Breakdown Of Liquid Film Research Articles

Formation of thermocapillary structures in heated liquid film

Data on the formation of thermocapillary structures and breakdown of a heated liquid film flowing down a vertical surface with Reynolds number variation from 0.1 to 500 have been analyzed and generalized. Small-scale thermocapillary structures are distinguished. It is shown that the distances between temperatures heterogeneities (rivulets) of thermocapillary structures do not depend on the Reynolds number. It is determined that the interaction of waves with thermocapillary structures leads to an increase in the critical heat flux corresponding to the liquid film breakdown as compared with the data of publications.

Effect of thermocapillary instability on liquid film breakdown

The film of water flowing down along a vertical surface with a heater was studied experimentally at Re = 10–50. The initial temperature of the water film varied from 15 to 70 °C and heat fluxes on the heater varied from 0 to 6.5 W/cm2. Simultaneous measurements of the film thickness and surface temperature carried out. The effect of development of thermocapillary instability type A on wave amplitudes, deformation of the liquid film surface, and formation of the first stable dry spot on the heater was investigated. It is shown that when the longitudinal temperature gradients reach values larger then 7–10 K/mm formation of thermocapillary structures begins. At the leading edge of the heater, X/mm < 10–15, the thermocapillary structures in the form of a series of rivulets with a thin film between them are formed on the surface of residual liquid film after wave front propagation. The distance between the rivulets is λ/mm = 10. It is shown that the formation of the first stainable dry spots occurs in areas where deformation liquid film reaches its maximum value, and the value of the wave’s amplitudes decreases. The interaction of waves with thermocapillary structure type A leads to an increase in the critical heat flux corresponding to the liquid film breakdown on 75% in comparison with the data known in literature. A new mechanism of action on the film flow was first identified and studied in detail.

Analysis of fluctuation and breakdown characteristics of liquid film on corrugated plate wall

Corrugated plate dryer is a very vital separation equipment for nuclear engineering and it is meaningful to study the flow, fluctuation and breakdown characteristics of the liquid film on corrugated plate wall. Liquid film thickness of steady flow is measured based on plane laser induced fluorescence (PLIF) technique and time series of liquid film thickness are obtained. Besides, amplitude and frequency domain information of liquid film fluctuations are analyzed by probability density function (PDF), power spectral density (PSD) and wavelet method. A two-dimensional liquid film breakdown model is established. Critical airflow velocity when the water film breakdown is measured experimentally. Based on theoretical results, equations for calculating critical airflow velocity of the corrugated plate under different liquid film thicknesses are fitted in the form of nike function. Fitting equations for calculating critical airflow velocity in the form of nike function agree well with experimental results.

Stability of the film flow passing round the cross-section elements in the area of high density irrigation

The paper studies the loss of stability of a liquid film using regular surface roughness. The model of the breakdown process is described and the conditions for the formation of a liquid film breakdown are determined experimentally. It was found that there are two characteristic modes, namely drop and film regimes. Using the methods of thermal anemometry, data were obtained on the distribution of the droplet sizes after a film breakdown and the dependence of the We number on the projection arrangement height was plotted.

Waves, Instabilities, and Rivulets in High Quality Microgap Two-Phase Flow

Two-phase flow in sub-millimeter microgap channels offers highly potent thermal management capability and is the foundation for the emerging "embedded cooling" paradigm of electronic cooling. While the thermofluid characteristics and operational limits of such microcoolers are intimately tied to distinct forms of vapor-liquid aggregation in the microgap channel, insufficient attention has been paid to the formation of distinct wave patterns and instabilities on the thin liquid films associated with high-quality microgap channel flow. This paper focuses on the results of visualization and heat transfer studies of such two-phase flows, under both adiabatic and diabatic conditions, for FC-72 flowing in a 184 micron microgap channel at a mass flux of 230 kg/(m2.s). The study has revealed the existence of a post-annular, high-quality Rivulet flow regime, in which the liquid film breakdown and local wall dryout drives large surface anisothermalities and limits the heat transfer rate from the wall.As predicted by the prevailing flow regime models, annular flow is found to be the dominant flow regime for this microgap configuration.. For the adiabatic conditions, flow qualities ranged between 27% and 81%, and widely spaced, 3D waves, with a wavelength that decreases with increasing flow quality, were observed on the liquid-vapor interface. For the diabatic condition, the inlet flow quality was maintained at 36% and the exit flow quality varied between 47% and 97%. For exit qualities greater than 61%, the liquid film would periodically rupture into rivulet of varying width and length. The spacing, length, and width of the rivulets varied considerably, and can easily stretch well into dryout region. The axial variation of the wall heat transfer coefficient was found to reflect and confirm the expected axial propagation of the two- phase flow regimes and the onset of local dryout associated with the newly-defined Rivulet regime.

