Shear-Driven Flow of Locally Heated Viscous Liquid Film in a Minichannel

This paper considers a flow of a liquid sheared by gas in a flat mini-channel with two identical heaters arranged in a row one after another in a streamwise direction at the bottom wall. The present study is focused on the investigation of influence of local heaters arrangement and size on thermocapillary deformations in a viscous film, gravity effect is also investigated. 3D one-sided model is considered, viscosity of the liquid is supposed to be temperature dependent. Numerical analysis reveals that interaction and mutual influence of 3D structures takes place. Film pattern changes qualitatively depending on the heaters arrangement and form. For rectangular heaters a middle stream exists. Minimum film thickness value increases and its location moves to heater edges for rectangular heaters. A critical backlash between two heaters, at which film thinning is the largest, exists. Gravity significantly affects on the film deformations. Decreasing of gravity level leads to a flow destabilization and film deformations, especially film thinning, essentially increases.     

Effect of artificial perturbations on the formation of structures in a nonisothermal liquid film

The effect of artificial perturbations on the formation of structures in a nonisothermal liquid film flowing down a vertical plate with a 150 × 150 mm heater has been experimentally studied. The action of heat flux on the wave flow leads to the formation of periodic flowing rivulets separated by thin film regions. Artificial perturbations in a certain interval of wavelengths produce a change in the number of rivulets formed on the heater surface. The “most dangerous” wavelength of artificial perturbations altering the flow structure is determined.     

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.     

Heat Transfer from a Local Heat Source to Subcooled Liquid Film

An experimental investigation is performed of heat transfer from a local heat source to films of water and low-boiling dielectric liquid that flow down a vertical plate by gravity. The liquids are substantially subcooled. In the case of perfluorotriethylamine flow, regular structures are formed in the film at the threshold value of the heat flux density. After the heated layer of liquid comes to the film surface, three characteristic modes of heat transfer are observed, which are associated with the variation of the modes of liquid flow caused by thermocapillary convection. At low values of the Reynolds number of the film, a specific form of critical heat transfer is observed, which is characterized by disintegration of the jet into droplets and their separation from the heater.     

Gravity Effect on the Locally Heated Liquid Film Driven by Gas Flow in an Inclined Minichannel

Thin nonisothermal liquid film flowing under action of gravity force and co-current gas flow, which create the tangential force on the gas-liquid interface, in an inclined minichannel is considered. 3D time dependant mathematical model has been developed. Effects of surface tension, temperature dependent viscosity and thermocapillarity are taken into account. The effect of gravity as well as the effect of gas speed has been studied to define main features of the film dynamics. In calculations vector of gravitational acceleration is oriented along the flow and is equal to the normal Earth gravity and Lunar gravity. Our investigations have shown that gravity has a significant effect on the film deformations. At the lower gravity conditions 3D liquid film pattern changes noticeably in spanwise direction and a middle stream between two main lateral waves appears. Also speed of film deformation is higher and stabilization time is longer. Variation of gas Reynolds number from 543 to 2000 does not change noticeably film pattern at normal gravity. At lower gravity conditions increasing of gas Reynolds number decreases significantly the width of the thermocapillary deformations and leads to a film stabilization.     

The Effect of Three-Dimensional Deformations on Local Heat Transfer to a Nonuniformly Heated Falling Film of Liquid

An experimental investigation is performed of heat transfer under conditions of flow of a water film on a vertical surface with a heater 150×150 mm in size in the range of the Reynolds number values from 1 to 45. A map of modes of flow of the liquid film is plotted, and regions of heat transfer are identified. Data are obtained on the longitudinal coordinate dependence of the heater wall temperature and of the local heat flux on the symmetry axis of the heater. Local coefficients of heat transfer are measured. The experimental data are compared with the results of numerical calculations for a smooth film. The effect of the forming of jet flows on heat transfer to the liquid film is analyzed.     

