Active Nucleation Site Density Research Articles

Investigation of the Influence of Elevated Pressure on Subcooled Boiling Flow—Model Evaluation Toward Generic Approach

Modeling subcooled boiling flows in vertical channels has relied heavily on the utilization of empirical correlations for the active nucleation site density, bubble departure diameter, and bubble departure frequency. Following the development and application of mechanistic modeling at low pressures, the capability of the model to resolve flow conditions at elevated pressure up to 10 bar is thoroughly assessed and compared with selected empirical models. Predictions of the mechanistic and selected empirical models are validated against two experimental data at low to elevated pressures. The results demonstrate that the mechanistic model is capable of predicting the heat and mass transfer processes. In spite of some drawbacks of the currently adopted force balance model, the results still point to the great potential of the mechanistic model to predict a wide range of flow conditions in subcooled boiling flows.

Effect of channel height and mass flux on highly subcooled horizontal flow boiling

Experiments of highly subcooled flow boiling of water in horizontal macrochannels with orthogonal cross-section are performed. Explored parameters are channel height (3 and 10mm) and mass flux (330–830kg/m2s). The range of applied heat fluxes is 200–1000kW/m2. Aging of the copper boiling surface is examined during a 48h continuous operation and is found to gradually reduce heat transfer rates compared to a polished surface. Yet, aging reaches a steady condition already at 24h of operation with about 10% lower heat transfer rates than for the polished surface. Using the steady aged surface in the main experiments, three heat transfer regions are identified: (a) forced convection region, before the onset of boiling, depending highly on mass flux but for the most part being channel height independent, (b) nucleate boiling region depending highly on channel height but for the most part being mass flux independent, and (c) a transition region in-between depending on both mass flux and channel height. The 3mm channel promotes initiation of boiling at lower wall superheats leading to better heat transfer efficiency compared to the 10mm channel, except for the highest mass fluxes where their performance is comparable. The largest enhancement in heat transfer coefficient provided by the 3mm channel compared to the 10mm channel is ∼15–20% and is found at the lowest mass flux 330kg/m2s. Analysis of the present data supports the notion that heat transfer is dictated by the thickness of the thermal boundary layer and the density of active nucleation sites.

Computational fluid dynamics and population balance modelling of nucleate boiling of cryogenic liquids: Theoretical developments

The main focus in the analysis of pool or flow boiling in saturated or subcooled conditions is the basic understanding of the phase change process through the heat transfer and wall heat flux partitioning at the heated wall and the two-phase bubble behaviours in the bulk liquid as they migrate away from the heated wall. This paper reviews the work in this rapid developing area with special reference to modelling nucleate boiling of cryogenic liquids in the context of computational fluid dynamics and associated theoretical developments. The partitioning of the wall heat flux at the heated wall into three components – single-phase convection, transient conduction and evaporation – remains the most popular mechanistic approach in predicting the heat transfer process during boiling. Nevertheless, the respective wall heat flux components generally require the determination of the active nucleation site density, bubble departure diameter and nucleation frequency, which are crucial to the proper prediction of the heat transfer process. Numerous empirical correlations presented in this paper have been developed to ascertain these three important parameters with some degree of success. Albeit the simplicity of empirical correlations, they remain applicable to only a narrow range of flow conditions. In order to extend the wall heat flux partitioning approach to a wider range of flow conditions, the fractal model proposed for the active nucleation site density, force balance model for bubble departing from the cavity and bubble lifting off from the heated wall and evaluation of nucleation frequency based on fundamental theory depict the many enhancements that can improve the mechanistic model predictions. The macroscopic consideration of the two-phase boiling in the bulk liquid via the two-fluid model represents the most effective continuum approach in predicting the volume fraction and velocity distributions of each phase. Nevertheless, the interfacial mass, momentum and energy exchange terms that appear in the transport equations generally require the determination of the Sauter mean diameter or interfacial area concentration, which strongly governs the fluid flow and heat transfer in the bulk liquid. In order to accommodate the dynamically changing bubble sizes that are prevalent in the bulk liquid, the mechanistic approach based on the population balance model allows the appropriate prediction of local distributions of Sauter mean diameter or interfacial area concentration, which in turn can improve the predictions of the interfacial mass, momentum and energy exchanges that occur across the interface between the phases. Need for further developments are discussed.

