Active Nucleation Site Density Research Articles

Enhanced nucleate pool boiling heat transfer on CNTs-Cu nanoparticles-coated surfaces: Effects of sintering temperature and CNTs composition on pool boiling behavior

The present study involves the experimental investigation of pool boiling heat transfer characteristics on CNTs-Cu nanoparticles-coated heating surfaces. The advantages of coating the mixture of infinitesimal CNTs with Cu particles on the copper surface are to generate the active nucleation site density by porous structures and enhance the thermal conductivity of the heating surfaces. To explore the effects of sintering temperatures and CNTs composition on pool boiling behavior, we fabricated the boiling test sections under the sintering temperatures of 700, 750, and 800 °C for four different CNTs compositions of 0.0CNTs-100Cu, 0.2CNTs-99.8Cu, 0.3CNTs-99.7Cu, and 0.5CNTs-99.5Cu in wt.%. We first analyzed the surface morphologies, that is, the average pore size, porosity, and pore density, to characterize each tested sample. Pool boiling experiments were later conducted under atmospheric pressure using water to obtain the pool boiling curves and bubble dynamics for different sintering temperatures and CNTs composition. Based on the experimental analysis, nucleate boiling heat transfer enhancement was interpreted in relation to the sintering temperature with surface morphologies and CNTs composition in relation to improved pore density. Compared to the bare copper surface, the achieved maximum enhancement for boiling heat transfer coefficient and critical heat flux were 3.86 and 1.49, respectively, when a 0.5CNTs-Cu-coated surface sintered at 800 °C was applied.

Investigation of nucleate boiling and bubble dynamics at multiple nucleation sites in subcooled boiling flow

Accurate prediction of nucleate boiling heat transfer and phase change is crucial for subcooled boiling flow modeling. Among the nucleate boiling parameters, departure diameter, departure frequency, and active nucleation site density are considered the most important. To fulfill the modeling requirements, the present study established a comprehensive experimental database with simultaneous measurement of departure diameter, departure frequency, and active nucleation site density at multiple nucleation sites. The experimental data were collected in a vertical annular test section using a microscopic high-speed imaging system. Twenty-two experiments were performed at a system pressure of 150 kPa, liquid flow rate of 0.5 m/s, local subcooling ranging from 10 to 30 °C, and heat flux varying from 250 to 700 kW/m2. Stochastic characteristics of departure diameter and departure frequency at single and multiple nucleation sites were investigated. A correction approach was developed to minimize the impact of upstream bubbles on departure frequency measurement. The sample size of single-site measurement was determined based on the analysis of data dispersions. Large variations between nucleation sites were observed, indicating that single-site measurement is insufficient to characterize all nucleation bubbles. A parametric analysis of nucleate boiling parameters over operating conditions was performed based on the multi-site data. The experimental database generated in this study will be utilized for model evaluation and development in future research.

Numerical simulation of boiling behavior in vertical microchannels

High heat flux electronic devices put forward new requirements for heat dissipation, and boiling heat transfer technology is widely used because of its higher heat dissipation capacity. In this study, the volume of fluid method was employed, along with the incorporation of the Lee phase-change mass transfer model, to investigate two-phase flow and heat transfer in vertical upward rectangular microchannels. The heat flux was varied within the range of 10–40 kW/m2, while the mass flux was varied within the range of 200–600 kg/m2 s. With the increase in heat flux, bubble flow, slug flow, churn flow, and annular flow were found successively. A phase diagram was established to predict the flow pattern transition during the boiling process. When the flow pattern changes to the churn and the annular flow, the active nucleation site density increases obviously with the Boiling number (Bo). A new correlation was proposed for two-phase flow boiling heat transfer, suitable for vertical upward channels in microscale fluids. The friction factor obtained using the Darcy friction factor equation agrees well with the simulation results at a high-pressure drop. The instability in microchannels increases with the increase in heat flux, particularly in annular flow, resulting in more severe wall temperature fluctuations.

