Bubble Departure Diameter Research Articles

Single bubble dynamics during subcooled nucleate boiling on a vertical heater surface: An experimental analysis of the effects of surface characteristics

In the present experimental study we investigated the effects of surface characteristics, such as wettability and roughness, on nucleate boiling in de-ionized water at a vertical heater. In the experiments, bubbles were generated from an artificial nucleation site on a stainless steel heater surface. High-resolution optical imaging has been used to capture the bubble life cycle, that is, departure, sliding, and lift-off. We found, that the lower wettability leads to larger departure diameter, longer sliding and larger lift-off diameter of bubbles. Also surface roughness effects have been analyzed and it was found that bubble departure and lift-off diameters are smaller and departure period is longer for a smooth surface. Bubble sliding velocity was found faster for a rough surface compared to a smooth surface. It was also found that the roughness is very influential to bubble growth and departure, which can be explained by considering its interaction with the microlayer underneath the bubble. An “optimal roughness”, which accelerates the bubble growth, was found. The knowledge gained from this study shall be particularly useful to improve nucleate boiling models for numerical simulations.

An analytical model for predicting growth rate and departure diameter of a bubble in subcooled flow boiling

Bubble formation in flow boiling undergoes an ebullition cycle in which nucleation is followed by bubble growth, departure and a waiting period. Bubble growth rate plays an important role in bubble dynamics and also affects the whole ebullition cycle. It is difficult to make analytical calculation of bubble growth rate and departure diameter due to involvement of various forces and heat transfer processes on a nucleating bubble. In this paper, an analytical technique is proposed to calculate the growth rate and departure diameter of a nucleating bubble in subcooled flow boiling. Both force and energy balance approaches have been applied to calculate the growth and departure diameter of the bubble. Heat transfer contributions from evaporative microlayer beneath the bubble, superheated liquid surrounding the bubble and condensation from the bubble cap have been considered to calculate the bubble growth rate. In place of using a constant coefficient to represent the fraction of bubble in subcooled bulk liquid by existing analytical models, the present model correlates the coefficient with the operating parameters of flow boiling. An effort has also been made for accurate calculation of temperature distribution in heated surface and flowing liquid. Bubble growth rate and departure diameter have been calculated for different values of coolant mass flux, system pressure, applied heat flux, surface orientation angle and liquid subcooling. The heat transfer contribution from individual layer towards the growth rate of the bubble has been quantified and analyzed. The present analytical model has been validated with a wide range of experimental data and it produces better agreement with the experimental data compared to existing analytical models.

Detachment forces on Spherical bubble during formation

The present work focuses on the analytical determination of forces acting on a single spherical bubble during detachment due to pool boiling conditions in water and liquid nitrogen (LN2). Forces acting in X & Y directions have been evaluated and compared with that of bubble formation in LN2. Net vertical force existing with the LN2 bubbles after the detachment from the nucleation site has been found to be about one-third of the net force obtained by the vapour bubble in case of water as a working fluid. Also, in case of water, the net horizontal force is about 400% more than the detachment forces obtained for liquid nitrogen bubble. In addition to detaching forces, bubble departure diameter has also been evaluated using different models. It was found that the bubble diameter remains nearly equal for water and liquid nitrogen.

Bubble departure diameter at transient heat release

This paper presents results of the experimental study of the subcooled water boiling on the surface of the cylinder heater. The initial stages of formation of the developed nucleate boiling were investigated. The field temperature of heated liquid layers at the moment of bubble departure was described by numerical modelling at non-stationary condition. A high-speed digital video camera was used to capture the bubble dynamics. The experimental data on the bubble dynamics was compared with Prodanovic’s and Song’s models. A poor agreement between the predicted departure diameter and the experimental one was obtained for both of the models.

Numerical simulation of nucleate boiling in shallow liquid

This paper presents a numerical investigation on nucleate boiling in a condition of low liquid level, where the liquid height is comparable with the bubble departure diameter. A coupled volume-of-fluid and level-set method (VOSET) was used to deal with the moving liquid-vapor interface, and a microlayer model was adopted for the thin liquid film around the three-phase contact line. The heat flux obtained by numerical simulation was compared with those calculated by experimental correlations, and the outcomes matched. The numerical results provide a reasonable explanation for the augment of heat transfer coefficient of nucleate boiling as the liquid level decreases to a certain value.

