Nucleation Site Density Research Articles

Dynamics of water condensation on a switchable surface originated from molecular orientations.

In heat transfer systems, how water condenses on the surface is critical to the energy efficiency of the system. With fixed surface wettability, hydrophilic surfaces enhance the nucleation rate but result in filmwise condensation due to pinning effect, which impedes the heat transfer between water vapor and surface during droplet growth. A hydrophilic surface with high drop mobility is realized with static tailored wettability surfaces, while tunable surfaces have potential in more comprehensive manipulation in condensation with different scale in time and scale. However, the mechanism has rarely been investigated and elucidated. In this paper, we investigate water condensation on a tunable surface originated from surface tension distribution control. The surface tension distribution under applied electric field is modeled and tested. We demonstrate that the surface tension manipulated by liquid crystal orientation alters the nucleation site density. Also, the periodic surface tension distribution aligns condensed water drops and decelerates the radius growth of droplets. The mechanism of active water condensation manipulation can be further applied to other tunable surfaces for various applications such as atmospheric water generator, heat transfer systems, and desalination systems.

Domain boundaries in incommensurate epitaxial layers on weakly interacting substrates

There has been increasing interest in the fabrication of thin film materials with mixed dimensions, in particular, 2D to 3D and 3D to 2D heterostructures. Often, if the interface interaction is weak, the lattice matching criterion between the substrate and overlayer can be lifted. If the overlayer lattice is completely relaxed, it can form an incommensurate film on the mismatched substrate. In this work, we show that domain boundaries are inherent in the incommensurate epitaxial films due to random nucleation sites of domains in an overlayer. The nature and origin of the incommensurate domain boundaries are different from the conventional dislocation boundaries that come from the relaxation of strain due to film–substrate lattice mismatch. We propose that the formation of such domain boundaries can be studied through Voronoi tessellation. Using a case study of monolayer WS2 on sapphire (2D on 3D), we show the formation of domain boundaries that compared well with a recent experiment reported in the literature. In the Voronoi tessellation, we also show quantitatively that the average domain size depends on the density of nucleation sites. The conclusion of this case study may be generalized to any incommensurate epitaxial films when the interface interaction is weak.

Numerical study of bubble behaviors and heat transfer in pool boiling of water/NaCl solutions using the lattice Boltzmann method

Through modification of the equation of state applicable to the non-volatile salt solution and characterization of the effect of solute concentration on the physical properties of the solution, a lattice Boltzmann pseudopotential model was developed and used to study the pool boiling processes of pure water and NaCl solutions. The model quantitatively verified the equilibrium vapor pressure reduction and boiling point rise of the NaCl solution with solute concentration. The complete pool boiling curves of pure water and the NaCl solution were simulated, and the differences in bubble behaviors and heat flux evolutions in pure water and the NaCl solution at different superheats were analyzed. The results revealed that the difference of heat flux between water and NaCl solution in the nucleate boiling regime is dominated by the nucleation site density and bubble departure frequency at lower superheats, while dominated by the bubble interactions at larger superheats. As a result, the NaCl solution presents a lower nucleate boiling heat flux than that of water when the imposed superheat is smaller than a critical superheat, while it exhibits a larger heat flux even a higher critical heat flux than water when the superheat is higher than the critical superheat. Furthermore, the lower bubble departure frequency in NaCl solution film boiling was captured by simulation. The simulated film boiling heat flux of the NaCl solution was smaller than that of water, and both the simulated results fitted the Berenson correlation well.