Dryout of counter-current two-phase flow in a vertical tube

Dryout of counter-current two-phase flow is one of key issues in chemical and/or industrial systems. The aim of the present study is to provide insights into the dryout phenomena of such counter-current two-phase flow of binary gas and liquid in a vertical tube. Experimental results suggest a significant influence of the subcooling of water and the binary gas flow rate on the dryout characteristics. The observed dryout was classified into three modes, i.e., film breakdown, dryout due to flooding and dryout due to critical flooding. At the low inlet air flow rate, dryout characteristics were affected by subcooling of water. The test section was partially overheated owing to the liquid film breakdown due to the Marangoni effect. At the moderate inlet air flow rate, the dryout is induced by the reduction on the falling liquid film due to flooding. At the high gas flow rate, most of the water was carried over due to the critical flooding. In the latter two cases, there was no influence of subcooling of water on dryout. These dryout phenomena were successfully realized in the numerical simulation based on the one-dimensional heat and mass transfer model, i.e., energy balance of a heat input, a heat release rate from the heating surface and a mass transfer rate due to phase change.

A model of liquid film breakdown formed due to impingement of a two-phase jet on a horizontal surface

The present work aims to provide an explanation to the phenomenon of breakdown of the thin liquid film created by impinging two-phase, liquid–gas jet. Existing in the literature models describe merely the breakdown of single phase liquid films. The model presented here is based on examination of mass and energy equations under the applied criterion of the minimum of total energy. That allows to determine the minimum thickness of isothermal, thin liquid film created by impinging two-phase jet on a solid surface. The mechanical energy of the system consists of kinetic energy of liquid film and surface energy of all physical surfaces consisting for the control surface. An analytical expression for the minimum thickness of such liquid film is derived. The liquid film thickness at the breakdown is a function of the contact angle and shear stresses on the liquid–gas interface. Some comparisons with the experimental data are shown exhibiting a good performance of the postulated model.

Heat transfer in a subcooled evaporating liquid film

An investigation is performed of heat transfer in films of water and FC-72 liquid falling down a 60×120 mm heater. Heat transfer mode maps are constructed. Zones of structure formation and regions of emergence of breakdown of liquid film are identified, as well as regions of boiling in jets. An averaged coefficient of heat transfer was used in analyzing the experimental data. It is demonstrated that thermocapillary forces have a complex and ambiguous effect on heat transfer. The emergence of extensive stable dry spots causes a decrease in the average coefficient of heat transfer. On the other hand, an increase in the path length of film and in the amplitude of wave motion leads to the washing out of the dry spots and to an increase in the relative intensity of heat transfer. The regularities of heat transfer in the region of flow of film with breakdowns for weakly and intensely evaporating liquids differ significantly. An enhancement of heat transfer is observed under conditions of significant evaporation during structure formation in the thermocapillary mode.

Bubble departure in the direct-contact boiling field with a continuous liquid–liquid interface

Behavior of vapor bubbles was experimentally investigated in the boiling field where a volatile liquid layer of per-fluorocarbon PF5050 (boiling point 306 K) was directly in contact with an immiscible hot liquid layer of water above it. Heat was supplied to the continuous liquid–liquid interface by the impingement of the downward hot water jet. Vapor bubbles were generated not only from this continuous interface but from a large number of PF5050 droplets floating on it. According to precise observation, incipience of boiling did not occur at the liquid–liquid interface but in the PF5050 liquid close to the interface in both cases of continuous and dispersed interfaces. As a result, the bubbles broke up the thin PF5050 liquid film above them and rose up into the water layer. This bubble departure phenomenon, which does not occur in the ordinary pool boiling field on the solid heating wall, is very important to evaluate the heat transfer performance in the present direct-contact boiling system. For modeling this behavior, sizes of the bubbles were measured at the moment just after they were released into the water pool. Volumes of the bubbles were larger in the case of departing from the continuous liquid–liquid interface than from the droplets. This tendency could be explained by taking into account the buoyancy force acting on unit area of the thin PF5050 liquid film above the bubble before departure, which was one of the most important parameters for the liquid film breakdown.