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.

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.

Effect of extreme wetting scenarios on pool boiling conditions

This study focuses on the detailed description of the heat transfer and bubble dynamics processes occurring for the boiling of water on surfaces with extreme wetting regimes, namely hydrophilicity and superhydrophobicity. The wettability is changed by modifying the surface chemistry and without significant variations in the mean surface roughness. Under these conditions and for the range studied here the effect of the extreme wetting regimes was dominant, thus the influence of surface topography was not addressed. A particular trend is observed for the boiling curve obtained with the superhydrophobic surfaces, as the heat flux increases almost linearly with the superheat, although with a much lower slope than the hydrophilic surfaces. This occurs due to the formation of a large stable vapour film over the entire surface just at around 1K superheat, because of the almost immediate coalescence of the bubbles generated on the surface. This behaviour agrees with the so-called “quasi-Leidenfrost” regime recently reported in the literature and with a theoretical prediction of the heat flux that is presented in this study. Furthermore, a comprehensive analysis of bubble dynamics, useful for comparison with numerical simulations is given. Such analysis is based on the temporal evolution of the bubble diameter together with bubble contact angle and with the velocity of the contact line. The results suggest that the existing models and correlations can predict the trends of the bubble growth using a modified contact angle value, called the bubble contact angle (or its supplemental value), for the hydrophilic surfaces, even if they cannot accurately predict bubble sizes. Approximating the modified contact angle with the quasi-static contact angle, obtained during surface characterization is practical for a qualitative evaluation, but the results obtained here do not support for its use when estimating the bubble departure diameter. On superhydrophobic surfaces, the effect of the vapour film must be considered, since although this is not the starting point of the boiling process, it represents the actual working conditions when using this kind of surfaces.

A three-dimensional lattice Boltzmann model for numerical investigation of bubble growth in pool boiling

In this paper, a three-dimensional lattice Boltzmann model is proposed to simulate pool-boiling phenomena at high-density ratios. The present model is able to predict the temperature field inside the bubble. The three-dimensional multiphase model is validated against the analytical solution of evaporation d2law problem and Laplace's law. In addition, effects of different parameters including, Jacob number, gravitational acceleration (g) and surface tension (σ) on bubble departure diameter are presented for further validation. The bubble departure diameter is found to be proportional to g−0.354 and σ0.5, and has a linear relation with Jacob number. These results are more consistent with previous experimental correlations when compared with available lattice Boltzmann literature. Furthermore, the dynamic behavior of multiple bubble formation sites such as micro convection and vortex ring mechanism are presented to show the capability of presented model for capturing more complex physical phenomena. To sum up, the proposed three-dimensional lattice Boltzmann model is feasible and accurate for numerical simulations of pool boiling.

Boiling Heat Transfer Performance in a Spiraling Radial Inflow Microchannel Cold Plate

ABSTRACTThis study presents an experimental exploration of flow boiling heat transfer in a spiraling radial inflow microchannel heat sink. The effect of surface wettability, fluid subcooling, and mass fluxes are considered. The design of the heat sink provides an inward radial swirl flow between parallel, coaxial disks that form a microchannel of 300 microns. The channel is heated on one side, while the opposite side is essentially adiabatic to simulate a heat sink scenario for electronics cooling. To explore the effects of varying surface wetting, experiments were conducted with two different heated surfaces. One was a clean, machined copper surface and the other was a surface coated with zinc oxide nanostructures that are superhydrophilic. During boiling, increased wettability resulted in quicker rewetting and smaller bubble departure diameter, as indicated by reduced temperature oscillations during boiling, and achieving higher maximum heat flux without dryout. The highest heat transfer coefficients were seen in fully developed boiling with low subcooling levels as a result of heat transfer being dominated by nucleate boiling. The highest heat fluxes achieved were during partial subcooled flow boiling at 300 W/cm2 with an average surface temperature of 134° Celsius. Recommendations for electronics cooling applications are also discussed.

