Film Flow Rate Research Articles

Modelling of evaporative falling-film heat transfer over a horizontal tube

This article focuses on evaporative falling film heat transfer through numerical simulations. A two-dimensional symmetric liquid film flowing outside a single horizontal tube was simulated using the open-source CFD software OpenFOAM. The Tanasawa model was employed to consider mass transfer at the liquid–vapour interface, and the isoAdvector VOF method was used for interface tracking. The simulations explored the impact of film flow rate, tube surface heat flux, and tube diameter on the liquid film thickness and surface heat transfer coefficient. Additionally, the critical heat flux and operational limits were considered. The results demonstrate the accurate reproduction of evaporative falling film heat transfer performance by the present model. The film thickness exhibits a similar profile to adiabatic and sensible falling film heat transfer but provides smaller values. Similarly, the evaporative heat transfer coefficient exhibits significantly higher values and a gentle peripheral variation. The influence of heat flux on evaporative falling film heat transfer is contingent on the film flow rate; it can either enhance or weaken the process. Notably, a larger tube diameter negatively affects heat transfer, suppressing the impact of heat flux, and also increases the risk of liquid film dryout. For a given film flow rate, the average heat transfer coefficient increases initially, reaches a maximum value, and then decreases as the wall heat flux is increased. The peak heat flux, termed critical heat flux, increases with film flow rate but decreases with tube diameter. An operational limit exists where the heat flux is lower than the critical heat flux and the film flow rate is higher than the critical film flow rate; under these conditions, evaporative falling film heat transfer operates in a fully wetting status.

Simulation Analysis of Non-concentric Squeeze Film Damper

Squeeze film dampers are components that provide additional damping for rotating motors. In general, in order to achieve the best results of the squeeze film damper, it is necessary to appropriately increase the damping value, reduce the stiffness and set a reasonable eccentricity. In the process of designing the squeeze film damper, we can apply the rotor dynamics module of COMSOL software to model and simulate it, so as to achieve better simulation results and lay a foundation for the subsequent development and design of the product. In COMSOL software, in order to simplify the modeling of rotor components, we usually model squeeze film dampers based on the damping coefficient. The model calculates the damping coefficient for a short squeeze film damper and compares the results with the analytically calculated values. In addition, the damping coefficient depends on the position of the journal in the damper, i.e., the magnitude of the eccentricity. In this paper, by analyzing the dimensionless damping coefficient, bearing stiffness, journal displacement, and fluid load at different eccentricity, and plotting the fluid pressure distribution and the average oil film flow rate of the squeeze film damper, we can find the most suitable range of eccentricity and the damage-prone part of the damper, which can provide an important reference for the actual design of the squeeze film damper.

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.

Numerical and experimental investigation on droplet entrainment mechanism and closed equations of two-fluid model

As a clean and low-carbon energy source, natural gas is an important force in achieving carbon peaking and carbon neutrality goals. Affected by the strong force of gas and liquid during mining and transportation, it exhibits the phenomenon of droplet entrainment. Entrained droplets change the properties of gas and liquid phases, which is crucial for accurate measurement of liquid film thickness, gas holdup, pressure drop and flow rates in natural gas pipelines. The measurement of entrained droplets has always been a focus and difficulty in the two-phase flow research field. Therefore, numerical simulation combined with experimental measurement are presented to study the droplet entrainment mechanism and closed equations of two-fluid model. The specific parameter settings for simulating horizontal gas–liquid annular flow based on numerical method are discussed, and effective simulation of droplet entrainment and deposition in annular flow is achieved. By comparing liquid film thickness and wave velocity from the simulation results with the experimental results, the correctness of the simulation method is verified. Then, the mechanism and dynamic evolution process of entrained droplets in annular flow are revealed, and the generation and deposition characteristics of entrained droplets are studied. As a result, the closed equations of two-fluid model such as droplet velocity, gas–liquid interface velocity, and gas–liquid interface friction factor are established. The proposed two-fluid model is validated using dual mode ultrasound and differential pressure sensors, demonstrating its good applicability and extrapolation.

