Bubble Growth Characteristics Research Articles

Modeling of confined bubble growth and heat transfer characteristics in parallel microchannel heat sinks

The current study aims to present the modeling and numerical analysis of parallel microchannel heat sinks accounting for transient phenomena during confined bubble growth in one of the microchannels, considering dynamic aspects for the bubble growth and heat transfer characteristics to identify flow instabilities. The model incorporates the estimative of the inlet velocities and pressure drop due to parallel channels’ interaction. Therefore, the model can predict conditions that result in reversal flow, as well as the main trends and values of the main parameters during this process. A parametric analysis is performed numerically aiming to infer the effects of the number of channels, heat flux, inlet mass velocity, and location of the nucleation site on the occurrence of reversal flow.

Numerical simulation of single bubble growth and heat transfer of R32+R1234yf binary mixtures during saturated pool boiling

As a promising alternative refrigerant for meeting carbon reduction targets, R32+R1234yf binary mixtures combine the thermal performance of R32 with the environmental performance of R1234yf. Since the degree of heat dissipation during the boiling process is closely related to bubble behavior, the bubble growth and heat transfer characteristics of R32+R1234yf mixtures must be understood in depth. Accordingly, this study proposed a numerical model for single bubble growth during nucleate pool boiling of mixtures. Numerical simulations of single bubble growth in R32, R1234yf, and four binary mixtures thereof were conducted. Owing to the preferential evaporation of the more volatile component, a low concentration region of R32 appeared in the liquid near the heating surface, while the concentration of R32 in the vapor was generally higher than that in the liquid. The evaporation heat flux at the bubble interface in the mixtures was significantly lower than in pure R32, indicating that the existence of mass transfer resistance considerably reduced the heat transfer capacity of mixtures. In contrast to a pure fluid, the microlayer evaporation in a mixture might terminate owing to an increase in the local bubble point temperature, and two modes of microlayer evaporation—called “bubble-point-controlled mode” and “thickness-controlled mode”—were identified accordingly. The total heat transferred during bubble growth first decreased and then increased with an increase in the bulk concentration of R32; a minimum value was observed at an R32 mol fraction of approximately 0.4. This result is consistent with the observed variation in the difference between the vapor and liquid phase concentrations. Overall, this study provides a detailed description of the mass and heat transfer characteristics during single bubble growth in binary mixtures, contributing to a deeper understanding of the associated boiling heat transfer mechanisms.

Bubble Dynamics and Heat Transfer Characteristics of Flow Boiling in a Single Pentagonal Rib Channel

Abstract Two-phase flow boiling is the preferred method for efficient heat dissipation in electronic equipment, so the bubble dynamics and heat transfer characteristics of flow boiling in a single pentagonal rib channel are numerically analyzed by the volume of fluid (VOF) model. Three typical rib structures are obtained, whose block ratios are 0.4, 0.75, and 0.25, and the corresponding length-diameter ratios are 1.72, 0.59, and 12.70, respectively. The Reynolds number is 14,000 and 28,000. The heat flux is set from 50 to 150 kW/m2. The results show that the starting position of the bubble is at the cone and sides of the rib, and its moving direction is related to the configuration of the rib, the heat flux, and fluid velocity. Additionally, the bubbles are shedding at the corner of the rib, and the short rib does not appear to be the vortex shedding phenomenon. The wall temperature and temperature fluctuation increase with the increase of heat flux and with the decrease of fluid velocity. The heat transfer coefficient decreases with the increase of heat flux and with the decrease of fluid velocity. The short rib channel has a higher heat transfer coefficient, approximately 14% and 50% higher than that of the median and long rib channels, respectively. Compared with the single-phase flow, the main frequency of the lift coefficient increases with the increase of the fluid velocity while decreasing with the increase of heat flux. The main frequency of the median rib without harmonic signal keeps constant, and that for long rib decreases by 38%. The study of the instantaneous characteristics of bubble growth in pentagonal rib channels is instructive to the efficient heat dissipation in electronic equipment.

