Bubble Lift-off Research Articles

Numerical study on the growth characteristics of bubble in a petal-shaped fuel rod channel

Based on the VOF multiphase flow model and bubble growth model, a numerical analysis model of bubble growth of subcooled flow boiling in a petal-shaped fuel rod channel was established. The growth characteristics of a single bubble and the influence of thermal parameters were analyzed. Firstly, the growth rate, shape, and trajectory of the bubble were investigated under typical working conditions. Secondly, the influences of inlet subcooling, inlet velocity, and wall superheat on the bubble growth characteristics were clarified. Finally, the existing bubble growth rate prediction correlation was evaluated based on the results of numerical calculations. The results show that the bubble growth rate is fast at the early stage of bubble growth when the evaporation heat flow from the microlayer plays a major role. With the disappearance of microlayer evaporation, the bubble diameter increases slowly. The bubble diameter decreases continuously when the evaporation heat flow in the overheating layer is less than the condensation heat flow after the bubble lift-off. Compared to the rectangular channel, the bubble diameter in a petal-shaped fuel rod channel has a larger equivalent diameter and more obvious lateral movement. With the increase of inlet flow velocity and inlet subcooling, the total evaporation heat flow of the bubble decreases, while the condensation heat flow increases, which results in the decrease of bubble diameter. The influence of increasing wall superheat on bubble diameter is opposite to that of increasing inlet flow velocity and inlet subcooling. Based on the numerical results, a new prediction correlation for bubble growth rate in a petal-shaped fuel rod channel was developed, whose prediction error is within ±20 %. The related research results help to further reveal the subcooled boiling heat transfer mechanism in a petal-shaped fuel rod channel.

New correlations for bubble departure and liftoff diameters in vertical subcooled boiling flow under low-pressure conditions

The bubble departure and liftoff diameters are crucial parameters affecting wall boiling heat transfer in subcooled boiling flow. This study aimed to develop new correlations for these diameters of a growing bubble on the heated wall in vertical subcooled boiling flow. By analyzing a force balance model, the major non-dimensional parameters were derived to express the bubble diameters. Especially, it revealed that, in addition to the Reynolds number, the Capillary number should be considered to better account for the effect of mass flux on the bubble departure diameter. Finally, the correlations of the two bubble diameters were developed by utilizing the available experimental data. The mean absolute percentage errors of the new correlations were 18.57% and 22.16%, respectively for the bubble departure and liftoff diameters when compared with experimental data. The applicable ranges of correlation of bubble departure diameter cover the superheated Jacob number of 0.0056−0.035, subcooled Jacob number of 0.0081−0.086, liquid Prandtl number of 1.006−1.76, product of Capillary and Reynolds numbers of 181.6−575.23, and density ratio of 0.0006−0.05. For the bubble liftoff diameter, the applicable ranges are 0.0014 – –0.045 for the superheated Jacob number, 0.0024–0.111 for the subcooled Jacob number, 1.29–1.75 for the liquid Prandtl number, 2883.4–99,716.21 for the Reynolds number, and 0.000615–0.00175 for the density ratio.

Experiments to understand bubble base growth mechanism(s) on hydrophobic surfaces under the influence of bulk flow inertia during nucleate boiling regime

Wetting characteristics of substrate surface is one of the parameters that strongly influence the phenomena of boiling heat transfer. In this direction, importance of hydrophobic surfaces has been emphasized in the literature due to their potential to offer high boiling heat transfer rates at low heat fluxes. For such surfaces, it has been shown that microlayer ceases to exist with contact line evaporation being the dominant bubble base growth mechanism during nucleate pool boiling regime. The present work investigates the plausible mechanism(s) of bubble growth on hydrophobic surfaces under nucleate flow boiling conditions. Possibility of formation of microlayer even on low wettability surfaces under the influence of bulk flow inertia has been examined. Thin-film interferometry, in conjunction with high-speed videography, has been configured to record the microlayer dynamics and the bubble growth process in tandem. For a direct comparison, experiments under identical flow and bulk subcooled conditions have also been performed on a hydrophilic substrate, which is known to promote microlayer formation during the bubble growth process. Observations made through thin-film interferograms confirmed the formation of microlayer on hydrophobic surfaces. Experiments further revealed that the rate of growth of dry patch is higher on hydrophobic surfaces than that in the case of hydrophilic substrates. Vapor bubbles have been observed to be pinned on the hydrophobic surface whereas the phenomenon of bubble lift-off has been distinctly observed on hydrophilic substrate surface.

