapor core flow on the droplet generation process in diabatic annular two-phase flow. The two types of boiling entrainment mechanisms called the jet-type and the filament-type were observed in the present work. The jet-type boiling entrainment commenced at relatively low wall heat flux and produced small droplets. The filament-type was observed at higher wall heat flux and it produced larger droplets. Hence, the filament-type was supposed to have a greater impact on the liquid film flow rate in diabatic annular two-phase flow. The experimental data accumulated in this work were used to develop correlations for the onset conditions of the jet-type and filament-type boiling entrainments.

A numerical study on fluid flow and heat transfer characteristics of oily wastewater falling film outside an elliptical tube

Falling film heat exchangers have recently been widespread as a heat transfer technique for oily wastewater. Researching the characteristics of fluid mechanism for heat transmission is crucial. In this research, numerical models are applied to simulate the heat transmission and flow of falling film outside ellipse tube. In order to emulate fluid flow and heat transmission, the oily wastewater falling film is analyzed utilizing the VOF approach under circumstances of constant thermal flow. On the flow properties and heat transfer coefficient distribution of liquid film, the impacts of the input height, contact angle, liquid film flow rate, and oil content are examined. These computational outcomes are attained: (1) More so than film thickness, according to the simulation results, increasing liquid film flow significantly affects velocity;(2) One of the main causes of how oil concentration affects heat transmission is the complex disturbance process of liquid film and the force change around tube structure;(3) Around 40°, inlet height has the greatest impact on local heat transfer coefficient;(4) The overall heat transmission will deteriorate due to the hydrophobic impact brought on by increased contact angle.

Numerical study on the effect of co-current gas flow on the falling film flow characteristics outside the horizontal tube

Gas-liquid interactions are a ubiquitous feature of falling film exchangers in the nuclear industry. In the current research, a 2D model is established to explore the impact of co-current gas flow on the hydrodynamic characteristics of the falling film outside the horizontal pipes. This study examines different factors, including film flow rate, tube diameter, co-current gas flow velocity, transverse pitch ratio, distributor height, and inlet liquid temperature. The results revealed that the co-current gas flow significantly reduces the film thickness in the strong gas-liquid interaction zone and intensifies the fluctuating motion in the weak portion. Lead to the location of the minimum film thickness shifting upwards to around 90°, which highlights the need to monitor this region to prevent the occurrence of film rupture or dry spots that negatively impact heat transfer performance. When the gas flow velocity is constant, the lower liquid film flow rate, the larger tube diameter, and the distributor height enhance the impact of gas flow velocity on the falling film, resulting in reduced liquid film thickness. A film thickness correlation applicable to both with and without co-current gas flow is established, and 91% of 900 prediction data fell within ±15% of the numerical results.

Parametric analysis of interfacial friction factor for liquid film dynamics sheared by turbulent gas flow

Water droplet impingement on a low-pressure steam turbine blade causing erosion has been recognized as a crucial issue. It is essential to elucidate a comprehensive droplet detachment mechanism, not only from the trailing edge but also from the liquid film surface. In the present paper, we investigate the influence of interfacial friction factor against liquid film dynamics on a wall sheared by a turbulent gas flow, including the liquid film thickness, liquid film velocity and entrained droplet detached from liquid surface for both pipe flow and plate flow conditions. We conduct the analyses by using a liquid film dynamics model, recently established, considering the three-dimensional destabilized waves and droplet entrainment from the liquid surface. As a result, the film thickness and velocity greatly depends on the interfacial friction factor. Interestingly, the rate of entrained droplet to initial liquid film has a minimum value when the interfacial friction factor equals to the inverse of the liquid film Reynolds number, while the remaining liquid film flow rate becomes maximum.