Experimental investigation on the characteristics of bubble growth and slide on a downward-facing heater surface in flow boiling

Previous studies have shown that the bubble behavior has significant influence on boiling phenomena. However, studies of downward-facing bubble behavior were rather scarce compared to that of the upward-facing or vertical surface. In this study, a downward-facing flow boiling experiment was conducted to investigate bubble moving speed and bubble expanding speed under different heat flux and mass flux. A high-speed camera was used to observe the boiling surface conditions, images of which were later processed by a computer vision code written by python. Bubble moving speed and bubble expanding speed were analyzed. In the experiment, bubble moving speed was close to the liquid velocity and prone to follow a normal distribution. Bubble expanding speed increased with heat flux and could be regarded as constant for each bubble in the study regime. A parameter called heater factor was proposed and the average of which was observed to be rather constant under different heat fluxes and mass flux. Moreover, the heater factor was likely to follow a lognormal distribution. The distribution information could be used for building boiling models in future works.

Experimental study on single bubble growth of R32 + R1234yf binary mixtures during saturated pool boiling

R32 + R1234yf mixtures are considered highly promising refrigerants. Understanding the bubble growth characteristics of mixtures is vital for understanding their boiling heat-transfer performance. In this study, pool boiling experiments were conducted to investigate the single-bubble growth characteristics of R32, R1234yf, and their binary mixtures. The bubble growth curves for each test refrigerant were obtained in the superheat range of 1.3–6.2 K. The results showed that the asymptotic bubble growth rates of the pure refrigerants were slower than those predicted using previous theoretical models, which were developed for isolated bubbles growing in an infinite superheated liquid. The bubble growth constants of the mixtures were lower than those of the pure components, whereas the growth exponents were greater than the “ideal growth exponent.” The sudden decrease in the effective wall superheat and sharp increase in mass transfer resistance slowed the initial bubble growth rate. In the late stage of growth, the microlayer that had not yet dried out provided stable heat input to the bubble, resulting in a higher asymptotic growth rate than that of pure components. A set of dimensionless correlations for the bubble growth rate of mixtures is proposed, and they predicted the data in this study within a ± 15% deviation band.

Mass transfer mechanism during bubble evolution on the surface of photoelectrode

The limitation of the transfer of gas products within the photocatalytic system is the key to the efficiency of the water splitting, which is directly influenced by the bubble evolution process on the catalyst surface. In this study, the entire process of bubble evolution on the surface of a superhydrophilic TiO2 photoelectrode was simultaneously measured visually and electrochemically. The bubble growth and detachment characteristics were investigated under different laser powers and constant bias voltages. An expression for the bubble coverage characterized by the bubble radius and photocurrent, which is applicable to the photoelectrocatalytic water splitting, was proposed to accurately obtain the mass transfer coefficient during bubble evolution. The mass transfer mechanism of gas product transfer in the low current density range dominated by single-phase free convection was revealed. The mass transfer coefficient was found to increase in the inertia-controlled phase and decrease in the chemical reaction-controlled phase of bubble growth. Furthermore, a method to accelerate bubble detachment from the photoelectrode surface by periodic illumination was proposed, which could significantly enhance the gas product transfer process.

Visual experimental study on bubble behavior from nucleation site in vertical rectangular channel