DETERMINATION OF WALL HEAT FLUX BASED ON BUBBLE DEPARTURE AND LIFT-OFF DIAMETERS FOR VARYING PRESSURE AND FLOW VELOCITY CONDITIONS

In this study, the bubble departure and lift-off (BDL) boiling model has been improved by implementing the subcooling suppression factor and flow-related parameters, with the aim of developing an accurate heat transfer model for various engineering applications. The BDL model has been exploited using a MATLAB program to determine the variation of wall heat flux that controls the boiling curve. Three different parameters, namely, bulk flow velocity, operating pressure, and bulk flow temperature have been considered in this study. Experimental conditions were simulated as per Chen's BDL model algorithm, and the resulting wall heat flux was compared with the measured values. Additionally, different boiling curves for bubble departure and lift diameters were generated and their effect on varying operating conditions was studied. The resultant boiling curves were compared with the experimental data by means of mean absolute error (MAE) and regression coefficient (<i>R</i><sup>2</sup>) values. In general, with a reasonable margin of error, i.e., MAE = 3.35%, and <i>R</i><sup>2</sup> = 0.99, the algorithm performed well in comparison with the experimental data. The bubble departure and lift diameters were also correlated with the experiment and the predictions were satisfactorily close.

Development and assessment of five-component wall boiling heat flux partitioning model

A mechanistic framework of wall boiling model with the total heat flux partitioned into five components, i.e. solid quenching, sliding-induced transient conduction, lift-off induced transient conduction, evaporation and single-phase convection, is developed and implemented into the unsteady two-fluid solver in the OpenFOAM. As important sub-models, bubble departure diameter, lift-off diameter and sliding length are calculated based on force balance analysis. A two-phase wall function is employed to account for the effect of nucleated bubbles on the near-wall turbulence. The developed model framework is assessed with the DEBORA experiment with the R12 as work fluid at the pressure of 1.46 MPa and 2.62MPa. The assessment indicates that the two-phase wall function improves prediction of liquid and vapor velocity profile and that the void fraction, the liquid and wall temperature can be well predicted. Under the condition of lower pressure and higher surface heat flux, bubble grows faster, which increases bubble departure and lift-off diameter. Higher liquid mass flux increases quasi-steady drag force and shear lift force, which consequently reduces the bubble departure and lift-off diameter. Due to large nucleation site spacing, relatively large sliding length appears in the lower-pressure cases. Nucleation awaiting time dominates the bubble departure frequency which increases steeply when the near-wall liquid approaches saturated. Comparing with the RPI model, the current model predicts relatively small evaporation heat flux. The predicted solid quenching and transient conduction heat flux induced by bubble lift-off are less significant in all the cases. The calibrated algebraic bubble size model can be adequate in the simulation of subcooled boiling at high pressure condition.

The Modeling of Bubble Lift-Off Diameter in Vertical Subcooled Boiling Flow

Bubble lift-off diameter is characterized as the size of a bubble rising from a wall, which is vital in the boundary condition of heat transfer model and interfacial area transport equation. In this paper, mechanistic force analysis was conducted to explore a predictive model for bubble lift-off diameter in a vertical channel of subcooled boiling flow. Specifically, the component of growth force normal to the wall and the shear lift force lead to the lift-off of a bubble on the vertical surface. Through force analysis, we found that bubble lift-off diameter is arranged to be related to wall superheat, latent heat, liquid velocity, fluid properties, bulk liquid subcooling, etc. To account for the contribution of the influencing factors, the dimensionless bubble lift-off diameter was correlated with dimensionless parameters, including the Prandtl number, the Reynolds number, the Jacob number, and dimensionless subcooling. The proposed correlation was assessed according to experimental data and the predictions showed good agreement with the data.

Bubble Lift-Off Diameter and Frequency in Ferrofluid Subcooled Flow Boiling

Due to the importance of the boiling phenomena and the need of studying the bubble nucleation in different conditions, a series of experiments have been performed in the present study to investigate a magnetic nanofluid subcooled flow boiling phenomenon inside an annulus. The test section is made of outer glass and inner stainless-steel tubes. The effect of three important parameters, including mass flux, inlet subcooling temperature, and pressure on the bubble liftoff diameter and frequency in the absence and presence of a magnetic field caused by quadrupole magnets, have been considered. The experiments were visualized by using a superzoom camera to study the bubble behaviors on the heating surface. The obtained results show that increasing the mass flux, subcooling temperature, and pressure contributes to decreasing bubble liftoff diameter both in the absence and presence of the magnetic field during the flow boiling. The bubble liftoff diameter decreases by applying the magnetic field, and the bubbles lift from the surface faster. Moreover, the bubble liftoff frequency showed stochastic behavior such as dependence on heat transfer surface, bubble nucleation site properties, and fluid velocity. The obtained results show that the bubble liftoff frequency decrease with increasing the fluid mass flux.

