Flow boiling in a microchannel is numerically investigated by using an incompressible multiphase flow solver modified from the Open-source CFD solver OpenFOAM. The effects of inlet flow rate and applied heat flux on the bubble dynamics and heat transfer characteristics of flow boiling in microchannels are systematically analyzed. The simulations capture key features of microchannel boiling, including bubble nucleation, growth, coalescence, and the formation of local dry-out regions. The results indicate that heat transfer enhancement is primarily governed by the boiling phase change, with significant contributions from the three-phase contact line and microlayer regions. The interaction between bubble dynamics and liquid flow is found to strongly influence the spatial distribution of heat transfer. At low flow rates, bubble growth is promoted and latent heat transfer plays a dominant role, whereas at higher flow rates, bubble growth and its contribution to heat transfer are suppressed. Localized heat transfer enhancement is observed due to the interaction between liquid flow and bubble interfaces. Under high heat flux conditions, very rapid vapor generation leads to strong volumetric expansion and apparent flow acceleration, so the flow with a very high predicted velocity should be treated as a critical flow problem, and incorporating compressibility effects in the simulation is essential.

Over the past decade, two key technologies—high-speed imaging (including cameras and lighting) [...]

Gas–liquid two-phase flows are typical in engineering systems involving heat and mass transfers. The cross-sectional geometries and sizes of these flow channels vary, exerting a notable impact on the thermofluid behavior. To develop thermofluid models, it is imperative to obtain local measurements of two-phase flows and gain insights into their characteristics from these measurements. Numerous experimental studies have investigated the local flow characteristics in circular pipes and rectangular channels; however, few studies on square channels have been conducted, particularly for flow regimes beyond bubbly flows, such as cap-bubbly flows. Given the importance of two-phase flows in large square channels for advanced nuclear reactors, such as the economic simplified boiling water reactor (ESBWR), we experimented with upward cap-bubbly flows in a large square channel. Detailed cross-sectional distributions of the void fractions, axial gas velocities, and interfacial area concentrations for two bubble-size groups were obtained at three axial locations using local measurements of a four-sensor optical probe. Based on the database, the cap-bubbly flow characteristics were understood, including flow development in a large square channel. In addition, the existing drift-flux correlations for large circular pipes predicted the measured void fraction with an accuracy of approximately ±10%, whereas the existing interfacial area concentration correlations reasonably correlated with the measured interfacial area concentration with an accuracy of approximately ±30%. Furthermore, the database was used to calculate the covariances of the void fractions. The correlations for large circular pipes significantly underestimated the void fraction covariance of the larger bubble group, probably because of the peculiar void fraction distribution of the group in a large square channel. The other covariances were well predicted.

This study investigates the coalescence of two equal-sized bubbles with air injection in distilled water experimentally, numerically, and theoretically. The neck expansion is the most prominent characteristic within a few milliseconds after bubbles coalesce. The effects of needle configurations and air injection flow rates are mainly discussed. Air injection during bubble coalescence is identified as an additional driving force for neck expansion in addition to the capillary pressure. The neck expansion time increases linearly with the bubble size at contact while decreasing with the air injection flow rate. The dimensionless neck expansion time follows a consistent linear decrease with the air injection speed. A novel theoretical expression for the dimensionless neck radius is derived, by quantifying the air injection effects as a multiplier (alpha) of the Laplace pressure at the neck. alpha demonstrates a consistent linear increase in air injection speed at all the needle configurations and flow rates. Moreover, numerical results of velocity vectors reveal the mechanism of air injection promoting neck expansion.

The coalescence of two growing bubbles presents unique characteristics compared to static bubble coalescence. The gas injection flowrate significantly affects the different stages of bubble evolution, which is poorly understood. In this study, we investigate the flowrate effects on the lateral coalescence of two growing bubbles experimentally. The synchronous bubbling from adjacent needles is achieved using water to push air. During the bubble growth process, we find that the initial nonlinear evolution of bubble volume is because the bubble emerges as a small spherical cap with a large curvature radius and apparent contact angle. As the neck expands after bubble coalescence, the injection flowrate accelerates the neck evolution compared to the case without air injection. We find the neck expansion time decreases linearly with increasing flowrate, while the expansion speed increases with flowrate, but only in the early stage. Moreover, we propose a new theoretical expression that predicts the neck radius well at all the flowrates. At the post-coalescence oscillation stage, the average projection area of the coalesced bubble increases linearly with time, except for periodic oscillations. Besides, we find that the injected air primarily influences the coalesced bubble's height, which in turn affects the projection area.

