Coalescence phenomenon of liquid drops of equal and unequal volumes over a hydrophobic surface is reviewed and presented using images from experiments and numerical simulations. Bond number based on the combined volume is in the range of 0.01-0.2. As the drops coalesce, a great variety of interface shapes appear as the merged drop approaches equilibrium. These interface shapes are visualized using a high-speed camera and followed by an examination of the timescales, axisymmetry, bridge formation, dynamic contact angle variation, energy budget, and shear rate.

In this paper, we present the visualization results obtained on the liquid flow in a pulsating heat pipe. Relative to their position in the condenser section, we measured the volume variations of bubbles as they moved through capillary tubes. For small heat fluxes, the bubble volume weakly depended on the distance. However, with an increase in the supplied power, the bubble volume changed significantly as the bubbles moved through the evaporator part due to evaporation of the liquid inside these bubbles. In addition, we employed an infrared scanner to perform a temperature distribution analysis and demonstrate the heat transfer processes.

The interaction of a liquid drop with a copper surface is studied. The substrate is assumed to be superhydrophobic with a wetting angle of 150^∘ . Based on the volume of the drop, the Bond and Weber numbers are approximately 0.23 and 1.6, respectively. The temperature of the surface and the surrounding air is 298 K, and the temperature of the liquid drop is 5K lower. Simulation of conjugate heat transfer is performed using an axisymmetric coordinate system. The Kistler contact line model is used to determine the dynamic contact angle of a drop during spreading. The change in the shear stress on the substrate and the heat flux induced during the propagation of the drop as a function of time is studied.

Evaporative microchannel cooling is a perspective approach to be implemented in future 3D integrated high-performance processors and in the next generation of power electronics. In the present work, numerical and theoretical approaches for the verification of the methodology of the experimental investigation of heat transfer in a flat microchannel have been applied. The experimental methodology for a broad range of conditions, beginning from the convective heat transfer up to the experiment with shear-driven liquid film under intense heating, has been verified. The advantages of the shear-driven liquid films in terms of the critical heat flux against the saturated and the subcooled pool boiling have been demonstrated.

Experiments on coalescence are carried out with two water drops over a chemically textured superhydrophobic surface (equilibrium contact angle ~ 150° and contact angle hysteresis ~ 10°), with one drop initially above the other, under atmospheric conditions. Drops are of equal volumes with a combined Bond number of 0.11. The coalescence events are imaged using two synchronized high-speed cameras in orthogonal directions. As the drops coalesce, a great variety of interface shapes appear as the merged drop approaches equilibrium. The velocity components of the combined drop, and hence the wall shear stress, are estimated by curve fitting the centroidal position data. Two distinct timescales are clearly revealed. These are related to the balance of inertia forces and surface tension on one hand and a balance of inertia and viscous forces on the other. The interface shapes in the two orthogonal views are seen to be similar, indicating the process to be practically axisymmetric during the entire time period. The drop shape gradually evolves over the shorter timescale with the appearance of a positive as well as negative curvature and leads to recoil. Wall shear stresses are estimated to be quite large during the early time period. At later times, it is characterized by small amplitude damped oscillations of a merged drop dominated by viscous dissipation. The scaling law for curvature seen during neck formation corroborates the one reported for drops coalescing in free space. Further, the bulge and neck curvatures as well as the drop shapes seen in experiments are in good agreement with numerical simulation carried out in an axisymmetric coordinate system.

The paper presents an experimental study of heat transfer in a liquid rivulet flowing down a vertical heated plate under the action of gravity. With the use of the optical schlieren method and high-speed shooting, regimes of rivulet flow of liquid over a heated plate were visualized. By means of infrared thermography, the process of heat transfer between the rivulet of liquid and the plate was studied. The heat transfer coefficient was measured for the rivulet flow of intensely evaporating low-boiling liquid. The results obtained can be used in development of cooling systems for various heat-loaded equipment.