The Effect of Wave Formation and Wetting Angle on the Thermocapillary Breakdown of a Falling Liquid Film

An experimental investigation is performed of the breakdown of a liquid film flowing down a vertical plate with a heater sized 150×150 mm. The main parameters which are varied in the experiment are the Reynolds number Re = 0.47 to 331 and the heat flux q = 0 to 1.92 W/cm2. It is found that the effect of the heat flux on the wave motion of the liquid film causes the formation of a jet flow. Dry spots are formed in the region of thin film between the jets. For the purpose of investigating the effect of wave formation on the film breakdown, the distance between the nozzle and heater is varied from 41.5 to 200 mm. It is found that the distance between the nozzle and heater defines the hydrodynamics of the liquid at relatively low heat fluxes, but has no appreciable effect on the heat flux at which the film breakdown occurs. Different working liquids and coatings of the working surface are used in the experiments to investigate the effect of the wetting angle on the film breakdown. The equilibrium wetting angle is measured by the "bubble" method. No effect of the equilibrium wetting angle on the nonisothermal breakdown of the film was revealed.

A211 液-液直接接触蒸発時の界面における液膜破断に関する研究

A211 液-液直接接触蒸発時の界面における液膜破断に関する研究

Breakdown of Evaporating Falling Films as a Function of Surface Tension Gradient

This study has found that liquid film breakdown in isopropanol, ethylene glycol, and water mixtures occurred only in systems with negative surface tension gradients. These systems were characterized by a decrease in local surface tension with a decrease in film thickness. Thus, the direction of surface tension gradient with respect to film thickness can be used as a criterion for predicting falling film breakdown in certain mixture systems. Visual observations of an evaporating falling film gave insight into the mechanism of film breakdown. It appeared that a lateral surface shear force created by the surface tension gradient caused lateral expansion of valleys of the wavy film liquid film as it flowed over the heating surface. This valley expansion caused localized gradual thinning of the wavy film, so that the film thinned to an unstable thickness and broke down.

Generation of a Wave on a Liquid Film Flowing Down a Heated Inclined Plane. 2nd Report, Disturbance and Marangoni Waves.

An investigation was conducted into the generation of a disturbance wave on a liquid film flowing down a heated inclined plane. Theoretical analysis shows that the Marangoni effect due to the surface tension gradient contributes to reduction of the critical flow rate of the liquid on the generation of a disturbance wave. Theoretical results also suggest the existence of two boundaries for generation of a disturbance wave, at a fixed inclined angle. The lower boundary, however, could not be observed for thin water film because liquid film breakdown occurred before the lower critical flow rate was achieved. In this paper, silicone oil with high viscosity was used as the working fluid instead of water. Small waves were observed under the lower critical flow rate. The surface temperature gradient along a wave is one order of magnitude larger than that of a disturbance wave under the higher critical flow condition. We defined a wave under lower critical condition as a Marangoni wave. The measured velocity of a Marangoni wave is almost the same as the liquid surface velocity.

Generation of a Disturbance Wave on a Liquid Film Flowing Down a Heated Inclined Plane.

An investigation was made into the generation of a disturbance wave on a liquid film flowing down a heated inclined plane. Theoretical analysis based on the instability caused by both dynamic and kinematic waves shows that the Marangoni effect due to surface tension gradient contributes to reduce the critical flow rate of liquid at the occurrence of a disturbance wave. Theoretical result also suggests the existence of two boundaries of generation of a disturbance wave at a fixed inclined angle. The flow pattern of a liquid film flowing on a heated brass duct was examined. As a result, one of the boundary which corresponds to the existence of the disturbance wave is qualitatively consistent with the theoretical result; the lower boundary, however, could not be observed because the liquid film breakdown occurred before reaching the lower critical flow rate. The measured velocity of a disturbance wave and its wave hight are also discussed in comparison with those of an isothermal liquid film flow.

The Effect of a Flat-plate-Type Obstacle on the Thin Liquid Film Accompanied by a High Speed Gas Flow

A flatplate-type obstacle, which simulates a grid-type spacer in a nuclear reactor, is set in an air-water cocurrent stratified flow to investigate liquid film breakdown occurring near the obstacle. We made detailed visual observations and measurments of the velocity profile of the air flow and the axial distributions of liquid film thickness and static pressure near the obstacle. Experimental parameters were the inclination of the rectangular duct, the configuration of the obstacle, i. e., with and without a projection and a hole, which is bored in order to delay the onset of dry patch formation near the obstacle and the gap between the plate and the lower-wall surface. The results show that the plate itself does not promote dry patch formation but the projection, even if it is in contact with the wall surface at only one point, has a strong effect on the liquid film breakdown. In general the film breakdown occurs in front of the projection in a wide range of flow conditions due to the leading edge down-wash of the stream and due also to the rejection of water by gravitational force in the case of the upward flow in the inclined duct. By setting a hole in or in front of the projection the occurrence of the dry patch formation is delayed.

