In this study, the water entry of a high-temperature sphere was experimentally investigated to explore the dynamics of the inverse Leidenfrost phenomenon. During this process, oscillations were observed on the gas-liquid interface, which originated at the sphere's windward surface and propagated upward. To capture flow details that could not be obtained experimentally, a numerical model was developed by solving the complete formation of the governing equations. The numerical results matched well with the experimental data and revealed that the oscillations resulted from the collective coupling among fluid flow, heat transfer, and phase change. To further reveal the physical mechanism of these oscillations, a reduced-order theoretical model was established by employing the potential flow assumption and the Karman-Pohlhausen method. Based on mass and momentum conservation, an expression for the oscillation period of the gas-liquid interface was derived from this model. The theoretical predictions showed excellent agreement with both experimental and numerical results, validating the proposed model.

When it comes to droplet evaporation, most evaporation models assume a spherical droplet. However, gravity causes a droplet to deviate from a spherical shape in practice. In this study, the equilibrium shapes of gravity-induced non-spherical droplets are obtained using the energy minimization method. Based on these shapes, a three-dimensional numerical model incorporating vapor diffusion, natural convection, conjugate heat transfer, and evaporative cooling is developed to examine the differences in evaporation characteristics between spherical and non-spherical water droplets on horizontal and vertical hydrophobic substrates. The results demonstrate that gravity-induced deformation reduces the thermal resistance in the horizontal non-spherical droplet from its bottom to the top. This results in a higher minimum interfacial temperature in the horizontal non-spherical droplet than in its horizontal spherical counterpart. In contrast, the vertical non-spherical droplet exhibits a lower minimum interfacial temperature than the vertical spherical droplet. These different temperature distributions and geometric shapes drive different internal flow fields, enabling the horizontal non-spherical and vertical spherical droplets to exceed the average interfacial flow velocity of their counterparts by 7.27% and 9.91%, respectively. Furthermore, the local evaporation flux, governed by temperature and gas-phase convection, also exhibits distinct spatial distributions in these droplets. Surprisingly, despite causing notable changes in heat and mass transfer, gravity-induced deformation does not significantly alter the total evaporation rate. This might be attributed to the minimal difference in surface area of the droplets and a stable diffusion resistance. These findings imply that the evaporation rate of gravity-induced non-spherical droplets may be approximated by spherical droplets.

A three-dimensional transient numerical model incorporating coupled mass, momentum, heat, and species transport was developed to investigate the evaporation dynamics of binary sessile droplets. The evaporation behavior and the temporal evolution of the internal flow structures were systematically examined, revealing four distinct stages based on internal flow pattern: (i) the initial stage of coexistence of solutal and thermal Marangoni convection, (ii) solutal Marangoni-dominated stage with complex multi-vortex structures, (iii) solutal Marangoni-dominated stage with imbalanced macroscopic vortex flow, and (iv) stable low-velocity convection stage. The flow characteristics at each stage were delineated, and the underlying physical mechanisms driving the transitions were elucidated. Particular attention was given to the interplay between thermal and solutal effects and their influence on the evolving flow field, temperature and concentration distributions, and local evaporation rates. These results provide new insight into the coupled fluid dynamics governing the evaporation of multi-component droplets.

Seawater desalination and distributed water treatment demand efficient and controllable interfacial evaporation technologies. Micro-scale channels offer a robust platform for achieving stable and high-efficiency evaporation. In this work, a multiphysics model was developed for solar-driven brine evaporation within capillaries, coupling heat transfer, gas-liquid convection-diffusion and interfacial solute transport. The microscale flow and mass transfer behaviors near the meniscus were numerically reconstructed, exposing the interactions among temperature gradients, solute concentration and phase-change convection, along with their spatial patterns. A detailed examination of interfacial phase change, vapor diffusion and local flow clarified the physical mechanisms behind evaporation and energy transfer pathways. The heat distribution within system is governed by the competition among physical processes of solid conduction, gas convection and interfacial phase change. In this process, heat flux distribution exhibits high sensitivity to geometric conditions. The heating width and liquid column height govern the radial heat-mass distribution and axial transport scale, serving as key geometric factors regulating overall evaporation performance. A wider heating region strengthens interfacial heat input, raising the normalized peak flux from about 0.25 to 0.88, but also intensifies convective and conductive losses, reducing evaporation efficiency to about 20%. While a taller liquid column impedes downward heat conduction, further raising the normalized peak flux to about 0.9, and enhancing evaporation efficiency by roughly sixfold under non-overcooled conditions, it also increases salt accumulation and crystallization risks. In addition, based on a balance between evaporation rate and efficiency, the optimal operating range in this study was identified. These findings provide quantitative guidance for structural design of interfacial evaporators and offer theoretical support for long-term operation in distributed seawater desalination and thermal management systems.

