Bubble Merger Research Articles

Boiling crisis is a consequence of thermal runaway in the substrate due to degradation of nucleate boiling heat transfer

Chaotic bubble interactions during vigorous boiling have foiled attempts at an accurate mechanistic understanding of this important industrial transport process. Prediction of the boiling crisis (dryout) that occurs due to the spontaneous merger of bubbles into an insulating vapor film at critical heat flux remains an unsolved challenge. This work diagnoses the dryout process using synchronized high-resolution spatiotemporal heat flux and phase data, obtained via through-substrate infrared and visual inspection. We examine prevailing theories for the boiling crisis and provide evidence to rule out all but one. The boiling crisis is found to be a consequence of a peak in the nucleate boiling curve, past which the degradation of boiling heat transfer and concomitant increase in superheat are caused by replacement of the thermally efficient contact line region with the inefficient vapor-covered region for the surface-fluid combination studied. This results in substrate thermal runaway and dryout. Rather than seeking a separate dryout trigger mechanism, nucleate boiling models must instead inherently capture this peak. The heat flux partitioning employed demonstrates that critical heat flux can indeed be predicted by capturing this peak. A generalized framework is suggested for predicting the boiling curve as part of a multidimensional surface, a boiling manifold.

The bubble merger in rectangular microchannels during boiling processes based on conjugate heat transfer

In recent years, boiling heat transfer in microchannels has become an effective solution for managing high heat flux in electronic devices. Numerical simulations offer details on flow characteristics that experimental methods cannot capture, deepening our understanding of boiling mechanisms within microchannels. In this study, we developed a self-programmed solver, “interMultiRegionFoam,” based on the volume of fluid (VoF) method, phase-change mass transfer models, and conjugate heat transfer. Analyzing the force acting on individual bubbles, a model for two bubbles' merger was constructed, encompassing three stages: bubble bridging, bubble retraction, and bubble contraction. For isolated bubbles, during the restricted growth stage, the dry patch length increases by only 25% with an increase in the Capillary number, while in the elongation growth stage, it grows fivefold. Furthermore, the investigation into four typical bubble merger cases within microchannels demonstrated that the bubble merger model effectively describes the process. During the restricted and free growth stages, bubble merger was observed to enhance heat transfer, and the heat transfer coefficient in the bottom region of the macrolayer increased by a maximum of 16%. However, when bubbles merge in the elongation growth stage, the maximum increase in the heat transfer coefficient at the bottom of the macrolayer is only 7%. Several factors contribute to the enhanced heat transfer during bubble merger, including the evaporation and rewetting of macrolayer liquid, as well as the replenishment of the microlayer. Moreover, the disturbance caused by bubble merger and transient conduction arising from bubble retraction and contraction contributes to the enhancement in heat transfer. With an increase in the contact angle, the peak heat flux gradually decreases, with a maximum reduction of 27%. As the heat flux increases, the Nusselt number gradually increases, reaching a maximum peak increase of 52%. These studies provide theoretical guidance for the design of wall modifications and nucleation in microchannels.

Boiling heat flux partitioning model with bubble tracking method considering bubble merger and stochastic characteristics

This study presents the development and validation of a numerical tool of heat partitioning model for boiling heat transfer using a bubble tracking method. It was aimed to account for the effect of stochastic phenomena in the boiling process by simulating individual bubbles. Saturated pool boiling conditions on a horizontal plate heater were analyzed to demonstrate the effectiveness of the proposed model. The model was devised based on the observations from high-resolution pool boiling experiments, assuming a truncated sphere for bubble shape and using simplified microlayer size and thickness models. The developed heat partitioning model was verified by comparisons with the conventional heat partitioning model. Afterward, the validation of the model was conducted by comparing the result of the simulation with the experimental data of the saturated pool boiling. In this validation, measured values of nucleation site density, departure diameter, single-phase heat transfer coefficients, etc. were applied. Additionally, the effect of stochastic phenomena on boiling heat transfer was evaluated using the bubble tracking method, focusing on nucleation timing and nucleation site distribution. Stochastic nucleation was found to reduce the bubble merger probability compared to simultaneous nucleation, leading to increased microlayer evaporation. Stochastic site distribution, on the other hand, decreased active nucleation sites due to the site suppression effect, resulting in reduced microlayer evaporation compared to uniform site distribution.

