This experimental research focuses on boiling development, formation and evolution of dry spots, and non-uniform heat transfer during spray cooling of an ultra-smooth, thin sapphire heated substrate using a combined diagnostic approach based on infrared thermography, internal-reflection high-speed visualization, and machine-learning and tracking algorithms. The tests were performed with subcooled water at initial temperature of 25-75 degrees C and a spray nozzle with an orifice diameter of 1.2 mm and a liquid flow rate of 10 g/s, at various nozzle-to-heater distances. The experiments and data analysis show that nucleation sites and locations of large-scale dry spots are highly non-uniformly distributed across the heater surface, and that their spatial organization depends on the nozzle-to-surface distance and the initial liquid temperature. A joint analysis of high-speed visualization and IR thermometry reveals that regions of bubble nucleation and large-scale dry spots coincide with local temperature maxima, whereas zones of enhanced heat removal suppress nucleation activity. Two distinct mechanisms of large dry-spot formation are identified: (i) coalescence of multiple neighboring dry spots generated by boiling in the liquid film, and (ii) evaporation in thin liquid sublayers formed at the base of craters induced by the impact of large droplets on the film surface. Based on a comparison of experimental heat transfer and visualization data, it is shown that the development of boiling and the formation of large coalescing dry spots during spray cooling with subcooled water at an optimal distance do not exert a significant influence on the overall heat transfer performance. The study demonstrates the significant potential of combining infrared thermography, internal-reflection visualization, and AI-based post-processing for elucidating two-phase heat transfer mechanisms in spray-cooling systems. Furthermore, the proposed experimental framework enables the quantitative characterization of key parameters-including dry-spot area evolution, maximum size and lifetime, bubble growth and collapse dynamics, as well as spatial distributions and nucleation site density-thereby providing critical input for modeling the heat transfer and thermal stability of dry spots, elucidating trigger mechanism of crisis phenomena development in spray cooling.

Dielectric liquids possess unique physical and chemical properties, making them promising candidates for application in electronic cooling systems. However, available heat transfer and critical heat flux (CHF) data in the literature exhibit significant variability, complicating the design of such systems. This study presents experimental results on dry spot dynamics, heat transfer and crisis phenomena during boiling of dielectric liquids HFE-7100 and Novec 649, obtained using IR thermography and high-speed video recording with multiple illumination configurations, including LED reflection from the bottom side of a transparent sapphire heated substrate. Analysis of the boiling curves revealed that the maximum heat transfer coefficients and CHF for HFE-7100 are higher by factors of 1.47 and 1.56, respectively, compared to those for Novec 649. High-speed visualization with reflected LED light and a CNN-based model enabled quantitative analysis of the dependence of dry spot density, contact line length per unit area, void fraction, and dry spot sizes on the heat flux up to the onset of boiling crisis. In particular, it was shown that at CHF the structure of the two-phase flows near the heated wall changes drastically, which leads to a change in the dynamic picture of dry spots evolution and a bimodal distribution of their areas, prior to the appearance of irreversible dry region. The growth rate of the irreversible dry spot for both liquids was measured and compared against existing predictive models, demonstrating good agreement with the analytical traveling thermal wave model. The experimental findings support the concept of the boiling crisis as a conjugate problem, where the interplay between the two-phase flow dynamics near the heated wall and the thermal stability of dry spots must be considered comprehensively. The obtained results are important not only for verification of theoretical model and numerical simulations, but also for the development of effective prototypes of two-phase cooling systems using the HFE-7100 and Novec 649 as a coolants.

The results of an experimental investigation on heat transfer and critical heat flux during surface cooling with a dispersed flow of deeply subcooled liquid are presented. The study was carried out using a pressure nozzle with a mass flow rate of water of 24.2 g/s. A record critical heat flux of 13.2 MW/m2 was achieved in these experiments. The findings indicate that the onset of boiling within the liquid film formed on the impact surface during spraying leads to a notable reduction in the temperature non-uniformity across the heater.