Heat pipes are a good solution for temperature stabilization, for example, of microelectronics, because these kinds of systems are without any moving parts. Experimental research of the effect of operating parameters on the heat transfer in a cylindrical heat pipe has been conducted. The effect of the working fluid properties and the porous layer thickness on the heat flux and temperature difference in the heat pipe has been investigated. The temperature field of the heat pipe has been investigated using the IR-camera and K-type thermocouples. The data obtained by IR-camera and K-type thermocouples have been compared. It is demonstrated the power transferred from the evaporator to the condenser is a linear function of the temperature difference between them.

The configuration of a pair of liquid droplets of unequal temperatures with one placed initially above the other is encountered in thermal spray applications and investigated in the present study. The lower drop is initially cold and placed above a horizontal superhydrophobic surface of equilibrium contact angle 150 deg. The second drop placed above the first is initially at a higher temperature before coalescence. The two drops merge and spread over the substrate. Three liquids of interest are Cs-alloy, water, and glycerin with Prandtl numbers of 0.036, 6.64, and 7188.6, respectively. Coalescence process takes place under atmospheric conditions while thermal interaction between the liquid medium and the substrate (copper, alpha similar to 10(-4) m(2)/s and Teflon, alpha similar to 10(-7) m(2)/s) makes it a conjugate heat transfer process. With reference to the volume of the combined drop, Bond number is close to 0.2. Flow and heat transfer simulations are performed for the coalescence process of the two drops as they exchange energy with the substrate and approach thermal equilibrium. The interfacial shapes generated in time, temperature distribution, and the wall heat flux are primary quantities of interest. An axisymmetric coordinate system has been adopted for numerical simulations with the Kistler's model representing contact line motion. Water and Cs-alloy show drop recoil followed by oscillatory spreading over the surface. Thermal convection is visible in water while it is suppressed in glycerin and Cs-alloy owing to high viscosity and thermal diffusivity, respectively, in these media. The instantaneous surface-averaged wall heat flux is initially zero, increases quickly to a maximum, and then gradually decreases to small values. The evolution of wall heat flux in water shows time-dependent oscillations while it is monotonic in glycerin as well as Cs-alloy. Among the three liquids, Cs-alloy displays the highest instantaneous peak in wall heat flux.

The paper is devoted to an experimental study of the heat transfer dynamics during evaporation of a single liquid drop on a heated horizontal surface, which is a sapphire glass coated with a high heat-resistant black graphite paint. The method employed in research can be used to study the heat and mass transfer processes in the gas-liquid-solid contact line region with maximum heat transfer coefficient. Its particular feature as compared to the previously known methods is the solution of the initial-boundary problem for the heat conductivity equation, which in terms of mathematics is a correct problem. Using the thermography method, the sapphire surface temperature fields after single drop impingement are determined. The data obtained will be used to calculate the heat flux density in the region of the contact line of the drop.

The work is devoted to an experimental study of the heat transfer when a water drop falls on a heated horizontal surface, which is a thin constantan foil coated with a fluoropolymer film. Using infrared thermography data, the temperature distributions on the opposite side of the foil from the drop are obtained. The graphs of the temperature distribution along the line passing through the center of the foil with the drop is plotted. The data obtained will be used to calculate the heat flux density in the region of the dynamic contact line of the drop.

An experimental study was carried out to investigate heat transfer during the coalition of two drops of the same volume. The falling of a water drop to other drop sitting on a heated horizontal foil made of constantan with a thickness of 25 mu m from a distance of 10 mm was recorded with a high-speed video camera. An infrared scanner was used to measure the temperature field on the surface of the foil. Measurements taken with the infrared scanner will be used to determine the heat flux density in the region of the dynamic contact wetting line at the coalition of drops.

The flow regimes and pressure drop in a slit microchannel with a height of 164 μm and width of 10 mm are studied experimentally. The boundaries between the regimes are precisely determined using the developed procedure. The homogeneous flow model and the separated flow model are considered for determining the frictional pressure drop. Experimental data are compared with theoretical models. For the homogeneous flow model, the Dukler correlation gives good agreement with experimental data with a mean absolute error of 12%. A new correlation, which describes the experimental data with a mean absolute error of 8.1%, is proposed for the homogeneous flow model. For the separated flow model, the Hwang and Kim correlation gives the best agreement with a mean absolute error of 12.8%. The dependence of the pressure drop in the film flows (annular and stratified regimes) on the mass gas quality has been investigated. It is shown that the minimal pressure drop for the film flows is achieved in the stratified regime; thus, it is the most promising for the use in technical applications.

