Active Nucleation Sites Research Articles

Vapor stem bubbles dominate heat transfer enhancement in extremely confined boiling

Boiling has long been sought as the heat dissipation mechanism for a wide variety of compact thermal management applications owing to low-resistance heat transport, high heat flux limits, and surface isothermalization. This work aims to elucidate the thermofluidic transport mechanisms of boiling in extremely confined gaps through experimental measure of the temporal evolution of heat fluxes and surface temperatures during deionized water boiling, as well as high-speed visualization of bubble formation. The flow visualizations reveal small residual pockets of vapor, termed ‘stem bubbles’ herein, that remain on the boiling surface through a pinch-off process where vapor escapes through the edges of the confined heated region. These stem bubbles act as seeds for vapor growth in the next phase of the boiling process. These bubbles dictate the boiling performance for extremely confined boiling as defined based on a dimensionless ratio of the gap spacing to capillary length (Bo≤ 0.35 - 0.5). This conclusion is supported by the enhanced thermal response of the surface compared to nucleate boiling. Because activation of nucleation sites is not required for stem bubble boiling, phase change occurs at a reduced surface superheat at a given heat flux compared to nucleate boiling. Criteria for the dimensionless confinement gap spacing are identified to harness this improved heat transfer rate of the stem bubble boiling regime. This new understanding of boiling in extremely confined gaps offers a new direction to design compact two-phase thermal management solutions through using the unique enhancements provided by the vapor stem bubble boiling regime.

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.

Immobilization of Pt nanoparticles on hydrolyzed polyacrylonitrile-based nanofiber paper

The electrochemical activity of catalysts strongly depends on the uniform distribution of monodisperse Pt nanoparticles without aggregates. Here, we propose a new hydrolysis-assisted smearing method for Pt loading on a free-standing paper-type electrode. Polyacrylonitrile (PAN)-based nanofiber paper was used as the electrode, and it acted as a Pt support. Hydrolysis of the electrode tripled the number of active nucleation sites for Pt adsorption on the PAN nanofibers, thereby significantly enhancing the wettability of the nanofibers. This facilitated the uniform distribution of Pt nanoparticles without aggregate formation up to 40 wt% (about 0.8 mg/cm2) with a particle size of about 3 nm. The catalytic current of the hydrolyzed Pt electrode in CH3OH/H2SO4 solution exceeded 213 mA/cm2 Pt mg, which was considerably greater than the current was 148 mA/cm2 Pt mg for an unhydrolyzed electrode.

The Effect of Naphthalene‐Based Additives on the Kinetics of Tin Electrodeposition on Boron‐Doped Diamond Electrodes

AbstractThe effect of naphthalene‐based additives: naphthalene (NPT), naphthalenesulfonate (NPTS) and hydroxynaphthalenesulfonate (HNPTS) on the kinetics of tin electrodeposition on a boron‐doped diamond (BDD) electrode has been studied by means of chronoamperometry and scanning electron microscopy (SEM). Potentiostatic current transients in the absence and the presence of naphthalene‐based additives are analyzed by using the Scharifker‐Hills model. A strong decrease of the kinetics of tin nucleation on BDD was observed in the presence of naphthalene‐based additives, NPT showing the smallest effect and HNPTS showing the largest effect. From the long‐term Cottrell behavior of the transients, similar values of tin(II) diffusion coefficients were obtained for all additives, suggesting that there is no complexation of Sn(II) by the additives and that the charge‐transfer kinetics itself is not substantially influenced by the presence of the additives. In the absence of additives, tin deposition on BDD displays a progressive nucleation and growth mechanism at the least negative potentials, switching to instantaneous nucleation and growth at more negative potential. In the presence of NPTS and HNPTS, progressive nucleation and growth transients are observed. The growth mode results are confirmed by the tin features observed in the scanning electron micrographs. In conclusion, NPT, NPTS and HNPTS mainly decrease the rate of the nucleation of tin deposition, most likely by blocking or reducing access to active nucleation sites. In comparison, ethoxylated α‐napthalenesulfonic acid (ENSA, a commonly used additive in the tin plating industry) inhibits tin deposition process on BDD even more strongly. These observations show a striking similarity to our previous study of tin deposition on gold electrodes.

