Bubble Departure Characteristics Research Articles

Numerical Simulation of Departure from Nucleate Boiling in Rod Bundles under High-Pressure Conditions

In subcooled boiling flows beyond a certain heat flux, heat transfer is hampered due to a phenomenon known as Departure from Nucleate Boiling (DNB). Conducting DNB experiments at one-to-one nuclear reactor operating conditions is highly challenging and expensive. Another alternative approach is to use Look-up table data. However, its applicability is limited due to its dependence on rod bundle correction factors. In the present investigation, a state-of-the-art Eulerian-Eulerian two-fluid model coupled with an extended heat flux partitioning model is used to predict DNB in tubes and rod bundles with square and hexagonal lattices (relevant to Pressurized Water Reactors). In this approach, bubble departure characteristics are modeled using semi-mechanistic models based on force balance analysis. The predicted DNB values are compared with experimental and Look-up table data and found out to be within 1.8% to 20%.

Nucleate pool boiling heat transfer enhancement in saturated Novec 7100 using titanium dioxide nanotube arrays

The effect of titanium dioxide (TiO2) nanotubes on pool boiling heat transfer is experimentally investigated. Four surfaces including plain or nanotube coated copper or titanium substrates are tested in the saturated Novec 7100. Both the nanotube coated surfaces enhances the critical heat flux (CHF) at around 40% compared to the relative plain surfaces. The nanotube coated titanium substrate effectively improves the heat transfer coefficient at around 65% over the plain surface, while the nanotube coated copper substrate shows the average heat transfer coefficient at about 20% lower than its plain surface. From the coincident boiling curves after reducing a thermal interface resistance from the nanotube coated copper substrate and bubble departure characteristics of the two coated surfaces, it is inferred that the nanotubes are effective in enhancing both the heat transfer coefficient and CHF. The observation shows both the improvements of the nanotube surfaces could be attributed to the modified bubble dynamics and effective rewetting of the dry spots, related to the faster liquid replenishment due to capillary wicking of the tube-like structure. The identification of the TiO2 nanotube influence on pool boiling with the dielectric fluids can help the further applications of the nanotube coating in two-phase cooling systems.

Whole Field Measurements to Understand the Role of Varying Depths of Nucleation Site on Vapor Bubble Dynamics and Heat Transfer Rates

Abstract This work studies the possible effects of varying depths of cavity on bubbling features and the associated heat transfer rates in nucleate pool boiling regime. A single vapor bubble has been generated on a substrate with a cylindrical cavity at its center that acts as the nucleation site. Experiments have been conducted for three cavity depths (250, 500, and 1000 μm), while keeping its throat diameter constant at 200 μm. With the bulk fluid maintained under saturated conditions, for each cavity depth, surface superheat level has been varied in the range of ΔTsuperheat = 8, 10 and 12 °C. A gradient-based visualization technique, coupled with a high speed camera, has been employed to simultaneously map the changes in thermal gradients during the formation of the vapor bubble as well as bubble dynamic parameters. The image sequence obtained has been qualitatively and quantitatively analyzed to elucidate the dependence of bubbling features and various heat transfer processes on cavity depth. With an increase in the depth of cavity, the net effect of reduction in the available thermal energy due to the increased convection effects and significant depletion of superheated layer are identified as the dominant heat transfer processes that influence the bubbling features. Furthermore, based on the statistics of bubble departure characteristics, the cavity with higher depth (1000 μm) showed a much stable bubble formation with minimal variation in the bubble departure frequency as compared to the bubbling features from a cavity with smaller depth (250 μm). Evaporative heat transfer process has been identified as the primary cause for increased inconsistency of bubbling features at high superheat conditions for experiments performed for low cavity depths.

Pore-scale investigation on flow boiling heat transfer mechanisms in gradient open-cell metal foams by LBM

Flow boiling heat transfer mechanisms inside gradient open-cell metal foam are investigated based on the newly proposed metal foam model. This new model considers thermal responses of metal skeletons. The structural effects of gradient metal foam, including porosity gradient, thermal conductivity gradient and wettability gradient, on flow boiling heat transfer performance are studied by pseudopotential lattice Boltzmann method. Reynolds number effect on bubble departure characteristics, such as bubble departure diameter, bubble departure frequency and the corresponding probability density distributions, are investigated based on statistical analysis. The results show that bubble confinement weighs heavily over bubble movement inside pore cell during bubble escaping process in porosity-gradient metal foams. Bubble nucleation is depressed by the highly thermal conductive gradient metal foam at high heat fluxes. The interaction between escaping bubble and pore throat shifts from “complete contact” mode to “thin liquid film” mode in the positive wettability-gradient metal foam. Shear force dominates over buoyant force with increasing Reynolds number, resulting in the sliding motion of premature bubbles.