Mechanism of the Initial States of a Bubble Formation and Departure from a Heated Surface in a Subcooled Flow

Growth of a nonisothermal bubble on a heated horizontal surface in a subcooled flow is studied to determine the significance of different heat transfer mechanisms on the bubble growth. The heat transfer mechanisms that are considered are (1) microlayer evaporation, (2) transient thermal boundary layer conduction, and (3) bubble surface evaporation and condensation. The results indicate that a different heat transfer mechanism dominates the bubble growth at different stages of the bubble growth. And, the temperature gradient inside the bubble decreases after bubble liftoff in high Reynolds numbers. The results also show an oscillatory heat flux during the initial stages of the bubble growth.

On the Development of Correlations for Bubble Liftoff Parameters During Subcooled Nucleate Flow Boiling Using Nonintrusive Dynamic Measurements

Abstract Accurate prediction of bubble dynamic parameters is essential to improve boiling heat transfer models. Considering the complexities and challenges associated with performing a large number of boiling experiments, researchers have realized the importance of experimental correlations for predicting bubble dynamic parameters. In this direction, we report an experimental work concerned with the development of correlations for various bubble liftoff parameters during nucleate flow boiling regime. As a definite advancement, the experimental measurements have been performed in a purely nonintrusive manner, thereby minimizing the errors arising due to the interaction of any external probe with the process under study. The measurement approach makes use of a gradient-based imaging technique to simultaneously map the bubbling features and thermal field around a single vapor bubble generated under subcooled flow boiling conditions. Experiments have been performed in a rectangular channel for a wide range of heat fluxes (q" = 20–50 kW/m2), subcooling level (ΔTsub = 2–9 K), and Reynolds numbers (Re = 600–6000) with water as the working fluid. Results show a strong dependence of bubble liftoff parameters on Reynolds number, subcooling level, and applied heat flux. Based on the experimental measurements, empirical correlations have been developed for various bubble liftoff parameters as a function of Jacob number and Reynolds number. Predictions made through the developed correlations are found to be in good agreement with the measured values as well as with the values reported in the available literature. Of all the bubble parameters, maximum deviation between the predicted and measured values (≈23%) was found to be in bubble release frequency.

Whole field measurements to quantify the thermal impact of single vapor bubble under nucleate flow boiling regime

The work reports on simultaneous measurement of bubble dynamics and thermal field to understand the local effects of bubble motion on thermal boundary layer behavior and heat transfer characteristics under subcooled flow boiling conditions. Single vapor bubble was generated using a specially designed ITO glass heater to avoid the phenomenological complexities and to obtain reproducible data. Flow boiling experiments were carried out in a vertically oriented rectangular channel with water for varying Reynolds numbers (Re = 600–4800), keeping constant subcooling level and power input. The thermal field around the vapor bubble along with the dynamics has been visualized using a non-intrusive optical technique; rainbow schlieren deflectometry, which was configured with a high-speed color camera to acquire high spatial and temporal resolution. The recorded experimental data have been analyzed qualitatively as well as quantitatively and the obtained results have been presented in the form of whole field temperature distribution, the dynamics of thermal boundary layer, local heat transfer coefficients etc. It was observed that the effect of bubble formation on the wall temperature variation, thermal boundary layer and heat transfer coefficient was relatively higher during the growth period (until the bubble lift-off) compared to the waiting period after the lift-off of the bubble. The experimental results showed that the bubble-induced heat transfer significantly enhance the overall heat transfer rate from the heated surface while the degree of enhancement was observed to be reducing with increasing Reynolds number.

An improved mechanistic model for predicting bubble characteristic size in subcooled flow boiling

Bubble nucleation and its life cycle in subcooled flow boiling follows a sequence of processes known as bubble ebullition cycle, where the nucleating vapor bubble grows, followed by sliding and departure from the heated surface before the cycle is repeated. In the present work an analytical model has been developed to predict bubble departure and lift-off diameter in subcooled flow boiling. Present model is based on mechanistic force balance approach and it considers few unresolved issues of existing analytical models. Wall contact diameter and relative velocity of sliding bubble with respect to coolant flow have been identified as key parameters which have been either ignored or used as constants in existing analytical models. In the force balance approach dominant forces responsible for bubble departure have been considered while others are ignored. Relative velocity of the vapor bubble is premeditated during bubble lift off predictions and its value is used for optimizing the wall contact diameter while predicting the departure diameter. Present model has been validated with reported experimental data of subcooled flow boiling for a wide range of operating conditions. Compared to available analytical models, present model further reduces the prediction error in estimating bubble characterisctic size.