Falling film evaporation and pool boiling heat transfer of R1233zd(E) on thermal spray coated tube

• Falling film evaporation heat transfer of R1233zd(E) was experimentally investigated. • Up to 4.2-fold heat transfer enhancement could be obtained by thermal spray coating. • R1233zd(E) achieved a lower heat transfer coefficient than R134a at 20 °C. Falling film evaporators have been expected as an alternative to flooded evaporators for reducing the refrigerant usage. It is important for the enhancement of the film evaporation heat transfer to keep a liquid film and promote nucleate boiling in the liquid film. This study focused on nucleate boiling heat transfer in the liquid film by thermal spray coating that can be applied regardless of the material of heat transfer tubes. The heat transfer performance of falling film evaporation on a horizontal copper tube was experimentally evaluated. Since nucleate boiling phenomena, such as activation of nucleation sites and bubble behaviors in the liquid film, strongly depend on the physical properties of the refrigerant, the experimental results for R1233zd(E), one of the alternative refrigerants with low GWP, were compared with those for R134a. Two types of tubes were used: the smooth tube and the thermal spray coated tube fabricated by an arc wire spraying method, with the outer diameter of 19.05 mm and the heating length of 50 mm. Pool boiling experiments were also conducted to confirm the wall superheat for nucleate boiling. The heat transfer coefficients were evaluated with a film mass flow rate range of 8.5 × 10 −3 to 6.3 × 10 −2 kg/(m·s), a heat flux range of 10 to 85 kW/m 2 , and a saturation temperature at 20 °C. The effects of the structure of the heating surface, heat flux, and thermophysical properties of the refrigerant on the heat transfer performance are discussed. Results show that the heat transfer of the falling film became the nucleate boiling dominant with a sufficient heat flux and liquid film flow rate. The thermal spray coated surface enhanced not only the nucleate boiling but also the liquid spreading by the vapor bubbles in the liquid film, and thus the coated tube produced higher heat transfer coefficients of 2.1 to 4.8 times those for the smooth tube. The wall superheat at the onset of nucleate boiling was higher for R1233zd(E) than for R134a, because R1233zd(E) has a higher surface tension and lower vapor density. Thus, the pool boiling and falling film evaporation heat transfer coefficients were lower for R1233zd(E) than for R134a.

Composition dependence of cyclohexane‐extracted gangue drying at ambient conditions

AbstractThe drying of residue cyclohexane in a series of reconstituted gangue samples at ambient conditions was studied. The reconstituted gangue samples contained controlled amounts of residual bitumen, water, and fines (solid particles <45 μm). Regardless of the gangue composition, the initial drying flux was constant, and more than 90% of the residual cyclohexane was removed in this stage. The initial drying was driven by the liquid film flow induced by the differences in liquid film's radii of curvature along the pore channels. The liquid was a diluted bitumen solution that was transported to the surface, where cyclohexane evaporated with a rate comparable to that of pure cyclohexane evaporation. Upon the solvent evaporation, the dissolved bitumen was deposited on the sample surface. The residual bitumen content exerted an adverse effect on the initial drying flux because the dissolved bitumen significantly increased the solution viscosity, thereby reducing the liquid film flow rate. There existed an optimum water content range (3.9‐6.0 wt%) that facilitated the formation of bitumen solution film in the pores, favouring drying. When there was a high concentration of water or no water, the initial drying flux was decreased remarkably. The initial drying flux was insensitive to the fines content. Adding fines may have two counteracting effects on the drying process. Fines decreased the mean pore size, thereby lowering the liquid film flow rate. However, the hydrophobicity of the pore surfaces was increased as fines are usually covered by asphaltenes. This favours liquid film formation.

Effectiveness of viscous hydrogel in extinguishing burning solid substances

Introduction. When extinguishing burning solids, the use of water due to its high consumption and low insulating capacity of water films is ineffective. As a result of using viscous hydrogels as a fire extinguishing liquid, its consumption is reduced and a hydrogel film is formed on the burning surface, which has a greater thickness and better insulating ability compared to water. Aims and purposes. The aim of this study was to compare the effectiveness of using water and viscous hydrogels as a fire extinguishing liquid in the extinguishing of solid substances. The following tasks were solved: assessment of the impact of the concentration of gelling additives on the viscosity and evaporation of hydrogels; determination of the effect of viscosity of the fire extinguishing liquid film thickness and flow rate of liquid flowing from a vertical wall; a quantitative comparison of the effectiveness of fire suppression water, and various hydrogels according to their flow rate and film thickness. Methods. The influence of the concentration of gel-forming additives on the viscosity and evaporation of hydrogels was analyzed. An expression is obtained for the viscosity of the “Firesorb” hydrogel depending on the concentration of the polymer additive. The dependences of the evaporated mass of the hydrogel “Firesorb” from time to time at different concentrations of polymer additives. Evaluate the effectiveness of the use of hydrogels in their consumption and the thickness of the hydrogel films. The dependence of the film thickness of the extinguishing liquid flowing from the vertical wall on its viscosity and density is obtained. The coefficient of efficiency of hydrogel use is proposed, which characterizes the efficiency of hydrogel by its consumption and the insulating ability of the hydrogel film by its thickness. The efficiency of using hydrogels of different viscosities in comparison with water is calculated. Results and discussion. By changing the concentration of the gel-forming additive, you can vary the viscosity and evaporation of the hydrogel. The higher the viscosity of the hydrogel flowing from the vertical wall, the thicker the hydrogel film and less its consumption. Compared with water, the efficiency of using hydrogels is 2…4 times higher. Conclusions. The results obtained during the study indicate that the use of viscous hydrogels is effective in extinguishing burning solids.