The bubble behavior is a fundamental issue to deep understanding the boiling heat transfer, especially in rectangular channels where the bubble may be confined by the channel walls. In this paper experiment is performed and the bubble behavior is studied by a high-speed camera in the rectangular upward flow channels with a gap of 3.2 mm. The characteristics of bubble growth, bubble sliding velocity and coalescence are discussed in detail by analyzing the captured boiling behavior, including the effect of heat flux and system pressure. It is found that bubble originating from a single nucleation site experiences two stages, the namely initial rapid growth stage and the subsequent coalescence stage. The effect of heat flux on the growth rate of sliding bubble looks insignificant before coalescence, but it has an important impact on bubble growth rate at nucleation site. The bubble is easier to coalesce under higher heat flux, and also higher heat flux results in higher sliding bubble velocity. Besides, the system pressure has a significant effect on the bubble growth and coalescence. Under higher system pressure, bubble departing from the nucleation site slides along the heated wall and frequently coalesces with the previous bubble. The sliding bubble velocity after coalescence is approximately twice than before. A predictive correlation for sliding bubble velocity is presented in this paper and good agreement is achieved when comparing with experimental results.

Experimental Investigation on the Characteristics of Bubble Growth and Slide on a Downward-Facing Heater Surface in Flow Boiling

Experimental Investigation on the Characteristics of Bubble Growth and Slide on a Downward-Facing Heater Surface in Flow Boiling

Numerical modeling of single droplet flash boiling behavior of e-fuels considering internal and external vaporization

In recent years, significant efforts have been made to develop e-fuels from renewable electricity and carbon sources for enabling highly efficient and advanced propulsion systems. Compared to conventional fuels, such fuels can have very different thermo-physical properties depending on their molecular structure. Particularly, fuels with high vapor pressures are highly susceptible to flash boiling depending on boundary conditions, which can significantly alter the spray formation and mixing behavior. Thus, it becomes imperative to develop a fundamental understanding of the underlying physics associated with the flash boiling of these fuels in a single droplet configuration. In this work, oxymethylene ethers (OMEx) are chosen as a generic example to study the flashing behavior of newly developed e-fuels. This study employs the Lagrangian Particle Tracking (LPT) approach considering both internal and external vaporization of flash boiling single droplets. The internal vaporization model includes several sub-models that compute bubble number density, bubble growth rate, and droplet bursting criterion. External vaporization is modeled considering heat transfer from the droplet interior to the droplet surface and from the surrounding gas to the droplet surface. The study reveals that the formation and subsequent growth of vapor bubble nuclei is the primary source causing the transition of the metastable liquid phase into the stable state. We found that for moderate to high superheating degree, the bubble growth characteristics indicate three distinct growth phases: (1) surface tension-controlled, (2) transition, and (3) inertia-controlled, whereas, for low superheating degree, only two of these were present, namely (1) surface tension-controlled, and (2) transition phase. It is also observed that the chain length of OMEx has significant impact on bubble dynamics. OME4 is found to have a larger critical nucleus, a longer time delay in bubble growth, and a slower growth rate compared with dimethyl ether (DME). Furthermore, a quantitative analysis shows that droplets burst earlier with increasing superheating degree. In addition, it is found that the system pressure has a negligible influence on the initiation of the bursting process, except when the superheating degree is very low.

Experimental investigation on bubble growth and detachment characteristics on vertical microelectrode surface under electrode-normal magnetic field in water electrolysis

The effects of current density and electrode-normal magnetic fields on the growth and detachment characteristics of a single bubble on vertical microelectrode surface have been investigated. A high-speed camera was used to capture the bubble evolution behavior and the bubble contact characteristic parameters were extracted and analyzed with OpenCV-Python program. The results reveal that an apparent bubble coalescence behavior occurs at low current densities and can be gradually inhibited with increasing current density. With the increase of current density, the bubble growth rate, departure diameter, working electrode potential and potential fluctuations increase, while the bubble growth time first increases and then decreases continuously. The upper microelectrode surface is more easily covered than the lower microelectrode surface. The whole microelectrode can be completely covered when the current density exceeds a certain limit with and without magnetic fields. The external magnetic fields can obviously promote the bubble detachment behavior within a relatively large current density range.