In this paper, the water-entry process of high-temperature spheres was examined. Due to the intense phase change, the cavities formed by high-temperature spheres differed significantly from those formed by ambient-temperature spheres. Based on distinct cavity evolutionary characteristics, two cavity types were identified: quasi-static cavities occurring at lower initial impact velocities and deep seal cavities occurring at higher initial impact velocities. The variation trends of pinch-off time and characteristic length with respect to initial impact velocity differed among these cavity types due to their unique evolutionary behaviors. Further analysis revealed that although no significant differences in the motion of the sphere were observed among different cavity types, the initial impact velocity significantly influenced it. Higher initial impact velocities resulted in greater drag forces but lower drag coefficients. Next, higher initial impact velocities also induced higher phase change rates in the early stages of the sphere's water entry. Furthermore, the distinct cavity types led to differences in vapor distribution during the process. At the pinch-off time, the quasi-static cavities contained higher vapor content due to their smaller cavity volume. Finally, the cooling process of the sphere also varied with cavity type: for quasi-static cavities, heat flux gradually increased as the gas–liquid interface progressively enveloped the sphere, whereas for deep seal cavities, heat flux remained stable most of the time because the area of the sphere surface near the interface remained stable.

The growing thermal management demands of high-performance electronics have intensified research on microchannel flow boiling, driven by its exceptional heat transfer efficiency and cost-effectiveness. The present work develops a 3D computational framework integrating Volume-of-Fluid interface tracking with Saturated Interfacial Volume phase-change modeling to analyze saturated boiling dynamics in microchannel flows containing a heated square column obstruction. The research focuses on elucidating the intricate interactions between the heated square column and the vapor-liquid interfacial film, revealing that thin-film evaporation serves as the dominant mechanism for enhanced heat transfer efficiency. Additionally, increasing Reynolds number leads to liquid film thickening that reduces heat transfer efficiency, while larger initial bubble diameter enhances thermal performance through film-thinning effects. Additionally, increasing Reynolds number leads to liquid film thickening that reduces heat transfer efficiency, while larger initial bubble diameter enhances thermal performance through film-thinning effects.

In this work, the bubble merger process in a heated symmetric micro-fluidic T-junction is numerically studied with the variations of heat flux and seed bubble volume. Detailed bubble behaviors and phase change heat transfer characteristics are revealed. Results show that the bubble experiences slipping and colliding merger regimes at small and large seed bubble volumes, respectively. The bubble grows faster at large heat flux and seed bubble volume. Obvious peak in evaporation rate during the bubble merger can be seen. The bubble behaviors significantly affect the phase change heat transfer. The asymmetry of bubble behaviors under slipping merger regime leads to the heat transfer difference between two main channel walls. Besides, the squeezing effect during bubble merger could promote the heat transfer. Generally, the heat transfer enhancement becomes stronger as the heat flux and seed bubble volume are increased. The present study?s findings could improve the understanding of bubble behaviors and transport details in micro-fluidic T-junction structures.

In this work, the bubble merger process in a heated symmetric micro-fluidic T-junction is numerically studied with the variations ofheatflux and seed bubble volume. Detailed bubble behaviors and phase change heat transfer characteristics are revealed. Results show that the bubble experiences slipping and colliding merger regimes at small and large seed bubble volumes, respectively. The bubble grows faster at large heat flux and seed bubble volume. Obvious peak in evaporation rate during the bubble merger can be seen. The bubble behaviors significantly affect the phase change heat transfer. The asymmetry of bubble behaviors under slipping merger regime leads to the heat transfer difference between two main channel walls. Besides, the squeezing effect during bubble merger could promote the heat transfer. Generally, the heat transfer enhancement becomes stronger as the heat flux and seed bubble volume are increased. The present studys'findings could improve the understanding of bubble behaviors and transport details in micro-fluidic T-junction structures.