An experimental modeling of gravity effect on rupture of a locally heated liquid film

This study deals with the formation of dry patches in a subcooled liquid film flowing over a locally heated plate at small positive and negative plate inclination angles with respect to the horizon. Prior to film rupture appreciable thermocapillary deformations of the film surface appear, growing with the heat flux. Upon reaching a threshold heat flux the film rupture occurs. By means of high speed imaging it is found that the process of rupture involves two stages: 1) abrupt film thinning down to a thin residual film on the heater; 2) rupture and dryout of the residual film. As the plate inclination angle is reduced the threshold heat flux required for film rupture weakly decreases, however when the angle becomes negative the threshold heat flux begins to rise dramatically, which is associated with an increase of the stabilizing hydrostatic effect due to the growth of the film thickness. The characteristic time of rupture decreases as the threshold heat flux increases. At nucleation of the dry patch the speed of contact line can be as high as 220 mm/s. The results obtained, apart from their intrinsic importance for ground-based applications, can also be of interest for microgravity research as a film flow with different relative contribution of inertia, hydrostatic and thermocapillary forces is considered.     

The wave characteristics of a nonisothermal film of liquid under conditions of jets forming on its surface

An eight-channel capacitive sensor is used for the first time, which enables one to investigate the dynamics of three-dimensional wave flows and the variation of the transverse profile of a nonisothermal film of liquid during formation of jets. Measurements are performed of the wave characteristics of the flow of a film of water on a vertical plate with a heater 150 × 150 mm in size. During the heating of falling liquid, the thermocapillary forces cause the formation of jets and of a thin film between them. The film thickness and wave amplitude in the interjet region decrease with increasing heat flux. Two ranges of the effect of the heat flux on the characteristics of wave flow are identified. Under conditions of low heat fluxes, the film flow hardly differs from isothermal. Under significant heat loads, an intensive formation of jets occurs. Three-dimensional waves propagate over the jet crests, where the film thickness and wave amplitude increase with increasing heat flux. In the interjet region of the film being heated, the average relative amplitude of waves increases with decreasing average thickness, and in the isothermal region this amplitude decreases. Comparison of the obtained results with experimental data for isothermal film reveals that the values of relative amplitude differ significantly in the interjet region at high densities of heat fluxes. Transverse temperature gradients cause a decrease in the liquid film thickness, and longitudinal gradients cause an increase in the relative amplitude of waves compared to isothermal flows. In the end, this leads to the emergence of dry spots and breakdown of film. The relative amplitude of waves on the jet surface decreases with increasing heat flux; this is true of isothermal film flows.     

Study of thermocapillary film rupture using a fiber optical thickness probe

Rupture of a subcooled water film flowing down an inclined plate with a 150×150 mm heater is studied using a fiber optical thickness probe. The main governing parameters of the experiment and their respective values are: Reynolds number (3.2–30.2), plate inclination angle from the horizon (3–90 deg), heat flux (0–1.53 W/cm²). The effect of the heat flux on the film flow leads to the formation of periodically flowing rivulets and thin film between them. As the heat flux grows the film thickness between rivulets gradually decreases, but, upon reaching a certain critical thickness, the film spontaneously ruptures. The critical film thickness is practically independent on the film Reynolds number as well as on the plate inclination angle and lies in the neighborhood of 60 µm (initial film thickness varies from 93 to 368 µm). The heater surface temperature prior to rupture is also independent of Re and Θ, and is about 45°C (initial film temperature is 24°C). The process of rupture involves two stages: 1) abrupt film thinning down to a very thin residual film remaining on the heater; 2) rupture and dryout of the residual film. The threshold heat flux required for film rupture is scarcely affected by the plate inclination angle but grows with the Reynolds number.     

Measurement of the transient heat transfer to liquid helium from a thin metal film

A method to measure transient heat transfer to liquid helium from a thin metal film heater under the condition of pulsed heating during τ ≤ 400 ns is proposed. The experimental equipment used for the measurements is described. The method is based on the comparison of heat pulses transfered from the heater into a monocrystal substrate which is surrounded either by vacuum or by liquid helium. The method can also be used to investigate the heat flux density transmitted into liquid helium over a wide region of thermal loads. Experimental results showing the heat flux density radiated from a Cu heater into liquid helium at 3.8 K as a function of the electric power fed into the heater by pulses of 200–400 ns duration are demonstrated.     

