O. A. Kabov

Determination of surface tension and contact angle by the axisymmetric bubble and droplet shape analysis

The algorithms of solution to the Young–Laplace equation, describing the shape of an axisymmetric droplet on a flat horizontal surface, with various ways of setting the initial data and geometric parameters of a droplet, were derived and tested. Analysis of the Young–Laplace equation showed that a family of curves that form the droplet surface is the single-parametric one with the accuracy of up to the scale factor, whose role is played by the capillary length, and the contact angle determines the curve turn at a contact point, but it does not affect the shape of the curve. The main natural parameter defining the family of the forming curve is the curvature at the droplet top. The droplet shape is uniquely determined by three independent geometric parameters. This fact allows us to calculate the physical properties, such as the capillary length and contact angle, measuring three independent values: height, droplet diameter, and diameter of the droplet base or the area of the axial cross section of the droplet or its volume.

Experimental investigation of the temperature field in the gas-liquid two-layer system

Results of an experimental investigation of the temperature field across the liquid-gas two-layer system are presented. The liquid layer is locally heated from the bottom substrate, and the intensive liquid evaporation is observed. A technique for measuring the temperature profile across the liquid and gas layers (including their interface) is developed. To do these measurements, the microthermocouple is moved across the layers with the help of precision micropositioner with a step of 1 μm. The temperature jump at the liquid-gas interface is measured, and its value increases with the temperature increase. Detailed information on the temperature field near the interface is obtained by using the precise thermocouple displacement with a small step.

Effect of viscosity on thermocapillary breakdown of a falling liquid film

Thermocapillary breakdown of a liquid film flowing due to gravity over a vertical plate with a heater of 150×150 mm is studied in a wide range of liquid properties (in particular, dynamic viscosity at the initial temperature varies from 0.91·10-3 to 16.9·10-3 Pa·s) and film Reynolds number (Re = 0.15-53.5). It is found that liquid viscosity has a significant effect on the threshold heat flux corresponding to film breakdown. To take into account the effect of liquid properties, the breakdown criterion traditionally used in literature was modified. This allowed successful generalization of all data obtained.

Experimental investigation of heat transfer in a rivulet on the inclined foil

Heat transfer at rivulet water flow over the constantan foil with the length of 80 mm, width of 35 mm, and thickness of 25 mm was studied experimentally. The foil surface temperature was measured by an IR-scanner. Distributions of heat flux density on the surface of the foil, where the liquid flowed, were obtained. To determine the heat flux density from the foil to liquid near the contact line, the Cauchy problem was solved for the stationary heat equation using the thermographic data. Calculation results showed that the maximal heat flux occurs in the area of the contact line and exceeds the average heat flux from the entire foil surface by several times. This is explained by the influx of heat from the periphery of foil to the rivulet due to the relatively high value of heat conductivity coefficient of the foil material and high evaporation rate in the region of the contact line.

Capillary waves at microdroplet coalescence with a liquid layer

The quickly damped capillary waves generated at coalescence of microdroplets (diameter of up to 100 µm), formed in a gas atmosphere at water vapor condensation, with the horizontal layer of water are studied experimentally. Evaporation takes place at intensive local heating of liquid from the substrate side. To visualize and measure the deformations, the Schlieren technique and high-speed video (up to 54000 f/s) are applied. The measured wave amplitude varies within 1-6 μm, and this is consistent with the magnitude of the surface energy of coalescing microdroplets. The waves are generated by the energy of droplet surface.

Heat flux density in the region of droplet contact line on a horizontal surface of a thin heated foil

The evaporating water droplets on a horizontal heated substrate were experimentally studied. The constantan foil 25 μm thick with a size of 42×35 mm2 was used as a substrate. The experiments were carried out with a single droplet or with an ensemble of two or three droplets on the foil. The temperature of the lower surface of foil was measured by an IR scanner. To determine the heat flux density at evaporation of liquid near the contact line, the Cauchy problem for the heat conduction equation was solved using the thermographic data. The results of calculations showed that the maximal heat flux density takes place in the region of the contact line and exceeds the average heat flux density from the entire surface of foil. This is explained by the heat inflow from the foil periphery to the droplet due to relatively high value of the coefficient of heat conductivity of the foil material and high evaporation intensity in the contact line region.

Critical heat flux in locally heated liquid film moving under the action of gas flow in a mini-channel

Thin and ultra thin liquid films driven by a forced gas/vapor flow (stratified or annular flows), i.e. shear-driven liquid films in a narrow channel, is one of the promising candidate for the thermal management of advanced semiconductor devices with high local heat release. In experiments performed in this paper with locally heated shear-driven liquid films of water the effect of various conditions, such as flow rates of liquid and gas and channel height, on critical heat flux (CHF) was investigated. In experiments the record value of CHF as high as 540 W/cm2 has been achieved. The heat spreading into the substrate and the heat loses into the atmosphere in total dont exceed 30% at heat fluxes higher than 200 W/cm2. Comparison of shear-driven liquid films and gravity-driven liquid films showed that CHF in shear-driven films up to 10 times higher than in gravity-driven liquid films. Thus, prospect of using shear- driven films of water in modern cooling systems of semiconductor devices was confirmed.

Effect of heater sizes on evaporation of a liquid film entrained by the gas flow in a microchannel with local heating

The numerical study found the effect of changes in the longitudinal and transverse dimensions of the heater with keeping constant the area of heating on evaporation intensity, dynamics, and extreme thicknesses of a thin liquid film moving in a microchannel under the influence of the gas flow at local heating. It is shown that for relatively small heating, evaporation is more intense for the wide heaters than for the narrow ones. For the narrow and long heaters, evaporation process is slower, but then it is intensified in a short time.

Thickness measurement in a horizontal liquid layer when heated from a localized hot-spot

Thermocapillary breakdown of thin (0.3 - 0.7 mm) horizontal layers of liquid (ethanol) when heated from a localized hot spot was investigated experimentally. The effect of layer thickness on the breakdown dynamics was studied. Visualization and control of the liquid layer were carried out using schlieren and confocal techniques. Main steps of the breakdown process were determined with the help of both systems. Evolution of the layer thickness in the center of substrate was observed and the critical thickness of the layer was measured using confocal sensor.