Abstract Current theoretical studies of parallel channel instability are restricted to the full development assumption, and the entry effect of the existing developmental stage in miniaturized heat dissipation scenarios is not taken into consideration, leading to some deviations in the predicted results of the stability in practical applications. Using a kinetic model of phase transition and pressure drop, we study microchannels with an L/D ratio of 50 and a hydraulic diameter of 200 μm. The significant influence of the entry effect during both the development and fully developed stages is revealed by our results, which demonstrates a narrowing of the overall flow discrepancy and, consequently, an enhancement in the system’s stability. Particularly, the mitigating effect of the entry effect on the flow instability and maldistribution within the parallel channels was gradually enhanced when the qin increased from 20 W/cm2 to 100 W/cm2.

Abstract The inverse heat transfer problem is vital for scientific research and engineering applications. This paper introduces a method using the Nonlinear Autoregressive with Exogenous Inputs (NARX) neural network to identify heat boundary conditions in nonlinear transient heat transfer processes in real time. This method has two notable advantages: (1) It relies solely on surface temperature time series to obtain inversion results; (2) Even in the absence of knowledge regarding the system’s state equations, it can estimate heat flux density. The NARX neural network is trained by using Bayesian regularization with surface temperature and heat flux data. (3) As per the inversion results, the NARX neural network’s accuracy in predicting the boundary heat flux density (BHFD) increases as the temperature measurement points approach the heat flux boundary. This neural network calculates the current heat flux density by incorporating both present and past surface temperature measurements as inputs. Through numerical simulation experiments, the efficacy of the NARX method is confirmed, showcasing its exceptional accuracy, robustness against noise, and broad suitability.

In this study, the evaporation of ethanol-water binary mixture within heated capillary is experimentally and numerically investigated. The ratio of ethanol and water evaporation rates was found to be equal to that of their initial concentration in the mixture. This observation contradicts the widely accepted belief of selective evaporation, where the ratio of ethanol-to-water evaporation rates is expected to be considerably higher than the concentration ratio owing to the higher volatility of ethanol. We refer to this novel phenomenon as non-selective evaporation. Subsequently, numerical analysis revealed that, within the heated capillary, changes in component concentration caused by ethanol preferential evaporation appear solely in a small area known as the diffusion layer near the meniscus. When the diffusion layer was fully developed, the evaporation process shifted from selective to non-selective. Because the duration of the selective evaporation stage was short, the evaporation process exhibited distinct non-selective characteristics. Furthermore, by considering the coupled effects of convection and diffusion, an analytical model was proposed, and the Peclet number criteria were established to determine whether the evaporation process of binary mixture exhibits selective or non-selective characteristics.

The high surface area of porous media enhances its efficacy for evaporative cooling, however, the evaporation of pure substances often encounters issues including local overheating and unstable heat transfer. To address these challenges, a volume of fluid (VOF) model integrated with a species transport model was developed to predict the evaporation processes of ternary mixtures (water, glycerol, and 1,2-propylene glycol) in porous ceramics in this study. It reveals that the synergistic effects of thermal conduction and convective heat transfer significantly influence the mixtures evaporation, causing the fluctuations in evaporation rates. The obtained result shows a significant increase in water evaporation rates with decreasing the microcolumn size. At a pore size of 30 μm and a porosity of 30%, an optimal balance between capillary forces and flow resistance yields a peak water release rate of 96.0%. Furthermore, decreasing the glycerol content from 70% to 60% enhances water release by 10.6%. The findings in this work propose the approaches to optimize evaporative cooling technologies by controlling the evaporation of mixtures in porous media.

