Liquid Film Velocity Research Articles

Effect of geometric and operating parameters on the spray characteristics of an open-end swirl injector

To study the influence of geometric and operating parameters on the spray characteristics of an open-end swirl injector, seven injectors with different tangential inlet diameters ( D p) and injector length to injector orifice diameter ( L/D) ratios were tested and simulated. Using high-speed backlight, the evolution laws of liquid film thickness, discharge coefficient, spray cone angle, breakup length, and velocity distribution in the swirl chamber under different geometric and operating parameters were captured after unified image processing. Low-injection pressure drop is directly proportional to the discharge coefficient and the spray cone angle. When the injection pressure drop approaches or reaches a critical value of 0.4 MPa, the discharge coefficient and spray cone angle remain nearly constant with maximum fluctuations of 1% and 5%, respectively. With an increase in the geometric characteristic constant A, the liquid film thickness, discharge coefficient, breakup length, and velocity in the swirl chamber decrease, whereas the spray cone angle increases. As the viscous effect increases for increasing L/D, the discharge coefficient and breakup length increase, whereas the spray cone angle decreases. Based on experiment results, empirical formulas for the discharge coefficient, spray angle, and breakup length were put forward as reference for engineering applications, including the effect of the geometric and operating parameters.

Experimental investigation of heat transfer of flowing liquid film with inserted metal foam layer subjected to air jet impingement

In this paper, coupling the air jet impingement and the copper metal foam above flowing liquid film were employed to enhance the heat transfer. The thickness of flowing liquid film can be controlled owing to the application of the metal foam above the film, and its solid matrix extends the air-liquid-solid interface of heating surface. The evaporated water can be supplied by the capillary force in the porous layer. The experiments were conducted to investigate the performances of the flowing liquid film with inserted porous layer subjected to impinging jet air. The air jet velocity, the flow rate and thicknesses of the liquid film as well as the porosity of metal foam influence the surface temperature of heated wall and the corresponding local heat transfer coefficient greatly. The change ratios of heat transfer coefficient due to the above factors were presented. More cooling can be obtained on the heated wall in the flowing liquid film with inserted porous layer subjected to impinging jet air while the higher liquid film velocity and air jet velocity, the thinner liquid film and the lower porosity of metal foam occur.

Heat transfer mechanisms of single droplet impacting onto flowing film

The experimental study of heat transfer and film dynamics of droplet impacting onto flowing film using Novec-7100 as the cooling liquid was presented in this article. The droplet velocity and the liquid film velocity could be adjusted in this experimental study. The Reynolds number of the flowing liquid film varied from 1257 to 6290. In this study, the substrate of heater was Zinc Selenide (ZnSe), and the heating element was a thin layer of Indium Tin Oxide (ITO). The overflow plate was used to make the liquid film thickness fixed at 1 mm. The heat transfer mechanisms of flowing liquid film (flowing film only), droplet impinging on the dry surface (droplet-dry surface) and droplet impinging on the flowing liquid film (droplet-flowing film) were discussed in this paper respectively. The results showed that when the averaged temperature of the heated surface does not reach the liquid saturation temperature, bubbles appear in the liquid film, and the bubbles are more dense downstream. When the averaged heat flux of the heated surface ranges from 6.2 × 103 W/m2 to 3.4 × 104 W/m2, the averaged heat transfer coefficient of the droplet-flowing film can be calculated by the superposition of the averaged heat transfer coefficient of the flowing film only and the averaged heat transfer coefficient of the droplet-dry surface, the maximum error is 14%. When the averaged heat flux of the heated surface ranges from 3.4 × 104 W/m2 to 6.8 × 104 W/m2, the Reynolds number of the flowing liquid film and the effect of the coupling between droplet impingement and flow boiling should be considered in the calculation of the averaged heat transfer coefficient of the droplet-flowing film. In addition, boiling heat transfer is the main heat transfer mechanism of droplet impinging on thin liquid film when the averaged heat flux of the heated surface ranges from 2 × 104 W/m2 to 6.8 × 104 W/m2.