Basic study on heat rejection system using high heat flux micro channel evaporator

Thermal management system which can reject very high amount of heat by small thermal devices will be required for future space systems. Our purpose is to develop miniaturized heat rejection system that can reject more than 100 W/cm². In the evaporator, thin liquid film vaporization which can dissipate very high heat flux, was utilized. The liquid film is stabilized in micro-channels by capillary forces. The microchannels are fabricated by chemical etching on silicon and copper plate. Also miniaturized condenser which utilized droplet condensation was tested. Droplets were produced on a cooled plate covered by non-wetting coating. After we built a heat rejection system constructed by above mentioned evaporator and condenser, influence of heat flux, coolant flow rate, and inlet temperature on the temperature of the heater element were investigated. Water is used as working fluid. Heat flux of 100 W/cm² could be achieved for water inlet temperature in flow rate of 3.0 mL/min. The temperature of the heater element is kept constant at about 120°C.     

Experimental Study of Subcooled Flow Boiling Heat Transfer on a Smooth Surface in Short-Term Microgravity

The flow boiling heat transfer characteristics of subcooled air-dissolved FC-72 on a smooth surface (chip S) were studied in microgravity by utilizing the drop tower facility in Beijing. The heater, with dimensions of 40 × 10 × 0.5 mm³ (length × width × thickness), was combined with two silicon chips with the dimensions of 20 × 10 × 0.5 mm³. High-speed visualization was used to supplement observation in the heat transfer and vapor-liquid two-phase flow characteristics. In the low and moderate heat fluxes region, the flow boiling of chip S at inlet velocity V =?0.5 m/s shows almost the same regulations as that in pool boiling. All the wall temperatures at different positions along the heater in microgravity are slightly lower than that in normal gravity, which indicates slight heat transfer enhancement. However, in the high heat flux region, the pool boiling of chip S shows much evident deterioration of heat transfer compared with that of flow boiling in microgravity. Moreover, the bubbles of flow boiling in microgravity become larger than that in normal gravity due to the lack of buoyancy Although the difference of the void fraction in x-y plain becomes larger with increasing heat flux under different gravity levels, it shows nearly no effect on heat transfer performance except for critical heat flux (CHF). Once the void fraction in y-z plain at the end of the heater equals 1, the vapor blanket will be formed quickly and transmit from downstream to upstream along the heater, and CHF occurs. Thus, the height of channel is an important parameter to determine CHF in microgravity at a fixed velocity. The flow boiling of chip S at inlet velocity V =?0.5 m/s shows higher CHF than that of pool boiling because of the inertia force, and the CHF under microgravity is about 78–92% of that in normal gravity.     

Experimental study of the wave flow of a liquid film on a heated surface

The wave flow of a water film over the surface of a vertical plate with a 150×150-mm heater has been experimentally studied. The action of heat flux on the wave flow of the liquid film is manifested by the formation of periodic flowing rivulets separated by thin film regions. The thickness of the film between rivulets was measured using a fiber optical reflection probe. As the heat flux grows, the average film thickness h continuously decreases. However, when the thickness reaches h≈0.5 h₀, where h₀ is the value given by the Nusselt formula for a laminar liquid film, the film exhibits spontaneous rupture. It was found that, as the local flow rate decreases, the wave amplitude in the region between rivulets drops more rapidly than expected according to the laws of “cold hydrodynamics.”     

Heat exchange processes near the boundary of a film boiling nucleus

Results are presented of an experimental investigation of the change in temperature, heat flux to the liquid, and rate of displacement of the isotherms near a film boiling nucleus propagating over a plane surface. The experiment was carried out in a liquid nitrogen bath at atmospheric pressure on the saturation line. The heater was a sapphire plate 1.2 mm thick having a heat transfer surface area of 77×22 mm². The following facts were established: 1) near the boundary of the film boiling nucleus a new heat exchange mechanism takes place caused by the instability of the liquid microlayer; 2) the maximum heat flux to the liquid is considerably greater than the critical heat flux; 3) the vapor film in the film boiling region grows gradually with increasing distance from the boundary, i.e., there is a smooth transition in terms of heat exchange intensity before the equilibrium film boiling level is reached. Pis’ma Zh. Tekh. Fiz. 25, 39–46 (November 12, 1999)     

Features of thin liquid film flowing down a substrate over local heat source of variable size

Numerical simulations show that the character of flow and the extremal values of thickness of a locally heated liquid film flowing down a substrate under the action of gravity depend on the heater width. It is shown that there are certain critical values of the heater width. The effect of the Reynolds number of the flow on the process under consideration is established.     