Effect of heating surface morphology on active site density in subcooled flow nucleated boiling

This paper presents a combination of experimental work and the data analysis used to develop a set of morphology-dependent correlations, in order to determine the active nucleation site density, in a heat flux partitioning model for subcooled flow boiling. Three copper parts with different surface finishes (fine sanding, electrical discharge machining and a combination of both) were tested under several experimental conditions: bulk temperature, 76.5–93.5[°C]; absolute pressure, 110–190[kPa]; mass flux, 96.9–871.8[kg/sm2]; and wall heat flux, 400–650[W/m2]. Automatic high-speed video was processed using third-party image recognition libraries. Functional dependencies for the nucleation site density were presented for the tested range after data processing and analysis. The inclusion of additional morphological parameters was found to considerably reduce error when compared to values obtained with the best previously available correlations and models, in which the contact angle was the sole parameter for modelling the surface-fluid interaction.

Subcooled Flow Boiling Over Microstructured Plates In Rectangular Minichannels

ABSTRACTMicrostructures offer enhancements in boiling heat transfer by increasing bubble departure frequency, active nucleation site density, critical cavity size, and surface area. Integration of microstructures to surfaces alters significant surface parameters such as porosity of the microstructured plates, contact angle, and configuration of microstructures on the surface, which all affect boiling heat transfer. The goal of this study is to investigate boiling heat transfer on different microstructured plates and the effect of various microscale surface morphologies on boiling heat transfer. The microstructured surfaces were formed on aluminum alloy 2024 sheets with the use of a simple and environmentally friendly technique of random mechanical sanding (grits of #36, #60, #400, and #1,000). Distilled water was pumped using a micro gear pump to the rectangular minichannel test section at flow rates of 100, 180, and 290 ml/min, which correspond to mass fluxes of 5.46, 10.58, and 16.15 kg/m2.s, respectively. It was observed that surfaces with low grit (grit #36) showed no considerable enhancement, whereas the use of higher grit counts considerably enhanced boiling heat transfer up to a critical grit count. The results were supported by the images from the performed visualization of flow boiling.

Bubble characteristics in time periodic saturated flow boiling of R-134a in a narrow annular pipe due to heat flux oscillation

In this work, experiments have been conducted to investigate how the imposed time periodic heat flux oscillation affects the bubble characteristics of saturated flow boiling with refrigerant R-134a in a horizontal narrow annular pipe. The test section for the horizontal annular duct consists of an outer pipe made of Pyrex glass and an inner heated copper pipe, intending to facilitate the visualization of boiling processes. A cartridge heater is installed inside the inner pipe to provide the required heat flux to the refrigerant flow in the narrow annular duct. In particular, attention is focused on the time periodic saturated flow boiling characteristics affected by the mean levels, amplitudes, and periods of the heat flux oscillation. The results show that the bubble departure diameter, bubble frequency and active nucleation site density are found to oscillate periodically in time as well and at the same frequency as the imposed heat flux oscillation. Furthermore, in the boiling the resulting oscillation amplitudes of the bubble parameters, such as the bubble departure diameter, bubble frequency and active nucleation site density, get larger for a longer period and a larger amplitude of the imposed heat flux oscillation and for a higher mean imposed heat flux.

