Liquid Film Thickness Distribution Research Articles

Dryout Quality Prediction in Helical Coils Based on the Nonuniform Liquid Film Thickness Distribution: A Model Study

Dryout Quality Prediction in Helical Coils Based on the Nonuniform Liquid Film Thickness Distribution: A Model Study

Flow behavior and film thickness of gas-oil two-phase flow in the single screw expander

The objective of this work is to study the flow behavior and oil film thickness in the vertical helical channel to improve the efficient operation of the single screw expander. The volume-of-fluid (VOF) model is adopted to examine various effects, including flow direction, oil viscosity, surface tension, flow rate, and oil inlet width on the liquid film thickness distribution and liquid holdup. An interesting phenomenon is found that the gas axial velocity is accelerated in the flow, and this has a profound influence on the liquid film thickness distribution and liquid holdup. The liquid film thickness exhibits a significant improvement as the downward flow is considered. The liquid film thickness becomes more symmetrical with increasing the oil viscosity, surface tension, oil flow rate, and inlet width, and more liquid are concentrated at the two endpoints of the channel. A higher viscosity oil or larger surface tension coefficient will result in a higher liquid holdup. These results and analysis are crucial for improving the performance of single screw expanders.

Growth of dry patches in an evaporating film flowing around a horizontal circular tube

The falling film flows over solid surfaces have been exploited extensively, but the modeling of film dry-out and the dry patches' formation poses a considerable challenge. Here, we presented a simple mathematical model that accounts for the evaporation to predict the evolution of dry patches in a falling film. Besides, we also analyzed the effect of flow rate, spray temperature, and tube diameter on the growth of dry patches. The distribution of liquid film thickness is asymmetrical, and the film thickness decreases firstly and then increases after a central angle of 90°. The liquid film covered the tube initially under the action of gravity, then evaporation thinned liquid film and induced small dry patches. With the increasing flow rate, decreasing spray temperature, or decreasing tube diameter, the dry-out time increases, and the dry patche area decreases. It is more effective to suppress dry patches' appearance by controlling both flow rate and spray temperature rather than tube diameter.

Improvement of offshore adaptability of main cryogenic heat exchanger in FLNG dual mixed refrigerant liquefaction process

The dual mixed refrigerant liquefaction process and its key equipment applied in FLNG (floating liquefied natural gas) suffer performance deterioration due to the offshore conditions. In order to improve the offshore adaptability of FLNG dual mixed refrigerant liquefaction process, the experimental device of FLNG dual mixed refrigerant liquefaction process, the numerical simulation model and the visualization experimental device of falling film flow are developed. Multi-scale marine adaptability is studied from the aspects of the liquefaction process and its key equipment (meter scale) and liquid film thickness distribution (micron to millimeter scale). The results show that the effect of offshore conditions is more obvious on the spiral wound heat exchanger (SWHE) than the plate-fin heat exchanger (PFHE) of precooling circuit and the low temperature throttle valve of natural gas. It is recommended to use the corrugated tube with the diameter difference of 2 mm in the FLNG spiral wound heat exchanger to improve the offshore adaptability of dual mixed refrigerant liquefaction process.

Effect of part-length rods for different BWR subchannel geometries

Despite the beneficial effects of part-length rods (PLRs) in BWR fuel, they also introduce inhomogeneities to the coolant flow. These effects are hard to simulate and might affect cooling efficiency, dryout margin and CRUD deposition. So far, no fundamental experimental research was conducted to explore the details of their influence to the liquid film behavior in the dominating annular flow regime.To investigate the fluid dynamical effects of PLRs, we analyzed three different types of subchannel geometries. We varied the positions of PLRs in a pair of adjacent subchannels as well as the liquid and gas flow rates, staying within the annular flow regime. The adiabatic air-water test section was equipped with two types of sensors measuring the liquid film thickness distribution on half a fuel rod and on a fuel element wall. The measurement frequency was 10 kHz and the lateral resolution in the two-dimensional measuring domain was 2 mm. This corresponds to 64 × 16 and 16 × 32 measuring points, for rod and wall sensor respectively.Closely downstream of a PLR, we found a recirculation zone with less shear driving the liquid film. This is reflected by an increase of the average film thickness and a decrease of the wave velocity. A cross-flow of gas and liquid towards the open space downstream of the PLR was detected. The side of neighboring fuel rods facing this open space sees an increase of liquid flow. On the PLR-averted side, the coolant flow decreases, reducing the dryout margin. Further downstream, more gas tends to traverse into the open space left behind the PLR, which again increases the driving shear stress on the liquid film. The film becomes thinner and faster. The precise channel geometry featuring PLRs have a strong and complex influence on the local average liquid film thickness and the wave velocities. Their qualitative distribution at different flow conditions however, is similar.