The crisis of heat transfer in a FC-72 liquid film moving under the action of a gas flow along a smooth or micro-finned heater was experimentally investigated. It is shown that the use of micro-finning leads to an increase in the critical heat flux, since additional washing of the heated surface occurs due to the capillary effect.

In modern microprocessor the heat flux reaches about 200 W/cm. An air cooling system is not efficient for good performance of the microprocessor due to high temperature (close to 75o C). One promising method is to use thin liquid film driven by gas flow in the miniand microchannel. Various studies show the effectiveness of the application of liquid film flow. It is experimentally demonstrated that the critical heat flux for a gasdriven liquid film moving along a surface with a smooth heater can exceed three times that of a liquid film freely flowing down the surface for the same fluid flow rate [1]. Additive technologies are widely used for aerospace industry, in medicine, in the car industry [2]. It is promising for the heat transfer processes intensification. To increase the efficiency of heat removal, it is proposed to use highly developed surfaces with microstructure to enhance the heat transfer processes. The experimental setup has two circuits: liquid circuit and gas circuit. Liquid gear pump is used to pump the working fluid to the test cell. Ultrapure water is used as a working fluid. Dry nitrogen gas is supplied from gas bottle with pressure 200 bars and the gas flow rate is measured and regulated by the Bronkhorst mass flow meter and controller in a range of gas flow rates from 10 to 100 l/min is used. The two-phase flow is formed in a minichannel with a rectangular cross-section of 30x0.9 mm and a length of 50 mm. Heating element is produced from aluminum powder by EOS M 290 device. The surface of the produced element has periodic cylindrical vertical projections with a diameter and height of 400 μm, the distance between the centers of the projections is about 1 mm. The schlieren method is used to visualize the processes inside the minichannel. In the case of superficial gas velocity 0.018 m/s and 0.026 m/s the critical heat flux is about 70 and 101 W/cm, respectively in the range of superficial gas velocity between 7 and 12 m/s. Heat losses does not exceed 30% of the total heat that is released by the electrical heater. The experiments were carried out at the expense of the grant of the Russian Foundation for Basic Research (agreement No. 18-48-543034)

Two-phase cooling systems have a high potential for solving problems of heat removal from the surface of powerful components of microelectronics (chips, microprocessors), LEDs, and solar cells. In this context, a stratified flow regime is of great interest for heat transfer enhancement in such systems. With the development of additive technologies, new opportunities are given to intensify heat transfer by creating highly intelligent surfaces. The paper presents a study of heat and mass transfer of a two-phase system in a minichannel with a local heating source. The surface of the heating element has a periodic columnar structure with characteristic size (diameter and height) equal to 300 µm, produced using additive technologies.

AbstractWe have experimentally studied a two-phase flow in a microchannel with a height of 150 μm and a width of 20 mm. Different liquids have been used, namely, a purified Milli-Q water, an 50% aqueous-ethanol solution, and FC-72. Before and after the experiment, the height of the microchannel was controlled, as well as the wettability of its walls and surface tension of liquids. Using the schlieren method, the main characteristics of two-phase flow in wide ranges of gas- and liquid-flow rates have been revealed. The flow regime-formation mechanism has been found to depend on the properties of the liquid used. The flow regime has been registered when the droplets moving along the microchannel are vertical liquid bridges. It has been shown that, when using FC-72 liquid, a film of liquid is formed on the upper channel wall in the whole range of gas- and liquid-flow rates.

The heat transfer in a shear driven rivulet flow at isothermal conditions in a minichannel has been studied. The experiments were conducted using the setup, specially designed for the shear driven rivulet flow. The effect of liquid flow rate and substrate temperature on the width of the rivulet was studied experimentally using Laser induced fluorescence technique (LIF). The temperature distribution for different time moments of the rivulet surface was measured by the FLIR infrared (IR) camera. IR-measurements served to demonstrate that rivulet surface temperature is lower than substrate one due to liquid evaporation from the rivulet surface.