Bubble Growth in NaCl Solution: Influence of Superheat Temperature and Solution Concentration

An in-depth understanding of vapor bubble growth characteristics is of great importance for the design and optimization of gas–liquid two-phase flow instruments, which are widely utilized in chemical and thermal engineering processes. The effects of dissolved salts on vapor bubble growth remain unclear at present. In this work, we developed a surface-based bubble growth model to analyze the bubble growth rates and interface hydrodynamics in sodium chloride solution by considering the influence of dissolved salt on bubble growth. The effects of the mass fraction of NaCl and the superheat temperature on bubble growth rates and interface parameters were studied. Further, the characteristics of three typical bubble growth stages were discussed, and the roles of interfacial forces under different operating conditions were demonstrated. The results indicate that the bubble growth rates increased with the increase of the superheat temperature and mass fraction. The effects of the superheat temperature on the bubble duration time are more significant than the NaCl concentration. Besides, the validated surface-based bubble growth model could be extended to other electrolytes by modified solution properties and interface evaporation parameters.

Effect of Heat Transfer on the Growing Bubble with the Nanoparticles/Water Nanofluids in Turbulent Flow

This paper is devoted to study the effect of heat transfer on the temperature distribution in a superheated liquid during the growth of vapour bubbles immersed in different types of nanoparticles/water nanofluids between two-phase turbulent flow. The mathematical model is formulated and solved analytically depending on Scriven's theory and using the modification of the method of the similarity parameters between two finite boundaries. The characteristics of vapour bubble growth and temperature distribution are obtained by using the thermo-physical properties of nanoparticles nanofluids. The results indicate that the nanoparticle volume concentration reduces the bubble growth process under the effect of heat transfer. The better agreements are achieved, for bubble dynamics in turbulent nanofluid using the appropriate numerical and theoretical data for the values of concentration rate of nanoparticles χ=0,0.2,0.4. The temperature distribution surrounding the regime of bubble growth in pure water is more intensive than in other cases of Al2O3/H2O, Fe3O4/H2O and CuO/H2O nanofluids in turbulent flow. A Comparison of the current solution with previous works is carried out and discussed.

Applying the Lattice Boltzmann Method to Simulate Bubble Growth in Porous Media for PEM Water Electrolysis

Polymer electrolyte membrane water electrolysis (PEMWE) is one of the most appealing options to produce hydrogen and oxygen from water. Understanding bubble growth and flow transport behavior is essential to improve the cell performance and reduce the cost of PEMWE devices, especially at high current densities. This work presents and discusses the modeling results obtained for PEMWE under different operating conditions. In this work, the Lattice Boltzmann Method (LBM) [1-3] of computational fluid dynamics (CFD) is used to investigate the characteristics of bubble growth and transport through the porous transport layer (PTL) used in PEMWE. Studies of the evolution, growth profile, size distribution, and velocity distribution of propagating bubbles are explored. The model geometries of PTLs used in this study were obtained via X-ray computed tomography (CT). Figure 1 shows the bubbles in the PTL after they have detached from the electrode surface. This approach is able to examine the bubble characteristic regarding cell operating condition changes within different PTL samples. These findings should be useful for improving the modeling of bubble transport and enhancing our understanding of processes in PEM water electrolysis.

Numerical Simulation on Bubble Growth and Micro-explosion Characteristics of Biodiesel/ethanol Fuel Droplets

In this paper, the biodiesel/ethanol gas-containing fuel droplet was numerically simulated at room temperature. The growth characteristics of the bubble growth process were obtained, and the influencing factors affecting the bubble growth and micro-explosion process were explored. Studies have shown that bubble growth and droplet micro-explosion at room temperature are dominated by the pressure difference between the inside and outside of the bubble. Within a certain range, reducing the initial radius ratio of droplets as much as possible can also promote the bubble growth and micro-explosion of droplets. The proportion of fuel in the droplet will also affect the bubble growth and micro-explosion characteristics.