Numerical Simulation for Falling Film Thickness around Horizontal Tube in MVC and MED Evaporators

Falling film on horizontal tube evaporators, of both Mechanical Vapor Compression (MVC) and the Multi-Effect Distillation (MED) desalination systems, plays an important role in the heat and mass transfer (evaporation) and accordingly the systems productivity. Falling film thickness is mainly influenced by the intertube space, circumferential angle and the film’s Reynolds number. This paper presents two-dimensional numerical study of falling film thickness around horizontal tube in MVC and MED evaporators. The study is based on computational fluid dynamics (CFD) using volume of fraction (VOF) as a multi-phase technique in ANSYS Fluent. The numerical model is developed in order to study the heat and mass transfer charactristics, the liquid falling film behaviour and thickness distribution around circular horizontal. Four CFD study cases are developed to simulate the falling film behaviour at circumferential angle range from 150 to 1650 with inter-tube spacing of 10 mm, 16 mm, 33 mm and 40 mm and for constant value of flow rate and at the same surrounding conditions. Simulations are conducted using a domain of only two tubes with 20 mm outer diameter.The results from the numerical models are compared with the published experimental correlations, showing a comparatively reasonable agreement. In addition, a parametric study is carried out to illustrate the effect of flow Reyonlds number (Re) and intertube space on the average circumferential film thickness and heat transfer rates.

Liquid film dispersion on horizontal circular tubes under spray impingement

The evaporative cooling equipment using spray films can improve the thermal and anti-scaling characteristics owing to the advantages of simple structure, effective removal of the blockage and expanded liquid coverage. This paper explores the liquid dispersion and film phenomena over the horizontal circular tubes under spray impingement at varied water flow rates and positions. In the experimental study, a high-speed camera is used to capture the optical images of spray film flows for viewing the impingement of ejected fan-shaped liquid sheets, development of falling spray films exhibiting three basic fluid modes as well as formation and distribution of liquid films over the tubes. To simulate the moving interface progression, the theoretical analysis implements the unsteady three-dimensional volume of fluid (VOF) method and continuous surface force (CSF) scheme with the stress transport (SST) k-ω turbulent model for turbulence closure. The liquid film flows are predicted and compared with the measured liquid film thickness distributions around the tubes to validate the computational model. Both the experimental measurements and CFD simulations are then extended to investigate the film flow patterns and liquid film thickness distributions of falling sprays on the tubes at varying water flow rates and locations. At a low water flow rate of 3 l/min, the reduced ejection velocities develop a lesser spreading coverage over the horizontal tubes, causing the completely dryout results on the tube surfaces at a distance of 95 mm from the centerline. The scenarios at high water flow rates of 5 and 7 l/min can cover broader, more uniform spray areas, with the film thicknesses ranging 0.28–0.85 mm and 0.22–0.75 mm, respectively.

Liquid film transport around Taylor bubble in a microchannel with gas cavities

In Taylor flow, the low velocity in liquid film around the bubbles limits the mass transfer between the bubble body and liquid phase. To overcome this deficiency, a cross-junction Microchannel with gas Cavities (MGC) is designed and processed. The restricted inert nitrogen in the gas cavities provided partial slip boundaries, which makes it distinctive from the conventional Straight Microchannel (SM). The liquid film thickness and velocity distribution in the liquid film are investigated. It is found that the bubble shape in MGC is more sensitive to the capillary number. A thicker liquid film and a sharper bubble shape can be observed under the same operating conditions. These changes lead to a larger specific surface area (about 110 % of SM) and a remarkable increment of leakage flow rate (132.2–181.1 % of SM). Then, the intensifying mechanism of liquid transport in the liquid film is analyzed by numerical simulation. The results reveal that the slip and radial fluctuation of velocity at the gas cavity interface prominently improved the liquid transport. The high specific surface area and strong renewal ability of liquid film demonstrates a great potential for mass transfer in MGC.