The paper presents the results of the experimental study of the structure of a liquid microlayer at the base of vapor bubbles during water pool boiling using the method of light-emitting diode (LED) interferometry and a transparent design of the heating surface. Microlayer profiles were obtained at different time moments and the effect of heat flux density on its characteristics was analyzed. It is shown that the applied technique with a fairly simple optical scheme allows obtaining of up-to-date information on the structure and dynamics of the microlayer under bubbles during boiling.

The paper is concerned with an experimental study of heat transfer and boiling crisis development on a biphilic silicon surface made using a set of methods, including chemical vapor deposition and laser texturing. It is shown that the use of a biphilic surface with the proposed configuration of hydrophobic zones on a superhydrophilic base leads simultaneously to an increase in heat transfer by 60

Subatmospheric flow boiling heat transfer is a promising method for electronics cooling due to lower saturation temperatures. However, pressure is a crucial parameter that affects surface tension and vapor density. In this study, the effect of surface mixed wettability configuration on bubble dynamics and flow boiling was investigated under atmospheric and subatmospheric pressure conditions. Superhydrophilic, superhydrophobic, and mixedwettability surfaces were prepared and tested at various heat fluxes and three system pressures of 48 kPa, 68 kPa, and 101 kPa. The channel dimensions were 50 mm x 15 mm, and the channel had a depth of 1 mm. The results showed that biphilic surfaces enhanced the performance up to 28% compared to superhydrophilic surfaces at high heat fluxes for subatmospheric boiling. Flow visualization efforts reveal that mixed-wettability surfaces improve heat transfer by extending the efficient slug regime to higher heat fluxes by preventing dried spot formation. These surfaces benefit from high density nucleation sites at low and medium heat fluxes, resulting in a noticeable performance improvement compared to the superhydrophilic surface. The obtained experimental data in this study will be helpful for the development of thermal-fluid systems operating under subatmospheric conditions.

To date, using biphilic surfaces is one of the most promising methods for enhancing heat transfer and critical heat flux during boiling simultaneously. However, most of studies on the effect of biphilic surfaces on boiling performance have been carried out under atmospheric pressure conditions. In this context, the issues of heat transfer enhancement and stabilization of the boiling process at subatmospheric pressures are particularly critical due to the interesting characteristics of boiling heat transfer and bubble dynamics at subatmospheric pressures and their practical significance including aerospace applications. This paper investigates the effect of the pitch size between hydrophobic spots on a biphilic surface on heat transfer and bubble dynamics during boiling at subatmospheric pressures (from 11.2 kPa up to atmospheric pressure). The data analysis using infrared thermography demonstrated that the maximum heat transfer rate was achieved on a surface with a uniform pitch size (6 mm) at all pressures. In this case, the heat transfer enhancement, compared a bare surface, reached 3.4 times. An analysis of the departure diameters of bubbles based on high-speed visualization indicated that the optimal configuration of the biphilic surface corresponds to the pitch size equal to the bubble departure diameter. Using high-speed visualization also demonstrated that an early transition to film boiling was evident for configurations with a very high density of hydrophobic spots (pitch size of 2 mm).

This paper presents the results of an experimental study on the evolution of a nonstationary temperature field during ethanol pool boiling in a pressure range of 12–101.2 kPa. Experimental data were obtained using infrared thermography with high temporal and spatial resolutions, which made it possible to reconstruct the distribution of the heat flux density and to study the influence of pressure reduction on the local heat transfer rate in the vicinity of the triple contact line under vapor bubbles for the first time. It is shown that, for all studied pressures, a significant heat flux density is removed from the heating surface due to microlayer evaporation, which exceeds the input heat power by a factor of 3.3–27.7, depending on the pressure. Meanwhile, the heat transfer rate in the area of the microlayer evaporation significantly decreases with the pressure reduction. In particular, the local heat flux density averaged over the microlayer area decreases by four times as the pressure decreases from 101.3 kPa to 12 kPa. Estimates of the microlayer profile based on the heat conduction equation were made, which showed the significant increase in the microlayer thickness with the pressure reduction.