Deep learning-based approach to R-134a bubble detection and analysis for geothermal applications

This study utilized deep learning to analyse bubble images of R134a fluid produced on bare and novel oleophilic coated stainless steel heat exchanger plates to compare the heat transfer performance in terms of bubble parameters. This work was carried out as a part of the GeoHex project. The vapour bubbles of the boiling R134a fluid, produced on two types of heat exchanger plates, were captured with a high-speed camera during a boiling experiment. A deep learning algorithm based on the Mask R–CNN model was trained for bubble detection in the experimental videos. The algorithm achieved a bubble detection accuracy of 61.22% and 78.6% for the coated and uncoated plates, respectively. The lower accuracy for the coated plate could be attributed to the challenge of distinguishing bubbles from the complex coating pattern. The algorithm determined the number of detected bubbles, bubble sizes, and centroids per frame, tracking each bubble frame by frame. The tracking results were used to calculate the bubble parameters such as departure frequency, departure diameter, and active nucleation site density. The calculated departure diameter for both plates exhibited a difference of 3%, which corresponded closely to the experimental results showing a 4% variation in heat transfer coefficient, which was measured under identical experimental conditions utilizing the GeoHex rig. However, the calculated departure frequency and active nucleation site density exhibited a 17% disparity between the coated and uncoated plates, primarily stemming from the lower detection accuracy for the coated plate. Some limitations of the trained model were identified in accurately detecting very small, out of focus, and mutually occluded bubbles. Future research should focus on improving experimental rig designs to capture better side-view images and develop advanced machine learning or other techniques to address these limitations.

Enhancement in pool boiling performance of GNP/Cu-Al2O3 nano-composite coated copper microporous surface

An experimental analysis of pool boiling is conducted on the GNP/Cu-Al2O3 (GNP-graphene nano particle) nanocomposite coated copper surfaces at atmospheric pressure using DI (distilled water) as boiling fluid. A two-step electro-deposition technique is used for fabrication of these micro/nano porous coated surfaces. A modified surface structure is formed due to this deposition and various surface morphology parameters such as surface structure, roughness, wettability are analysed. Results show the enhancement in BHTC (boiling heat transfer coefficient) and CHF (Critical heat flux) in nano composite coated surfaces compare to the bare copper surface. The maximum CHF of 2415 (kW/m2) and BHTC of 171.28 (kW/m2K) is found in the NCCS (nano-composite coated superhydrophilic) surfaces which are 122% and 239% more than the bare copper surfaces. The boiling performance is augmented mainly because of the improvement in roughness, wettability and also for the high thermal conductivity of the GNP/Cu- Al2O3 nano coating. Quantitative study of the bubble dynamics like active nucleation site density, bubble emission frequency, departure diameter of bubbleand their impact on heat transfer performance in the case of NCCS surfaces is performed with high-speed visualization.

Numerical Investigation on Pool Boiling Heat Transfer of Silica and Alumina Nanofluids

Surface modification through pool boiling of nanofluid is an efficient technique in delaying critical heat flux and attaining high heat flux at lower wall superheat temperature. The method of involving phase interaction, bubble evolving and dynamics phenomenon, and distribution of vapor void fraction altogether is the motivation behind current topic. The present work will contribute to the recovery and reuse of the wastewater and incinerator heat, cooling of high heat flux generating micro-electronics chips, and cooling of fast breeder test reactors. In the present study, the nucleate pool boiling of SiO2/water and Al2O3/water nanofluid are studied numerically and the effect of wall superheat temperature and heat flux on active nucleation site density, bubble departure diameter, and bubble waiting time are studied. The effect of various heater surface material and nanofluid concentration on wall superheat temperature and heat transfer coefficient are also investigated. A two-phase Eulerian approach, heat flux wall-partitioning model, and Rensselaer Polytechnic Institute boiling model are considered to simulate the current model. The pool boiling of nanofluids resulted in the formation of nano-porous layer on heater surface because of microlayer evaporation process that enhanced the heat transfer coefficient and reduced the wall superheat temperature.