Surface Modification Utilizing Photolithography Process for Pool Boiling Enhancement: An Experimental Study

Pool boiling can be mentioned as one of the most effective processes of heat transfer regarding the resultant phase change. In this research study, heat transfer in nucleate pool boiling is modeled experimentally with the consideration of surface modification methods utilizing surface photolithography in saturated water at atmospheric pressure. Further, the effects of etching material, confined etched surface, surface area and wettability comparison which is used in fabrication copper surfaces were examined. The images of scanning electron microscopy as well as atomic force microscopy are utilized for obtaining the phase structure and surface morphology in the prepared samples. The experimental results showed that the heat dissipation increases with higher additional area and active nucleation sites densities. Sample with checkered pattern and width of 60 created by photolithography process exhibited the maximum enhancement among other samples and the highest heat transfer coefficient, up to 178% compared to the plain surface. Different parameters, including the bubble dynamic, slide and scrape of the bubbles, area of the heat transfer as well as the capillary flow can be considered as the reasons for improved heat transfer. In addition, the bubble dynamic and enhancement mechanisms are investigated based on the images.

Dynamics of droplet impact and evaporation on the regular micro-grooved and irregular microstructured surfaces

The dynamics of droplet impact and evaporation on the regular micro-grooved and irregular microstructured surfaces are experimentally investigated in this study. The regular micro-grooved and irregular micro structured surfaces are fabricated by a method combining ultraviolet lithography and Ni micro-electroforming process, and electrochemical etching, respectively. It is found that the surface temperature has little effect on the curves of spreading factor in the first spreading phase, as the first spreading phase is governed by the impact of the kinetic energy. At low surface temperature, the total evaporation time for three surfaces is almost same. When the surface temperature exceeds 100 °C, the total evaporation time for the three surfaces shows the differences, and in this regime the droplet mass can mainly be accelerated by the formation of bubbles. At the surface temperature of 104 °C, the micro-cavities of irregular microstructured surface provide more active nucleation sites than that of flat and regular micro-grooved surfaces, which contributes to shorten the total evaporation time. At the surface temperature of 165 °C, the large amount of secondary droplets eject outward on the flat surface, which has not been seen on the regular micro-grooved and irregular microstructured surfaces.

The microchannel combined hydrophobic nanostructure for enhancing boiling heat transfer

Abstract The boiling heat transfer performance of hydrophobic nanostructure decorated microchannel was investigated experimentally to reveal the subtle balancing issues of bubble nucleation sites increase and bubble coalescence delay. Meanwhile the bubble behaviors including bubble nucleation, growth and departure were also studied by high-speed visualization. Microchannel, nanostructure and micro-nano structure were fabricated on pure nickel substrates through the UV-LIGA and electrodeposition techniques. The incipience temperature at the onset of nucleate boiling for smooth surface, microchannel, nanostructure and micro-nano structure were 6.7 K, 3.5 K, 3.5 K and 3.0 K, respectively. Micro-nano structure and smooth surface exhibited the minimum and maximum bubble departure diameter of 1 mm and 2.25 mm at heat flux of 4 W/cm2. To sum up, nanostructure significantly increased the bubble nucleation sites by reducing the energy barrier of the phase change at low heat flux, while microchannel decreased bubble growth time, and delayed bubble coalescence due to capillary pressure at high heat flux. The utilization of micro-nano structure resulted in to the combined effects of active nucleation sites improvements and bubble coalescence delay, achieving better heat transfer performance.

Multiwalled carbon nanotubes synthesis from methane using a stainless steel foils as a catalyst

In this study, a thin stainless-steel foil was used as a catalyst for carbon nanotubes (CNTs) using methane as a carbon source via the chemical vapor deposition (CVD) method. Our results revealed that pre-treatment step of the catalyst plays an important role in CNT formation. In our experiments, a catalyst pre-treatment temperature of 850 oC have been found to facilitate the surface roughness and provide more active nucleation sites for CNTs formation. Multiwalled CNTs with 6 layers, their diameters of 10 – 20 nm and their length of app. 300 nm were grown. This finding might lead to a process for improving the quality of MWCNTs grown on steel foil as catalyst.

Active sites for ice nucleation differ depending on nucleation mode

The nucleation of ice crystals in clouds is poorly understood, despite being of critical importance for our planet's climate. Nucleation occurs largely at rare "active sites" present on airborne particles such as mineral dust, but the nucleation pathway is distinct under different meteorological conditions. These give rise to two key nucleation pathways where a particle is either immersed in a supercooled liquid water droplet (immersion freezing mode) or suspended in a supersaturated vapor (deposition mode). However, it is unclear if the same active sites are responsible for nucleation in these two modes. Here, we directly compare the sites that are active in these two modes by performing immersion freezing and deposition experiments on the same thin sections of two atmospherically important minerals (feldspar and quartz). For both substrates, we confirm that nucleation is dominated by a limited number of sites and show that there is little correlation between the two sets of sites operating in each experimental method: across both materials, only six out of 73 sites active for immersion freezing nucleation were also active for deposition nucleation. Clearly, different properties determine the activity of nucleation sites for each mode, and we use the pore condensation and freezing concept to argue that effective deposition sites have size and/or geometry requirements not of relevance to effective immersion freezing sites. Hence, the ability to nucleate is pathway dependent, and the mode of nucleation has to be explicitly considered when applying experimental data in cloud models.