Comparison between modelled and measured heat transfer rates during the departure of a steam bubble from a solid surface

This paper presents analysis of the heat transfer attendant upon the departure of a single steam bubble during pool boiling of water at atmospheric pressure. The flow of heat from a solid surface to liquid water during and immediately after bubble lift-off has been extracted from micro-scale measurements of the spatial and temporal variation of the temperature at the solid surface beneath the bubble. The numerical procedure used to extract the heat flux from the temperature variations at the solid surface has been assessed and verified, and applied to investigate the heat transfer during the bubble departure phase, and after the eventual bubble lift-off. Results confirm that fluid motion activated by a departing bubble is the cause of heat transfer enhancement. The phenomenon can be characterised as the process of rewetting, by an advancing liquid front, of a dry portion of wall area at the base of the bubble. The portion of wall area that is affected by the observed heat transfer augmentation mechanism has been found to be that of a circle of diameter roughly equal to half the bubble departure diameter. The current measurements enabled validation of interface-capturing numerical simulation of the hydrodynamics and heat transfer of single bubble formation and departure from a surface, including conjugate heat transfer in the solid substrate. From simulation results, the spatial and temporal variation of the heat flux at the solid surface beneath the bubble has been computed and monitored during bubble departure and after the eventual bubble lift-off. Heat transfer rates at bubble departure extracted from simulation have been found in good agreement with measurements. The simulation correctly captured experimental trends and was found to give an accurate estimate of the magnitude of the flows of heat to the liquid due to the bringing of cold fluid in the vicinity of the wall caused by the bubble departure process.

Schlieren-based simultaneous mapping of bubble dynamics and temperature gradients in nucleate flow boiling regime: Effect of flow rates and degree of subcooling

Bubble dynamics and the local thermal field associated with single nucleating bubble under subcooled flow boiling conditions have been investigated in a one side-heated rectangular channel with water as the working fluid. A real-time non-intrusive technique, namely rainbow schlieren deflectometry has been employed to simultaneously map the full bubble cycle and temperature gradients field near the heated surface as well as around the bubble. Influence of varying subcooled conditions and Reynolds number on bubble dynamic parameters, various sub-processes associated with subcooled flow boiling phenomenon and the resultant temperature gradients have been studied. Schlieren images for each data set have first been qualitatively interpreted to develop an understanding of sub-processes such as bubble inception, bubble departure/sliding, bubble lift-off, bubble reattachment and their effects on the thermal and flow fields. Experimental observations recorded in the form of color schlieren images clearly highlighted the possible influence of process parameters on a range of phenomena that include enveloping of the superheated liquid layer during bubble inception, micro-layer formation and scavenging of the superheated liquid layer during bubble lift-off. The bubble dynamic parameters observed under different experimental conditions have been quantified in terms of equivalent bubble diameter, aspect ratio and contact angle and bubble trajectory in the field of view. The equivalent bubble diameter is found to be decreasing with increasing Reynolds number and degree of subcooling. The results show that for all the experimental conditions, the bubble grew in a flattened shape, gradually became more spherical during sliding and elongates after lift-off. Regardless of the experimental conditions, the occurrence of bubble lift-off was found in the aspect ratio range of 0.75–0.85. Local thermal field observed over the full bubble cycle has also been captured and presented in the form of temperature gradients field in quantitative terms.

A study on the lift-off diameter of bubbles generated on horizontal tube

In this study, experiments and a theoretical analysis on the lift-off diameter of bubbles generated on a horizontal tube were conducted. A force balance analysis in the direction normal to the heated surface at the moment of the bubble lift-off was performed to develop the model. According to the developed model, the bubble lift-off diameter strongly depends on the azimuthal position of the horizontal tube, the relative velocity between a bubble and the surrounding liquid, and the properties of the bubble and liquid. To validate the prediction performance of the proposed model, the dynamics of the bubble growth and sliding process was visualized using a high-speed digital video camera. The proposed model agrees well with the experimental data within an averaged relative deviation of 19.6%.