微圆管内R1234ze(E)流动沸腾的环状流特性模拟研究

本文建立了制冷剂R1234ze(E)在微圆管内流动沸腾过程中的环状流模型,对传热和气液两相流动压降进行了模拟研究。综合考虑重力、表面张力及气液界面剪切力的影响,模拟分析了周向液膜不均匀分布特性及该特性对流动与换热的影响,经验证,计算结果与已有实验结果吻合较好,此外还研究了不同因素对环状流区域表面传热系数与压降的影响。模拟结果表明：在流动起始区域,截面液膜厚度的分布受重力作用影响,随着流动沸腾过程的进行,该影响作用开始减弱,且有重力作用时的环状流平均表面传热系数高于无重力作用时的环状流平均表面传热系数,随着重力加速度的增加,环状流的平均表面传热系数不断增大;随着质量流速的增大,表面传热系数与压降均随之增大;随着管径增大,表面传热系数与压降均随之减小。     

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.     

Marangoni effect on wave structure in liquid films

Liquid films are encountered in space systems as well as in numerous industrial processes and everyday life. The present work is a part of the preparation of the SAFIR experiment of the European Space Agency onboard the International Space Station. Wave characteristics of the water film flow over a vertical or an inclined plate with a heater have been studied. The fluorescence method was used to measure local instantaneous film thickness. In addition to measure wave characteristics eight capacitance sensors were located as a line across the flow. The process of rivulet formation in a heated film was registered. Two zones of the heat flux effect on liquid film deformation were distinguished. At low heat fluxes, the film flow is hardly deformed. At high heat fluxes the thermocapillary forces provide formation of rivulets and a thin film between them. The measured values of the distance between rivulets depend slightly on the plate inclination angle. It was found that for a positive temperature gradient along the flow, heating may increase the wave amplitude because thermocapillary forces are directed from the valley to the crest of the wave. This effect was shown in the interrivulet zone, where relative wave amplitude and Marangoni number increase with a rise of the heat flux density.     

The effect of mutual location of heaters on the falling film dynamics

Thin nonisothermal liquid film flowing down under action of gravity is considered. Investigation of the influence of the spanwise and streamwise arrangement of the rectangular heaters on 3-D structures, occurring at the film surface, is the main objective of the present work. Three-dimensional time-dependant mathematical model for calculation of gas-liquid interface deformations and evolution of temperature fields was developed. Our numerical investigations have shown that interaction, imposition and mutual influence of the 3D structures (bumps, lateral waves ...) takes place. In the case of streamwise arrangement of the heaters film rupture is most likely on the second heater. There is a critical backlash between the heaters, at which film deformations, including film thinning, are the largest. For the spanwise arrangement of the heaters distance between them practically do not effect on the minimum film thickness, but mutual imposition of the lateral waves and film thickening exists.     

Numerical Study on the Effects of Gravity and Surface Tension on Condensation Process in Square Minichannel

A volume of fluid (VOF) method is adopted to simulate the condensation of R134a in a horizontal single square minichannel with 1 mm side length. The effect of gravity, surface tension and gas-liquid interfacial shear stress are taken into account. The result denotes that condensation is first appeared at the corner of channel, and then the condensation is stretched at the effect of surface tension until the whole channel boundary covered. The effect of gravity on the distribution of the liquid film depends on the channel length. In short channel, the gravity shows no significant effect, the distribution shape of steam in the cross section of the channel is approximately circular. In long channel, due to the influence of gravity, the liquid converges at the bottom under the effect of gravity, and the thickness of the liquid film at the bottom is obviously higher than that of the upper part of the channel. The effect of surface tension on condensation is also analysed. The surface tension can enhance the condensation heat transfer significantly when the inlet mass flux is low. Whilst, at high mass flux, the enhancement of surface tension on heat transfer is unobvious and can be neglected.