HYPOTHESIS:Induced shape oscillations during bubble bouncing on substrates have been predicted to enhance mass transfer. However, the underlying physical mechanisms, including how bubble shape oscillations enhance mass transfer and whether this is governed by amplitude (low-order modes) or frequency (high-order modes), remain insufficiently understood. In this work, we hypothesize that bubble bouncing with shape oscillations enhances mass transfer by promoting interface renewal induced by circulation and concentration boundary layer separation. Within this framework, we further propose that the enhancement is primarily governed by low-order oscillation modes. EXPERIMENTS:A combination of optical methods with high spatiotemporal resolution - planar laser-induced fluorescence, particle image velocimetry and shadowgraphy - is employed to quantify the dissolved oxygen concentration field, the surrounding flow field and the bubble morphology. Additionally, a shape decomposition method is developed to analyze the oscillation modes of the bubble. FINDINGS:Bubble bouncing accompanied by shape oscillations enhances mass transfer by approximately 20%, compared with the predictions of the classical model dominated by convection for moving bubbles. This enhancement results from the circulation and the concentration boundary layer separation, both driven by relatively large-amplitude oscillations of low-order modes during bubble bouncing, which promote interface renewal, as revealed by the spatiotemporal evolution of the flow and concentration fields. Building on these findings, an extended Sherwood number formulation is proposed, which takes bubble bouncing with shape oscillations in the low-order mode into account.

The bubble departure process is of great importance in boiling heat transfer as it directly influences the surface rewetting of a heated surface. Prior experiments have shown that surface modifications can enhance boiling performance, yet the detailed effects on bubble departure dynamics remain less understood. In this work, pool boiling experiments were performed on smooth and micro-pillar arrayed surfaces with varying pillar heights (0, 3, 6, 9 & micro;m) to investigate bubble departure dynamics. Two distinct bubble departure mechanisms were identified. On the smooth surface and on the micro-pillar arrayed surface with a low pillar height (3 & micro;m), bubbles undergo an oblate-to-prolate shape transition during departure. This transition is accompanied by a significant increase in the apparent bubble contact angle, indicative of substantial microlayer depletion and the associated contact line dissipation. In contrast, bubbles on micro-pillar arrayed surfaces with higher pillar heights (6 and 9 & micro;m) remain oblate and exhibit a nearly constant apparent contact angle during departure, suggesting a more persistent microlayer beneath the bubble that mitigates contact line dissipation. These surface-dependent bubble departure behaviors can be interpreted as a competition between interfacial relaxation and contact line dissipation. To characterize the complex bubble departure dynamics, a capillary relaxation timescale based on bubble shape deformation is introduced. The closer match between relaxation and departure durations on surfaces with higher pillars indicates a capillary-relaxation-controlled rapid bubble departure. These findings provide new insights into bubble departure dynamics and can be leveraged to improve heat transfer performance in nucleate boiling.

Droplet impact on solid surfaces plays a critical role in a wide range of applications, including inkjet printing, spray cooling, surface coatings, and microdroplet chemistry. Precise control of droplet-surface interactions is essential, but the fundamental mechanisms governing this process are still not fully understood. In this study, we demonstrate that large contact angle hysteresis (CAH) on hydrophobic nanoporous surfaces significantly amplifies post-impact droplet oscillations. This reveals the critical influence of CAH on the redistribution of impact energy and the modulation of droplet-surface interactions. Using shape mode decomposition via Legendre polynomials and fast Fourier transform spectral analysis, we show that surfaces with larger CAH excite and sustain higher-order droplet shape mode oscillations, leading to persistent capillary waves even after contact line pinning. The observed amplitude modulation and multiple frequency components within individual shape modes reveal nonlinear energy transfer between different modes. These amplified and coupled oscillations are shown to promote daughter droplet coalescence. This study presents a framework for understanding the role of CAH in storing and redistributing impact energy through nonlinear mode excitation and establishes CAH as a critical design parameter for controlling fluid dynamics on solid surfaces.