Bubble dynamics of water-ethanol mixture during subcooled flow boiling in a conventional channel

In this paper, bubble dynamics in subcooled flow boiling of water-ethanol mixture in horizontal rectangular channels is investigated through visualization. The subcooled flow boiling heat transfer coefficient of water ethanol mixtures are determined for various heat flux, mass flux and ethanol volume fraction. A new empirical correlation is proposed to predict the heat transfer coefficient of pure water based on the parameters like heat flux, bubble departure diameter, waiting period and the growth period. Two types of bubble behaviours are observed after nucleation: (i) Sliding for a distance along the bottom wall of the channel surface before lift-off and (ii) Lift-off from the bottom wall of the channel surface without sliding. Force balance analysis is carried out to determine the reason for bubble lift-off and bubble sliding. The bubble lift-off without sliding is observed at higher ethanol volume fraction, lower heat flux and higher channel inlet temperature. The bubble sliding and lift-off are observed at higher heat flux and lower channel inlet temperature for water and water-ethanol mixture of 25% ethanol volume fraction. However, the effect of mass flux on the bubbles sliding or bubble lift-off is not significant.

サブクールプール沸騰における鉛直伝熱面からの気泡離脱

サブクールプール沸騰における鉛直伝熱面からの気泡離脱

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.

Bubble dynamics at boiling incipience in subcooled upward flow boiling

Bubble dynamics in water subcooled flow boiling was investigated through visualization using a high-speed camera. The test section was a vertical rectangular channel, and a copper surface of low contact angle was used as a heated surface. Main experimental parameters were the pressure, mass flux and liquid subcooling. Although all the experiments were conducted under low void fraction conditions close to the onset of nucleate boiling, no bubbles stayed at the nucleation sites at which they were formed. Depending on the experimental conditions, the following two types of bubble behavior were observed after nucleation: (1) lift-off from the heated surface followed by collapsing rapidly in subcooled bulk liquid due to condensation, and (2) sliding along the vertical heated surface for a long distance. Since the bubble lift-off was observed only when the wall superheat was high, the boundary between the lift-off and the sliding could be determined in terms of the Jakob number. Based on the present experimental results, discussion was made for the possible mechanisms governing the bubble dynamics.

ICONE19-43556 Bubble behavior in subcooled flow boiling in a vertical rectangular channel

In this study bubble behavior in subcooled flow boiling in an upward rectangular channel was discussed. Filtrated and deionized tap water was used as a working fluid. Heated surface is produced by cupper with high wetability or low contact angle (18°). Subcooled flow boiling is beginning with onset of nucleate boiling (ONB) which surface temperature is higher than saturation temperature while liquid temperature is less than saturation temperature. At various pressure, mass fluxes and inlet temperatures ONB heat flux is detected within observation incipient nucleate bubbles through high speed video camera. Comparing with available ONB correlations, it is revealed that surface wettability is a significant parameter. Among ΔT_<w,ONB> correlation only Basu et al. (2002)'s correlation has a good consistency with experimental data, and other correlations are most underpredicted. Nucleation rate of bubbles after ONB remain in very small amount until a specific condition nominated onset of significant void (OSV) or incipient point of net vapor generation (IPNVG) that void fraction drastically increased. Levy (1967) was the first to postulate bubble departure mechanism for OSV and many researchers believed this assumption and analytically and empirically proposed OSV correlation. However, in all the experimental conditions even at incipient bubble nucleation, bubbles are observed depart or lift-off from activated nucleation site on the heated surface. Bubble nucleation behavior as whether bubble Lift-off from hot surface or sliding along it are investigated. It is performed for low void fraction condition. It is concluded that lifting-off condition occurred when bubble size and subcooling temperature is high and sliding phenomena in the opposite condition happened. Bubble size dramatically drop down after atmospheric pressure. This categorization is useful for developing new OSV mechanism regarded to actual situation.

Photographic Study on Bubble Motion in Subcooled Pool Boiling

Using a static contact angle of a vertical heated wall as a main experimental parameter, a photographic study was carried out to elucidate the mechanisms to determine the vapor bubble dynamics during subcooled pool boiling. The test fluid was distilled water and the experiments were performed under the atmospheric pressure; liquid subcooling was set to around 5 K. To enable clear observation of bubble behavior with a high speed camera, the experiments were conducted in an isolated bubble regime near the onset of nucleate boiling. Distinctly different bubble behaviors were observed on hydrophobic and hydrophilic surfaces: the bubbles were adhered to the surface for a long period of time when the contact angle was large while lifted-off the surface within a short period of time after the nucleation when the contact angle was small. Since buoyancy does not remove the bubble from the vertical surface, the mechanisms of bubble lift-off were investigated. It was indicated that the change in bubble shape induced by the surface tension force, unsteady growth force, and local liquid flow induced by heterogeneous condensation around the bubble are considered to promote the bubble lift-off while the surface tension force acting on the three-phase common line prevented the lift-off. Effects of the surface wettability on the lift-off bubble diameter, the elapsed time from the nucleation at the lift-off, and the condensation rate after the lift-off were also investigated.