Measurement of Liquid Film Thickness in Vertical Multiphase Slug and Churn Flows Using Distributed Ultrasonic Method

A distributed ultrasonic method for measuring the liquid film thickness of vertical multiphase slug flow and churn flow widely existed in industrial production is proposed in this paper. First, according to the proportional relationship between transit time of ultrasonic echo and liquid film thickness, we use the twin-crystal ultrasonic probe to measure the local thickness of liquid film. In view of the non-uniform circumferential distribution of the liquid film, three ultrasonic probes are installed on the same pipe section to measure the circumferential distribution of liquid film thickness. Then, a high-speed data acquisition system suitable for multi-channel and a data transmission system based on gigabit network are designed. The experiments of gas-liquid two-phase flow and oil-gas-water three-phase are carried out in a vertical pipe with a diameter of 20 mm. The fluctuation of local liquid film thickness has been measured by the distributed ultrasonic sensor, and the average thickness of liquid film of slug flow and churn flow under different flow conditions is obtained. The result indicates that the average liquid film thickness of slug flow and churn flow gradually thickens with the increase of liquid superficial velocity, and the distributed ultrasonic sensor can accurately measure the change of film thickness.

The spatial distribution of liquid film thickness outside the horizontal falling film tube

The visualization method in 3D simulation is established to investigate the spatial distribution of film thickness and local heat transfer coefficient to avoid the dryout phenomenon and improve the heat transfer performance. The simulated results are in good agreement with the experimental data in the literature. The results indicate that the spatial distribution of liquid film is peak-valley-peak, and there exists a minimum thickness. The change path of the hazardous area is a space curve rather than a plane curve. The film thickness with dimensionless axial length of 0.375 is closest to the average film thickness. The hazardous area approaches the inlet column as the tube diameter increases in the axial direction. The tube diameter has a great influence on the hazardous area, and the large tube diameter promotes it to move to the inlet column. The spatial distribution rule of the local heat transfer coefficient is the same as that of film thickness. The place with the worst heat transfer performance is approximately in the same position as the place with the thinnest liquid film. The results provide a good guidance for further study on falling film process.

A study of two-phase annular flow using unsteady numerical computations

Flow behavior of thin liquid films in an annular flow regime is an important element of the thermal hydraulics of a BWR – it controls the heat transfer from the fuel rods to the coolant. Despite its relevance, the flow behavior of dynamic liquid films is not well understood even in adiabatic conditions. To investigate it, unsteady numerical simulations with an interface tracking model were performed in a double subchannel geometry with a P/D ratio of 1.325. Different turbulence models, namely the large eddy simulation (LES) and linear/nonlinear eddy viscosity unsteady-RANS (URANS) models, were tested. A novel approach for generating turbulent inlet conditions in a periodic flow domain was developed to reduce computational efforts. Validation against experimental data revealed shortcomings of the linear eddy-viscosity RANS model in predicting key flow parameters. By capturing the effects of the secondary flow structures in a subchannel geometry, improved predictions were obtained with a non-linear SST k–ω (QCR) turbulence model. Time-averaged liquid film thickness (LFT) and tracer distribution obtained with LES were found to have the best agreement with experimental data. Instantaneous and time-averaged velocity profiles were analyzed to understand the influence of the gas-liquid interface. Secondary flow structures in the subchannel gap region were found to enhance the turbulent mixing of the passive scalar in the liquid film. This finding is relevant towards the prediction of thermal-hydraulic parameters in a multichannel flow assembly by accounting for inter-channel mixing phenomena.

Hydraulic Characteristics of Liquid Film Generated by Slit Injector

Liquid film generated by a jet slit injector on a wall was analyzed experimentally. Liquid film characteristics were investigated by measuring the liquid film thickness distribution for various experimental conditions. At first, the distributions were qualitatively analyzed using backlight imaging. Furthermore, quantitative measurements were performed using a liquid film thickness sensor based on the electric conductance method, which was applied to the high-speed wire-mesh method. The injection angle (15, 30, 45 °), jet velocity (7, 10.5, 13.5 m/s), and injection distance (45, 55, 65 mm) were varied, and the responses of the liquid film were observed. Having increased the injection angle, the main stream thickness decreased and the position of the maximum film thickness moved upstream. A faster jet velocity corresponded to a wider liquid film area and larger perturbation of the liquid film. The injection distance did not affect the characteristics of the liquid film considerably. A hydraulic jump effect was also observed, and a large hydraulic jump affected the film thickness distribution of the main stream. The results of this study can be applied to the development of liquid rocket engines or thrusters using the film-cooling method.