Impact of micro-fins on a heated cylinder submerged in a nanofluid saturated medium

Flow and thermal features of nanofluid pool boiling on a circular heat sink with mirco-fins attached were numerically investigated. The initial geometry consisted of a horizontal cylinder with an outside diameter of 20 mm submerged in a saturated water/nanofluid for validation cases. Physical micro-fins with various configurations were added to the outside surface of the cylinder for the purpose of this study. Due to the presence of nanoparticles, three phases were considered in modelling as liquid, vapour and particles. Unsteady Eulerian- Eulerian from multiphase two-phase flow approach was combined with wall boiling model to identify the heat and mass transfer at the hot surface, as well as interaction forces between the liquid and vapour phases. For the case of nanofluid, thermo-physical properties were modified based on particles concentration. To track the fate of nanoparticles, discrete phase modelling was employed from the Lagrangian frame. To solve the boiling heat transfer at the hot surface and fins, nucleate site density and bubbles diameter were modified and implemented to account for the presence of nanoparticles and corrected roughness. All the equations were discretised and solved by CFD commercial software, ANSYS-Fluent v19.5. The results showed that the impacts of fin aspect ratio on heat transfer are higher compared to the number of fins. However, the role of these geometrical parameters decreased as the nanofluid concentration increased. In addition, it was found that the best enhancement in heat transfer was obtained for nanofluid volume fraction around 1% vol.

Decrypting the boiling crisis through data-driven exploration of high-resolution infrared thermometry measurements

We develop a neural network model capable of predicting the margin to the boiling crisis (i.e., the departure from nucleate boiling ratio, DNBR) from high-resolution infrared measurements of the bubble dynamics on surfaces with different morphologies and wettability (or wickability). We use a feature ranking algorithm, i.e., minimum redundancy maximum relevance, to elucidate the importance of fundamental boiling parameters, i.e., nucleation site density, bubble departure frequency, growth time, and footprint radius, in predicting the boiling crisis. We conclude that these parameters are all necessary and equally important. This result has profound implications, as it undermines the general validity of many observations and mechanistic models that attempt to predict the critical heat flux (CHF) by describing how a single boiling parameter changes with the heat flux or from one surface to another. Notably, the neural network model can predict the DNBR on CHF-enhancing surfaces of different wickability without using any input information related to the surface properties. This result suggests that, at least on the considered surfaces, surface wickability enhances the CHF by modifying the bubble dynamics, i.e., the aforesaid boiling parameters, rather than acting as an additional heat removal mechanism.

Effect of Reduction Rate on the Recrystallization Behavior in AA3003 Aluminum Alloy Fabricated by DC Casting

The recrystallization behavior of the cold-rolled AA3003 aluminum alloy with the reduction rate of 20%, 50% and 90% during annealing at the temperature ranging from 300°C to 400°C was investigated. As increasing reduction rate, the cold rolled specimens exhibit deformation bands with elongated grain microstructure consisting of straight grain boundary parallel to rolling direction. Therefore, large density of nucleation sites for recrystallization would be expected with increase of strain energy. The grain size of the cold-rolled specimens decreased with increase of reduction rate, c.f., as the rolling reduction increased to 90%, grain size along the direction normal to the sheet decreased to about 8μm in thick. When the sample annealed at 350°C for 5s, the first recrystallized grains were observed in the vicinity of the grain boundary. The relaxation and recrystallization kinetics under different annealing conditions were characterized in terms of the Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. The apparent activation energies of recrystallization for the cold-rolled specimens with reduction rate of 20%, 50% and 90% were determined as 332 kJ/mol, 239 kJ/mol and 115 kJ/mol, respectively. XRD analysis by using modified Williamson–Hall plots revealed that the tendency of the change in dislocation density is varied depending on reduction rate. These results indicate that the apparent activation energy for recrystallization and the crystallites size decrease with increase of the reduction rate, which leads to a decrease in the size of the recrystallized grains.

An experimental investigation on bubble dynamics of alternating saturated flow boiling of R-407C with oscillating refrigerant flow rates

In this work, an experimental investigation is accomplished on the bubble dynamics of alternating saturated flow boiling of refrigerant R-407. The refrigerant flow rate is oscillated with time approximating the form of the inverted triangular wave to study bubble behaviors of saturated flow boiling. The effects of different parameters, including average mass flux G¯, period of oscillating mass flux tp, amplitude of oscillating mass flux ΔG, and mean saturation temperature T¯sat of refrigerant R-407, on bubble structures, mean bubble departing size and frequency, and mean density of active nucleation site are investigated. Measured results show that the times in which the bubble nucleation phenomenon is started or terminated are associated with the oscillatory flow conditions. The bubble departure size is raised considerably with the decreasing mass flux in the first half of the alternating cycle. However, an opposite trend is observed in the second half of the temporal alternating cycle where the mass flux is raised. The period of oscillation in the mass flux has negligible influence on the bubble dynamics. Finally, more changes in the bubble departing frequency and the densities of the active bubble nucleation sites are observed as the amplitude of oscillation in the mass flux increases.