Dynamics of the wall thermal boundary layer at the initial stage of the nucleate boiling

This paper presents the results of the experimental study of the water nucleate boiling on the surface of the cylinder heater. We investigated the initial stage of formation of the thermal boundary layer with rise and departure of bubbles with the departure diameter of 50 μm. To build the field temperature at the moment prior to the start of boiling, we used a numerical model implemented in the COMSOL multiphysics software. The experimental data on bubbles dynamics was compared to the numerical calculation of the temperature, and the relationship between the temperature growth rate, the bubble departure diameter and the Jacob number was established. The outcome results demonstrated that the non-transient heat influences the departure diameter and we need to employ the modified Jacob number to build predictive models.

Nucleate boiling performance evaluation of cavities at mesoscale level

Nucleate boiling heat transfer (NBHT) from enhanced structures is an effective way to dissipate high heat flux. In the present study, a 3D multi-relaxation-time (MRT) phase-change lattice Boltzmann method in conjunction with conjugated heat transfer treatment is proposed and then applied to the study of cavities behaviours for nucleation on roughened surfaces for an entire ebullition cycle without introducing any artificial disturbance. The bubble departure diameter, departure frequency and total boiling heat transfer rate are also explored. It is demonstrated that the cavity shapes show significant influence on the features of NBHT. The total heat transfer rate increases with the cavity mouth and cavity base area while decreases with the increase in cavity bottom wall thickness. The cavity with low wetting can enhance the heat transfer and improve the bubble release frequency.

A CFD based model to predict film boiling heat transfer of cryogenic liquids

Following an accidental spill of cryogenic liquid (e.g., LNG) on a solid substrate (e.g., concrete), the vapor generation corresponds to different boiling regimes i.e., film boiling, transition boiling, and nucleate boiling. As film boiling phenomena dictate the vapor generation in the early stage of the spill, it is considered as the most important boiling regime in the context of cryogenic (e.g., LNG) source-term estimations. This paper presents CFD simulations of cryogenic film boiling for liquid nitrogen (LN2) and LNG as pure methane. Different aspects of CFD modeling such as vapor-liquid interface morphology, the behavior of heat flux at the heated surface, the effect of wall superheats on bubbles generation frequency and bubbles departure diameter are presented. Based on the results of CFD simulations, a first principle model is applied to correlate the wall heat flux in the film boiling regime. This model can be used to enable a faster estimation of wall heat flux when CFD simulations and use of empirical correlations are not feasible.

Investigation and experimental analysis of the bubble departure diameter in pure liquids on horizontal cylindrical heater

In this study, partial pool boiling heat transfer and bubble departure diameter on horizontal cylindrical heater in heat flux range between 103 and 105 w m−2 were investigated. Pure liquid water, Ethanol and Acetone were utilized as the fluids for the system. Aluminum, stainless steel 316A, copper and brass were considered as the materials for the heater’s surface rod. Different degrees of roughness were applied for copper and aluminum surface. Bubble departure diameter and heat transfer coefficients were chosen for the system measurement. The empirical model for bubble departure diameter was estimated by using experimental data. This model is based on dimensionless numbers that through which experimental data are shown from literature and the present the study is in good agreement with the model.

Accuracy of Eulerian–Eulerian, two-fluid CFD boiling models of subcooled boiling flows

Boiling flows are frequently found in industry and engineering due to the large amount of heat that can be transferred within such flows with minimum temperature differences. In the nuclear industry, boiling affects in different ways the operation of almost all water-cooled nuclear reactors. Recently, the use of computational fluid dynamic (CFD) approaches to predict boiling flows is increasing and, in the nuclear area, CFD is being developed to solve thermal hydraulic safety issues such as establishing the critical heat flux, which is perhaps the major threat to the integrity of nuclear fuel rods. In this paper, the accuracy of an Eulerian–Eulerian, two-fluid CFD model is evaluated over a large database of subcooled boiling flows, avoiding the rather popular case-by-case tuning of descriptive models to a limited number of experiments. The model includes a Reynolds stress turbulence model, the method of moments-based Sγ population balance approach and a boiling model derived using the heat flux partitioning approach. The database covers a large range of conditions in subcooled boiling flows of water and refrigerants in vertical pipes and annular channels. Overall, a satisfactory predictive accuracy is achieved for some quantities of interest, such as the void fraction and the turbulence and liquid temperature fields, but results are less satisfactory in other areas, more specifically for the average bubble diameter and the mean velocity profiles close to the wall in annular channels. Agreement may be improved with advances in the treatment of large bubbles and bubble break-up and coalescence, as well as in improved modelling of the boiling region close to the wall, and more specifically the bubble departure diameter, the wall treatment and the contribution of bubbles to turbulence.