On Rayleigh–Taylor Dynamics

In this work, we theoretically and numerically investigate Rayleigh–Taylor dynamics with constant acceleration. On the side of theory, we employ the group theory approach to directly link the governing equations to the momentum model, and to precisely derive the buoyancy and drag parameters for the bubble and spike in the linear, nonlinear, and mixing regimes. On the side of simulations, we analyze numerical data on Rayleigh–Taylor mixing by applying independent self-similar processes associated with the growth of the bubble amplitude and with the bubble merger. Based on the obtained results, we reveal the constituents governing Rayleigh–Taylor dynamics in the linear, nonlinear, and mixing regimes. We outline the implications of our considerations for experiments in plasmas, including inertial confinement fusion.

Unified 2D/3D bubble merger model for Rayleigh-Taylor mixing

Unified 2D/3D bubble merger model for Rayleigh-Taylor mixing

Study on the Fluidization Quality Characterization Method and Process Intensification of Fine Coal Separation in a Vibrated Dense Medium Fluidized Bed.

Uniform and stable bed density is the basis of efficient coal separation by a gas–solid dense medium fluidized bed. The traditional air dense medium fluidized bed (ADMFB) is a kind of bubbling bed. By introducing vibration energy, a vibrated dense medium fluidized bed (VDMFB) with uniform and stable bed density can be formed, where the bubble merger is suppressed, the gas–solid contact can is strengthened, and the fluidization quality is also improved. In this paper, the transfer process of vibration energy in a fluidized bed is studied in detail. By calculating the coherence of pressure signals induced by vibration energy and bubbles at different bed heights, the suppression effect of vibration energy on bubble merger is analyzed. The coefficient Rimp to quantitatively evaluate the improvement effect of vibration energy on the fluidization quality is proposed. The differences and incentives of density uniformity and stability in different height bed areas have been clarified under different vibration parameters and gas flow parameters. It is proposed that the optimal separation bed height area of VDMFB is about H = 40–150 mm. The separation effect of the ADMFB and the VDMFB on 1–6 mm fine coal was compared. The results show that, compared with the ADMFB, the VDMFB reduces the separation probable error, E, from 0.134 to 0.083 g/cm3, and the ash content of the clean coal is reduced from 18.83 to 14.97%. The vibration energy significantly improves the fluidization quality of the ADMFB and the separation effect of fine coal.

Advanced boiling heat transfer model for a horizontal tube with numerical analysis of bubble behaviours

The boiling heat transfer model for a horizontal tube is developed herein, with numerical investigations to realistically simulate bubble nucleation site distributions and bubble behaviours. Accordingly, a new heat partitioning model was proposed to reflect the bubble sliding phenomena more realistically than the extant heat partitioning models for horizontal tubes. The bubble trajectories, merging, lift-off, and distribution of nucleation sites were numerically analysed based on the force balance model, bubble tracking method, and Latin-hypercube sampling. The proposed approach can overcome the limitations of the arithmetic heat partitioning model, which does not reflect bubble mergers and the random nature of the nucleation site distribution. Distinctive bubble behaviours on a horizontal tube can be reproduced by the present model, including bubble sliding, departure, lift-off, and merger. The numerical model presented here considers bubble sliding based on a mechanistic force balance model and bubble merger based on the bubble tracking method. The Monte Carlo method was applied to determine the spatial distribution of the nucleation sites, and statistical information on the wall heat transfer and contribution of each heat transfer mechanism were obtained via extensive calculations. The method developed herein was applied to evaluate the wall heat transfer in horizontal heaters. Further, the proposed numerical boiling heat transfer model was validated against an extant experimental database of horizontal heat exchanger tubes, and the model was observed to predict the wall heat flux with satisfactory accuracy.

