A comprehensive methodological approach is used to study bubble dynamics under the downward-facing substrate including high-speed imaging and interferometry simultaneously with shadow technique and infrared thermometry. A transparent indium-tin-oxide heater deposited on a sapphire substrate is utilized. This approach allows bubble and dry-spot dynamics, bubble coalescence, and the formation and evaporation of the microlayer to be investigated. The fluorocarbon liquid FC-72 is used as the working fluid. The study revealed significant nonlinear bubble and dry-spot growth rates with increasing heating power. The motion of the bubble relative to the heated substrate facilitates the formation of a microlayer at the bubble's periphery. It was observed that bubbles moving at higher velocity along the substrate at the same heating power experienced faster growth, apparently due to an increase in the area of the microlayer. It is established that the coalescence of bubbles results in the formation of a microlayer under the resulting bubble. The dynamics of the microlayer after coalescence is studied using interferometry. A significant increase in the evaporation rate of the microlayer with increasing heating power is observed. The considered processes play an important role in the microgravity conditions where, due to the lack of buoyancy force, the bubble does not detach from the heater. The presented experimental results can be useful for a deep understanding of bubble dynamics under the heated downward-facing substrate and for the future model development.

The paper investigates a levitating array of microdroplets above a heated layer of distilled degassed water with an area of 40×40 mm2. The droplets are distributed unevenly, and most of them accumulate at the edges of the test section. The coalescence of droplets with the liquid layer is studied. It is shown that with increasing liquid layer temperature, the average diameter of the coalescing microdroplets increases.

Numerical solutions of the three-dimensional equations for Rayleigh-Benard convection in extremely thin layers of different liquids (water and Fluorinert Electronic Liquid FC-72 coolant) uniformly heated from below are presented. The relative size of the liquid layer Gamma = D/H varied from 8 to 400. The sizes of ordered Rayleigh-Benard convective cells were determined, which for water are about 0.42 mm with a layer diameter of 40 mm and a layer height of 0.4 mm, which is comparable with the minimum experimentally recorded transverse size of a monolayer of hovering micro droplets above an evaporating liquid layer in the atmosphere. The effect of temperature difference and liquid layer height on heat transfer was studied. The relative contribution of buoyancy forces and the thermo capillary effect was investigated. The temperature difference at which the transition to non-stationary flow regimes occurs was determined for water. Analytical approximations of the integral heat flux from the free surface of the liquid layer are presented.

The capital expenditures to energy capacity ratio (capex) stands as a key competitive metric for energy storage systems. This paper presents an evaluation of this indicator for an aboveground suspended weight energy storage system. For the first time, an analytical foundational correlation was found between capital expenditures of gravity energy storage, its energy capacity, and storage power. The correlation reveals that capex can be expressed as the sum of three components: one inversely proportional to discharge duration, another inversely proportional to the square root of energy capacity, and a constant term. By using established construction and power element prices the study demonstrates that capex can be reduced to less than 600 $/kW & sdot;h for discharge durations of 4 h or more, and can decrease to nearly 450 $/kW & sdot;h for a 10-h discharge duration. Employing the computed capex an evaluation of the total cost of ownership was conducted and juxtaposed with lithium-ion energy storage. The gains from adopting gravity technology become significant starting from the initial replacement of degraded Li-ion batteries. This scenario results in nearly a twofold savings in the ownership cost of gravity energy storage system over a 20-year operational span with further prospects for enhanced economic benefits.

The evolution of the flow structure of thermal gravity-capillary convection in thin cylindrical water layers with a free surface, heated from below, was numerically studied. The layer diameter ranges from 10 mm to 40 mm, and the height ranges from 0.1 mm to 5 mm. The influence of the layer height and temperature difference on heat transfer was investigated, and the boundaries for the transition to nonstationary flow regimes were determined. The relative contributions of buoyancy and thermocapillary effects were examined, and analytical dependencies for heat dissipation from the free surface as a function of temperature difference and layer height were constructed. The dimensions of the ordered Rayleigh-B & eacute;nard convective cells were determined to be approximately 0.42 mm for a diameter of 40 mm and a layer height of 400 mu m, which is comparable to the size of monolayer levitating microdroplets above an evaporating liquid layer in the atmosphere.

According to the American Council for an Energy-Efficient Economy, transition from conventional wire ropes to PU-coated multiple-rope belts has significantly increased energy efficiency of lifting mechanisms, so expanding this experience to the design of gravity energy storage systems seems very promising. In the present paper, an algorithm to calculate the round-trip efficiency (RTE) of gravity energy storage systems with a rope traction mechanism using PU-coated multiple-rope belts is presented. The algorithm includes a mathematical model describing belt/hoisting unit interaction. Efficiency calculation for a specific design of a gravity energy storage system is given as an example. High sensitivity of the system's RTE to the mechanical parameters of the lifting mechanism is demonstrated. The estimated RTE has comprised 86 % for a 900-kW lifting system that transports a weight at a nominal speed of 1.5 m/s. Such multiple systems working together as a single facility can become a powerful gravity storage.

Gravity Energy Storage (GES) is an emerging renewable energy storage technology that uses suspended solid weights to store and release energy. This study is the first to investigate the feasibility of using unstabilized Compressed Earth Blocks (uCEBs) as a cost-effective and sustainable alternative for weight manufacturing in GES systems. The analysis demonstrates that uCEBs offer competitive advantages over conventional materials in terms of density and cost. The relatively low strength requirements of weights allow for CEB production without stabilizers, reducing manufacturing time and capital costs. The study highlights potential of uCEBs for GES and describes the manufacturing method. Overall, uCEBs represent a promising option to enhance GES efficiency and affordability.