Effect of surface tension, gravity and turbulence on condensation patterns of R1234ze(E) in horizontal mini/macro-channels

The condensation patterns of R1234ze(E) in circular mini/macro-channels with diameters ranged from 0.493 to 4.57 mm was numerically investigated. The effects of mass flux, vapor quality and tube diameter on heat transfer coefficients were analyzed. The relative role of surface tension, gravity and shear stress in mini/macro-channels was explored and the turbulence effect of liquid phase was discussed. The heat transfer coefficients increased with the mass flux, vapor quality and the decrease of tube diameter. The liquid film thickness decreased significantly when reducing the tube dimension for the increased shear stress. The surface tension was found to play a role to increase the heat performance by enhancing the heat convection between liquid film and wall surface, which was only obvious in mini-channels. Contrary to the effect of surface tension, the gravity effect was negligible in mini-channels with a diameter less than 1 mm, while had an important effect on heat transfer coefficients for macro-channels. The gravity affected both liquid film distribution and velocity field, which can either enhance or weaken the heat transfer coefficient during condensation flow. The turbulence in liquid film played an important role in reducing the local thermal resistance and was more important in tubes with larger diameter. The onset of turbulence inside the liquid film may occur at much lower liquid Reynolds numbers for mini-channels.

Development of wall and interfacial friction models for two-dimensional film flow with local measurement methods

Abstract Based on the advancement of computational capability, high-precision analyses of multi-dimensional thermal hydraulic phenomena in nuclear power plants have been carried out. One of the multi-dimensional phenomena is two-phase flow in the upper downcomer. The quantification of momentum transfer between a downward liquid flow and a transverse gas flow in the reflood phase is significant because it determines the bypass flow rate of the emergency core coolant (ECC). Multi-dimensional modules of thermal-hydraulic analysis codes have been used to simulate this multi-dimensional two-phase flow. However, the model and correlations incorporated in the multi-dimensional modules are ones developed from one-dimensional experimental results. Because the characteristics of multi-dimensional phenomena are different from one-dimensional conditions, the implemented models and correlations might produce inaccurate results. Therefore, new models based on multi-dimensional experimental data should be developed. This study involved the twodimensional, two-phase experiments performed using 1/10 and 1/5 scale facilities. These were simplified unfolded downcomers of the Advanced Power Reactor 1400 (APR1400). The local measurement methods, pitot tubes, ultrasonic thickness gauge, and depth-averaged PIV, were applied to measure local variables: air velocity, liquid film velocity, and liquid film thickness. Local experimental data on the two-dimensional film flow were used to produce the wall and to inform interfacial friction factors from two-dimensional, two-phase momentum conservation equations. New wall and interfacial friction models were developed incorporating newly proposed physical concepts.

Numerical study of falling film flow on a horizontal rotating tube

In this paper, a CFD simulation of falling film on a horizontal rotating tube is illustrated. The objective of this simulation is to explore the influence of various parameters on this kind flow. The simulation results show that in a certain range, the offset angle of the liquid column increases with the increase of the rotational speed, which agrees well with the experimental values. Compared to a stationary tube, the liquid film thickness at the left side of the liquid column is thinner but the right gets larger when the tube is rotating counterclockwise. And the liquid film velocity at the left side of the liquid column is equivalent to the linear superposition of the velocity at the standstill and the rotational velocity denoted by ω. Rotation has a great influence on the velocity distribution at the right side of liquid column. Increasing the rotational speed and the diameter, decreasing the inlet velocity and the inlet hole diameter will make the offset angle of the liquid film larger. The high temperature reduces the offset angle of the liquid column, and the gradient of the liquid column offset angle to temperature becomes smaller. When the temperature is above 65 °C, the effect of temperature on the offset angle of liquid column is negligible. When ω = 200 rpm, the offset angle increases first and then decreases with the increase of the inlet height, but when ω = 300 rpm, the offset angle increases with the increase of the inlet height.