The influence of bubble departure characteristics on CHF prediction at high-pressure conditions

The two-fluid Eulerian model, coupled with the heat flux partitioning model, is being widely used to predict subcooled flow boiling characteristics and critical heat flux (CHF). The heat flux partitioning model relies on essential parameters like bubble departure diameter, departure frequency, and nucleation site density. Mainly, performing experiments to determine bubble departure diameters and frequencies is challenging. Another approach is to use semi-mechanistic models; however, even such models rely on some crucial parameters like advancing contact angle, receding contact angle, contact diameter, and many others. In this study, the influence of such parameters, and the role of different forces in determining bubble departure diameters and departure frequencies, at high-pressure conditions is studied. Moreover, a parametric analysis is carried to understand the influence of such parameters on CHF prediction.

Enhanced boiling heat transfer on micro-structured surfaces via ultrasonic actuation

Abstract Emerging issues in boiling heat transfer include enhancing heat transfer uniformity/stability, and critical heat flux (CHF). Microcavity structures improve heat transfer uniformity/stability by departing bubbles with arranged formation due to pinning effects and regular pitch. Ultrasonic actuation induces an acoustic field on departed bubbles, and this enhances contact line instability between the bubble and microcavity structures, resulting in an increased dissipation rate of smaller bubbles. In this study, we demonstrate that synergetic effects from microcavity structures and ultrasonic actuation can enhance CHF and thermal stability while also improving temporal/spatial temperature uniformity. Applying microcavity structures with ultrasonic actuation, we observe smaller and faster bubbles' departure with the proposed formation. These bubble departure characteristics on the microcavity surface with ultrasonic actuation enhance CHF and thermal stability by delaying bubble coalescence and ensuring liquid paths between smaller and faster-departed bubbles. Thus, when ultrasonic actuation is applied to the microcavity structure, CHF increased by 20%, and temporal/spatial temperature variations near CHF were reduced to less than 1/2 and 1/3, respectively, compared to no actuation case. This research will help to understand the interaction of ultrasonic wave and bubbles, and to show the way to overcome CHF limitations of passive methods using microsized structures.

Experiments on pool boiling regimes and bubble departure characteristics of single vapor bubble under subcooled bulk conditions

An experimental study for understanding the effects of subcooling of bulk fluid on the phenomenon of nucleate boiling heat transfer was carried out. The whole field experimental data was recorded in a purely non-intrusive manner using rainbow schlieren deflectometry as the optical diagnostic tool. Boiling experiments were conducted over a range of heat flux conditions (1⩽q′′⩽40 kW/m2) and for varying levels of subcooling (0 K ≤ ΔTsubcooling ≤ 10 K). The recorded images were first been interpreted to understand the effect of subcooling on various regimes of boiling phenomenon, which include convection, isolated nucleate boiling and vertical coalescence of the departing bubbles. Observations on convection regime showed an increasing influence of natural convection heat transfer. Based on the formation of vapor bubble and its departure characteristics, the isolated nucleate boiling regime of the classical boiling curve was further segmented into three sub-regimes namely, stable vapor bubbles, oscillating vapor bubbles and oscillating-departing vapor bubbles. A mechanism explaining the oscillations of vapor bubbles under subcooled condition was proposed that takes into account the relative dominance of condensing apex region and evaporating microlayer region. Various bubble dynamic parameters such as equivalent bubble diameter, departure frequency, aspect ratio and oscillation frequency were determined quantitatively to understand the effect of subcooling on the bubble departure characteristics. Based on the experimental results, an empirical correlation for predicting the bubble departure diameter and evaporative heat flux was developed and proposed.

Numerical study of bubble growth and merger characteristics during nucleate boiling

This paper numerically investigates bubble nucleation, growth, merger, and departure characteristics as well as the heat transfer distributions on the surface, and compared the simulation results with experimental results. Micro-heaters are utilized in experiments to provide constant temperature as well as measure the time-resolved heat flux variations. Bubble images and corresponding heat flux distributions under bubbles are obtained in experiments. A 3-dimensional computational domain with two symmetry planes is built and 5 heating elements are arranged at the bottom surface of the domain to simulate the real heaters used in experiments. The temperatures of these elements are set constant and the local heat fluxes were calculated. Liquid-vapor interface is captured with a volume of fluid (VOF) method in numerical simulations. Bubble growth and merger characteristics and heat flux distributions under bubbles calculated in numerical simulations are analyzed and compared to experimental observations. The heat transfer contributions of different heat transfer modes are calculated. The heat transfer mechanisms during nucleate boiling are better interpreted by the coupling of bubble dynamics and time- and space-resolved heat flux variations on the surface.