CFD modeling of liquid film reversal of two-phase flow in vertical pipes

The phenomenon of two-phase flows is characterized by its wide range of presence in nature and industrial applications. In gas and oil production, the simultaneous two-phase flow of gases and liquids often occurs in gas wells. The produced gases flow rate decreases over the years until reaching a critical flow rate when they lose their ability to lift the associated liquids upward and the liquid film reverses direction, which triggers liquid loading. Liquid loading causes the produced liquids to accumulate in the bottom of the wellbore, which causes a high back pressure that reduces the well production rate till production is ceased eventually. The critical gas velocity exists in the churn flow regime which is mainly characterized by the oscillatory behavior of the liquids flow field. This study employs CFD techniques to model the churn flow in a 3-inch-diameter vertical pipe near the critical gas flow rate for different liquid flow rates. This work utilizes the two-fluid Eulerian model along with the RNG (Re-Normalization Group) k-ε turbulence model to investigate the behavior of the flow field in a two-dimensional axisymmetric computational domain. Simulations were carried out using the commercial software ANSYS Fluent 18.2 with air and water as the two working fluids. The model results showed a good agreement with the experimental data and proved the mesh and time independence of the model. Oscillatory behaviors of the liquid film flow rate, shear stress, and pressure were observed along with the formation of interfacial waves. Detailed information about the velocity, shear stress, and pressure behaviors is presented. Accordingly, recommendations are suggested for future considerations.

Application of different advanced oxidation processes for the removal of chloroacetic acids using a planar falling film reactor

Advanced oxidation processes (AOPs) are considered as an effective and promising method for the degradation and mineralization of aqueous recalcitrant organic pollutants. In this study, application of ozonation and various types of AOPs including photocatalysis, Fenton alone and their combinations were investigated and compared for the degradation and mineralization of chloroacetic acids (CAAs) in aqueous solutions, using a planar falling film reactor. CAAs are widely available in water treated by chlorination processes and are resistance against ozonation in the darkness. The results of the present work showed that the plain ozonation was inefficient method for the destruction of the CAAs as only about 2% degradation was observed after 90 min treatment. However, the best results were achieved by ozone in combinations with other oxidation processes. Furthermore, a synergistic effect on the removal rate was observed when these processes were exposed to the UVA light. Among the examined processes, combination of photo-Fenton with ozonation was found to be the fastest one for CAAs degradation. The effects of different parameters such as initial concentration of Fe2⁺, H₂O₂ and CAAs in photo-Fenton combined with ozonation were investigated. The optimum ratio of 0.12 of Fe2⁺/H₂O₂ concentration was found to give the best result for CAAs degradation. The degree of CAAs mineralization, measured by the total organic carbon removal, as well as the effect of falling liquid film flow rate on the removal of CAAs were also studied and discussed.