Modeling and validation of interfacial area transport equation in subcooled boiling flow

The first comprehensive validation of the interfacial area transport equation in subcooled boiling is presented and shown to perform exceptionally when compared with experimental data. The formulation and closure of the bubble layer averaged interfacial area transport equation is reviewed along with the treatment of the two-fluid model in subcooled boiling. Interfacial area concentration source and sink terms in subcooled boiling are presented including the bubble interaction mechanisms (random collision and turbulent impact), as well as phase change terms (wall nucleation and condensation). Additionally, the volume source terms from phase change are described and discussed in terms of their significance to the interfacial area transport equation. The validation of the interfacial area transport equation with a recently proposed wall nucleation source term is shown to have excellent prediction at low and elevated pressure, as well as a wide range of mass flux. With new confidence in the wall nucleation source term, the interfacial area concentration in subcooled boiling can be accurately predicted. Due to its strong dependence in the modeling of active nucleation site density, bubble departure frequency, and departure diameter, the calculation is shown to be very sensitive to wall temperature.

Enhanced pool-boiling heat transfer on laser-made hydrophobic/superhydrophilic polydimethylsiloxane-silica patterned surfaces

This study presents the application of hydrophobic polydimethylsiloxane-silica coating used for the development of biphilic surfaces that are designed to enhance the heat transfer during boiling. Surface analyses showed that this coating exhibits a high hydrophobicity due to its hierarchical structure and the use of hydrophobic polymer. An appropriate thermal treatment leads to the oxidation of the methyl groups and a formation of silicon oxide and silicon carbide that result in a wettability transition from hydrophobic to superhydrophilic. On this basis, we manufactured hydrophobic/superhydrophilic patterns on stainless-steel foils using a pulsed Nd:YAG laser. The uniform, superhydrophilic surface exhibited a 350% larger critical heat flux (CHF) than bare stainless-steel foil. High-speed IR thermography revealed that the increased wettability reduced the bubble contact diameter, allowed a higher density of active nucleation sites, and delayed the dry-out. The biphilic surfaces with differently sized hydrophobic spots exhibited the highest heat transfer coefficients, with an up to 200% higher CHF compared to the bare stainless steel. Smaller hydrophobic spots reduced the bubble diameter and increased the nucleation frequency. However, surfaces with larger hydrophobic regions promoted boiling incipience and exhibited higher heat transfer coefficients at low heat fluxes. These results suggest that the optimal biphilic pattern could only be determined for a particular operating point. Our data provide a new insight into the complex phenomena of nucleate pool boiling on chemically and mechanically heterogeneous surfaces.

Saturation boiling of PF-5060 on rough Cu surfaces: Bubbles transient growth, departure diameter and detachment frequency

Investigated is the transient growth of discrete bubbles in saturation nucleate boiling of PF-5060 liquid on rough Cu surfaces at an applied heat flux of ∼0.5W/cm2. The uniformly heated 10×10mm surfaces have an average roughness, Ra=0.039μm (smooth Cu), and 0.21–1.79μm. The transient growth rate, departure diameter and detachment frequency of the discrete vapor bubbles are determined from images captured at 210fps using high-speed video camera. On smooth Cu, the bubble departure diameter and detachment frequency are 655±40μm and 31±4Hz. On the other Cu surfaces (Ra=0.21–1.79μm), the measured values of 438±17μm and 38±3Hz, respectively, are independent of surface roughness. Results, in conjunction with the experimental nucleate boiling curves, estimate the surface average density of active nucleation sites as a function of wall superheat. For smooth Cu, the density of active sites ranges from 100 to 2000cm−2, compared to 650–10,000cm−2 for the rough Cu surfaces. For all surfaces, the active nucleation sites density increases with increasing the wall superheat and/or surface roughness.

Electrochemical studies of sol-enhanced Zn–Ni–Al2O3 composite and Zn–Ni alloy coatings

Zn–Ni–Al2O3 nano-composite coatings were electrodeposited on mild steel using a novel sol enhanced electroplating method. The effect of alumina sol on the electrodeposition process, and coating properties was investigated using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The results indicated that the electro-crystallization processes of Zn–Ni and Zn–Ni–Al2O3 were governed by a three-dimensional nucleation process controlled by diffusion. Evaluation of nucleation mode in the presence and absence of alumina sol showed that the progressive nucleation was predominant for the Zn–Ni alloy deposit. However, for the Zn–Ni composite coating, the nucleation mode was closer to instantaneous nucleation. Nucleation parameters such as density of active nucleation sites and nucleation rate were increased in the presence of alumina nano-particles in the bath. Zn–Ni–Al2O3 nano composite coatings produced more uniform and compact deposits, with fine grained microstructure when compared to Zn–Ni coatings. XRD results showed that the phase structure of both alloy and composite coatings was single Ni5Zn21-γ phase, and that the incorporation of alumina in the Zn–Ni coating refined the crystal grain size.