Gas-liquid two-phase flows frequently occur in various flow channels in engineering systems with heat and mass transport processes. The channel geometry affects thermo-fluid behavior generally. Extensive research is done on gas-liquid two-phase flows in circular and annular channels with a wealth of local two-phase flow measurement data. Square channels are used in various engineering disciplines, but there has only been limited experimental research on two-phase flows in square channels with local flow parameter measurements. For two-phase flow analyses in square channels, knowledge of the flow characteristics and model validation against some key parameters, such as the drift-flux parameters and the interfacial area concentration, is crucial. An experiment for upward bubbly flows was conducted in a large square channel with an inside cross-section of 0.136 x 0.136 m. A four-sensor optical probe and a hotfilm anemometer were used to measure local two-phase flow parameters at 66 points in an octant symmetric triangular cross-section at three axial locations. The measured two-phase flow parameters were the void fraction, axial gas velocity, axial liquid velocity, interfacial area concentration, and bubble Sauter mean diameter. The flow characteristics through flow development were investigated based on the local measurement data. The local measurement data directly determined the drift-flux parameters through their definitions. The distribution parameters determined through experimentation could be reproduced by the existing correlation for large circular pipes indicating that the flow characteristics in large square channels may be similar to those in large circular pipes. Based on existing drift-flux correlations for large circular pipes, the void fractions in the current experiment could be fairly predicted. Furthermore, the interfacial area concentrations could be predicted by existing correlations with reasonable accuracy. (c) 2023 Elsevier Ltd. All rights reserved.

Divertor systems of fusion devices are exposed to intense heat loads from plasmas, which degrade solid plasma-facing components. Fast liquid metal (LM) flow divertors may be more advantageous for this purpose but have risk of piling due to intense magnetohydrodynamic (MHD) drag. However, severe deceleration of the flow could be countered with the injection of currents that are transverse to external magnetic fields, allowing to thrust the flow with j × B (Lorentz) forces. Given that the injection of currents as an approach to propel LM-divertor flows has remained experimentally understudied, this article focuses on the evaluation of j × B -thrust and finding its drawbacks. j × B -thrust was experimentally tested with free-surface-LM flows, a vertical magnetic field and an externally applied current. Experiments were reviewed with a theoretical model, showing agreement in the trends of theory and experiments. Full 3D-MHD-free-surface-flow simulations were also performed with FreeMHD and confirmed the sensitivity to unstable flow behavior in LM systems when applying external currents. Furthermore, excessive power requirements are expected for the implementation of j × B -thrust at the reactor scale, making these systems inefficient for commercial devices. This paper evidences that the simple operation of a LM-flow divertor with j × B -thrust, without any of the instabilities caused from reactor plasmas or parasitic currents, already presents intrinsic challenges.

Whether a droplet slides or not on inclined solid surface is mainly influenced by a balance between the adhesion force at contact area and the gravitational force exerted on the droplet. Especially as the adhesion force is a key parameter for the determination of the sliding behavior of droplets. The adhesion force is mainly estimated by experimental observation for the sliding motion of the droplet. However, at present it is unknown whether the adhesion force is a constant value regardless of the droplet size or not. In the present study, focused on the onset for sliding of water-ethanol binary mixture droplets on inclined solid surface, experimental investigation on the sliding droplets is performed by considering the droplet volumes ranging from 7 to 600 μL in order to understand the effect of the size of the droplet on the adhesive property. The results are discussed using the existing analytical models. From the results, it is found that the adhesion force increases in the case of large droplet volume, while the force reaches constant value in the case of small droplet volume. This difference is related to the degree of the droplet shape deformation, which leads to a change in the contact angle. Finally, a simple empirical model for the adhesion force including the size effect is proposed.

Droplet splashing behaviors of water-ethanol binary mixture liquids on roughened solid surfaces were experimentally observed in order to investigate the effects of surface tension, viscosity, and wettability/surface roughness on the splashing occurrence. The range of the droplet volumes was from 1.7 μL to 32.6 μL. The ranges of the surface tension and the viscosity were from 21.1 mN/m to 71.9 mN/m, and from 1 mPas to 2.91 mPas, respectively. The surface roughness range was from 0.03 μm to 1.25 μm for Ra. The present experimental data were evaluated on the basis of the existing models. Resulting from these experiments, a simple model using the Ohnesorge number evaluated by the capillary length was proposed and the accuracy of the predicted critical values such as the critical Weber and Reynolds numbers were discussed. The result indicated that the liquid properties and the quantification of the surface condition such as surface roughness are important factors for the prediction of the splashing behavior.