Numerical Investigation on Bubble Growth and Merger in Microchannel Flow Boiling With Self-Rewetting Fluid

Abstract There exist some problems such as flow instability and critical heat flux (CHF) caused by the local dryout phenomenon, which is an obstacle to the application of microchannel flow boiling heat sink. Utilizing self-rewetting fluid is one of the promising ways to minimize the dryout area, thus increasing the heat transfer coefficient and CHF. To investigate the heat transfer performance of self-rewetting fluid in microchannel flow boiling, a numerical investigation is carried out in this study using the volume of fluid (VOF) method, phase-change model, and continuum surface force model with surface tension versus temperature. A three-dimensional numerical investigation of bubble growth and merger is carried out with water and 0.2 wt % heptanol solution. The single bubble growing cases, two x-direction/y-direction bubbles' merging cases, and three bubbles' merging cases are conducted. Since the bubbles never detach the heated walls, the dryout area and regions nearby the contact line with thin liquid film dominated the heat transfer process during the bubbles' growth and merger. The self-rewetting fluid can minimize the local dryout area and achieve larger thin liquid film area around the contact line due to the Marangoni effect and thermocapillary force, thus resulting in higher wall heat flux. The two x-direction bubbles' merging case performed best for heat transfer in the microchannel, in which self-rewetting fluid achieves heat transfer enhancement for over 50%.

Bubble coalescence in electrolytes: Effect of bubble approach velocity

The coalescence of air bubbles in aqueous solutions of electrolytes was studied experimentally. The bubbles were produced from two vertical parallel capillaries in a still liquid and video recorded. Several quantities were evaluated: bubble shape and size, bubble distance, bubble expansion rate, time of the first touch, time of bubble merger, time of bubble detachment. The three state variables were: bubble coalescence efficiency E, transition electrolyte concentration Ct, bubble contact time T. The effect of four control parameters was investigated: electrolyte type (3 inorganic salts: NaCl, Na2SO4, CaCl2), electrolyte concentration C (5 values sparsely covering a broad interval, 0.001–2 mol/L), bubble approach velocity V (9 values, from 0.03 to 40 mm/s), bubble diameter D (2 values, 1 and 1.5 mm). The following results were obtained. Coalescence efficiency E decreases with both C and V. Transition concentration Ct is not a material property but depends on the hydrodynamic conditions, Ct decreases with V. Contact time T decreases with V, as T ∼ V−1, and increases with C, roughly as T ∼ C1.9. Bubble approach velocity V plays a dual role: it reduces the coalescence region but accelerates the coalescence process. The transition from coalescence to noncoalescence proceeds via a minimum in the T(V) graphs. Bubble size D reduces the value of Ct, whence the extent of the coalescence region. The three electrolytes behave in a similar way. A scale-estimates model was developed to describe the results. The effect of electrolytes on the coalescence is similar to the effect of liquid viscosity.

Bubble merger in initial Richtmyer-Meshkov instability on inverse-chevron interface

Inverse-chevron interfaces with various initial amplitudes and wavelengths are used in shock tube experiments to study bubble competition in the Richtmyer-Meshkov instability. We find that width growth of the large interface is enhanced and that of the small interface is more affected and inhibited.

A numerical study on dynamics behaviors of multi bubbles merger during nucleate boiling by lattice Boltzmann method

In this paper, bubble dynamics and multi bubbles merger associated with heat transfer are studied numerically by a three-dimensional hybrid lattice Boltzmann model. The dynamics behaviors of the vapor bubbles including growth, merger and departure on the superheated wall are simulated. Compared the numerical simulation results with experimental results available in references; it is found that the LBM simulations are in good agreement with existing results. The processes of multi bubbles merger show formation of vapor bridges with liquid trapped underneath the bridges and vapor neck before departure. The merger bubble is found to cause higher heat transfer rate compared to a single bubble. The highest wall heat transfer is obtained for four bubbles merging on the superheated wall. Bubble merger process increases the overall wall heat transfer by trapping a liquid layer between the bubble bases and by pulling cool liquid towards the wall during contraction.