Spray cooling is a highly effective method of heat removal that has broad practical applications, including use in modern cooling systems designed for microelectronics and microchips. It is known that spray cooling performance is influenced by a huge number of factors. This experimental research is devoted to the study of the influence of a liquid flow rate in the range of 15.1–24.2 cm3/s, heat flux up to 6.4 MW/m2, and nozzle-to-surface distance on the heat transfer rate in non-boiling mode and the distribution of the local temperature of the heat exchange surface during spray cooling. It is shown that the heat transfer coefficient weakly depends on the heat flux for all studied nozzle-to-surface distances. It is demonstrated that the nozzle-to-surface distance has a significant influence on the heat transfer and the temperature distributionon the heating surface during spray cooling in non-boiling mode. At the same time, there is an optimal distance at which the maximum heat transfer rate and uniformity of the temperature are achieved. Criteria and a ratio for determining the optimal distance from the spray nozzle to the heated surface are proposed.

Today, neural networks have increasingly gained the attention of researchers and become an effective instrument for a wide range of scientific applications, including issues related to the boiling. However, there are no universal tools in the literature that would allow detecting the life cycle of individual vapor bubbles and automatically measure a wide range of main boiling characteristics based on the high-speed visualization data.In this study, the U-net and Mask R-CNN convolutional neural networks were used to detect and segment bubbles obtained by visualization from the bottom side of a transparent heater during water boiling at various subatmospheric pressures. The key feature of the trained CNN architectures is the ability to detect bubbles located on a heated wall, while ignoring the bubbles that lift-off, and to determine the moment of their de-parture. The verification of various neural networks demonstrated that the Mask R-CNN architecture is more preferable to measure dynamic boiling characteristics.Through trained convolutional neural networks, a wide array of data on local boiling characteristics, including the nucleation site density, bubbles growth rate, life-time and departure diameters, waiting time between mo-ments of bubbles departure and nucleation frequencies were automatically obtained for water boiling at various heat fluxes and pressures in the range of 42-103 kPa. Based on the parameters obtained, heat transfer simulation was carried out using various heat flux partitioning approaches and the ranges of their applicability were demonstrated.

The possibility is shown of measuring the temperature field, the droplets sizes, and the irrigation pattern of an impact surface during spray cooling with the use of high-speed IR thermography, video visualization, and a transparent design of the heating element. New experimental data on the heat transfer intensity are obtained. In particular, it is shown that the heat transfer coefficients during spray cooling are 1.5 times higher than the maximal heat transfer under multijet cooling regime. The liquid flow rate for the spray nozzle is lower by almost 5 times compared to the multijet nozzle.

The article is devoted to the research of transient heat transfer, explosive vaporization and condensation in subcooled ethanol on microheater during pulsed heating. Experiments were performed using high-speed infrared thermography and visualization for various initial liquid temperatures 30-60 degrees C and heat fluxes in the range of 3.3-7.8 MW/m(2). The evolution of temperature field of the microheater and liquid during transient heating and the nucleation temperatures for different heat fluxes and liquid subcooling were determined. It was shown that the nucleation temperature was almost independent on heating power and amounted to 194 degrees C, which is close to the calculations based on homogeneous nucleation theory. New experimental data were obtained on the times of convection and boiling onset, evolution of vapor bubbles, including the dependence of their maximum sizes on the heat flux and initial liquid temperature. It was shown that the maximum bubble size decreases with an increase in the heat flux and liquid subcooling, varying in the same range as for water. It was demonstrated that at low heat fluxes, during condensation, a liquid funnel was formed at the bubble top, which resembled the formation of a liquid jet during the collapse of cavitation bubbles.

Spray cooling today is one of the most effective methods for high heat flux application. Since there are numerous factors influencing the heat transfer in spray cooling, some issues related to the effect of the nozzle-to-surface distance, heat flux and liquid subcooling on the heat transfer efficiency in non-boiling mode remain debated. This study is dedicated to a comprehensive experimental investigation of the impact of various factors, including the distance from the nozzle to the heater (2–35 mm), heat flux density (up to 6.9 MW/m2), liquid flow rate (8.8–25.2 mL/s), and initial liquid temperature (20 and 80 °C) on heat transfer in non-boiling spray cooling using different nozzles with varying spray angles (30–90°). Heat transfer results were obtained based on field temperature measurements using high-speed infrared thermography. It is shown that the nozzle-to-surface distance has a significant effect on the spatial distribution of the temperature field and the heat transfer rate in spray cooling. It is shown that for each nozzle and heating surface there is an optimum distance at which the maximum heat transfer rate at non-boiling spray cooling is observed and relationship for their determination are proposed. The heat transfer coefficient for deeply subcooled liquid (80 K) is practically independent of the heat flux. At low subcooling levels (20 K), the heat transfer coefficient increases with the increase in heat flux associated with the influence of evaporation from the liquid film surface on the total heat transfer. The results obtained are analyzed, compared and generalized with data from the literature, and a correlation is proposed for determining the heat transfer in non-boiling spray cooling.