HYPOTHESIS:Recent experiments have shown that tuning the initial wetting state of a hydrophobic surface before boiling gives rise to bubble behavior resembling that on hydrophilic surfaces. This contrasts with typical bubble behavior on hydrophobic surfaces, where bubble growth initiates from a residual vapor seed and results in slow growth and delayed departure. Here, we hypothesize that such wetting-state tuning modifies the near-surface liquid-vapor interfacial structure beneath the bubble, possibly through the formation of a near-surface thin liquid film, thereby altering bubble dynamics on hydrophobic surfaces. METHODS:Synchrotron X-ray imaging with high spatial resolution (2.44 μm) and a large field of view (∼5 × 5 mm) was employed in pool boiling experiments to visualize the near-surface liquid-vapor interface and bubble evolution under different surface initial wetting states. Non-dimensional analysis of bubble growth and departure was performed to characterize bubble dynamics. FINDINGS:We identify a unique bubble mode on the PDMS-coated hydrophobic surface in which the near-surface liquid-vapor interfacial structure resembles that on hydrophilic surfaces, suggesting the possible presence of a near-surface thin liquid film. In addition, the associated non-dimensional parameters indicate rapid bubble growth similar to that observed during microlayer evaporation-assisted growth on hydrophilic surfaces. Notably, the bubble departs rapidly and smoothly, maintaining an oblate shape and a nearly constant apparent contact angle during departure without obvious hysteresis. A capillary relaxation timescale defined from bubble-shape deformation is comparable to the actual departure duration, suggesting that capillary forces contribute significantly to the rapid bubble departure.

Recent years have seen porous metal foams gaining research interest for boosting boiling heat transfer in sustainable energy systems because of their high surface area, nucleation sites, and thermal conductivity. To keep electronic devices compact, surface coatings must be thin to lower thermal resistance and optimize space, with components often placed vertically. Despite extensive research, most studies have focused on millimeter foams in horizontal orientations, leaving the pool boiling behavior of sub-millimeter foams in vertical configurations largely unexplored. In this work, the pool boiling performance of vertically oriented copper foams with thicknesses of 50, 100, and 200 mu m and pore densities ranging from 90 to 130 PPI is investigated. The influence of foam thickness and pore size on boiling characteristics such as onset of nucleate boiling (ONB), heat transfer coefficient (HTC), and critical heat flux (CHF) is quantitatively analyzed. All copper foam surfaces exhibit enhanced CHF compared to the bare silicon surface, while HTC trends vary depending on foam thickness and pore density. Two distinct CHF and HTC trends are observed based on foam thickness, indicating two different mechanisms at work. In one case, at 200 mu m cooper foam thickness, higher wall superheats and poor HTCs are seen, suggesting vapor sublayer dryout caused by coalescence of vapor channels induced by hydrodynamic instabilities. In the other case, at 50 mu m, a non-monotonic CHF trend with lower superheats is observed, similar to micro-pillared surfaces, which results from dryout of the liquid sublayer beneath vapor bubbles. These findings on CHF trends in porous metal foams highlight the unique characteristics of microporous foam structures in a vertical orientation and guide future research aimed at optimizing foam designs for improved boiling heat transfer.

In the evolving energy landscape, there is an increasing demand for efficient and reliable heat transfer methods to prevent overheating in renewable energy systems. Pool boiling presents viable solutions, and the surface orientation of the heated surface is a key parameter which affects its performance. This research investigates the effect of surface orientation on critical heat flux (CHF) in pool boiling using silicon (Si) and silicon dioxide (SiO 2 ) surfaces. Experiments were conducted across seven preset orientation angles ranging from 0° to 180°. The experimental results indicated that these conditions had a notable effect on heat transfer performance, with the highest CHF observed at a 60° orientation for both types of surfaces. At 180°, a significant reduction in CHF was exhibited at the SiO 2 surface, with CHF values less than 5% of those at 0°. Si surfaces exhibited larger bubble departure angles and smaller bubble sizes at higher orientation angles compared to SiO 2 surfaces. These findings, in which CHF peaks at 60°, challenge the predictions of many existing models that predict a steady decrease of CHF as the surface orientation increases. This research involves a detailed analysis of vapor bubble dynamics, and the interactions between bubbles and the heating surface across different surface orientations. Through the examination of bubble detachment, coalescence, and liquid‐vapor interactions, this study aims to provide a clearer understanding of the mechanisms driving CHF variations.