Effects of Nozzle Geometry on the Fluid Dynamics of Thin Liquid Films Flowing down Vertical Strings in the Rayleigh-Plateau Regime.

Thin-liquid films flowing down vertical strings undergo instability, creating wavy film profiles and traveling beads. Previous studies assumed that the liquid film thickness and velocity profiles within the healing length from a nozzle were specified by the Nusselt solution, independent of the nozzle geometry. As a result, the influence of the nozzle diameter on the flow characteristics, such as the liquid bead size, spacing, and traveling speed, was largely overlooked. We report an experimental and numerical simulation study on liquid-film flows in the Rayleigh-Plateau regime while systematically varying the nozzle diameter from 0.5 to 3.2 mm at different mass flow rates (0.02, 0.04, 0.06, and 0.08 g/s). We find that the nozzle diameter does have a strong influence on the flow regime and the flow characteristics. We identify the thickness of a nearly flat portion of a liquid film that precedes the onset of instability, which we term the preinstability thickness, as a critical flow parameter that governs the size, spacing, and frequency of liquid beads that develop downstream. By defining the liquid film aspect ratio α in terms of the preinstability thickness, we capture a flow transition from the Rayleigh-Plateau (RP) instability regime to the isolated droplet regime. Improved understanding of the flow regimes and characteristics assists in the systematic design and optimization of a wide variety of processes and devices, including fiber coating and direct contact heat and mass exchangers.

Mechanistic modeling of critical sand deposition velocity in gas-liquid stratified flow

Among the different regimes in multiphase pipelines, stratified flow is most prone to sand deposition, owing to the low liquid velocity and the lack of mixing. The main goal of this study is to predict the critical sand deposition velocity under stratified flow. The critical sand deposition velocity is defined as the minimum velocity that keeps all particles moving at all time, above which there is no stationary bed at all.Two models with similar approaches are developed based on the forces acting on a particle, such as the drag, turbulent, apparent gravity, van der Waals and lift forces for prediction of the transitions between moving/stationary dunes and moving/stationary bed. Dune pilling up and collapsing mechanism considering a torque balance on a rolling particle located at the top of the dune is adopted for stationary dunes. Torque balance applied to a rolling particle located on the lowest stratum of the moving bed layer is used to predict the minimum liquid film velocity for particle to grow a stationary bed. Comparison between the predictions of the two mechanistic models and the experimental data show good agreement, with critical sand deposition velocity absolute relative errors at 16.4% for stationary dunes and 12.6% for stationary bed.

Research of vertical falling film behavior in scrubbing-cooling tube

Experimental results of the thickness and velocity distribution of the vertical falling film of a gas–liquid two-phase flow in a scrubbing-cooling tube are reported. The liquid Reynolds numbers ranges from 1.0×104 to 3.1×104. A 3D numerical model of the scrubbing-cooling ring and tube has also been established. The simulation results agree well with the experimental data. The research shows that the average liquid film thickness is largest at the 0° circumferential position. A part of the liquid is separated from the local liquid film and drips in the gas core when Reynolds number≥1.5×104. Circumferentially the liquid film thickness is distributed more evenly in the lower section of the tube. When the liquid Reynolds number increases, both of the global liquid film thickness and velocity increase, and the thickness distribution becomes unevener. The liquid film velocity increases with the increase of flow distance at small Reynolds number and decreases at large Reynolds number.