Development of bubble dynamic parameter models in subcooled flow boiling of saline solution

Concentrated saline solutions are inevitably used as coolants in the industry heat exchangers. From the aspect of simulation prediction, the closure models for RPI model, including the models of bubble departure diameter (Dw), departure frequency (f) and active nucleation site density (Nw), are of great importance for the overall success of heat transfer calculation. However, the applicability of the existing closure models for the nucleate flow boiling of saline solutions is yet to be answered. In this paper, the experiments of subcooled flow boiling of the NaCl solutions with concentration range of 0%–6% were conducted. The experimental data of the three bubble parameters, Dw, f and Nw, were obtained under different operating conditions, making up for the shortage of the bubble parameter data of saline solution in literature. The impacts of key influencing parameters for each of the three bubble parameters were discussed. For model development, the prediction performance of several existing representative Dw, f and Nw models on saline solution were studied. The results indicate that the available models for Dw, f and Nw show rather large deviations for saline solutions. Therefore, new models for Dw, f and Nw were developed according to the correlation method and the influencing parameter analyses. The parameters and dimensionless groups, which can be used to characterize the variations of Nw, Dw and f, were determined. The dimensionless parameters including Boiling number (Bo), density ratio (ρ⁎), dimensionless surface tension, Prandtl number and a suggested dimensionless temperature (ΛT), can be used to characterize the variation of Dw. And the dimensionless groups of Bo, ρ⁎, ΛT, and the non-dimensional bubble departure diameter, can be used to correlating with the non-dimensional departure frequency. As for Nw, the wall superheat and solution concentration are the most critical factors for model correlating. The new correlations for predicting the Dw, f and Nw during subcooled flow boiling in both the pure water and saline solutions were built and validated.

Boiling heat transfer enhancement on titanium through nucleation-promoting morphology and tailored wettability

• Cavity- and spear-type TiO 2 nanostructures are prepared by hydrothermal treatment. • Nucleation-promoting cavity morphology is superior to spear-like microstructure. • Hydrophobic surface with microcavities (CTH) provides HTC enhancement of over 200%. • High nucleation site density on CHT surface narrows surface temperature distribution. • ONB at 2.5 K and D b of 0.55 mm are observed on hydrophobic microcavity surface. Surface engineering aimed at tuning the wettability and morphology of the boiling surface is a facile approach to moderate and enhance the nucleate boiling process. Key issues include control over the active nucleation site density, bubble departure frequency and liquid replenishment of active nucleation sites while simultaneously reducing the bubble nucleation temperature. In this study, we fabricated spear-type (ST) and cavity-type (CT) TiO 2 nanostructures on 25 μm titanium foils via hydrothermal etching in an alkaline solution. High-speed IR and video cameras were used to detect local phenomena in terms of temperature and heat flux fluctuations and observe the bubble dynamics during saturated pool boiling of water. Intrinsically hydrophilic ST and CT surfaces provided a moderate overall enhancement of the heat transfer coefficient compared to an untreated surface due to increased nucleation site density and bubble frequency. The CT surface also decreased the bubble nucleation temperature due to effective vapor-entrapping and nucleation-promoting cavities. In a further step, both surfaces were hydrophobized through chemical vapor deposition of a fluorinated silane to tailor the wettability of the surface into a superhydrophobic state. This further reduced the average surface superheat by at least 40%, while the nucleation frequencies exceeded 200 Hz on the hydrophobized CT surface. In comparison with the untreated reference surface, the heat transfer coefficient on hydrophobized ST and CT surfaces was enhanced by 89% and 237% at 100 kW m −2 , respectively. Moreover, the full width at half maximum (FWHM) value of the surface temperature distribution was reduced by 73% and 95% at the same heat flux, respectively. The study confirms that hydrophobic surface treatment can significantly enhance the nucleate boiling process when combined with an appropriate surface structure. Despite the affinity between the vapor and the hydrophobic layer, the cavity-type and spear-type TiO 2 structures are able to maintain active nucleation sites well-separated, which prevents the undesirable vapor spreading that possibly leads to an early onset of critical heat flux.