A Molecular Dynamics Study of Heat Transfer Enhancement during Phase Change from a Nanoengineered Solid Surface

This study investigates the enhancement of the rate of evaporation from a nanoengineered solid surface using non-equilibrium molecular dynamics simulation. Four different types of surface modifications were introduced to examine the thermal transportation behavior. The surface modification includes: (1) transformation of surface wetting condition from hydrophobic to hydrophilic, (2) implementing nanostructures on the smooth surface, (3) cutting nano slots on the smooth surface and (4) introducing nano-level surface roughness. Evaporation behavior from the same effective surface area was also studied. The simulation domain consisted of three distinct zones: solid base wall made of copper, a few layers of liquid argon, and a vapor zone made of argon. All the nano-level surface modifications were introduced on the solid base surface. The few layers of liquid argon representing the liquid zone of the domain take heat from the solid surface and get evaporated. Outside this solid and liquid zone, there is argon vapor. The simulation began at the initial time t = 0 ns and then was allowed to reach equilibrium. Immediately after equilibrium was achieved on all three-phase systems, the temperature of the solid wall was raised to a higher value. In this way, thermal transportation from the solid wall to liquid argon was established. As the temperature of the solid wall was high enough, the liquid argon tended to evaporate. From the simulation results, it is observed that during the transformation from hydrophobic to hydrophilic conditions, enhancement of evaporation takes place due to the improvement of thermal transportation behavior. At the nanostructure surface, the active nucleation sites and effective surface area increase which results in evaporation enhancement. With nano slots and nano-level surface roughness, the rate of evaporation increases due to the increase of solid-liquid contact area and effective surface area.

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.

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.

Comparing the heat transfer coefficient of copper sulfate and isopropanol solutions in the pool boiling process: Bubble dynamic and ultrasonic intensification

In this work, a comparison study was implemented on the pool boiling heat transfer coefficient (BHTC) between the copper sulfate and isopropanol solutions. To precise investigate; the bubble dynamic including departure diameter, departure frequency and active nucleation sites were examined. In addition, ultrasonic waves’ irradiation (25 kHz) was employed to intensive the solutions and improve the dynamic parameters due to cavitation phenomenon. First, although the cooper sulfate had a bigger bubble departure diameter and more active nucleation sites than isopropanole, its heat transef coefficient was lower due to great bubble departure frequency of isopropanol. Finally, Calus model with an average error of 12.8% for isopropanol and Palen with 13.1% for copper sulfate had the lowest errors. Moreover, the isoropanole at 20 V.% with 15% power of ultrasonic irradiation and the cooper sulfate at 10.5 V.% with 35% had the maximum increase in the BHTC as 10.549% and 18.238%, respectively.

Comparative study on immersion freezing utilizing single-droplet levitation methods

Abstract. Immersion freezing experiments were performed utilizing two distinct single-droplet levitation methods. In the Mainz vertical wind tunnel, supercooled droplets of 700 µm diameter were freely floated in a vertical airstream at constant temperatures ranging from −5 to −30 ∘C, where heterogeneous freezing takes place. These investigations under isothermal conditions allow the application of the stochastic approach to analyze and interpret the results in terms of the freezing or nucleation rate. In the Mainz acoustic levitator, 2 mm diameter drops were levitated while their temperature was continuously cooling from +20 to −28 ∘C by adapting to the ambient temperature. Therefore, in this case the singular approach was used for analysis. From the experiments, the densities of ice nucleation active sites were obtained as a function of temperature. The direct comparison of the results from two different instruments indicates a shift in the mean freezing temperatures of the investigated drops towards lower values that was material-dependent. As ice-nucleating particles, seven materials were investigated; two representatives of biological species (fibrous and microcrystalline cellulose), four mineral dusts (feldspar, illite NX, montmorillonite, and kaolinite), and natural Sahara dust. Based on detailed analysis of our results we determined a material-dependent parameter for calculating the freezing-temperature shift due to a change in cooling rate for each investigated particle type. The analysis allowed further classification of the investigated materials to be described by a single- or a multiple-component approach. From our experiences during the present synergetic studies, we listed a number of suggestions for future experiments regarding cooling rates, determination of the drop temperature, purity of the water used to produce the drops, and characterization of the ice-nucleating material. The observed freezing-temperature shift is significantly important for the intercomparison of ice nucleation instruments with different cooling rates.

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.