Abstract As the need for effective heat dissipation in specialized systems intensifies, the study of flow boiling in microchannels has gained prominence due to its high efficiency and cost benefits. Utilizing a three-dimensional computational model incorporating the Volume of Fluid (VOF) and Saturated Interfacial Volume (SIV) methodologies, this research scrutinizes the behavior of saturated flow boiling in a microchannel featuring a centrally positioned, heated square cylinder. The analysis focuses on variations in bubble morphology, fluidic behavior, and thermal profiles, revealing that the evaporation of a thin liquid film contributes significantly to heat transfer efficacy. A systematic assessment of temperature gradients, heat transfer coefficients, and liquid film metrics uncovers the critical drivers behind effective heat transfer across different surfaces. Additionally, the study elucidates the role of Reynolds number and initial bubble size, indicating that higher Reynolds numbers thicken the liquid film, reducing heat transfer efficiency, while larger initial bubble diameters thin the liquid film, thereby boosting efficiency. Factors such as thermal boundary layer disruption and superheated fluid absorption further contribute to heat transfer optimization. This investigation enriches the current understanding of phase-change heat transfer dynamics and bubble interactions in microchannel environments equipped with a heated square cylinder.

Understanding the bubble dynamics is essential to various two-phase applications. The three-dimensional numerical study on bubble coalescence with phase change in different heated branching microchannels (15°, 90°, and 180°) is performed. The detailed bubble dynamics, flow structures, and phase change heat transfer characteristics are respectively presented. Results show that during coalescence process, the bubble successively experiences three stages including liquid film formation, neck growth, and oscillation. It is interesting to point out that the sandwich structure (droplet exists within bubble) may occur in large-angle branching microchannels due to the classical Rayleigh-Plateau instability. Besides, the unique encircling coalescence phenomenon is found to occur in 90° branching microchannel, leading to the sudden increase in neck growth rate. Two distinct flow structures can be recognized, namely the smooth flow occurring in small-angle branching microchannels and the colliding flow occurring in large-angle branching microchannels. The fluid flow becomes quite chaotic under colliding flow structure, which significantly facilitates the convective heat transfer. The heat transfer enhancement for main channel is found to be limited because of the counter-balance mechanism of phase change heat transfer involved. The main channel produces the better heat transfer enhancement than the branching channel, and the best heat transfer enhancement occurs on the outer wall of main channel. This fundamental research sheds new light on the physical mechanisms of bubble dynamics in confined branching structures.

The process of a sphere entering water at various temperatures and impact velocities was studied experimentally and numerically. The interaction states between the sphere and the fluid could be categorized into four distinct regimes: no-phase-change, nucleate boiling, transition boiling, and film boiling. This study reveals that pressure drag was dominant over friction drag in all regimes. Compared with spheres in the no-phase-change regime, those in the nucleate boiling regime experienced reduced pressure drag, while those in the transition and film boiling regimes experienced reductions in both pressure and friction drag. Further mechanical analysis showed that the pressure drag reduction resulted from the increase in static pressure on the leeward side of the sphere, while the friction drag reduction was due to the low viscosity of the vapor surrounding the sphere. In the nucleate boiling regime, the heat flux on the sphere surface could reach up 741 kW/m2, and then continuously decreased over time along with the sphere temperature. In contrast, owing to the insulating effect of the vapor film between the sphere and water in the transition and film boiling regimes, the average heat fluxes were 306 kW/m2 and 248 kW/m2, respectively, despite the higher temperature of the sphere.

In the pursuit of enhancing thermal management for miniaturized electronic devices, our study delves into the impact of entry effects on Ledinegg instability and flow maldistribution within parallel microchannels. Utilizing a coupled model that incorporates phase change and pressure drop dynamics in boiling flow, we examine microchannels characterized by a 50 length-to-diameter ratio and a 200 μm hydraulic diameter. Our findings unveil a significant influence of entry effects, which narrow the total flow excursion interval, thereby bolstering system stability. Specifically, as the heat flux escalates from 5 W/cm2 to 120 W/cm2, the entry effects increasingly mitigate flow instability and maldistribution in parallel channels, diminishing the total flow rate range susceptible to flow instability by 4.73% and 47.52%, while narrowing the total flow rate range corresponding to uneven flow distribution by 4.70% and 46.75%, respectively. Furthermore, entry effects expand the inlet subcooling range necessary for stabilizing the parallel channel system by 38.89% and 1000%. This research not only underscores the importance of considering entry effects in microchannel design but also opens avenues for further exploration into enhancing thermal management solutions.