Evaluation of various active nucleation site density models of subcooled flow boiling in a vertical tube using OpenFOAM

To evaluate the applicability of the active nucleation site density models in CFD simulations, the simulations of subcooled flow boiling process in a vertical tube are conducted with an open source CFD code, OpenFOAM. The effect of physical parameters on different active nucleation site density models is studied. The predictions of subcooled flow boiling under five active nucleation site density models have been compared with previous experimental data. Radial distributions of local void fraction, gas velocity and liquid velocity have been used for comparison of different models in different cases. It was found that the models show a significant influence on the prediction of void fraction, temperature distribution and the location of onset of nucleate boiling (ONB), while show little effect on the pressure distribution and bubble diameter. Comparing with other models, the simulation results of Lemmert and Chawla (LC) model are more suitable for predicting void fraction and liquid velocity for different conditions in simulations of subcooled flow boiling. The recommendations of different models for applicable scope are given by analyzing the simulation results. These works can provide reference for the selection of active nucleation site density models when analyzing subcooled flow boiling in vertical tube by CFD methodology.

Coupled two-fluid flow and wall heat conduction modeling of nucleate pool boiling

A new CFD modeling approach is developed and applied to a numerical study of pool boiling. It is based on a coupled simultaneous solving of the two-fluid model of two-phase flow in the boiling pool and the transient heat conduction within the heated wall. The applied two-fluid model consist in mass, momentum and energy balance equations for each phase and the closure laws for interface transport due to friction and phase transition. The boiling two-fluid mixture is thermally coupled with the heated wall by the heat transfer at the bubble footprint at randomly determined discrete locations of the nucleation sites and by the conjugate heat transfer from the heated wall to the wetting liquid on the surface outside the nucleation sites. The model is validated against detailed experimental data from the literature. Sensitivity analysis is performed to evaluate the influence of pool boiling parameters and conditions on the boiling curve, such as the nucleation site density, the wetting contact angle, the wall thermal conductivity, the wall thickness, the wall volumetric or bottom surface heating and the liquid volume in the pool. Temperature transients at the bubble nucleation sites are evaluated. The two-phase mixture pattern in pool boiling and the related swell level position can be well predicted with the nonuniform heat flux determined by the discrete nucleation sites of bubble growth and conjugate heat transfer from the heated wall to wetting liquid. The developed computational method is efficient and robust, while the reliability of its prediction is sensitive to the accuracy of nucleation site density and contact angle input data, as parameters that describe the coupled thermal and physical characteristics at the fluid-wall interface.

Laser texturing of silicon surface to enhance nucleate pool boiling heat transfer

• The laser texturing technique is used to modify the silicon-based heat exchange surface. • Modified surface is characterized by superhydrophilic properties and high stability. • The laser-textured surface demonstrates the heat transfer enhancement during water pool boiling. • The surface modification leads to the considerable change in the dynamics of vapor bubbles. 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.