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.

Self-Similar Multimode Bubble-Front Evolution of the Ablative Rayleigh-Taylor Instability in Two and Three Dimensions.

The self-similar nonlinear evolution of the multimode ablative Rayleigh-Taylor instability (ARTI) is studied numerically in both two and three dimensions. It is shown that the nonlinear multimode bubble-front penetration follows the α_{b}A_{T}(∫sqrt[g]dt)^{2} scaling law with α_{b} dependent on the initial conditions and ablation velocity. The value of α_{b} is determined by the bubble competition theory, indicating that mass ablation reduces α_{b} with respect to the classical value for the same initial perturbation amplitude. It is also shown that ablation-driven vorticity accelerates the bubble velocity and prevents the transition from the bubble competition to the bubble merger regime at large initial amplitudes leading to higher α_{b} than in the classical case. Because of the dependence of α_{b} on initial perturbation and vorticity generation, ablative stabilization of the nonlinear ARTI is not as effective as previously anticipated for large initial perturbations.

Bubble merger and scaling law of the Rayleigh–Taylor instability with surface tension

We present a theoretical model to study the effects of surface tension on the growth of single and multiple bubbles in the Rayleigh–Taylor instability. The asymptotic solution for a single bubble is obtained and is expressed in terms of the Eötvös number. The bubble merger process is also demonstrated from the model. We find the contrasting effects of surface tension: it reduces the growth of a single bubble, but enhances the mixing rate of multiple bubbles at a late time. The bubble merger of Rayleigh–Taylor instability follows the same scaling law of the growth of mixing zone even when surface tension exists, but the growth coefficient in the scaling law increases with surface tension. A comparison with an experimental result is in good agreement.

Experimental evidence of a bubble-merger regime for the Rayleigh-Taylor Instability at the ablation front

Under the Discovery Science program, the longer pulses and higher laser energies provided by the National Ignition Facility (NIF) have been harnessed to study, first time in indirect-drive, the highly nonlinear stage of the Rayleigh-Taylor Instability (RTI) at the ablation front. A planar plastic package with pre-imposed two-dimensional broadband modulations is accelerated for up to 12 ns by the x-ray drive of a gas-filled gold radiation cavity with a radiative temperature plateau at 175 eV. This extended tailored drive allows a distance traveled in excess of 1 mm for a 130 μm thick foil, a factor 3x larger than previously achieved on other laser facilities. As a consequence, we have measured the ablative RTI in transition from the weakly nonlinear stage up to the deep nonlinear stage for various initial conditions. A bubble merger regime has been observed and the ablative stabilization strength varied by changing the plastic dopant from iodine to germanium.

Numerical study of bubbles rising and merging during convective boiling in micro-channels

A three dimensional numerical study on bubble growth and merger in a micro-channel with diameter of 0.64 mm has been conducted. The working fluid is R134a and the wall material is steel. The inlet Reynolds number is set at 549 in order to keep the flow in laminar regime. Two different heat fluxes (14 and 28kWm2) are supplied to the wall to heat up the fluid. The coupled level set and volume of fluid (CLSVOF) method is used to capture the distorted two-phase interface. An evaporation model is also implemented through UDF (User defined function). The combination of these two methods has successfully eliminated spurious velocities which is a common problem in two phase flow simulation. The boiling and merger processes are well-predicted by the simulation. It is found that the whole process can be divided into three sub-stages: sliding, merger, and post-merger. The dynamics and heat transfer are found to be different in these stages. The evaporation rate is much higher in the first two stages due to the thermal boundary layer effects.

Mixing of spherical bubbles with time-dependent radius in incompressible flows.