The development of macrolayer thickness of water in the pool boiling regime

ABSTRACT The main purpose of this study is to analyse the phenomenon of pool boiling in water over a horizontally placed copper tube heater of 28 mm diameter. The experiment has been carried out to observe the bubble growth and departure characteristics for the heat flux range up to 40,000 W/m2. The entire process is being recorded by a digital camera at different time intervals. The measured parameters have been used to determine the initial layer thickness, macro layer thickness and critical heat flux and validated with models proposed in the literature.

An experimental study and comparitive validation of macrolayer thickness in nucleate pool boiling for horizontal copper tube heater

Nucleate Pool boiling is characterised by the progression of several nucleation sites (bubble formation), which surge from distinct points on a surface, whose temperature is slightly above the liquids. To investigate and evaluate this, a pool boiling setup was fabricated with a horizontal copper tube heater of 28mm diameter by imposing cartridge heater. An analytical expression, suggested by literature helps to determine the thickness of macrolayer based on bubble diameter was used. The macrolayer thickness for water was found by measuring the bubble diameter by using Photographic and CAD method. Experiment was carried out in a stainless steel container insulated with Teflon cover to observe the bubble growth and bubble departure characteristics for the heat flux range of 1000-42,000 W/m2. The bubble diameter were measured and the measured parameters were been used to determine the initial layer thickness, macro layer thickness and critical heat flux and validated through various models. The observed and calculated values are in good agreement as reported in various literature.

Bubble departure characteristics in a horizontal tube bundle under cross flow conditions

The visualization experiments for measuring departure diameter and frequency of vapor bubbles in the saturated nucleate boiling of distilled water have been carried out at atmospheric pressure. The experiments were carried out for an isolated single cylindrical tube and that for a tube bundle under pool and cross flow boiling conditions. The test setup was a stainless steel cuboidal tank of dimensions 400 mm × 200 mm × 180 mm provided with a 5 × 3 staggered tube bundle with an equilateral triangular pitch of 1.95. The departure diameter and frequency were measured using a high-speed camera to understand the parametric behavior of the bubble departure characteristics in a tube bundle under pool and cross flow condition. The range of operating conditions was: (1) heat flux q″=8−28kW/m2, (2) wall superheat ΔTsat=7−22K, (3) inlet mass fluxG=120−303.5kg/m2s. The departure diameter and frequency were found to increase with an increase in the heat flux, but decrease with an increase in the mass flow rate. The local departure diameter and frequency were observed to increase as the neighboring tubes in the bundle were successively heated. Empirical correlations have been developed for departure diameter and frequency for saturated nucleate boiling under cross flow conditions incorporating the effect of the tube bundle. The correlations were found to predict experimental data within acceptable limit.

Experimental study of bubble departure characteristics in forced convective subcooled nucleate boiling

ABSTRACTVisualization experiments in upward forced convective nucleate boiling flows were carried out. The bubble growth and collapse have been measured using high-speed photography technique with distilled water under atmospheric pressure. The experiments show that the bubbles depart from nucleating sites shortly after nucleation and slide along the heater surface. The bubbles grow while sliding, attain a maximum size, then lift from the surface sometime during condensation, and quickly vanish in the bulk liquid. Parametric studies show that bubble diameter and departure frequency increases with an increase in heat flux, a decrease in subcooling, and a decrease in mass flux.

Experimental investigation of variability in bubble departure characteristics between nucleation sites in subcooled boiling flow

Experiments have been conducted in a vertical square channel for investigation of wall nucleation characteristics in subcooled boiling flow. Bubble departures from multiple sites are measured simultaneously for nine conditions. Existing bubble departure diameter models are benchmarked and are shown to be satisfactory in predicting condition-average bubble departure diameter. However, significant variations are observed in the bubble departure frequency across different nucleation sites of a given condition largely due to intermittent periods of inactivity. A benchmark of the existing bubble departure frequency models shows that the models are generally applicable to an ‘active’ frequency but cannot account for the impact of these periods of inactivity. This finding highlights an important issue in the current modeling and understanding of the gas-phase boundary condition. This periodic inactivity, if not incorporated into the wall nucleation modeling, will result in a large overprediction of bubbles generated at the wall. A physical justification for this inactivity is discussed based on the modeling of the active nucleation site density.