Effective control of microstructure evolution in AZ91D magnesium alloy by SiC nanoparticles in laser powder-bed fusion

Laser Powder-Bed Fusion (LPBF) of Mg-Al alloys is a promising way to additively manufacture Mg-Al alloy components. However, the grains of the LPBF-processed AZ91D Mg-Al alloys could be coarse. The grains might undergo columnar growth, which is less resistant to hot tearing than equiaxed growth. Refining the microstructure by manipulating the processing parameters requires extensive experimental work. In this study, we used SiCnp-decorated AZ91D powder to additively manufacture AZ91D-SiCnp composites. The nanoparticles react with the melt, ensuring a high density of nucleation sites. The nanoparticles at the solid–liquid interface significantly blocked the diffusion of Al solute atoms, leading to solute enrichment at the interface. The remaining melt undercooled to a greater extent comparing with the nanoparticle-free melt, thus more potential nucleus were activated. As a result, columnar-to-equiaxed transition of the grains' growth has been achieved and the grains' size has reduced significantly by assembling SiCnp onto the feedstock AZ91D powder. In conclusion, AZ91D-SiCnp composites with desirable microstructure and mechanical properties have been successfully manufactured by LPBF.

Experimental investigation of the effect of cylindrical array structure on heat transfer performance during nucleate boiling

Using deionized water as the working fluid, a visualized pool boiling study was carried out on the surface of cylindrical pillar arrays of different sizes and the ordinary smooth surface under atmospheric pressure, and the boiling heat transfer curve and the change of heat transfer coefficient were studied. A high-speed camera is used to quantitatively measure the bubble dynamics, including bubble departure diameter and nucleation sites density. The results show that the onset of nucleate boiling temperature of all cylindrical pillar arrays has been significantly reduced, while the heat transfer coefficient is increased by 23.4%-163.4% compared to that of the smooth surface. The decrease of the pillar spacing helps to improve the performance of boiling heat transfer. The smaller gap optimizes the capillary flow of the liquid to a certain extent, thus facilitating the departure of bubbles, thereby inducing the working fluid to quickly replenish the dry area. The reduction of the diameter of the pillars is beneficial to improve the boiling heat transfer performance. More nucleation sites on the wall promote the bubbles to merge more quickly and reach the bubble departure diameter more easily, thus enhancing the evaporation of the micro-liquid layer and promoting the heat transfer. The correlation expressions as well as the upper and lower limits are obtained by fitting the change of heat transfer coefficient curve. Compared with the data in the literature, the S6 surface with the best performance shows obvious heat transfer advantages.

Experimental study on R-410A subcooled flow boiling heat transfer and bubble behavior inside horizontal annuli

The present research is concerned with the experimental work on R-410A subcooled flow boiling in horizontal annuli. Firstly, boiling curves and boiling heat transfer coefficients are used to explore effects of input heat flux (q), liquid mass flux (G), inlet refrigerant subcooling (ΔTsub), saturation temperature (Tsat) and annular gap (δ). The results indicate the obvious temperature drop occurs during onset of nucleate boiling (ONB) and increasing G and ΔTsub enlarges the temperature undershoot. With the increasing q, the heat transfer coefficient is raised apparently. Higher ΔTsub and larger δ both lead to lower boiling heat transfer coefficients, but varying G and Tsat has small influence on the boiling heat transfer. Secondly, the visualization of bubble behavior discloses increasing q improves bubble size, formation, and departure. With the increase in G, ΔTsub, Tsat and δ, the bubble growth is suppressed, leading to the reduction in bubble detachment diameter (dp). Moreover, the bubble detachment frequency (fb) and mean density of active nucleation site (Nac) are reduced with either the increase in ΔTsub and δ or the decrease in Tsat. Finally, empirical correlations are proposed for dp, fb, Nac, and total heat flux, which yields satisfactory agreement with our measured data.

Modeling and numerical investigation of hydrogen nucleate flow boiling heat transfer

Liquid hydrogen flow boiling heat transfer in tubes is of great importance in the hydrogen applications such as superconductor cooling, hydrogen fueling. In the present study, a numerical model for hydrogen nucleate flow boiling based on the wall partition heat flux model is established. The key parameters in the model such as active nucleation site density, bubble departure diameter and frequency are carefully discussed and determined to facilitate the modeling and simulation of hydrogen flow boiling. Simulation results of the numerical model show reasonably well agreement with experimental data from different research groups in a wide operation condition range with the means absolute error (MAE) of 10.6% for saturated and 5.3% for subcooled flow boiling. Based on the model, wall heat flux components and void fraction distribution of hydrogen flow boiling are studied. Effects of mass flow rate and wall heat flux on the flow boiling heat transfer performance are investigated. It is found that in the hydrogen nucleate flow boiling, the predominated factor is the Boiling number, rather than the vapor quality. A new simple correlation is proposed for predicting hydrogen saturated nucleate flow boiling Nusselt number. The MAE between the correlation predicted and experimentally measured Nusselt number is 13.6% for circular tubes and 12.5% for rectangular tubes. The new correlation is applicable in the range of channel diameter 4–6.35 mm, Reynolds number 64000–660,000, saturation temperature 22–29 K, Boiling number 8.37 × 10−5–2.33 × 10−3.