Surface modifications have demonstrated significant potential in enhancing heat transfer in nucleate boiling, yet their impact on microlayer evaporation-a key heat transfer mechanism-remains less understood. In this work, we performed isolated bubble nucleate boiling experiments with micro-pillar arrayed surfaces to study the microlayer heat transfer. We initially analyzed the bubble dynamics, including growth dynamics and shape evolution throughout the entire bubble life cycle on these surfaces in detail. The results show that bubble dynamics differ considerably across different surfaces under the same surface superheat, primarily due to differences in microlayer evaporation. We then statistically quantify the bubble growth dynamics to evaluate the microlayer heat transfer performance. Importantly, we found that the experimental results align closely with the inference on the microlayer heat transfer derived from our previous simulation results of initial microlayer morphology on similar surfaces. This alignment allowed us to experimentally confirm the existence of two distinctive microlayer morphologies on micro-pillar arrayed surfaces, as observed in our previous simulations: the disturbed and the disrupted microlayer. We demonstrated that the microlayer morphology governs its heat transfer performance and, consequently, bubble dynamics during the entire bubble life cycle. Notably, our findings suggest the existence of a critical microlayer thickness, which can be achieved through surface modifications, to sustain a high evaporation rate throughout the bubble life cycle. To optimize surface design, we proposed a microlayer morphology concept that links the microlayer morphology with the corresponding heat transfer performance on micro-pillar arrayed surfaces.

Subcooled nucleate flow boiling encompasses intricate simultaneous condensation and evaporation processes. It involves thin liquid microlayers trapped beneath growing bubbles, enabling high heat and mass transfer with fluxes exceeding 1 MW/m2. Understanding microlayer contribution to bubble growth is pivotal for developing reliable boiling models. Unlike previous studies, we account for condensation effects, important in the context of subcooled boiling regime, in estimating microlayer contribution by simultaneously obtaining microlayer dynamics from thin-film interferometry and whole-field temperature from rainbow schlieren deflectometry. We establish that the microlayer evaporation significantly influences bubble growth in flow boiling, contributing up to 60% (in growth phase) in the present study.

Understanding two-phase flow patterns in porous media is essential for estimating interfacial friction, and consequently, the coolability limit of debris bed - dry-out heat flux - in the event of a severe accident in nuclear power plants. The lack of clear visual data on two-phase flow in porous media led to the development of multiple maps to identify flow patterns based on void fraction and particle size. Hence, in this paper, to visualize two- phase flow patterns, adiabatic air/water experiments were conducted with a refractive-index matching tech-nique and high-speed photography. With single-point injection experiments, multiple distinguishable flow pat-terns, like, small distorted bubbles, Y-shaped bubbles, multi-dimensional slugs, and laterally interacting channels, were observed with an increase in air flow rate. Subsequently, the top flooding approach has been adopted for simultaneous estimation of void fraction and visualization of flow patterns. At a high void fraction (>0.75), laterally interacting multidimensional slugs were observed throughout the bed, while at lower void fraction (0.36-0.75), various flow patterns co-existed in different channels of the porous bed with 10 mm hydrogel particles. This investigation highlights the presence of complicated flow patterns at different void fractions, underlining the necessity of considering these identified flow patterns to reduce uncertainty in models estimating coolability

Nucleate boiling, an important heat transfer phenomenon, holds significance in the thermal management of numerous engineering applications. Understanding and predicting its heat transfer characteristics are vital for system optimization. The formation of a microlayer, resulting from rapid bubble growth on a heater substrate, subsequently facilitates further bubble growth through evaporation playing a critical role in overall bubble dynamics. This study examines the intricate relationship between bubble and microlayer dynamics and their impact on heat transfer rates. Experimental work is conducted for varying heat fluxes using rainbow schlieren deflectometry (RSD) and thin-film interferometry (TFI) in tandem for simultaneous mapping of key parameters: bubble dynamics, microlayer dynamics, and two-dimensional temperature field in vertical flow boiling. The analysis of these parameters has provided significant insights into some fundamental aspects of boiling. Firstly, this study emphasizes that the bubble growth rate models need to take into initial microlayer thickness in accurately predicting bubble growth dynamics. The iteratively obtained constant C-eff (describing the initial microlayer thickness) for bubble growth models matches well the value of C-eff obtained experimentally from thin film interferograms. Additionally, this study also validates the applicability of the kinetic theory-based model in predicting the evaporative heat transfer coefficient during the initial stage of flow boiling, while demonstrating that the accommodation coefficient is similar to 0.04 for the investigated range of heat fluxes. Subsequently, using the experimentally obtained evaporative heat transfer coefficient, further insights are provided into the dry patch dynamics. Experimental dry patch dynamics were in good agreement with theoretical predictions up to the rewetting phase. The results also indicate a substantial contribution of microlayer evaporation, constituting approximately 34 % of the total heat flux.