The internal flow within an evaporating sessile droplet has intriguing fluid mechanics important to various microfluidics applications. In the present study, a phenomenon is observed through numerical methods wherein the buoyancy-driven flow structure inside a droplet on a non-wetting substrate transitions from an axisymmetric toroidal vortex flow to a non-axisymmetric single vortex flow with increase in the substrate temperature. As the axisymmetric nature of the droplet flow field and evaporation characteristics are broken, the internal velocity accelerates significantly. The transition, which is attributed to a flow instability inside the droplet, is more prone to occur as the droplet volume or the contact angle increases. The onset of the flow transition is analysed as the amplification of a small perturbation, thereby establishing a correlation between the flow instability and the Rayleigh number (Ra). Specifically, when Ra exceeds some critical value, the onset of the flow transition is observed, which explains the effects of substrate temperature and droplet volume on the internal flow. Next, the influence of the droplet contact angle on the critical Ra was investigated, and the underlying reasons were analysed. Finally, we discuss the heat transfer efficiency within the droplet and analyse why the internal flow tends to transition to a non-axisymmetric flow pattern from an energy minimization perspective.

Bubble flow in confined geometries is a problem of fundamental and technological significance. Among all the forms, bubble breakup in bifurcated microchannels is one of the most commonly encountered scenarios, where an in-depth understanding is necessary for better leveraging the process. This study numerically investigates the non-uniform breakup of a bubble slug in Y-shaped microchannels under different flow ratios, Reynolds numbers, and initial bubble volumes. Overall, the bubble can either breakup or non-breakup when passing through the bifurcation and shows different forms depending on flow regimes. The flow ratio-Reynolds number phase diagrams indicate a power–law transition line of breakup and non-breakup. The bubble takes longer to break up with rising flow ratios yet breaks earlier with higher Reynolds numbers and volumes. Non-breakup takes less time than the breakup patterns. Flow ratio is the origin of non-uniform breakup. Both the Reynolds number and initial volume influence the bubble states when reaching the bifurcation and thus affect subsequent processes. Bubble neck dynamics are analyzed to describe the breakup further. The volume distribution after breaking up is found to have a quadratic relation with the flow ratio. Our study is hoped to provide insights for practical applications related to non-uniform bubble breakups.

As demand for managing high heat flux in specialized applications grows, flow boiling in microchannels has received escalating attention for its high efficiency and cost-effectiveness. The complex interaction between an evaporating bubble and a heated pillar in a microchannel is governed by a confluence of transport mechanisms, including bubble morphology, fluid convection, heat transfer, and phase change phenomena. This study develops a three-dimensional mathematical model, employing the saturated-interface-volume approach to simulate the complex interaction process effectively. The results indicate that the liquid film thickness between the bubble and the heated surface is the primary factor affecting heat transfer. A reduction in the Reynolds number as well as an increase in the initial bubble diameter lead to a decrease in the liquid film thickness and an increase in the temperature gradient within the thin liquid film, which enhance both the evaporation rate and heat transfer efficiency. The temperature of the surrounding fluid is also decreased. The bubble passage disrupts the flow structure, particularly impacting the boundary layer and vortex structure. These perturbations in temperature and flow structure constitute a secondary factor influencing heat transfer. The efficiency of heat transfer varies significantly across different surfaces; surfaces with a larger thin liquid film region exhibit the most significant improvement, followed by the downstream surface where the flow and temperature fields are most affected. This study advances the fundamental comprehension of the complex interaction between an evaporating bubble and a heated pillar in a microchannel, integrating a detailed analysis of the relevant transport mechanisms.

This work proposes a novel two-dimensional-structured radial microchannel heat sink (RMHS) to effectively dissipate high heat flux, which owns easily manufactured characteristics. The RMHS is formed by orderly arranged pin fins involving two branching microchannel structures. The fluid flows into the RMHS through the central inlet and then experiences the continuous split and mixing imposed by the branching structures. The generated vapor bubble experiences a frequent breakup and coalescence process with the boiling phenomenon, among which the thin liquid film thickness can be triggered. The RMHS is fabricated using the silicon etching technique. A series of visualization experiments are conducted to test the performance of RMHS and reveal the related physical mechanisms. The evolution of bubble dynamics within the RMHS is recorded with a high-speed photographic system. The features mentioned above of RMHS confer excellent boiling heat transfer performance. At 600 ml min-1 volumetric flow rate, the maximum dissipated heat flux reaches 1035 W cm-2 with an area of 1 cm2. The present study is hoped to provide valuable insights into designing a two-phase microchannel heat sink for the thermal management of electronic devices.