Effect of irradiation damage and indenter radius on pop-in and indentation stress-strain relations: Crystal plasticity finite element simulation

In this work, a crystal plasticity finite element model is developed to simulate the spherical nano-indentation of ion-irradiated metallic materials. Two critical features are addressed by the proposed theoretical framework, which include the pop-in event observed in the loading force-indentation depth (P−h) relations and irradiation hardening characterized by the transformed indentation stress-strain (ISS) curves. For the former, the pop-in event is dominated by the dislocation nucleation mechanisms, which are affected by both the density of irradiation-induced dislocation nucleation sites and indenter radii. For the latter, irradiation hardening is closely related to the heterogeneously distributed irradiation-induced defects within the irradiated layer with a limited depth. By applying the developed model to the spherical nano-indentation of helium-irradiated single crystal tungsten, it is informed that the simulated results can match well with corresponding experimental data, which include the unirradiated and irradiated P−h and ISS relations at different indenter radii. Moreover, the evolution of different hardening mechanisms during the indentation process is systematically analyzed, which can help comprehend the fundamental deformation mechanisms of ion-irradiated materials under spherical nano-indentation.

A unified statistical model for indentation pop-in: Effects of indenter radius and microstructure density on the transition from homogeneous nucleation to heterogeneous nucleation to bulk plasticity

Indentation pop-in, i.e. a sudden displacement burst on the measured load-penetration depth curves, has been known as the onset of elastic-plastic deformation for crystalline materials. Depending on the indenter radius or density of pre-existing dislocation nucleation sources, the evolution of pop-in shear stress can generally be categorized into three deformation stages, i.e. the homogeneous dislocation nucleation stage, heterogeneous dislocation nucleation stage and bulk plasticity stage. For the former, the fluctuation of pop-in stress is dominated by the thermally activated nucleation process. For the middle, a size-dependent pop-in behavior with stress fluctuation is informed over a wide range of indenter radius and microstructure density. For the later, the materials strength is determined by the critical resolved shear stress for the motion of existing dislocations. Here, we find two additional transition stages between the adjacent deformation stages that can be effectively addressed by a novel competition mechanism, and the transition points are affected by the density of dislocation nucleation sites. All these critical features are well characterized by a unified statistical model proposed in this work. Moreover, a mechanistic model with closed-form formulas is developed for the size-dependent pop-in behavior during the heterogeneous dislocation nucleation stage. By comparing both the calibrated and predicted theoretical results with different sets of experimental data, good agreements are achieved that can rationally verify the proposed model. This work presents a theoretical framework to characterize the fundamental pop-in mechanisms, and provides an avenue towards the prediction of transition points distinguishing different plasticity deformation regimes.

Thermodynamic analysis of entropy generation due to energy transfer through circular surfaces under pool boiling condition

Pool boiling is a well-known heat transfer mechanism which is accomplished by submerging a heating surface in a pool of stationary liquid. The role and share of different energy transfer mechanisms in pool boiling can be determined through entropy generation analysis. In this paper, entropy generation due to energy transfer through circular surfaces is analyzed and the optimized thermodynamic system is achieved. Furthermore, two models of total heat flux (linear and nonlinear) are developed for pool boiling heat transfer in which the effect of different parameters such as nucleation site density, contact angle, Prandtl number (Pr) as well as surface diameter are considered. The obtained results reveal the enhanced heat flux and entropy generation as the consequences of increased wall superheat temperature, contact angle, Prandtl number and surface diameter. The developed model is validated by comparing the obtained results with respective experimental data and semi-analytical results. In addition, results exhibit that the maximum entropy generation is observed at $$\Delta T = 15 \,{\text{K}}$$ and $$Pr = 3.5$$ which was equal to 4.06 W m−1 K−1 and 3.78 W m−1 K−1 for Model 1 and Model 2, respectively.