Today the problem of the boiling improvement is directly related to the heating surface modification at the micro- and nanoscale. As was shown by different authors the hemi-wicking surfaces demonstrate extremely high enhancement of heat transfer (HTC) and critical heat flux density (CHF) during boiling. In this paper, the laser ablation technique was used to fabricate textured hemi-wicking silicon surfaces. This surface modification technique is one of the most promising and discussed techniques today due to its high accuracy, implementation ease and high stability of the fabricated surfaces. The effect of the laser type on the silicon surface properties and boiling improvement was studied in detail. The texturing was performed using lasers with different wavelength - infrared (1064 nm) and visible (532 nm), but with the same number of pulses per unit area and laser spot diameter. The experiments showed that the water pool boiling performance differs significantly depending on the type of laser treatment. In particular, while the usage of infrared laser results in the HTC enhancement up to 78% compared to the untreated surface, the visible laser-textured surface in contrast shows the deterioration of the heat transfer rate up to 40%. At the same time, the visible laser-textured surface demonstrates the maximum CHF value of 1806 kW/m2, which is more than 2 times higher than for the untreated surface. The comparison with the models showed, that such an enhancement is explained by the highest capillary wicking of this surface. Based on the analysis of departure diameters, nucleation frequencies and the conditions for the vapor bubbles formation the observed difference in the heat transfer rate during boiling was revealed. Also using the high-speed thermography the dynamics of reversible and irreversible dry spots was studied during boiling on laser-textured hemi-wicking surfaces for the first time.

The paper presents the results of an experimental study of the effect of hydrophobic fluoropolymer coating on the multiscale characteristics of heat transfer at water boiling. New experimental data on dynamics of vapor bubble growth and detachment, evolution of contact line, nucleation site density, heat transfer coefficient were obtained using high-speed imaging techniques, including infrared thermography and video recording from the bottom side of transparent ITO heater. It was shown, that the using of hydrophobic fluoropolymer coating leads to heat transfer enhancement, to decrease of the superheat temperature at the onset of boiling, to increase of the active nucleation site density and to significant change in the dynamics of growth and departure of vapor bubbles and the evolution of the triple contact line.

To date the usage of biphilic surfaces is one of the most promising ways to simultaneously enhance heat transfer and increase critical heat fluxes during boiling. However, the vast majority of studies devoted today to the in-fluence of surfaces with mixed wettability on boiling performance refer to atmospheric pressure conditions. At the same time, the problems of heat transfer rate increasing and stabilizing the boiling process at subatmospheric pressures are particularly acute, which is associated with some features of boiling in a vacuum and its high practical relevance. The paper presents the results of experimental study on the local boiling characteristics, including the bubble departure diameters and emission frequencies, and the heat transfer rate during water boiling on a biphilic surface in the pressure range of 10-102 kPa. As a result of experiments, it was shown that the hydrophobic areas of the fabricated biphilic surface are the sites of continuous vapor bubbles generation in the entire range of the studied pressures. At the same time, the slight increase in the size of detached bubbles and their emission fre-quency from the hydrophobic spots was observed with pressure reduction. It was also demonstrated that in the range of low pressures (less than 40 kPa), the biphilic surface is characterized by noticeably smaller bubble departure diameters and much higher emission frequencies compared to bare surface. The analysis of boiling curves obtained using IR thermography revealed that the developed biphilic surface provides a significant heat transfer enhancement -up to 3.7 times during boiling at subatmospheric pressures compared to bare surface. Moreover, a significant decrease in the surface superheating and in the amplitude of integral temperature oscillations is observed, which represents the boiling stabilization at low subatmospheric pressures (less than 20 kPa) for the fabricated surface.