Assessment of wall friction model in multi-dimensional component of MARS with air–water cross flow experiment

Recently, high precision and high accuracy analysis on multi-dimensional thermal hydraulic phenomena in a nuclear power plant has been considered as state-of-the-art issues. System analysis code, MARS, also adopted a multi-dimensional module to simulate them more accurately. Even though it was applied to represent the multi-dimensional phenomena, but implemented models and correlations in that are one-dimensional empirical ones based on one-dimensional pipe experimental results. Prior to the application of the multi-dimensional simulation tools, however, the constitutive models for a two-phase flow need to be carefully validated, such as the wall friction model. Especially, in a Direct Vessel Injection (DVI) system, the injected emergency core coolant (ECC) on the upper part of the downcomer interacts with the lateral steam flow during the reflood phase in the Large-Break Loss-Of-Coolant-Accident (LBLOCA). The interaction between the falling film and lateral steam flow induces a multi-dimensional two-phase flow. The prediction of ECC flow behavior plays a key role in determining the amount of coolant that can be used as core cooling. Therefore, the wall friction model which is implemented to simulate the multi-dimensional phenomena should be assessed by multidimensional experimental results. In this paper, the air–water cross film flow experiments simulating the multi-dimensional phenomenon in upper part of downcomer as a conceptual problem will be introduced. The two-dimensional local liquid film velocity and thickness data were used as benchmark data for code assessment. And then the previous wall friction model of the MARS-MultiD in the annular flow regime was modified. As a result, the modified MARS-MultiD produced improved calculation result than previous one.

Experimental study on two-dimensional film flow with local measurement methods

In an accident condition of a nuclear reactor, multidimensional two-phase flows may occur in the reactor vessel downcomer and reactor core. Therefore, those have been regarded as important issues for an advanced thermal-hydraulic safety analysis. In particular, the multi-dimensional two-phase flow in the upper downcomer during the reflood phase of large break loss of coolant accident appears with an interaction between a downward liquid and a transverse gas flow, which determines the bypass flow rate of the emergency core coolant and subsequently, the reflood coolant flow rate. At present, some thermal-hydraulic analysis codes incorporate multidimensional modules for the nuclear reactor safety analysis. However, their prediction capability for the two-phase cross flow in the upper downcomer has not been validated sufficiently against experimental data based on local measurements. For this reason, an experimental study was carried out for the two-phase cross flow to clarify the hydraulic phenomenon and provide local measurement data for the validation of the computational tools. The experiment was performed in a 1/10 scale unfolded downcomer of Advanced Power Reactor 1400 (APR1400). Pitot tubes, a depth-averaged PIV method and ultrasonic thickness gauge were applied for local measurement of the air velocity, the liquid film velocity and the liquid film thickness, respectively. The uncertainty of the depth-averaged PIV method for the averaged velocity of the liquid film was discussed. Also the local velocity profiles of air and liquid film and the liquid film thickness distribution were presented. These local experimental data of two-dimensional film flow which simulated the two-phase cross flow can be used to validate the multidimensional models in the system analysis codes and CFD codes.

An investigation on the falling film thickness of sheet flow over a completely wetted horizontal round tube surface

An investigation was performed to analyze the liquid film distributions of sheet flow on the surface of a horizontal round tube in a falling film evaporator using the commercial computational fluid dynamic code Fluent 6.3.26. A two-dimensional multi-phase flow model was developed under adiabatic condition. The temporal variation of flowing process of sheet flow and the steady film thickness distribution in the circumferential direction were analyzed in detail. The main objective has focused on the effects of multiple factors on the film thickness distribution. It was found that the whole process of liquid film flowing along the horizontal tube surface includes the transient sub-process and steady-state sub-process, and the former can be divided into five stages to be analyzed. The obviously asymmetric distribution of film thickness, which is contradictory to the Nusselt theory, is captured by the simulation results. Multiple factors have effects on the film thickness distribution. The film thickness increases with the increase in film Reynolds number. The big inter-tube spacing is capable of inducing high momentum and increasing the impact velocity of liquid film on the top of the tube, which results in the thinner distribution of liquid film layer around the perimeter of the horizontal tube. The film thickness becomes thinner as tube diameter increases. The effect of tube diameter is minor and the further increment of tube diameter shows few values to the improvement of heat and mass transfer.