New concept of the experimental investigation has been proposed and successfully realized during the 73rd ESA parabolic flight campaign 2020.The evaporators' behavior is analyzed by evaluating the liquid level inside and by investigating the external condensation on the designed condenser's surface.Three evaporators made with different techniques have been tested.Analysis of the experimental data shows qualitative differences in considered evaporators.Aluminium foam provided uniform distribution of liquid close to its external surface.The evaporator with 3D-printed wick had a non-uniform distribution of liquid inside.Sintered brass wick showed the longest stabilization time.

The effect of a droplet ejection upward during the collapse of a droplet cluster is described for the first time. Using a high-speed camera, the dynamics of the droplet motion is studied, based on which the average velocity of the steam air flow along the droplet trajectory above the locally heated water surface is estimated. It was found that, for the largest observed droplets, the aerodynamic drag force by the Stokes' law at this average velocity does not exceed their weight. This confirms the fast decay of the flow velocity with the height over the layer. We hypothesize that the thin gas interlayer between the droplet and the evaporating water surface affects the droplet clusters levitation mechanism and ensures the resistance of such droplets to coalescing with the subjacent water layer.

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.

Thermocapillary deformations of horizontal self-rewetting layer of liquid (solution of 1-butanol of 5% concentration in water) when heated from a localized hot spot were studied experimentally. Measurements of the liquid layer deformations were performed using the confocal techniques with three-dimensional positioning system having high-speed linear actuator. Effect of equalizing the profile of the liquid surface over the heating area has been observed before the layer breakdown. The potential interest of the proposed studies induces by the large number of possible industrial applications, including space technologies and terrestrial applications, such as cooling of electronic components.

Breakdown dynamics was studied experimentally for the horizontal layers of various liquids (ethanol, water) with the thickness of 300 μm under the conditions of spot heating from the substrate. The main stages of the process of liquid layer breakdown were determined, and time of dry spot formation was measured. Time of dry spot formation for ethanol at the heat flux of 12.6 W/cm 2 was 7.85 s, and for water at the heat flux of 117 W/cm 2 , it was 0.13 s. It was found that for both working liquids, a residual layer appears in the region of spot heating before liquid layer breakdown. It is shown that together with the thermocapillary effect, evaporation is one of the main factors affecting dynamics of liquid layer breakdown and dry spot formation.

The evaporation of aqueous salt solution droplets from metal surfaces has been studied experimentally. The volumetric evaporation rate is found to decrease in time for any initial droplet volume due to an increase in salt concentration and the efforts of system to take a state of thermodynamic equilibrium. Crystalline hydrate film was being formed during desorption of CaCl2 10%, LiCl 10%, LiBr 30% salts. When NaCl 10% salt solution evaporated, there was no film. The average evaporation rate of NaCl salt is higher than for other salts. The lowest values of the average evaporation rate were found for LiBr 30% salt solution.

A two-dimensional problem of the fluid flows with a dynamic contact angle is studied in the case of an uniformly moving contact point. Mathematical modeling of the flows is carried out with the help of the Oberbeck-Boussinesq approximation of the Navier-Stokes equations. On the thermocapillary free boundary the kinematic, dynamic conditions and the heat exchange condition of third order are fulfilled. The slip conditions (conditions of proportionality of the tangential stresses to the difference of the tangential velocities of liquid and wall) are prescribed on the solid boundaries of the channel supporting by constant temperature. The dependence of the dynamic contact angle on the contact point velocity is investigated numerically. The results demonstrate the contact angle behavior and the different flow characteristics with respect to the various values of the contact point velocity, friction coefficients, gravity acceleration and an intensity of the thermal boundary regimes.

Vapor condensation of the HFE-7100 in loop heat pipe was studied experimentally and theoretically. Numerical calculations of the vapor condensation on the curvilinear fin have been performed. Numerical, theoretical and experimental data are in a good agreement. Minimal condensate film thickness on the top of the fin has been determined and increases monotonously with the increase in the temperature drop.

The problem of thermocapillary deformation of the locally heated horizontal liquid layer is considered. The numerical solution of the problem has been obtained in the lubrication approximation theory for two-dimensional axisymmetric thermocapillary flow. The model takes into account surface tension, viscosity, gravity and heat transfer in the substrate and liquid. Evaporation is neglected. The numerical algorithm for the joint solution of the energy equation and the evolution equation for the liquid layer thickness has been developed. Stationary solutions have been obtained by the establishment method. There have been measured and numerically calculated deformations in locally heated horizontal layers of silicone oils of different types and thickness. The dependencies of the depth of thermocapillary deformations on the layer thickness have been obtained for silicone oils of different viscosities. It has been found that the value of the relative deformation of the layer decreases nonlinearly with increasing the layer thickness, when other conditions being equal. It has been found a good qualitative agreement of numerical results and experimental data.

Mathematical model of liquid meniscus shape in cylindrical micro-channel of the separator unit of condensing/separating system is presented. Moving liquid meniscus in the 10 μm cylindrical microchannel is used as a liquid lock to recover the liquid obtained by condensation from the separators. The main goal of the liquid locks to prevent penetration of a gas phase in the liquid line at the small flow rate of the condensate and because of pressure fluctuations in the vapor-gas-liquid loop. Calculation of the meniscus shape has been performed for liquid FC-72 at different values of pressure difference gas - liquid and under normal and micro gravity conditions.