On icicle ripples

Natural icicles have an overall conical shape modulated by surface ripples. It has been noted from many observations of icicles formed in nature and in the laboratory that the wavelength of the ripples has a very narrow spectrum between about 8 and 12 mm and that, as time evolves, the phase of the ripples migrates upwards. In this pedagogical review, I explore some of the physical mechanisms that can cause and mediate the formation and migration of ripples on icicles using simple mathematical models. To keep the mathematics more straightforward and transparent, I confine attention to two dimensions. A key physical parameter is the surface tension between the film of water that coats an icicle and the air that surrounds it, which causes a phase shift between the film–air interface and the ice–film interface. I show that the wavelength of ripples is dominantly proportional to the cube root of the square of the gravity-capillary length times the thickness of the water film. At high film-flow rates, advection-dominated heat transfer coupled with the interfacial phase shift becomes the dominant driver of instability. Gibbs–Thomson undercooling provides an unexpectedly large stabilisation of small wavelengths at these large flow rates, sufficient to maintain wavelength selection at millimetre scales.

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.

Research on the lubrication performance of circular tilting pad thrust bearings for large wind turbines

AbstractIn view of the large temperature rise and impact load of thrust bearings in the main shaft system of large wind turbines, this article takes the circular tilting pad thrust bearings in large wind turbines as the research object, and analyses the basic theory of bearing lubrication. The lubrication performance calculation model established for this bearing includes the Reynolds equation, energy equation, film thickness equation, and elastic deformation equation. Theoretically analysing and calculating the lubrication performance of the circular tilting pad thrust bearing, the key performance parameters have identified such as minimum film thickness (hmin), temperature rise (ΔT), power consumption (W), and flow rate (Q). The calculation results show that the eccentricity ratio has a significant impact on the lubrication performance of the circular tilting pad thrust bearing. When both radial and circumferential eccentricity ratios are around 0.5, the bearing exhibits a high temperature rise, leading to a potential risk of bearing burnout. The results also show that at a radial eccentricity ratio of approximately 0.54, the minimum film thickness reaches its maximum value of 10.34 μm. Similarly, at a circumferential eccentricity ratio of about 0.60, the minimum film thickness reaches its peak value of 21.89 μm. This indicates that the eccentricity ratio plays a crucial role in the lubrication performance. In addition, the lubrication performance of the bearing under varying loads has been calculated at a speed of 9.5 r/min. The results demonstrated that the applied load significantly impacts the thrust bearing's performance. The findings elucidate the critical role of considering the eccentricity ratio and operational external load during the bearing design process. This study validates the potential of replacing rolling bearings with sliding bearings in wind turbine main shafts. It also provides a theoretical reference for the future design of sliding bearings.

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.

Lubrication analysis of hydrodynamic-static hybrid bearing with deep-shallow chambers considering thermal conduction

The oil film separates the shaft-bearing system to reduce the frictional force of contact surfaces with large temperature variations owing to thermal conduction. Multiple impact factors seriously affected the lubrication behavior of the bearing during actual working process. An applicable model regarding the lubrication analysis of a hydrodynamic-static hybrid bearing with deep-shallow chambers is proposed and validated with published literature. The lubrication behavior includes film pressure, load capacity, bearing stiffness and flow rate is numerically calculated with centered finite difference method by solving the Reynolds equation. The influence of film thickness and working temperature are considered, over a range of eccentricity ratio. Moreover, the temperature variation caused by thermal conduction is analyzed in details. To investigate the influence of thermal conduction on the lubrication behavior, the results show that the load capacity, bearing stiffness, friction power, and flow power significantly decline with the consideration of thermal conduction. This study proposes an accurate model that is helpful for the design of hydrodynamic-static hybrid bearings with deep-shallow chambers under low rotational speed, heavy load, and high temperature conditions.

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.

Research on Lubrication Characteristics of Cage-Free Ball Bearing with Local Functional Slot

The factors affecting the lubrication effect of a ball bearing without cage and containing a functional slot are analyzed, including the structural parameters of the functional slot, the speed of the rolling element, and the deformation of the contact surface, in order to establish the initial oil volume equation. Based on the multiple mesh method and Matlab programming, the established model is solved by obtaining the distribution rules of oil film pressure, oil film thickness, and oil film flow rate between the rolling element, the conventional raceway, and the functional slot under different speed conditions, and by determining the optimal functional slot depth. Finally, through an experiment performed to verify the lubrication effect of the lubricating oil in the functional slot, the results show that the lubricating oil in the functional slot can have a lubricating effect, and the initial amount of lubricating oil needed increases with an increase in speed.