Theoretical calculation of the characteristics of annular flow in a rectangular narrow channel

A mathematical separated flow model has been developed that is applicable to the annular two-phase flow in the rectangular narrow channels with peripheral heating. The theoretical annular flow model is based on the fundamental conservation principles: the mass, momentum and energy conservation equations of liquid film, and the momentum conservation equation of the vapor core. Through numerically solving the closed equations, boiling heat transfer coefficient, axial pressure gradient, liquid film thickness and velocity profiles in the liquid film are obtained. A good agreement has been found through comparing the experimental data and theoretically predicted results. The liquid film thickness will decrease with the increase of heat flux and channel width, while increases with increasing flow rate and channel height. The velocity profile in the liquid film is not linear when the thickness of the liquid film is not very thin and it increases with increasing heat flux, while decreases with the increase of mass flow rate and channel size. The effects of heat flux and channel width on velocity in the liquid film are larger than that of mass flow rate. The velocity in liquid film tends towards linear, when the liquid film is mainly in the laminar boundary layer.

Effect of thermal radiation on velocity and temperature fields of a thin liquid film over a stretching sheet in a porous medium

An analytical approach using the least-squares method (LSM) has been carried out to study the effect of thermal radiation on the flow and heat transfer in a thin liquid film over an unsteady stretching sheet embedded in a porous medium. Similarity transformations were used to convert the momentum, continuity and energy equations describing the problem, which are coupled nonlinear partial differential equations into a set of nonlinear ordinary differential equations. The resulting set of Ordinary Differential Equation (ODE) is then solved analytically by the least-squares method. By comparing the results of the applied method with previously published work the accuracy of the proposed method has been verified. Finally the effects of different outstanding parameters in this particular case such as the Darcy parameter, the radiation parameter and the Prandtl number on the flow velocity and temperature profiles are communicated.

Experimental study on the characteristics of air–water two-phase flow in vertical helical rectangular channel

Experimental investigations on the characteristics of air–water two-phase flow in the vertical helical rectangular channel are performed using the high speed flow visualization. The flow pattern map and the transition in the helical rectangular channel are presented. The flow pattern evolution in different positions of the helical rectangular channel is illustrated. The discussion on the coalescence of the bubble and slug is presented. The slug velocity, slug length distribution, liquid slug frequency, falling liquid film velocity and falling film thickness along the slug are investigated. The dimensionless liquid film thickness of the annular flow on the outer side of the channel is measured using the digital image processing algorithm.

Experimental research on breakup of 2D power law liquid film

On account of limited knowledge of the breakup of power law liquid film, the process of its disintegration and atomization was studied by using a planar liquid film. A linear stability analysis was adopted to predict the breakup characteristics of the power law film. The predicting formulas of stripping breakup length and diameter of ligament were put forward presently. Through high-speed photography and laser light sheet illumination, different breakup characteristics of flat power law film under different conditions were derived. The characteristic dimension of breakup regimes were defined and extracted. The effects of several parameters (injection pressure, ambient pressure, nozzle structure and fluid property) on the stripping breakup length and spray angle were investigated. The results revealed that increasing both the velocity of liquid film and the ambient pressure facilitated the breakup of film, reduced the stripping breakup length and enlarged the spray angle in different extents. The comparison between theoretical and experimental results was conducted to validate the feasibility of the linear stability theory.

Experimental and computational investigation of vertical downflow condensation

This explores downflow condensation in a circular tube both experimentally and computationally using FC-72 as a working fluid. A highly instrumented condensation module is used to map detailed axial variations of both wall heat flux and wall temperature, which are used to determine axial variations of the condensation heat transfer coefficient. The experimental results are compared to predictions of a two-dimensional axisymmetric computational model using FLUENT. The study provides detailed construction of the model, including choice of interfacial phase change sub-model, numerical methods, and convergence criteria. The model is shown to yield good prediction of the heat transfer coefficient. The computed temperature profiles exhibit unusual shape, with steep gradient near the annular liquid film interface as well as near the wall, and a mild gradient in between. This shape is shown to be closely related to the shape of the eddy diffusivity profile. These findings point to the need for future, more sophisticated measurements of liquid film thickness, and both velocity and temperature profiles, to both validate and refine two-phase computational models.