Computational fluid dynamics simulation of subcooled flow boiling in vertical rectangular 2-mm narrow channel

This work proposes and discusses a new wall boiling model for a 2-mm narrow rectangular channel with a copper heating surface at atmospheric pressure. The model includes a wall heat flux partitioning model and several sub-models that compute active nucleation site density, bubble departure diameter, and bubble departure frequency. This wall boiling model was developed based on our previous experimental work on narrow channel flow in order to account for the unique thermal-hydraulics that occur under these specific conditions. The model was implemented in CFX code to simulate the subcooled flow boiling in a narrow channel with a two-fluid model; we also considered the onset of nucleate boiling. The new model has been validated by the experimental data. Compared to the default model that had been developed in CFX code, the new model provides better predictions of the distribution of wall temperature in the 2-mm narrow channel.

Heterogeneous wetting surfaces with graphitic petal-decorated carbon nanotubes for enhanced flow boiling

Two-phase cooling is widely employed in temperature-sensitive devices for high heat flux dissipation owing to the benefits of latent heat exchange. The objective of this work is to elucidate the subcooled flow boiling characteristics of heterogeneous wetting surfaces using water as the working fluid. Heterogeneous wetting surfaces with alternating parallel stripes of superhydrophobic (SHo) and superhydrophilic (SHi) regions are fabricated with graphitic petal-decorated carbon nanotube (GPCNT) coatings. Graphitic petals are a carbon nanostructure comprising of stacks of graphene layers standing vertically on a substrate. GPCNTs are synthesized on bulk copper substrates by a two-step microwave plasma enhanced chemical vapor deposition technique. A combination of Teflon coating, shadow mask, and oxygen plasma treatment is utilized to create composite wetting surfaces with differing superhydrophilic area fractions and SHo to SHi channel width ratios. Boiling experiments reveal that heterogeneous wetting surfaces with higher superhydrophilic area fraction (0.66 and 0.85) exhibit significant reduction in surface superheat and higher heat transfer coefficients throughout the entire boiling regime as opposed to uniform SHo and SHi boiling surfaces. Flow visualization reveals enhanced active nucleation site density and preferential bubble nucleation in the superhydrophobic channels of composite wetting surfaces. Isolated near-spherical vapor morphologies on surfaces with higher superhydrophilic area fraction promote thin film evaporation and enhanced bubble ebullition cycles, thereby leading to improved two-phase thermal performance.

Prediction of bubble departure in forced convection boiling: A mechanistic model

In the context of computational fluid dynamic simulations of boiling flows using time-averaged Eulerian multi-phase approaches, the many sub-models required to describe such a complex phenomena are of particular importance. Of interest here, wall boiling requires calculation of the contribution of evaporation to global heat transfer, which in turn relies on determination of the active nucleation site density, bubble departure diameter and frequency of bubble departure. In this paper, an improved mechanistic model for the bubble departure diameter during flow boiling is developed. The model is based on the balance of forces acting on a bubble at a single nucleation site, with a new equation governing bubble growth proposed. The formulation accounts for evaporation of the micro-layer under the bubble, heat transfer from superheated liquid around the bubble surface, and condensation on the bubble cap due to the presence of sub-cooled liquid. Validation of the growth equation is provided through comparison against experiments in both pool boiling and flow boiling conditions. Introduction of condensation on the bubble cap allows reproduction of the growth of the bubble for different sub-cooling temperatures of the surrounding liquid. In addition, a sensitivity study guarantees dependency of the bubble departure diameter on relevant physical quantities such as mass flow rate, heat flux, liquid sub-cooling and pressure, with any inclination of the channel walls correctly accounted for. Predictions of bubble departure diameter and bubble lift-off are validated against three different databases on sub-cooled flow boiling with water and an additional database on saturated boiling with refrigerant R113. The whole data set guarantees validation is performed over a range of parameters and operating conditions as broad as possible. Satisfactory predictive accuracy is obtained in all conditions. The present formulation provides an appropriate starting point for prediction of the behaviour of vapour bubbles under more general conditions which include lift-off after sliding, the frequency of bubble departure, bubble merging and bubble shrinking and collapse due to condensation.