The motion of contracting and expanding bubbles in an incompressible chaotic flow is analyzed in terms of the finite-time Lyapunov exponents. The viscous forces acting on the bubble surface depend not only on the relative acceleration but also on the time dependence of the bubble volume, which is modeled by the Rayleigh-Plesset equation. The effect of bubble coalescence on the coherent structures that develop in the flow is studied using a simplified bubble merger model. Contraction and expansion of the bubbles is favored in the vicinity of the coherent structures. Time evolution of coalescence bubbles follows a Lévy distribution with an exponent that depends on the initial distance between bubbles. Mixing patterns were found to depend heavily on merging and on the time-dependent volume of the bubbles.

Three dimensional numerical simulation on bubble growth and merger in microchannel boiling flow

This paper presents a three dimensional direct simulation on boiling flow in a rectangular microchannel. A coupled volume-of-fluid and level-set method was adopted to track the liquid–vapor interface. An immersed boundary method was used to deal with the temperature around the interface. The numerical result was compared with a previous experimental result, and the outcomes matched. Growth and merger of the bubbles were simulated and the impact of the bubbles' merger on the heat transfer was analyzed. The results show that the merger can produce a temporal growth in heat flux, while the thin liquid film between the bubble and the wall holds the main reason for the high heat flux in microchannel boiling flow.

Long duration X-ray drive hydrodynamics experiments relevant for laboratory astrophysics

The advent of high-power lasers facilities such as the National Ignition Facility (NIF), and the Laser Megajoule (LMJ) in the near future, opens a new era in the field of High Energy Density Laboratory Astrophysics. These versatile laser facilities will provide unique platforms to study the rich physics of nonlinear and turbulent mixing flows. The extended laser pulse duration could be harnessed to accelerate targets over much larger distances and longer time periods than previously achieved. We report on the first results acquired on NIF with the ablative Rayleigh–Taylor Instability (RTI) platform. A 20-ns X-ray drive is tailored to accelerate planar modulated samples into the highly-nonlinear bubble merger regime. Based on the analogy between flames front and ablation front, highly nonlinear RTI measurements at ablation front can provide important insights into the initial deflagration stage of thermonuclear supernova of Type Ia. We also report on an innovative concept used to create even longer drive on multi-beam laser facilities. The multi-barrel hohlraum (Gattling Gun) approach consists, here, of three adjacent cavities, driven in succession in time. This novel concept has been validated on the Omega EP laser system. The three cavities were irradiated with three 6–10 ns pulse UV beams and a 30 ns, 90 eV X-ray radiation drive was measured with the time-resolved X-ray spectrometer μDMX. This concept is promising to investigate the pillar structures in the Eagle Nebula or for photoionization studies which require a steady light source of sufficient duration to recreate relevant physics.

Reshocked Richtmyer-Meshkov instability: Numerical study and modeling of random multi-mode experiments

The evolution of the three-dimensional planar Richtmyer-Meshkov (RM) instability during a two shock wave interaction (i.e., reshock) is investigated by means of comparing numerical simulations and analytical modelling with experimental results of low Mach numbers (M < 1.5) and fairly high Atwood numbers (A ∼ 0.7). The study discusses and analyses the differences in the evolution of the mixing zone for two different types of initial perturbations, namely, multi-mode random initial perturbation with a narrow or wide bubble size distribution. More specifically, the study is focused on the agreement between numerical simulations and experiments performed with an unknown random initial perturbation. Using a large set of experimental results with different reshock arrival times and Mach numbers, the numerical simulations results are compared to the experimental results for a variety of different scenarios. This methodology allows a constrained comparison, while requiring good agreement for all cases. A comprehensive parametric study is conducted, examining the evolution of the mixing zone (MZ) for different initial amplitudes and wavelengths. It is found that in order to achieve a good agreement, the numerical simulation must be performed using a wide enough initial spectrum, which enables a dominant, efficient bubble merging process to take place within the MZ. The numerical simulation results are compared to a model, based on classic single bubble RM evolution formulation, combined with high amplitude effects consideration and phase reversal treatment in case of heavy to light reshock passage. The model is also extended for the case of multi-mode fronts, accounting for a bubble merging process, determining that the MZ evolution after the reshock can be classified with high confidence as governed by an inverse cascade bubble merger, approaching self-similarity.