Numerical analysis on air entrapment during droplet impacting on a wetted surface

A numerical model is developed using the coupled level set and volume of fluid method including heat transfer and contact resistance to simulate air entrapment during a droplet impacting on a wetted surface. The dynamic characteristics of the phase interface are analysed. The mechanisms of deformation of the phase interface and formation of entrapped air are explored. The effects of impacting velocity and thickness of liquid film on characteristics of entrapped air are studied. The mechanism of heat transfer is also obtained in this article. The obtained results are as follows. The pressure difference between liquid and gas before the droplet impacting is the main factor determining the deformation of phases interface and the formation of air entrapment. The larger the impacting velocity, the larger the pressure inside the compressed air film is. When the droplet contacts the liquid film, the velocities of the droplet and liquid film increase to their maximum values, and at the impacting axis, they are approximately the same, nearly half the impacting velocity. The velocity distributions of phase interface of the droplet and liquid film are nearly the same in the area of impacting center. The impacting velocity has important effects on the dimensionless arc from bottom to breaking point and the dimensionless diameter of the air. The dimensionless arc and dimensionless diameter decrease with increasing impacting velocity. The dimensionless deforming heights of the droplet and liquid film are closely related to Stokes number: the larger the Stokes number, the larger the dimensionless deforming heights are, and they can be expressed as a power function with Stokes number. The initial thickness of liquid film also affects dimensionless deforming heights of the droplet and liquid film and dimensionless diameter of the entrapped air: the larger the dimensionless thickness of the liquid film, the larger the dimensionless deforming heights are, and the dimensionless diameter decreases with increasing dimensionless thickness of the liquid film. At the very initial stage of the impact, the entrapped air is important for surface heat flux distribution. The entrapped air presents contraction, breakup and detachment. The surface heat flux distribution changes closely with evolution of the entrapped air and tends to be uniform. The effect of the entrapped air on the surface heat flux distribution decreases gradually.

150 3次元光線追跡シミュレーションに基づく単一光ファイバープローブによる液膜厚さ・速度計測法の開発と高精度化

150 3次元光線追跡シミュレーションに基づく単一光ファイバープローブによる液膜厚さ・速度計測法の開発と高精度化

Reprint of: A multiphase, micro-scale PIV measurement technique for liquid film velocity measurements in annular two-phase flow

Prediction methods for two-phase annular flow require accurate knowledge of the velocity profile within the liquid film flowing at its perimeter as the gradients within this film influence to a large extent the overall transport processes within the entire channel. This film, however, is quite thin and variable and traditional velocimetry methods have met with only very limited success in providing velocity data. The present work describes the application of Particle Image Velocimetry (PIV) to the measurement of velocity fields in the annular liquid flow. Because the liquid is constrained to distances on the order of a millimeter or less, the technique employed here borrows strategies from micro-PIV, but micro-PIV studies do not typically encounter the challenges presented by annular flow, including very large velocity gradients, a free surface that varies in position from moment to moment, the presence of droplet impacts and the passage of waves that can be 10 times the average thickness of the base film. This technique combines the seeding and imaging typical to micro-PIV with a unique lighting and image processing approach to deal with the challenges of a continuously varying liquid film thickness and interface. Mean velocity data are presented for air–water in two-phase co-current upward flow in a rectangular duct, which are the first detailed velocity profiles obtained within the liquid film of upward vertical annular flow to the authors’ knowledge. The velocity data presented here do not distinguish between data from waves and data from the base film. The resulting velocity profiles are compared with the classical Law of the Wall turbulent boundary layer model and found to require a decreased turbulent diffusivity for the model to predict well. These results agree with hypotheses previously presented in the literature.