Revisiting the Corresponding-States-Based Correlation for Pool Boiling Critical Heat Flux

A corresponding-states correlation for predicting the critical heat flux (CHF) in pool boiling conditions is proposed, and only requires knowledge of physical property constants of the fluid at any fluid temperature: molar mass, critical temperature, critical pressure, and the Pitzer acentric factor. If a fourth corresponding equation of state (EoS) parameter is added, a more accurate CHF correlation is obtained and matches Kutateladze–Zuber prediction within ±10% in the reduced temperature range of 0.55–0.95. This way, CHF can be easily predicted for any reduced temperature within the range of correlation’s validity by only knowing basic properties of the fluid. Additionally, two corresponding-states correlations for determining the capillary length are proposed and also do not rely on any temperature- and pressure-dependent fluid properties. A simpler correlation only using the Pitzer acentric factor is shown to be imprecise, and a more complex correlation also accounting for the fourth corresponding EoS parameter is recommended. These correlations are fundamental for further developments, which would allow for accurate prediction of CHF values on functionalized surfaces through further studies on the influence of interactions of fluid properties with other parameters, such as wetting and active nucleation site density.

Assessment on population balance model and wall boiling model for subcooled boiling flow of steam generator of nuclear power plant

Assessments on the combinations of the population balance model (PBM) and wall boiling model are meaningful for the calculation of the steam generator (SG) of the nuclear power plant (NPP). Firstly, the non-dimensional analysis between the report experiment and the SG of the NPP are carried out. Then, the influences of the models of the active nucleation site density Nn combined with the PBM kernels are analyzed. Lastly, the error analysis on the combinations is illustrated. The results show that the one model combination performs relatively better for the calculation of the Sauter mean diameter (SMD) dp (maximum relative error 6.09%), void fraction α (maximum relative error 20.06%) and superficial liquid velocity jl (maximum relative error 2.06%) of the SG of the NPP. This study demonstrates an integral method and process for the selection of the model combinations in the subcooled boiling flow conditions.

Evaluation on Coupling of Wall Boiling and Population Balance Models for Vertical Gas-Liquid Subcooled Boiling Flow of First Loop of Nuclear Power Plant

An accurate prediction of the interphase behaviors of the vertical gas-liquid subcooled boiling flow is meaningful for the first loop of a nuclear power plant (NPP). Therefore, the interphase behaviors including the bubble size distribution in the first loop of the NPP are analyzed, evaluated, and validated using various wall boiling models coupled with the population balance model (PBM) kernels in this paper. Firstly, nondimensional numbers of the first loop of the NPP and DEBORA (Development of Borehole Seals for High-Level Radioactive Waste) experiment test cases are analyzed with approximation. Secondly, five active nucleation site density models Nn coupled with the PBM kernel combination, four kernel combinations (C1~C4) with the Nn models are calculated and analyzed. Lastly, various behaviors including the bubble size distribution Sauter mean diameter (SMD) dp, void fraction α, gas superficial velocity jg, and liquid superficial velocity jl are compared and validated with the experimental data of the DEBORA-1 (P = 2.62 MPa). The results indicate that the two Nn models are suitable for the calculations of thefirst loop of the nuclear power plant. For instance, for the bubble size distribution SMD dp, the specified Nn model with C1 (maximum relative error 9.63%) has relatively better behaviors for the first loop of the NPP. Especially, the combination C1 is applicable for the calculation of the bubble size distribution dp, void fraction α and liquid superficial velocity jl while C4 is suitable for the calculation of the gas superficial velocity jg. These results can provide guidance for the numerical computation of the subcooled boiling flow in the first loop of the NPP.