Nucleate flow boiling offers high heat transfer rates and is considered an effective mode of heat transfer in many systems involving high heat loads. The phenomenon is characterized by the inception of vapor bubble(s) and its growth, followed by its departure in a periodic manner. The evolution of the nucleating bubble's footprint-microlayer and dry patch dynamics-is important in understanding the heat transfer capability and limiting heat flux values. However, efforts toward developing a fundamental understanding of this phenomenon during the nucleate flow boiling regime under subcooled bulk conditions are scarce in the open literature. Toward bridging this gap, we report flow boiling experiments on a hydrophilic surface for investigating the plausible influence of subcooling and minimize the influence of the hydrodynamic movement of contact lines on the dry patch dynamics. Experiments have been conducted in a vertically oriented rectangular channel with water as the working fluid for a Reynolds number of Re = 2400. Real-time microlayer dynamics have been mapped using thin-film interferometry, while the bubble evolution has been captured using one of the gradients-based imaging approaches employed from the side view. Experiments revealed a noticeable influence of subcooling on dry patch and microlayer dynamics. The size of the dry patch and the radial spread of the microlayer showed a decreasing trend with increasing subcooling level. Experimental conclusions are also supported with theoretical considerations.

Critical heat flux (CHF) is one of the major safety limits in the nuclear power plant operation. Determining CHF for rod bundles at all operating conditions is essential; however, relying on experiments is expensive and challenging. Applicability of the empirical correlations available in the open literature to a new rod bundle is limited. In recent years, the two-fluid Eulerian approach, coupled with the heat flux partitioning model, is widely used to predict boiling flows. Especially at high-pressure conditions, computational fluid dynamics (CFD) models performance in predicting DNB in tubes are satisfactory. This work provides an overview of DNB modeling, and a state-of-the-art CFD approach for predicting the DNB is presented in detail. The complexity and limitation of the model are described while highlighting the areas requiring further efforts for understanding the physics of the same.

Wetting characteristics of substrate surface is one of the parameters that strongly influence the phenomena of boiling heat transfer. In this direction, importance of hydrophobic surfaces has been emphasized in the literature due to their potential to offer high boiling heat transfer rates at low heat fluxes. For such surfaces, it has been shown that microlayer ceases to exist with contact line evaporation being the dominant bubble base growth mechanism during nucleate pool boiling regime. The present work investigates the plausible mechanism(s) of bubble growth on hydrophobic surfaces under nucleate flow boiling conditions. Possibility of formation of microlayer even on low wettability surfaces under the influence of bulk flow inertia has been examined. Thin-film interferometry, in conjunction with high-speed videography, has been configured to record the microlayer dynamics and the bubble growth process in tandem. For a direct comparison, experiments under identical flow and bulk subcooled conditions have also been performed on a hydrophilic substrate, which is known to promote microlayer formation during the bubble growth process. Observations made through thin-film interferograms confirmed the formation of microlayer on hydrophobic surfaces. Experiments further revealed that the rate of growth of dry patch is higher on hydrophobic surfaces than that in the case of hydrophilic substrates. Vapor bubbles have been observed to be pinned on the hydrophobic surface whereas the phenomenon of bubble lift-off has been distinctly observed on hydrophilic substrate surface.

Nucleate boiling, a ubiquitous heat transfer mode, involves multiple vapour bubble nucleations on the heater surface and offers high heat transfer coefficients. The bubble growth process on a heating substrate involves the formation of microlayer, a thin liquid film trapped between the growing bubble and the heating substrate, and contributes to the bubble growth phenomenon through evaporation. Microlayer dynamics for a single bubble have been widely investigated in the pool and flow boiling conditions. However, the literature on multiple bubbles interactions and their associated microlayer information is scarce. Notably, in the case of flow boiling, where the microlayer dynamics are not symmetric due to the bubble's movement, the bubbles' interaction and its influence on associated microlayer dynamics have never been reported. Therefore, microlayer and bubble dynamics in multiple interactions have been investigated experimentally using simultaneous application of thin-film interferometry and high-speed videography techniques in flow boiling with water as the working fluid. Our experimental investigation revealed that the secondary nucleation could cause a reduction in lift-off time and may assist or hinder the movement of the first bubble. The experimental results also demonstrated that the secondary nucleation could deplete the microlayer of the first bubble hydrodynamically even when the bubbles are far apart. Furthermore, it has been found that the microlayer depletion rate depends on the growth rate and the location of the secondary nucleation. Hence, this experimental study emphasises the need to consider the interaction of bubbles while modelling boiling flows to avoid overestimating the contribution of microlayer evaporation.