Liquid Films Flowing Concurrently with Air in Horizontal Duct : 6th Report, Generation of a Disturbance Wave and its Role in the Breakdown of Liquid films

The generation of a disturbance wave on a liquid film flowing concurrently with a gas in a horizontal rectangular duct is analyzed theoretically based on the instability of both the dynamic and kinematic waves. The result shows that the occurrence of a disturbance wave strongly depends on a film thickness. The film thickness obtained theoretically by assuming both the liquid film and the gas flows being laminar is larger than the experimental value. Therefore, the experimental data on film thickness are used to predict the flow condition of transition from pebble wave to disturbance wave. A simple correlation for the P-D boundary is obtained which includes the effect of the duct height. Finally, the role of a disturbance wave in liquid film formation is also discussed.

Behavior of Gas-liquid Interface on a Liquid Film Jet : Instability of a Liquid Film Jet in a Co-current Gas Stream

Instability wave motion of the liquid film jet in a high speed co-current uniform gas stream was investigated experimentally an theoretically. The following results were obtained. The instability waves arose in both the flow direction and a perpendicular direction to it, and these waves have approximately the same wave number. Examination of the linear Kelvin-Helmholtz instability and the liquid film energy made it clear that the wave numbers of the instability waves agreed well with experimental results. It was found that the wave number of the liquid film jet could be estimated by a theoretical analysis. A close relation of the wave frequency to the liquid film breakdown was confirmed experimentally.

Free energy loss during the breakdown of liquid films

This study arose as an annexe to an ongoing investigation of the breakdown behaviour of liquid films traversing the stator blades of operating wet steam turbines. The plane surface of a horizonatal glass block is treated so that some areas are wetted by water and others are water-repelant. A circular area (wettable, ‘drop’) was surrounded by an annulus (non-wetting) in turn surrounded by a second narrow annulus (wettable, ‘ring’), all circles being concentric. A liquid film is formed over the whole area within the outer circle. This is diminished in thickness until perforation occurs and the residual water collects on and covers each of the wettable areas. Measurements of height and of liquid mass are made both for the original film and later for the drop and the ring. These measurements permit the calculation both of the original free energy and aggregate final free energies. An energy loss occurs which can be related to the mass of the film and the geometry of the prepared surfaces and can be calculated by suitably modelling the profiles of the ring and drop by geometric shapes which permit analysis. A circular segment gave the best fit. Contact angles calculated from the modelled profiles were checked by measurements on photographic silhouettes. Two new dimensionless numbers were introduced into the analysis: He=pgH c 1/2δ LG which is the ratio of the potential energy to the surface energy for a liquid component and Ae=pgAc/δ LG which is related to the ratio of the diameter of the solid surface wetted by the original film to the film thickness.

Liquid Films Flowing Concurrently with Air in Horizontal Duct : 3rd Report, The relation between occurrence of viscous wave and breakdown of liquid film

A thin liquid film flowing concurrently with an air stream was investigated with regard to waves generated when superficial velocity of air was less than about 10 m/s and water flow rate was considerably low. It was made clear theoretically and verified by an experiment that a special wave, named viscous wave by the authors, is generated on the thin liquid film when film thickness was less than a definite value which depended on liquid properties, i.e., 0.255 mm at 20°C for water. Theory shows that the occurrence of a viscous wave depends more on liquid viscosity than on surface tension, and that the wave velocity is lower than liquid surface velocity. The viscous wave occurs always in the vicinity of the nonwetting region, signifying that the occurrence of it reduces the minimum film thickness, which causes a breakdown of liquid film.

An Experimental Study of Falling Liquid Film Breakdown on a Horizontal Cylinder During Heat Transfer

In this study, an experimental investigation was conducted with subcooled water film flowing over an electrically heated horizontal cylinder. The combinations of film flow rate and heat flux at which film breakdown occurs (i.e., dry patches appear on the surface) were determined. At the conditions prior to dry patch formation, the heat transfer coefficient was determined as well. The results showed that the heat flux needed to cause a dry patch increases with film flow rate. Also, prior to dry patch formation, the heat transfer coefficient increases with film flow rate. The effects of the tube spacing and the liquid film inlet temperature on the breakdown heat flux and heat transfer coefficient were also studied.