Falling Film Evaporator for Desorption of Basic Aluminum Sulfate SO2-Rich Solution and Enhancement of Heat and Mass Transfer

This study was aimed to improve the SO2 desorption effect of basic aluminum sulfate SO2-rich solution. Based on the desorption mechanism, we first used a falling film evaporation method to intensify heat transfer and mass transfer. Based on the falling film desorption heat and mass transfer model, we analyzed the falling film evaporation and desorption of basic aluminum sulfate SO2-rich solution inside the converging–diverging tube and the smooth tube and investigated the heat and mass transfer rules under different conditions. It was found as the quantity of SO2-rich solution increased, the heat transfer coefficient and mass transfer coefficient of falling film evaporation both increased, but the SO2 desorption efficiency decreased. With the rise of the heating temperature, the three indices all increased. With the rise of the inlet sulfur concentration, the three indices all increased. With the rise of the aluminum concentration, the three indices all gradually declined. With the rise of the alkalinity, the three indices all gradually declined. Comparative analysis showed the heat transfer coefficient and mass transfer coefficient of falling film evaporation and SO2 desorption efficiency were 17%–29%, 33%–69%, and 6.7%–16.3% larger inside the converging–diverging tube than the smooth tube, respectively, indicating the basic aluminum sulfate SO2-rich solution significantly outperforms inside the converging–diverging tube in terms of heat transfer and mass transfer. At the heating temperature of 108 °C, liquid film flow rate of 0.005 kg/s, sulfur concentration of 0.06 mol/L, aluminum concentration of 20 g/L, and basicity of 20%, the SO2 desorption efficiency inside the converging–diverging tube could reach a high level 94.2%, compared with only 83.7% inside the smooth tube. Moreover, correlations were obtained to predict the heat and mass transfer coefficients.

Experimental analysis of Taylor bubble behavior and mass transfer during lateral oscillation of a vertical milli-channel

In this paper, we report on an experimental study on the influence of low-frequency horizontal vibration of a vertical millimeter-size channel with Taylor bubbles. We investigated the motion, shape and dissolution rate of individual elongated Taylor bubbles of air and CO2, which were freely rising in stationary water. Bubble size and dissolution rate were determined from microfocus X-ray radiographs. From the shrinking rate we calculated the liquid-side mass transfer coefficient. The rise velocity of bubbles and surface wave motion were analyzed using a videometric technique. The comparison of the results for the stationary and the oscillating channel showed that mechanical vibration of the channel is able to enhance the mass transfer coefficient from gas to the liquid phase by 80%–186%, depending on the frequency and amplitude of vibration. It was found that channel oscillation causes the increase of free rise velocity of bubbles which is mainly attributed to the development of propelling interfacial waves and increase of liquid film flow rate. Furthermore, analyzing the surface wave motion of bubbles revealed that the enlargement of contact area between the phases and the increased mixing enhances the mass transfer additionally up to 30% compared to non-agitated bubbles of similar Peclet number.

Entrained droplets in two-phase churn flow

The presence of droplets exerts a strong influence on liquid film flow rate, pressure drop, heat transfer and many other flow characteristics. Due to the complexity of churn flow, profound knowledge on the droplets in this flow pattern is not well documented in the existing literature. In the present paper, we study a gas-liquid two-phase churn flow in a 19-mm-inner-diameter pipe under atmospheric pressure and carefully design a shadow detection technique to capture the high-resolution images of the entrained droplets in churn flow. The images are subsequently binarized using an adaptive threshold. The results indicate that the amount of the entrained droplets is high in the chaotic churn flow regime and gradually decreases during the transition from churn flow to annular flow and finally reaches a minimum around the churn-annular flow transition. The detailed process of the droplet entrainment is discussed. Based on our experimental observations, it is concluded that the large droplets (chunks) are related to the breakdown of slugs and bag breakup mechanism, whereas the smaller droplets can be ascribed to the breakup of chunks, ligament breakup and impingement. A normalized size probability density function is implemented to analyze the effect of the entrainment mechanism on the range of droplet size. The Sauter diameter of the droplets in churn flow is compared with the existing empirical correlations in annular flow and a formula for the entrained droplet size in churn flow is proposed.