In subcooled boiling flows beyond a certain heat flux, heat transfer is hampered due to a phenomenon known as Departure from Nucleate Boiling (DNB). Conducting DNB experiments at one-to-one nuclear reactor operating conditions is highly challenging and expensive. Another alternative approach is to use Look-up table data. However, its applicability is limited due to its dependence on rod bundle correction factors. In the present investigation, a state-of-the-art Eulerian-Eulerian two-fluid model coupled with an extended heat flux partitioning model is used to predict DNB in tubes and rod bundles with square and hexagonal lattices (relevant to Pressurized Water Reactors). In this approach, bubble departure characteristics are modeled using semi-mechanistic models based on force balance analysis. The predicted DNB values are compared with experimental and Look-up table data and found out to be within 1.8% to 20%.

The bubble growth and its corresponding microlayer dynamics are strongly coupled from the point of bubble inception to its eventual liftoff. This paper discusses the complex and interesting interaction between a bubble and a microlayer through high-speed photography and thin-film interferometry in vertical flow boiling conditions. We analyzed existing force balance models and bubble growth rate models using experimental data. Our analysis revealed that the existing force balance models show severe limitations in predicting bubble dynamics, and the success of models reported by researchers is mainly due to over-parametrization and over-fitting. We show through our experimental results that the movement of the bubble in the flow direction and depletion of the upstream microlayer are strongly correlated with bubble diameter and growth rate. We discuss a non-dimensional approach based on forces acting on the bubble to predict the bubble movement in the flow direction. Furthermore, we report an interesting stage of the bubble ebullition cycle, where the bubble does neither liftoff nor contact the heater surface.

Boiling—a process widely used for its good heat transferability—is limited by a phenomenon known as critical heat flux (CHF). Our experiments revealed a new CHF mechanism that is different from previously believed theories; we refer to it as “sonneting CHF.” At CHF, the flow pattern changes from bubbly flow to slug/churn flow and then to an unusual reverse annular flow, leading to a significant rise in the heater surface temperature. The reverse annular flow, however, does not sustain but breaks down into a chaotic flow pattern, resulting in unprecedented quenching of the heater surface. The flow pattern shortly reverts back to bubbly flow; this entire process repeats for a few cycles, where the heater surface temperature rises and falls with amplitudes increasing in each cycle until the heater trips. The maximum removal-surface heat flux is significantly higher than the CHF. This new understanding will enable flexible and innovative boiling systems for several energy applications.

Subcooled flow boiling is widely used as a mode of heat transfer in many industries, especially in nuclear reactors. Despite its advantages, the heat transfer is hampered beyond a certain flux due to a phenomenon known as departure from nucleate boiling (DNB). It is important to determine the void fraction profiles, especially the near-wall void fractions, to evaluate the limiting heat flux conditions. The two-fluid Eulerian model, coupled with the heat flux partitioning model, is widely used to predict subcooled flow boiling characteristics. Over the years, many researchers have not considered lift and wall lubrication forces in their modeling of subcooled flow boiling. Few researchers have considered the Tomiyama model for lift force; however, their results were not encouraging. Moreover, there is no systematic study in evaluating the impact of lift and wall lubrication forces on subcooled flow boiling. In this paper, various lift and wall lubrication models are compared to understand the implications of these forces on void distribution. The advantages and limitations of the models are discussed in detail.

The two-fluid Eulerian model, coupled with the heat flux partitioning model, is being widely used to predict subcooled flow boiling characteristics and critical heat flux (CHF). The heat flux partitioning model relies on essential parameters like bubble departure diameter, departure frequency, and nucleation site density. Mainly, performing experiments to determine bubble departure diameters and frequencies is challenging. Another approach is to use semi-mechanistic models; however, even such models rely on some crucial parameters like advancing contact angle, receding contact angle, contact diameter, and many others. In this study, the influence of such parameters, and the role of different forces in determining bubble departure diameters and departure frequencies, at high-pressure conditions is studied. Moreover, a parametric analysis is carried to understand the influence of such parameters on CHF prediction.