Numerical study of the falling film thickness around the tube bundle with different spacings between spray holes and tubes under tilt and sloshing conditions

The shell-side flow patterns and liquid film thickness (δ) distribution of the falling film are extremely important for the comprehensive performance of the spiral-wound heat exchangers. In this paper, 3-D simulations calculated with volume of fluid (VOF) method are performed to study the inter-tube flow modes and the distribution characteristics of δ around the tube. At first, the effects of different spray densities, longitudinal inter-tube spacings (St) and spray holes arrangements (Sh) on the δ distribution are studied to determine the appropriate values. After that, the influence of tilt and sloshing conditions are examined based on the selected geometric parameters. The results show that although the value of St has a significant influence on the fluctuation range of δ in stable zone, the average δ in stable zone could remain in the same level. For the spray holes arrangement, the decrease of Sh does not thicken the δ in stable zone but promotes a 46% axial length expansion for the crest zone with a thicker δ. The tilt conditions significantly deteriorate the inter-tube flow modes transitions, which makes the column flow much easier convert to sheet flow. Under roll motion, a secondary crest zone emerges nearby the original main crest zone and its δ is 30%–40% thinner than that of the original main crest zone.

A numerical investigation of liquid film flow and film thickness distribution outside a horizontal tube

A three-dimensional model is applied to simulate the liquid film spreading process and liquid film thickness distribution. The volume-of-fluid (VOF) method is used to trace the gas–liquid interface. The mutual influences on the liquid film flow behavior between liquid columns are discussed in detail. It is found that the liquid film spreading and the distribution of the liquid film thickness are influenced by liquid column impact and the wave crest formation process. The wave crest forms in the vicinity of the center point of the adjacent liquid column. The Reynolds (Re) number is an important factor in the spread of the liquid film and the distribution of the liquid film thickness, but the tube spacer has almost no influence on it when the Re number is 188.

Combination of laser interferometric and laser extinction methods for precise thickness measurement of liquid film between coalescing twin air bubbles

A combined laser extinction and laser interferometric measurement system was developed for the measurement of the change in thickness of a liquid film formed between colliding twin bubbles. The liquid film thickness during the bubble coalescence process is measured based on the bubble approach velocity and the contact duration. The proposed method affords a ten-fold spatial resolution (∼6 μm) of the liquid film thickness distribution compared to the current laser extinction measurement method. Through the use of the proposed method, it was found that the liquid film in the vicinity of its thinnest area immediately before coalescence was wedge-shaped, with the minimum film thickness immediately before coalescence being 0.3–1.3 μm, which is ∼0.6-μm thinner than the value determined by the current laser extinction measurement method. Greater precision of the proposed method is enabled by its improved spatial resolution.

Experimental investigations of the circumferential liquid film distribution of air-water annular two-phase flow in a horizontal pipe

The circumferential film thickness distribution in an air-water horizontal annular two-phase flow was investigated in 26mm pipe diameter by using the conductance probe. The superficial gas and liquid velocities were 10–40m/s and 0.025–0.4m/s, respectively.In general, the film thickness decreases as the liquid velocity decreases and the gas velocity increases. For low superficial gas velocity, high asymmetry of liquid film thickness distribution is generally found. In addition, the liquid film at the upper half of the pipe mostly consists of surge of waves with frequency lower than that of the bottom. At the lowest superficial liquid velocity of the present experimental range, the mean film thickness at the bottom ranges from 0.24mm to 1.3mm. Meanwhile, at the highest superficial liquid velocity, it ranges from 1.0 to 4.29mm, depending on the superficial gas velocity. At the higher superficial gas velocity, a more uniform of liquid film thickness distribution is commonly found. Under this flow condition, the wave at the upper has a similar frequency to that of the lower half and a coherent liquid wave is normally found.To predict the film thickness at any circumferential position, an empirical correlation is proposed on the basis of the physics of the phenomena. In comparison to the available experimental data, the correlation has a good capability to predict the circumferential liquid film thickness distribution.

Bubbly, Slug, and Annular Two-Phase Flow in Tight-Lattice Subchannels

Abstract An overview is given on the work of the Laboratory of Nuclear Energy Systems at ETH, Zurich (ETHZ) and of the Laboratory of Thermal Hydraulics at Paul Scherrer Institute (PSI), Switzerland on tight-lattice bundles. Two-phase flow in subchannels of a tight triangular lattice was studied experimentally and by computational fluid dynamics simulations. Two adiabatic facilities were used: (1) a vertical channel modeling a pair of neighboring subchannels; and (2) an arrangement of four subchannels with one subchannel in the center. The first geometry was equipped with two electrical film sensors placed on opposing rod surfaces forming the subchannel gap. They recorded 2D liquid film thickness distributions on a domain of 16 × 64 measuring points each, with a time resolution of 10 kHz. In the bubbly and slug flow regime, information on the bubble size, shape, and velocity and the residual liquid film thickness underneath the bubbles were obtained. The second channel was investigated using cold neutron tomography, which allowed the measurement of average liquid film profiles showing the effect of spacer grids with vanes. The results were reproduced by large eddy simulation + volume of fluid. In the outlook, a novel nonadiabatic subchannel experiment is introduced that can be driven to steady-state dryout. A refrigerant is heated by a heavy water circuit, which allows the application of cold neutron tomography.