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.

Numerical investigation of pool nucleate boiling in nanofluid with lattice Boltzmann method

Due to significant improvement of thermal performance and other properties of nanofluids, this group of liquids is in high demand. According to the literature, the effect of nanoparticles on boiling heat transfer enhancement or degradation is not the same among different investigations. In the present article, the pseudo-potential multiphase lattice Boltzmann method is used to simulate nucleate pool boiling with two different fluids: a pure liquid and a nanofluid. The current results indicate that the contact angle is the same for both the fluid and nanofluid when the vapor bubble detachment occurs. Also, bubble departure diameter is greater in the base liquid while bubble release frequency is higher in the nanofluid. In brief, the present results demonstrate that using a nanofluid instead of its base fluid will increase the boiling heat transfer coefficient.

Theoretical analysis of pool boiling characteristics of Al2O3 nanofluid according to volume concentration and nanoparticle size

To investigate the effect of volume concentration and nanoparticle size on nanofluid boiling, a simulation with various concentrations of nanofluids and nanoparticle sizes was carried out using the HFP (heat flux partitioning) model with variations in the contact angle. The CHF (critical heat flux) of the nanofluid rose more than the base fluid. When the volume concentration was 0.025vol% and the size of the Al2O3-nanoparticles was 50nm, the maximum critical heat flux was 1515kW/m2 and gradually decreased with the concentration of the nanofluid. Since the Energy and K factor both improved as the Al2O3-nanofluid concentration increased, the frequency of bubble departure as well as the bubble departure diameter subsequently increased. However, the effect of nanoparticle size was not remarkable. The nucleate site density at the CHF increased from 32.97 to 30.53site/cm2 with the increase of volume concentration at 50nm-nanoparticle size but the increase was significantly lower than that of the base fluid, which was 65.90site/cm2. The temperature converted the natural convection boiling to the nucleate boiling increased and the dryout factor decreased as the concentration of the nanofluid increased. In addition, these parameters increased as the nanoparticle size became similar to the value for the surface roughness.

A modified force-balance model for prediction of bubble departure diameter in subcooled flow boiling

Experimental data by Sugrue et al., Klausner et al., Zeng et al., Prodanovic et al., and Situ et al. for bubble departure diameter in subcooled flow boiling in a wide range of orientation angle, subcooling, heat flux, mass flux, and pressure conditions were used to assess the predictive accuracy of the mechanistic force-balance models of Klausner et al. and Yun et al. The results suggested that both models capture the experimental trends correctly, but exhibit large average errors and standard deviations, i.e. 85.5% (σ=49.7%) and 43.9% (σ=23.1%) for Klausner’s and Yun’s models, respectively. Since the cube of the bubble departure diameter is used in subcooled flow boiling heat transfer models, such errors are unacceptable, and underscore the need for greater accuracy in predictions. Therefore, the databases were used to (i) identify the dominant forces determining bubble departure at various operating conditions, and (ii) optimize the empirical coefficients describing those forces in Klausner’s model. The modified model considerably lowers prediction error to 22.4% (σ=19.9%) for all data considered. Application of the modified model is demonstrated for the subcooled flow boiling conditions present in the hot channel of a typical Pressurized Water Reactor (PWR).

저압에서의 과냉각 비등 현상에 대한 CFD의 유효성 검토

Subcooled boiling under low pressure was numerically investigated using computational fluid dynamics(CFD). The wall boiling model was used for simulating the subcooled boiling; this model requires sub-models consisting of bubble departure diameter, nucleation site density and bubble departure frequency. The CFD code CFX provides the default models based on experimental data. Because these models are mostly developed under high pressure conditions, it would not be predicted well in low pressure conditions. Thus in this study, CFD validation for subcooled boiling under low pressure was analyzed. The numerical results were compared with experimental data from published paper. Simulations were performed with mass flux ranging from 250 to , heat flux ranging from 0.37 to and constant outlet pressure of 0.11 MPa. Employing the empirical correlation developed under low pressures could increase the accuracy of numerical analysis.