Experimental study on bubble departure characteristics in subcooled nucleate pool boiling

The boiling models use departure diameter and frequency in closure relations for the calculation of nucleate boiling heat flux. These parameters are normally derived from empirical correlations which depend heavily on experiments. While these parameters are studied mostly for saturated conditions, there is not sufficient data for the values of departure diameter and frequency in subcooled boiling. In this work, the bubble departure characteristics, i.e. the departure diameters and frequency have been measured using high speed visualization experiments with subcooled demineralized water at atmospheric pressure for nucleate pool boiling conditions. The water pool dimensions were 300 mm × 135 mm × 250 mm with four different heating elements to carry out the parametric studies of bubble departure behavior. The considered parameters were heater surface roughness, heater geometry and heater inclination along with the experimental conditions like degree of subcooling (ΔTsub = 5−20K), superheat (ΔTsat = 1−10K) and the heat flux. The departure diameters and frequencies were directly measured from the images captured. It was intended to generate the subcooled nucleate pool boiling data under a wide range of conditions which are not present in the literature. The departure diameter was found to increase with the wall superheat, heater size and the inclination angle while the liquid subcooling and surface roughness produced a damping effect on the diameter. The departure frequency was found to increase with the wall superheat and the inclination angle, but decreases with an increase in the heater size. The frequency increases with the degree of subcooling except very close to the saturation, and is unaffected by the surface roughness beyond a certain superheat value.

Wall nucleation modeling in subcooled boiling flow

The bubble departure characteristics are studied in flow boiling considering the available experimental data in the literature. The current bubble departure diameter and departure frequency models are reviewed and compared with the collected database, revealing the current modeling shortcomings in forced convective flows. Based on an energy balance approach at the heated surface, the important dimensionless groups are identified and correlated for the bubble departure diameter and departure frequency. These dimensionless groups are the Jakob number, Boiling number, density ratio, and Prandtl number. The bubble departure diameter data also suggests some effect of the flow geometry in mini-channels. The newly proposed semi-empirical models for the bubble departure diameter and departure frequency are shown based on the available database to be accurate to within ±22% and ±35% respectively. This work also identifies important future experimental considerations for the bubble departure characteristics including data at elevated pressures, larger Prandtl numbers, and around the boundary between mini and conventional channels.

Bubble departure size in flow boiling

Flow boiling experiments were conducted in a vertical annular channel to study bubble departure characteristics. Deionized water was used as the working fluid, and the tests were performed at atmospheric pressure. Bubble departure diameters were obtained from the images which were captured by a high-speed digital camera. The relationship between bubble contact diameter and departure diameter was discussed. A new model base on force balance analysis, taking bubble contact diameter into account for predicting bubble departure diameter is proposed in this study. A good agreement between predicted and measured results is achieved.

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.

A numerical study of quasi-static gas injected bubble growth: Some aspects of gravity

In the present investigation, adiabatic gas bubble growth from a submerged orifice has been numerically simulated. The growth of the bubble is described by the full Navier–Stokes equations which have been solved by the finite-volume method using the commercial software package TransAT. The numerical simulations have been validated against detailed experimental measurements including the position of the centre of gravity, the curvature profiles and the departure volume. Subsequent to this the CFD platform was used to study some aspects of the influence of gravity level on bubble growth dynamics. In particular, the influence of gravity in the range of 0.1⩽g/g∗⩽1.5 is investigated and the subsequent influence on the bubble formation and departure characteristics are discussed.

A Painting Technique to Enhance Pool Boiling Heat Transfer in Saturated FC-72

A benign method of generating a surface microstructure that provides pool boiling heat transfer enhancement is introduced. Pool boiling heat transfer results from an enhanced, horizontally oriented, rectangular surface immersed in saturated FC-72, indicate up to an 85 percent decrease in incipient superheat, a 70 to 80 percent reduction in nucleate boiling superheats, and a ∼ 109 percent increase in the critical heat flux (CHF = 30 W/cm2), beyond that of the nonpainted reference surface. For higher heat flux conditions (19 to 30 W/cm2), localized dryout results in increased wall superheats (8 to 48°C). The enhanced surface heat transfer coefficients are four times higher than those from the reference surface and similar to those from the Union Carbide High Flux surface. Photographs that identify differences in bubble size and departure characteristics between the painted and reference surfaces are presented.

Effect of surface material properties and surface characteristics inevaporative spray cooling

In the spray cooling of a heated surface, Variations in the surface contact angle cause a change in nucleation characteristics and, thereby, influence the heat transfer process; a higher contact angle shows an enhanced heat transfer due to the ease in nucleation caused by the lowered free energy associated with bubble formation. Results are presented for different surface coatings and spray configurations. The surface roughness variation influences the flowfield, altering the maximum liquid film thickness, the bubble diameter, vapor entrapment, bubble departure characteristics, and, thereby, the ability of the surface to transfer heat. The effect of surface roughness on spray cooling is also studied.