Surface wettability is one of the key parameters in the manipulation of the boiling phenomenon. Al-though there are a number of studies on the effect of surface wettability on boiling heat transfer, there are few research efforts to explain the boiling phenomenon on superbiphilic surfaces at sub-atmospheric pressures. In this study, pool boiling experiments were conducted to investigate the boiling heat trans-fer performance of surfaces with uniform (superhydrophobic and superhydrophilic) and mixed (super-biphilic) wettability. This study presents the results obtained from four different surfaces for both atmo-spheric (103.7 kPa) and sub-atmospheric (28.3 kPa) pressures and aims to provide an understanding of the wettability effect using saturated deionized water as the working fluid in the heat flux range of 7 -290 kW/m2. The experimental results show that the superbiphilic surface (superhydrophobic spots with a pitch size of 3 mm and diameter of 0.7 mm) offers improvements in boiling heat transfer at both atmo-spheric and sub-atmospheric pressures up to 98% and 54%, respectively. Due to bubble coalescence being more likely to occur at sub-atmospheric pressure, the enhancement effect of superbiphilicity on boiling heat transfer is more significant for atmospheric pressure.(c) 2022 Elsevier Ltd. All rights reserved.

This paper demonstrates the advantages and prospects of transparent design of the heating surface for the simultaneous study of the hydrodynamic and thermal characteristics of spray cooling. It was shown that the high-speed recording from the reverse side of such heater allows to identify individual droplets before their impact on the forming liquid film, which makes it possible to measure their sizes with high spatial resolution. In addition, such format enables one to estimate the number of droplets falling onto the impact surface and to study the features of the interface evolution during the droplets’ impacts. In particular, the experiments showed various possible scenarios for this interaction, such as the formation of small-scale capillary waves during impacts of small droplets, as well as the appearance of “craters” and splashing crowns in the case of large ones. Moreover, the unsteady temperature field during spray cooling in regimes without boiling was investigated using high-speed infrared thermography. Based on the obtained data, the intensity of heat transfer during spray cooling for various liquid flow rates and heat fluxes was analyzed. It was shown that, for the studied regimes, the heat transfer coefficient weakly depends on the heat flux density and is primarily determined by the flow rate. In addition, the comparison of the processes of spray cooling and nucleate boiling was made, and an analogy was shown in the mechanisms that determine their intensity of heat transfer.

In this paper, the results of numerical calculations of a vapor bubble growth in superheated water at different pressures are presented. Modeling is based on a previously developed by the authors semi-analytical solution. The results are verified by experimental data obtained at atmospheric and subatmospheric pressures. The presented simulation results and experimental data are in good agreement. The advantage of the solution over the earlier ones (based on the thermal growth model) is shown.

The surface modification is one of the most promising and discussed methods to improve the boiling performance. To date there are a lot of techniques to modify a heating surface, but the search for the optimal, simple and reliable one is still an actual problem. Recently, the surface texturing using laser ablation was applied in a number of studies, which showed its great potential for heat transfer enhancement and critical heat fluxes increase during pool boiling on the metal surfaces. In this paper, we modified a silicon surface by laser texturing and analyzed its effect on the heat transfer and bubble dynamics during water pool boiling using high-speed thermography and video recording. The experiments showed that the usage of laser-textured surface results in the heat transfer enhancement up to 49.5% compared to the reference rough silicon sample and up to 234% compared to the polished sample. Moreover, the modified surface is characterized with lower onset of nucleate boiling and lower bubble nucleation temperature. Dataset on the major characteristics of bubbles dynamics during boiling on the untreated and laser-textured surfaces was obtained using the high-speed video recording. Its analysis showed that the laser treatment leads to the significant increase in the nucleation site density and nucleation frequency, while the bubble departure diameter value dramatically decreases compared to the reference surface. On the basis of experimental data the relationship between nucleation frequency and departure diameter was found and analyzed both for untreated surface and for laser-textured one.