Modeling transient churn-annular flows in a long vertical tube

This work investigates the transient behavior of high gas fraction gas-liquid flows in vertical pipes (annular and churn flows). Hyperbolic balance equations for mass, momentum and entropy are written for the gas and liquid, which is split between a continuous film and droplets entrained in the gas core. Closure relationships to calculate the wall and interfacial friction and the rates of droplet entrainment and deposition were obtained from the literature. A finite-difference solution algorithm based on a coefficient matrix splitting method was implemented to deal with sharp variations in the spatial and temporal domains, such as pressure and phase holdup waves. The model results were compared with steady-state experimental data from eight different sources, totaling more than 1500 data points for pressure gradient, liquid film flow rate and void/core fraction. The absolute average deviation between the model and the data was 17% for the pressure gradient and 5.8% for the void fraction. A comparison of the model results with fully transient air-water data generated in a 49-mm ID, 42-m long vertical pipe is also presented. The experimental results consist of two outlet pressure-induced and two inlet mass flow rate-induced transient tests. Two main transient parameters are compared, namely the local void fraction and the pressure difference between selected points along the test section and the outlet (taken as a reference). The comparisons between the experiments and the numerical model indicate that the model was capable of describing the transient annular to churn flow transition with absolute average deviations of 14.5% and 7.9% for the pressure difference and void fraction, respectively.

Modeling of annular two-phase flow using a unified CFD approach

A mechanistic model of annular flow with evaporating liquid film has been developed using computational fluid dynamics (CFD). The model is employing a separate solver with two-dimensional conservation equations to predict propagation of a thin boiling liquid film on solid walls. The liquid film model is coupled to a solver of three-dimensional conservation equations describing the gas core, which is assumed to contain a saturated mixture of vapor and liquid droplets. Both the Eulerian–Eulerian and the Eulerian–Lagrangian approach are used to describe the droplet and vapor motion in the gas core. All the major interaction phenomena between the liquid film and the gas core flow have been accounted for, including the liquid film evaporation as well as the droplet deposition and entrainment.The resultant unified framework for annular flow has been applied to the steam-water flow with conditions typical for a Boiling Water Reactor (BWR). The simulation results for the liquid film flow rate show good agreement with the experimental data, with the potential to predict the dryout occurrence based on criteria of critical film thickness or critical film flow rate.

CFD model of diabatic annular two-phase flow using the Eulerian–Lagrangian approach

A computational fluid dynamics (CFD) model of annular two-phase flow with evaporating liquid film has been developed based on the Eulerian–Lagrangian approach, with the objective to predict the dryout occurrence. Due to the fact that the liquid film is sufficiently thin in the diabatic annular flow and at the pre-dryout conditions, it is assumed that the flow in the wall normal direction can be neglected, and the spatial gradients of the dependent variables tangential to the wall are negligible compared to those in the wall normal direction. Subsequently the transport equations of mass, momentum and energy for liquid film are integrated in the wall normal direction to obtain two-dimensional equations, with all the liquid film properties depth-averaged. The liquid film model is coupled to the gas core flow, which currently is represented using the Eulerian–Lagrangian technique. The mass, momentum and energy transfers between the liquid film, gas, and entrained droplets have been taken into account.The resultant unified model for annular flow has been applied to the steam–water flow with conditions typical for a Boiling Water Reactor (BWR). The simulation results for the liquid film flow rate show favorable agreement with the experimental data, with the potential to predict the dryout occurrence based on criteria of critical film thickness or critical film flow rate.

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.

Measurement of Liquid Film Thickness around Horizontal Tube Bundle by Optical Technique for Optimizing Evaporator Design and Manufacturing

This paper proposes the improvement of design and manufacturing of Falling Film Horizontal Tube Evaporators (FFHTE) through optimizing different parameters such as tubes pitch, tubes diameter and material and liquid film flow rate. These design and operational parameters have a significant influence on the hydrodynamic of the liquid film (eg: wetability of the tubes, scale deposition, heat transfer coefficient, etc.). Due to the complexity of the liquid film flow around the horizontal tube bundle, the experimental approach is preferred than modeling because it gives a better understanding of the phenomena occurring in the heat exchanger. In this paper one experiment was carried out to investigate liquid film flow around a single horizontal tube. A particular attention was taken for the measurement of liquid film thickness around the tube using a novel optical technique based on light reflection. The influence of the tubes pitch, tube diameter, height of the liquid distribution system and the liquid mass flow on the transitions between falling-film modes and film thickness is investigated and the results are compared to other data obtained from the literature. It was found that tubes wetability and heat transfer increased with increasing the vertical tube pitch. To account for fouling and heat transfer performance, a tube spacing value of 1.3 was recommended.