Experimental study on the saturated pool boiling heat transfer on nano-scale modification surface

Plain nickel foil surface, three kinds of plain nickel-based chemical treatment surfaces and four kinds of nickel-based electrochemical treatment surfaces with nano-cone array structure were used in the pool boiling experiment. The quantitative effects and the effect mechanism of the surface characteristic parameters, including nano-scale roughness (Ra), solid–liquid contact angle (CA) and effective heat transfer area ratio (r: the ratio of the actual heat transfer area and its projected area) on the heat transfer coefficient (HTC) on the modification plain surfaces and nano-cone array surfaces were studied and summarized. By improving the previous proposed the HTC prediction correlation for the common plain surfaces (without considering the effect of r), a new uniform correlation is proposed and it can well predict the HTC of the heated surface with simple convex structure (with considering the effect of r). In addition, the bubble dynamic characteristics including the bubble departure diameter, the bubble departure frequency, the active bubble nucleation site density and the combined bubble were investigated by visualization research. These results proposed some mechanism support for the new predicting correlation.

Experimental Investigation of Subcooled Vertical Upward Flow Boiling in a Narrow Rectangular Channel

Accurate models for the onset of nucleate boiling, density of active nucleation sites (Na), bubble departure size (Dd), and departure frequency (fd) are essential to the success of computational fluid dynamics analysis of two-phase thermal-hydraulics involving subcooled flow boiling in nuclear reactor systems. This work presents an experimental study of subcooled flow boiling in a vertical upward narrow rectangular channel that mimics the flow passage in the plate fuel assembly of boiling water reactors. The experiments are conducted over a range of mass flux (G = 122–657 kg/m2s), inlet subcooling (ΔTsub = 4.7–33.3˚C), and heat flux (q″ = 1.7–28.9 W/cm2). Based on the experimental data, empirical correlations are developed for the prediction of onset of nucleate boiling, Na, Dd, and fd for given flow conditions. These correlations are valid in the nucleate boiling regime when the wall superheat is less than 12°C and can be incorporated in the computational fluid dynamics codes to enable more precise simulation of subcooled flow boiling heat transfer and two-phase flow in nuclear energy applications.

Modeling subcooled flow boiling in vertical channels at low pressures – Part 1: Assessment of empirical correlations

Modeling subcooled flow boiling in vertical channels at low pressures requires not only the consideration of the dynamic behaviors of two-phase flow and bubbles undergoing coalescence, breakup and condensation in the bulk subcooled liquid but also the characterization of the single-phase and local boiling heat transfer phenomena in the near-wall region. The focus of this paper is the assessment of the heat flux partitioning model in handling the latter physics of subcooled flow boiling. In order to achieve closure to the model, the current prevailing approach has always been the utilization of empirical correlations particularly for the active nucleation site density, bubble departure diameter and bubble departure frequency. A comprehensive survey of existing empirical correlations is presented in the first part of this paper to assess the performance of these empirical models. Selected combinations of empirical correlations are compared and validated against axial and local radial experiments encompassing a wide range of different mass and wall heat fluxes and inlet subcooling temperatures for subcooled flow boiling at low pressures. Based on the comparisons made against the axial and local distributions of void fraction and bubble Sauter diameter, not one single combination of empirical correlations has shown the propensity of providing satisfactory predictions covering the entire axial and local conditions. For the modeling of subcooled flow boiling at low pressures to become an effective predictive tool, it must therefore be complemented with further consideration of first principal models of the underlying physical phenomena.