The impingement behaviours of droplets towards solid substrates depend on the liquid properties, impingement velocity and solid surface conditions, such as wettability and roughness. However, the prediction regarding whether the droplet splashes after the impingement, is still an open question. Here we show that the splashing can be predicted by the pressure balance of the liquid film appearing beneath the impingement droplet coupled with the modified energy balance equation. Hydrodynamic and hydrostatic pressures are the driving forces for the droplet's radial spreading, while the capillary pressure at the rim edge and viscous stress oppose the driving forces. Thus, splashing occurs when the driving forces overcome the opposing forces. Moreover, the splashing condition is affected by various surface factors, such as wettability and surface roughness. Our work would pave the way to understand the basic physics for rim or liquid film fragmentation and enabling advances in important for engineering field such as printing, sprays for cooling and pesticide.

The splashing of droplets impinging on solid substrates includes complex behavior that is dependent on numerous factors, including the liquid properties, impingement velocity, and solid surface properties, such as wettability and roughness. Quantitative understanding of the behavior of the unstable liquid film during the splashing process remains elusive. Here we propose new models in which the number of fingers and the size of the ejected secondary droplets can be characterized taking into account the liquid film thickness and the wettability and roughness of the surface. The experimental data regarding the size of the secondary droplet and the number of fingers is found to be in good agreement with the predictions of the proposed model. The results of the present study provide insight into the mechanism behind rim or liquid film fragmentation.

Wettability of a droplet on rough solid surface exhibits complex behavior compared with a droplet on smooth solid surface. On a hydrophilic / hydrophobic solid substrate, hydrophilicity / hydrophobicity of a droplet changes as the surface roughness of the solid substrate varies. This kind of wettability is mainly discussed on the basis of Wenzel and Cassi-Baxter models where the wettability of droplets on roughened surface is analyzed by the concept of roughness ratio and contact area fraction. However, in an actual situation, it is very difficult to quantify the structure of the roughened surface and the contact area between liquid and solid surface strictly. In the present study, wettability of water-ethanol binary mixture droplet on roughened low-surface-energy solid is experimentally investigated in order to understand the wetting behavior in wide range of surface tension of liquid. A simple morphological model to quantify the surface roughness and critical surface tension of roughened solid surface is proposed. In addition, analytical model to predict the wettability on roughened solid surface is developed on the basis of the concept of adsorption of liquid molecules at solid–liquid interface, which can be applied to wetting behavior on both hydrophilic and hydrophobic roughened solid substrates.

Distinct from solid particle, bubble is soft matter, which tends to adjust its shape according to motion, and according to the work of Tomiyama, for bubble with diameter larger than 1.3mm, the rising path would change from rectilinear to zig-zag or helical, which make the determination of the terminal velocity more difficult. Even for the bubble with diameter larger than 0.075 mm reported by Pawliszak, terminal velocity scatter started. The existence of surfactants or contaminants and different initial deformations of bubbles introduced by varying capillary or nozzle diameters were generally considered to be the two main reason account for the terminal velocity scatter. In this paper, the rising characteristics of small bubbles with 1<Re<100 were investigated. Especially the motion characteristic right after the departure. Comparisons were made between three bubbles with almost the same diameter (0.58 +/- 0.008mm), the same aspect ratio (0.98 +/- 0.01), it was found that the only differences lied on oscillation frequencies of aspect ratio. And the higher oscillation frequency of aspect ratio was, the higher steady-state velocity the bubble would achieve. Therefore, it was thought that the deformation is one of the ways for bubble to accelerate itself i.e., bubbles accelerated itself through the oscillations of aspect ratio. The oscillation of aspect ratio is of high possibility to be one reason for velocity scatter.