Analysis and Multi-target Optimisation of the Characteristics with Stepped Cavity of the Hybrid Bearing

In this paper, the peak pressure, stiffness, flow rate, and temperature rise of the oil film in the stepped cavity of the hybrid bearing are investigated. The pressure and flow equations for the central section of the stepped cavity and the axial section of the deep and shallow cavity are derived using the simplified Navier-Stokes equations and the boundary conditions of a liquid bearing. Using the control variable method, the influence of the geometrical parameters of the stepped cavity and the process parameters on the oil film characteristics was simulated, and the correctness of the pressure and flow equations was verified. The results show that: the spindle speed is positively correlated with the peak oil film pressure and flow rate; the oil film thickness is negatively correlated with the peak oil film pressure and positively correlated with the flow rate; the depth and width of the shallow cavity are correlated with the peak oil film pressure, but the correlation with the flow rate is low; the maximum error between the theoretical calculation and the simulation analysis is less than 7.3%. The pressure equation and the flow equation are used to establish the objective function with the maximum oil film stiffness, load-bearing capacity, and minimum temperature rise, and the ideal point evaluation function method is used to do a multi-target optimization design to optimize the ladder cavity and obtain the optimal oil cavity geometric parameters. And an 8.75% increase in peak pressure and a 4.1% reduction in temperature rise for the same process parameters.

A potential-responsive ion-pump system based on nickel hexacyanoferrate film for selective extraction of cesium ions

A nickel hexacyanoferrate (NiHCF) film electrode was prepared with NiHCF, conductive carbon black, and polyvinylidene difluoride, which was coated on graphite plate substrate for selective extraction of Cs+ ions by using electrochemically switched ion exchange (ESIX) technology. A potential-responsive ion-pump system for efficient extraction of Cs+ ions was designed, and the effect of wet film thicknesses, charging modes, flow rates, and chamber widths on Cs+ ions extraction performance was investigated. In the system, the adsorption capacity and removal percentage of Cs+ ions on the NiHCF film electrode reached as high as 147.69 mg·g−1 and 92.47%, respectively. Furthermore, the NiHCF film electrode showed high selectivity for Cs+ ions and stability. After seven cycles of adsorption/desorption, the desorption percentage could reach about 100%. The excellent Cs+ extraction performance should be attributed to the strong driving force produced by the potential-responsive ion-pumping effect in the ESIX process, as well as the low ion transfer resistance of the film electrode which is caused by the special crystal structure of NiHCF. In addition, the NiHCF film electrode was implemented to work together with the bismuth oxybromide (BiOBr) film electrode to accomplish the simultaneous extraction of Cs+ and Br–. And the adsorption capacity and removal percentage of Br– ions on the BiOBr film electrode reached 69.53 mg·g−1 and 77.32%, correspondingly. It is expected that such a potential-responsive ion-pump system based on NiHCF and BiOBr film electrodes could be used for the selective extraction and concentration of Cs+ and Br– ions from salt lake brine..

Experimental study of dripping, jetting and drop-off from thin film flows on inclined fibers

Gravity driven film flows on vertical fibers are known to exhibit a variety of flow dynamics including the formation of droplet trains induced by the hydrodynamic (Kapitza) and Plateau–Rayleigh instability mechanisms. Through an experimental study, it is shown how inclination of the fiber from the vertical influences these dynamics. The formation of waves, regime transitions from dripping to jetting regimes, as well as the onset of drop-off in the form of droplet detachment from the fiber are illustrated and described in dependence of the fiber inclination angle and the liquid mass flow rate. Additionally, the influence of fiber diameter and nozzle geometry on regime transitions and the onset of drop-off from the substrate are examined. It is shown that the onset of drop-off is strongly related to the transition from a regime characterized by a regular wave pattern to a regime characterized by an irregular wave pattern. It is also demonstrated that this regime transition depends not only on flow rate and fiber geometry, but also strongly on the inclination angle. Interestingly, a stabilizing effect of increasing the fiber inclination is detected for constant fiber geometry and film flow rate.

Experimental and Numerical Study of Droplet Impact on Radially Flowing Liquid Film

In this paper, a single droplet impact onto a radially flowing liquid film is investigated both experimentally and numerically. The interfacial evolution and dynamic features are studied in detail with regard to various droplet impact velocities and film flow rates. Also, the corresponding mechanisms are analyzed with the aid of numerical results using the coupled level set and volume of fluid method. Results show that the droplet impact on the radial film produces a larger crown radius in the direction perpendicular to the film flow compared with the unidirectional film. Increasing the impact velocity leads to increases in crown height and upstream radius, but it has a weak effect on the downstream radius. As for the effect of the film flow rate, its increase causes a rise in the secondary droplet number in the upstream but makes the finger jets associated with the crown splashing less developed. Besides, increasing the flow rate of the liquid film results in increases in the upstream crown height and downstream radius but decreases in the downstream crown height and upstream radius. Finally, a formula for predicting the threshold of splashing is built up based on the film flow Reynolds number and the droplet Weber number.

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.

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.