Bubble characteristics during alternating subcooled flow boiling with oscillated refrigerant flow rates – An experimental investigation

In this paper, a comprehensive experimental investigation is performed to study the bubble characteristics during the alternating subcooled flow boiling with the oscillated refrigerant flow rates. The R-407C refrigerant is selected as the refrigerant. Different parameters related to the bubble characteristics are investigated during the temporal alternating R-407C flow boiling. In addition, a number of relationships are proposed for these parameters as the function of R-470C mass flux. The flow regime map for this type of boiling is also investigated. The results indicated that the intermittent boiling can be observed for the wider ranges of the Boiling number. For the long oscillation period of the mass flux, the oscillation vibration of the subcooled flow boiling is quite strong. The bubble departure frequency boosts with increasing the mean mass flux. Eventually, the active nucleation site density is enhanced with increasing the mean saturation temperature of refrigerant.

Intensification of heat transfer during pool boiling of nitrogen on surfaces with capillary-porous coatings produced by 3D-printing

The study of heat transfer, critical heat fluxes (CHF) and evaporation dynamics at pool boiling of nitrogen at atmospheric and low pressures in a stationary heat generation regimes was performed. The two flat cooper heaters with porous coatings of various structural parameters obtained by additive 3D-printing as well as smooth one were used as working surfaces. According to obtained results such coatings significantly effect on pool boiling increasing up to six times the heat transfer coefficients (HTC) in comparison with uncoated sample. For all investigated heaters and pressures, visualization of boiling was performed using a video camera from which data on the bubble departure diameters and estimates of the active nucleation site density were obtained.

Experimental Study of Pool Boiling Enhancement Using a Two-Step Electrodeposited Cu–GNPs Nanocomposite Porous Surface With R-134a

Abstract The fabrication of porous metallic composite coating on the heating surface to improve pool boiling heat transfer (BHT) performance has received significant attention in recent years. In this work, Cu–GNPs nanocomposite coatings, which were prepared on a copper substrate using various current densities through a two-step electrodeposition technique, were used as heating surfaces to study the pool BHT performance of refrigerant R-134a. The surface morphology, elemental composition, thickness, surface roughness, and porosity of prepared Cu–GNPs nanocomposite coatings are studied and presented in detail. All Cu–GNPs nanocomposite coated surfaces exhibited improved boiling performance compared to the plain Cu surface. The heat transfer coefficient (HTC) values for Cu–GNPs nanocomposite coated Cu surfaces prepared at 0.1, 0.2, 0.3, and 0.4 A/cm2 were improved up to 1.48, 1.67, 1.82, and 1.97, respectively, compared with the plain Cu surface. The enhancement in the HTC is mainly associated with the increase in surface roughness, active nucleation site density, and micro/nanoporosity of the heating surface.

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.

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.

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.

Comparison of bubble parameters between boiling and hydrogen evolving systems

Bubble behavior in a hydrogen evolving system was compared to that under boiling conditions. Hydrogen was generated electrochemically in an aqueous solution of 1.5 M sulfuric acid (H2SO4). Hydrogen bubble parameters such as active nucleation site density, bubble departure diameter, and bubble frequency, were measured and compared to those observed during boiling. The hydrogen evolving system exhibited higher active nucleation site density that increased, and smaller departure diameters that gradually decreased, with increasing current density. Bubble growth time, which is related to bubble frequency, was shorter in the hydrogen evolving system due to smaller departure diameters. This study provides experimental evidence supporting the analogous relationship between bubble formation in boiling and that in hydrogen evolving systems.

Experimental study on bubble characteristics of time periodic subcooled flow boiling in annular ducts due to wall heat flux oscillation

The significant effects of imposed wall heat flux oscillation on bubble characteristics of subcooled flow boiling with R-134a in horizontal annular ducts are experimentally performed in details. Specific emphasis has been applied on the bubble dynamics of the time periodic subcooled flow boiling characteristics influenced by the mean levels, amplitudes, and periods of the wall heat flux oscillations. The obtained experimental results have disclosed that the bubble parameters like the departure diameter, frequency and active nucleation site density, do oscillate periodically in time and are at the same frequency as the imposed wall heat flux oscillation. Nevertheless, these parameters also increase at long periods, high amplitudes of imposed heat flux oscillation or high values of mean imposed heat flux. Furthermore, the bubble size reduced after the time lag when the imposed heat flux decreases with time.