To clear the effects of the hydrodynamic interactions among neighboring vapor bubbles on the saturated pool boiling heat transfer, artificial nucleation sites with several cavity arrangements were manufactured on the substrate surface and measured liquid temperatures near heated substrate by a miniature thermocouple probe with 10 kHz. The surface temperature of heated substrate was measured by an infrared camera with 1 kHz and bubble dynamics were simultaneously visualized by a high-speed video camera with 5 kHz and 10 kHz, respectively. The liquid temperature fluctuations, measured with micro thermocouple 100 mu m and 10 mu m above the substrate surface, were compared among each other and with the transient temperature fields obtained through IR measurements. Results showed that the hydrodynamic interaction among neighboring bubbles causes the liquid convection, that was only detected 100 mu m above the boiling surface. Generally, the degree of liquid temperature fluctuations at 10 mu m height and different arrangements of nucleation sites was under 1 K. Here the superheated sublayer in the vicinity of the boiling surface was generally not affected by the convection. Only when the coalescence from two neighboring nucleation sites occurred and the liquid-vapor interface nearly reached the substrate, a steep temperature drop over 2 K was detected. It was concluded that the viscous sublayer stably exists during boiling of water in lower heat flux conditions. To enhance the convective heat transfer in nucleate boiling, coalesced bubble needs to peel off the viscous sublayer during its deformation. Based on the measurements in this study, this peel-off effect only happens when the coalescence point is less than 1 mm above the heated surface.

It is important to investigate the characteristics of thermofluids for cooling blankets and divertors used in the high-temperature plasma of fusion reactors. Various coolant materials on the cooling problem such as a fusion reactor were widely considered from FLiBe to lithium. In the present study, we proposed pressurized water as the working fluid for the blanket design. We performed direct numerical simulation of pipe flow with Reynolds number of 2100 based on the pipe radius and friction velocity. The number of mesh points used were 4096 x 1024 x 1532 in the z-, r-, and phi-directions, respectively. The Prandtl number of pressurized water was set as 0.87. The results show that compared with air as a general coolant material, pressurized water has higher turbulent heat transfer.

Experiments reported till today have shown that narrower channel exhibits stronger evaporation. To clarify how this behavior comes, we proposed a new model for evaporation of wetting liquid film in a narrow channel. With considering heat conduction in liquid film, kinetics and diffusion, we described a local heat and mass transfer in the microscopic liquid-film region which length scale is characterized by minimizing free energy. The innovative point of our model is that we did not assume thermodynamic equilibrium in micro and macroscopic meniscus regions, and introduced an excess pressure term balancing a conventional stress term at gas/liquid interface, which acts as a source term of evaporation. Although a free parameter must be included because local thermal equilibrium was not assumed, our phenomenological model makes clear that non-isothermal driving force causes stronger evaporation in narrower channel. (C) 2019 Elsevier Ltd. All rights reserved.

In this study, a single nucleation site was made on the upper surface of heated substrate, and small tracer particles suspended in water for flow visualization. Single vapor bubble dynamics and particle movements were recorded by high speed camera, then the flow field was quantitatively grasped by particle tracking visualization. Resulting of the visualization, in earlier stage of nucleate boiling, rapid growth motion of bubble pushed liquid surrounding bubble interface outwardly. There was no retracting flow into root of boiling bubble though it needed to feed water into root of boiling bubble due to evaporation in case of lower heat flux condition. Liquid flow pushed by rapid bubble growth was explained by combining thermal diffusion equation with mass conservation law, moreover the pressure increase caused this liquid flow was estimated. In bubble departure process, liquid near substrate was horizontally pulled to nucleation site, but the eddy to enhance a vertical mixing of superheated liquid near substrate was not observed. After bubble departure, it was found that a hot liquid plume under a rising bubble was not mainly caused by entrainment of superheated liquid layer formed on heated substrate, but by a relative descending flow around a rising bubble induced by mass conservation law. (C) 2020 Elsevier Ltd. All rights reserved.

Critical surface tension (CST) is a measure of solid surface tension and is mainly determined by measuring the contact angle of a droplet on a target solid surface. The concept of CST makes it possible to determine solid surface tension without any unprovable assumptions such as the Fowkes hypothesis. However, it requires somewhat special devices and skills for measuring the contact angle. In this work, we propose a simple method to determine the CST of a solid by measuring the droplet spreading area. This method is developed by combining the conventional CST with a simple analytical droplet model. The difference in estimated CSTs between our method and the conventional one is within 3.0%. Our method enables a quick and simple evaluation of the solid surface tension without special devices for measuring the contact angle.