A deep learning method for estimating the boiling heat transfer coefficient of porous surfaces

Owing to the high nucleation site density and relatively robust behavior, sintered coated surfaces are of great interest for thermal management via pool boiling in many industries/applications such as desalination, electronics cooling, petrochemical, and power sector. The coated surfaces have been extensively used to improve the performance of the pool boiling process over the years. Regardless of a large amount of experimental data on the pool boiling of coated surfaces, no accurate mathematical/empirical approaches have been developed to estimate the heat transfer coefficient of these surfaces. The present study develops an AI-based method to estimate the pool boiling heat transfer coefficient for coated porous surfaces. The proposed AI method can handle the complex nature of the coating characteristics such as porosity, coating thickness, and particle size. Via using deep neural networks, the proposed method is applicable for highly wetting fluids (dielectric liquids), refrigerants, and low-wetting liquid (water). Correlation matrix analysis confirms that porosity, coating thickness, particle size, wall superheat, and surface inclination as well as the thermophysical properties of the working fluids are the best independent variables to estimate the considered parameter. Different deep neural networks are designed and evaluated to find the optimized model in terms of its predictive accuracy by experimental data (373 points). The best model with an input layer, three hidden layers, and an output layer (11–30–15–1–1) was able to predict the heat transfer coefficient with overall R2 = 0.976 and (mean absolute error) MAE% = 5.74. The proposed approach is simple and can be employed to optimize the sintered coated surfaces for different cooling applications.

Study on heat transfer and bubble behavior inside horizontal annuli: Experimental comparison of R-134a, R–407C, and R-410A subcooled flow boiling

Subcooled flow boiling (SFB) is a complicated phenomenon regarding the significant heat transfer and vapor bubble formation, which is extremely critical either in daily life or industrial application. The present work pertains to the experimental study on heat transfer characteristics and bubble behaviors in narrow horizontal annuli with emphasis on diverse refrigerants of R-134a, R–407C and R-410A. From experimental results, the significant temperature undershoot during ONB is observed and the level of temperature drop is R-134a > R–407C > R-410A. Enhancing the input heat flux increases boiling heat transfer coefficient (hr) for all three refrigerants. The hr of R-410A always remains higher than R-134a and R–407C stemming from the physical property of lower surface tension. The visualization of bubble behavior addresses that enhancing the input heat flux improves buoyancy force, resulting in the increases of bubble departure size and departure frequency. To achieve a quantitative analysis of bubble characteristics, three indexes of bubble detachment diameter (dp), bubble departure frequency (fb) and the active nucleation site density (Nac) are introduced to explore bubble dynamic behaviors in SFB. The results show that the refrigerant of R-410A exhibits the smallest dp but the highest fb and Nac among these three refrigerants.

Nucleate pool boiling heat transfer: Review of models and bubble dynamics parameters

Understanding nucleate pool boiling heat transfer and, in particular the accurate prediction of conditions that can lead to critical heat flux, is of the utmost importance in many industries. Due to the safety issues related to the nuclear power plants, and for the efficient operation of many heat transfer units including fossil fuel boilers, fusion reactors, electronic chips, etc., it is important to understand this kind of heat transfer. In this paper, a comprehensive review of analytical and numerical work on nucleate pool boiling heat transfer is presented. In order to understand this phenomenon, existing studies on boiling heat transfer coefficient and boiling heat flux are also discussed, as well as characteristics of boiling phenomena such as bubble departure diameter, bubble departure frequency, active nucleation site density, bubble waiting and growth period and their impact on pool boiling heat transfer.