Bubble dynamics in microchannel: An overview of the state-of-the-art

The inception of the boiling, in a pool or flow boiling, is the formation of the vapor bubble at an active nucleation site that plays a crucial role in the boiling process and it becomes critical and unfolds many facets when channel size reduces to submicron. The detailed knowledge of the bubble dynamics is helpful in establishing the thermal and hydraulic flow behavior in the microchannel. In the current paper, bubble dynamics that include bubble nucleation at the nucleation site, its growth, departure, and motion along the flow in a microchannel(s) are discussed in detail. Different models developed for critical cavity radius favorable for bubble nucleation are compiled and observe that models exhibit large deviation. The bubble growth models are compiled and concluded that the development of a more generalize bubble growth model is necessary that would be capable of accounting for inertia controlled and thermal diffusion controlled regions. Bubbles at nucleation sites in a microchannel grow under the influence of various forces such as surface tension, inertia, shear, gravitational and evaporation momentum. Parametric analysis of these forces reckoned that the threshold between macro- to microchannel could be identify through critical analysis of such forces. Eventually, the possible impact of the various factors such as operating conditions, geometrical parameters, thermophysical properties of fluid on bubble dynamics in microchannel has been reported.

Failure mechanism of zinc adipate as a β-nucleating agent for polypropylene in the presence of calcium stearate

Although zinc dicarboxylic acids are highly effective β-nucleating agents (β-NAs), they lose their β-nucleating ability in isotactic polypropylene (iPP) upon the addition of calcium stearate (CaStr2). In this study, we investigated the failure mechanism of these β-NAs in terms of the chemical and physical interactions of zinc adipate (ZnAA) with CaStr2. No chemical reaction occurs between CaStr2 and ZnAA; rather, it is the physical interactions between this β-NA and CaStr2 that are responsible for the loss of β-nucleating ability. Based on the experimental findings and molecular dynamics (MD) simulations, we suggest a blocking effect of CaStr2 on nucleation as a possible failure mechanism. MD simulations confirmed that the aggregation of CaStr2 on the β-NA surface, observed experimentally using polarized optical microscopy and scanning electron microscopy, resulted from the binding energy for the interaction between ZnAA and CaStr2 being higher than that for iPP/CaStr2. Because the active nucleation sites on the surface of the β-NA were covered, lattice matching between ZnAA and the iPP chains was hindered. Therefore, the addition of CaStr2 weakens the β-nucleating ability of ZnAA, leading to inhibition of the epitaxial crystallization process, thereby decreasing the degree of β-crystal formation and, consequently, lowering the toughness of the resulting iPP.

Gradient wick channels for enhanced flow boiling HTC and delayed CHF

Liquid supply to wick structures of flow boiling surfaces is fundamentally restricted by the capillary limit at which the pressure drop of the wicked liquid surpasses texture-amplified capillary forces. Gradient wick structures partially decouple permeability and capillary pressure, thereby delaying the capillary limit. In this study, gradient wick channels facilitating out-of-plane liquid delivery are introduced to postpone the capillary limit and thus enhance the two-phase flow boiling heat transfer coefficient (HTC) and delay critical heat flux (CHF). Here, the permeability of the gradient wick channels is augmented by large-pore-size meshes employed near the bulk fluid while capillary pressure is maximized by small-pore-size meshes utilized near the hot boiling surface. This combination of wick structures enables to preferentially guide the cooling liquid, deionized water, from the far-field cold liquid toward the bottom hot substrate. The spatial distribution of individual gradient wick channels promotes separate liquid-vapor pathways, thus facilitating the vapor escape process. Experiments conducted here reveal that the flow boiling performance metrics of the proposed heat sink leveraging the gradient wick channels outperform those of the homogenous wick channels and solid fin channels. The proposed heat sink demonstrates a strong liquid mass flux dependency due to a combination of convective boiling and amplified wickability effects. The enhanced convective boiling could be related to surface roughness, a high number of active nucleation sites, and a large surface area available through tortuous passages of wick channels. At higher mass flow rates, effective capillary pressure available for out-of-plane wicking action also increases, thereby further boosting the wickability effect and associated heat transfer processes. This could meaningfully delay the CHF. In fact, the CHF limit was not observed on the gradient wick channel surfaces in the mass flux and wall superheat range studied. The experimental results indicated a maximum heat flux of 870 W/cm2 with a gradient wick channel heat sink, a 60% improvement compared with a plain copper surface. Furthermore, a maximum HTC of 1000 kW/m2-K at a wall superheat of 3 °C was observed, a three-fold enhancement compared with that of the plain surface. The proposed gradient wick channel topology offers new pathways for designing innovative surface technologies with high heat removal capabilities, thereby potentially improving the energy economy in myriad modern energy applications.