Variation of the liquid film thickness distribution between contacting twin air bubbles during the coalescence process in water and ethanol pools

This study aims to investigate the characteristics of coalescence phenomena by precise measurement of the liquid film thickness distribution between approaching bubbles. This study targeted the coalescence of bubbles in pure water and ethanol pools to reveal the rupture characteristics of the liquid film formed between the approaching bubbles, which has a direct correspondence to the occurrence of coalescence phenomena. In this study, new and more precise measurements were performed for water and ethanol, whose physical properties are greatly different from one another. The basic experimental system, in which the approach velocity was easily controlled, was chosen to realize precise measurement of liquid film behaviors. That is, the coalescence of horizontally contacting twin bubbles was experimentally investigated using isothermal air–water and air–ethanol systems. The liquid film thickness formed between the contacting bubbles was measured using an improved laser extinction method. The variation of liquid film thickness between the bubbles at the rupture location and the distribution of the liquid film thickness were evaluated. The experimental parameters were the airflow rate and the measurement position for both water and ethanol. The bubble approach velocity and the contact duration, which is the time from the start of the bubbles’ collision to the commencement of coalescence, were measured. When relatively quick bubble coalescence (short contact duration) occurs, the liquid film thickness is thinnest near the film center. The thinnest film thickness appeared just before coalescence and was approximately 1.0–2.5μm and 0.3–1.0μm for water and ethanol, respectively. An annular-shaped thinner area in the liquid film emerged and shifted from the center toward the periphery of the liquid film with the increase in the bubble approach velocity and short contact duration. The thinnest liquid film thickness just before the commencement of rupture in the ring-shaped areas was approximately 0.2–0.9μm and 0.1–0.3μm for water and ethanol, respectively. Trends of film thickness variation were qualitatively similar to each other for water and ethanol.

Liquid film dynamics of two-phase annular flow in square and tight lattice subchannels

The flow behavior of a liquid film on the wall in square-lattice and tight-lattice double subchannels was studied by using a pair of flexible liquid film sensors based on the electrical conductance method. The liquid film sensor has a network of electrodes on the surface and the electrical conductance between transmitter and receiver electrodes is detected by using a high-speed wire-mesh measurement system. The sensors were fixed on rod simulators with 20mm diameter and the liquid film thickness distribution was estimated from the measured conductance data array. With this method, it is possible to observe the flow structure of the liquid film with a high sampling speed up to 10kHz and a spatial resolution of 2mm. In the present study, the spatial-temporal distributions of liquid film thickness in the annular flow in a square-lattice and a tight-lattice pair of adjacent subchannels were measured by using two liquid film sensors installed on the surface of opposing rods. The traveling of disturbance waves was observed in the time-series of two-dimensional film thickness distributions. The interaction of the liquid films on the opposing rods was investigated by correlating the signals of both sensors. As a result, the effect of the channel geometry on the averaged film thickness profiles and the correlation characteristics was analyzed for the first time.

MODELLING AIR AND WATER TWO-PHASE ANNULAR FLOW IN A SMALL HORIZONTAL PIPE

Numerical simulation using computational fluid dynamics (CFD) has been carried out to study air and water two-phase flow in a small horizontal pipe of an inner diameter of 8.8mm, in order to investigate unsteady flow pattern transition behaviours and underlying physical mechanisms. The surface liquid film thickness distributions, determined by either wavy or full annular flow regime, are shown in reasonable good agreement with available experimental data. It was demonstrated that CFD simulation was able to predict wavy flow structures accurately using two-phase flow sub-models embedded in ANSYS-Fluent solver of Eulerian–Eulerian framework, together with a user defined function subroutine ANWAVER-UDF. The flow transient behaviours from bubbly to annular flow patterns and the liquid film distributions revealed the presence of gas/liquid interferences between air and water film interface. An increase of upper wall liquid film thickness along the pipe was observed for both wavy annular and full annular scenarios. It was found that the liquid wavy front can be further broken down to form the water moisture with liquid droplets penetrating upwards. There are discrepancies between CFD predictions and experimental data on the liquid film thickness determined at the bottom and the upper wall surfaces, and the obtained modelling information can be used to assist further 3D user defined function subroutine development, especially when CFD simulation becomes much more expense to model full 3D two-phase flow transient performance from a wavy annular to a fully developed annular type.