Modeling subcooled flow boiling in vertical channels at low pressures – Part 2: Evaluation of mechanistic approach

In this paper, the improved heat flux partitioning model based on first principal models of the underlying physics in determining the active nucleation site density, bubble size and bubble frequency is evaluated for subcooled flow boiling in vertical heated channels at low pressures. Salient features of this model include the fractal approach in determining the active nucleation site density, the force balance model in determining bubble diameters at sliding and lift-off as well as bubble frequency through the consideration of growth and waiting times and the additional heat flux at the heated wall due to surface quenching of sliding bubbles. Model predictions, covering a wide range of different mass and heat fluxes and inlet subcooling temperatures, are compared against local and axial measurements. In comparison to the measured data and selected combinations of empirical correlations such as described in Part 1, the current model predictions clearly demonstrate the effect of the subcooling effect on the activation of nucleation sites at the heated wall. The selected combinations of empirical correlations consistently under-estimate the wall superheat temperature while the current model yields predictions that agree very well with experimentally measured temperatures. Bubble sliding along the heat wall and its influence on heat partitioning and surface quenching heat flux has been ascertained to play an important role during the subcooled flow boiling process.

INFANT MOTOR DEVELOPMENT PREDICTS DECLINE IN EXECUTIVE FUNCTION IN ADULT SCHIZOPHRENIA IN THE NORTHERN FINLAND 1966 BIRTH COHORT STUDY

In this paper, the improved heat flux partitioning model based on first principal models of the underlying physics in determining the active nucleation site density, bubble size and bubble frequency is evaluated for subcooled flow boiling in vertical heated channels at low pressures. Salient features of this model include the fractal approach in determining the active nucleation site density, the force balance model in determining bubble diameters at sliding and lift-off as well as bubble frequency through the consideration of growth and waiting times and the additional heat flux at the heated wall due to surface quenching of sliding bubbles. Model predictions, covering a wide range of different mass and heat fluxes and inlet subcooling temperatures, are compared against local and axial measurements. In comparison to the measured data and selected combinations of empirical correlations such as described in Part 1, the current model predictions clearly demonstrate the effect of the subcooling effect on the activation of nucleation sites at the heated wall. The selected combinations of empirical correlations consistently under-estimate the wall superheat temperature while the current model yields predictions that agree very well with experimentally measured temperatures. Bubble sliding along the heat wall and its influence on heat partitioning and surface quenching heat flux has been ascertained to play an important role during the subcooled flow boiling process.

An experimental investigation of enhanced pool boiling heat transfer from surfaces with micro/nano-structures

Experiments on subcooled and saturated pool boiling of ethanol are conducted on horizontal heated surfaces with micro- and nano-sized structures. The effects of structure size on bubble nucleation and departure characteristics as well as the heat transfer coefficient are discussed. It is found that microstructures can enhance bubble nucleation by significantly increasing the active nucleation site density at low heat fluxes, thus reducing the wall superheat and enhancing heat flux; while nano-structures can accelerate bubble departure by decreasing bubble departure diameter and increasing departure frequency. On the other hand, nano-structures will delay bubble mergence and prevent the vapor film from spreading at high heat fluxes during boiling crisis.

Wettability of Horizontal Stainless Steel Tube by Nanofluid Droplets

Wetting is the interaction of a liquid with a solid and is found in numerous natural processes as well as being of extreme importance to many industrial and engineering processes, such as absorption, distillation, flotation, gas scrubbing, condensation. Wetting is of key importance in boiling process, because contact angle, that is the measure of the degree of wetting, affects density of active nucleation sites, bubble departure diameter, and finally critical heat flux [. If the liquid will completely spread out on the solid surface and the contact angle will be close to zero degrees, the surface is called superhydrophilic. Less hydrophilic surfaces will have contact angle below 90o. If the surface is hydrophobic, the contact angle will be larger than 90o. Surfaces with contact angle larger than 150o are called superhydrophobic Fig. 1.