Experimental investigation of the heat transfer performance of an oscillating heat pipe with LiCl salt solution

The heat transfer performance of oscillating heat pipes (OHPs) with LiCl solution was investigated experimentally at the concentration of 5wt%, 10wt% and 15wt% with the heat input ranging from 10 to 100 W and the filling ratio (FR) of 45%, 55%, 62%, 70%, 80%, and 90%. OHP with different concentrations of LiCl solution can improve the heat transfer performance compared with deionized water (DW) in most conditions, because of the combined positive effects of bubble characteristics, absorption and generation mechanism, two-phase flow, mass transfer, and other behaviors caused by hygroscopicity of LiCl solution. At an appropriate filling ratio (62% and 70%), the addition of LiCl in DW reduces the thermal resistance of OHP. The thermal resistance of a 62% volume of 10wt% LiCl solution reduces up to 61% compared with DW. At low filling ratio (45% and 55%), LiCl solution can reduce the temperature of the evaporation section and improve the anti-dry-out ability; simultaneously, the higher the concentration, the better the effect. It is due to the low water activity and the characteristics of bubble generation and growth in LiCl solution. At increased filling ratio (80% and 90%), the reduction of the gas phase space inside the OHP and the slowing down of the working fluid make the effect of adding LiCl less on the heat transfer performance, 5wt% LiCl solution performs better than the other concentrations.

Analysis of bubble growth and motion dynamics in superheated liquid during flash evaporation

This study proposes a mathematical model for studying the growth and rise of a bubble in a superheated liquid during flash evaporation. The motion of the bubble is simulated based on force balance by considering various forces acting on the bubble, especially including the history force and the pressure force from a decreasing pressure field. The transformation of the rising bubble and its effect on interfacial heat and mass transfer are considered. Using this model, bubble growth and motion characteristics in the superheated liquid are analyzed in detail, and the effects of history and depressurization forces on both bubble motion and growth are examined. The results present a strong coupling of bubble growth with motion. A slow rise velocity of bubble at the late transition stage can significantly promote bubble growth, and the bubble growth even contributes more to the deceleration of bubble rise compared with the drag force. The history force is found insignificant for bubble rise and growth if the bubble has a relatively high growth rate in water at a superheated degree greater than 3.9 K in this study. The additional force from rapid depressurization provides a large pulse acceleration on the bubble. However, the bubble rise and growth are only sensitive to the depressurization rate for slow depressurizations. As the depressurization rate increases, the growth and rise curves of the bubble gradually converge to the case of instantaneous depressurization. In addition, the bubble rise velocity is lower for a higher overall pressure drop owing to a higher bubble growth rate. The proposed model is finally extended to simulate bubble growth and rise in superheated NaCl solution by revising the thermodynamic conditions at the bubble/solution interface based on the two-characteristic-parameter correlation model.

Pool boiling performance and bubble dynamics on graphene oxide nanocoating surface

The graphene oxide (GO) nanocoating surface was fabricated by GO nanoparticles self-assembly on plain copper surface under nucleate pool boiling. Saturated and subcooled pool boiling experiments on GO nanocoating surface with distilled water at atmospheric pressure were conducted. Saturated pool boiling performance indicated enhancements of 78% in critical heat flux (CHF) and 41% in maximum heat transfer coefficient (HTC) when compared with the plain copper surface. It was found that the enhancement was mainly attributed to the improved wettability and high thermal conductivity of GO nanocoating, as well as the increased surface roughness. Subcooled pool boiling results showed that the liquid subcooling had significant effect on heat transfer performance of GO nanocoating surface. The CHF increased linearly with the increased subcooling degree, and the maximum HTC increased almost linearly with the increased subcooling degree. The CHF reached 274 W/cm2 at subcooling degree of 19 K, which translated an enhancement of 128% when compared with the plain copper surface. This high CHF can address the heat dissipation bottleneck in ultra high-power density electronic devices. To better understand the subcooling effect on nucleate boiling, visualization studies on bubble growth characteristics at subcooled pool boiling were investigated with a high-speed digital camera. It was shown that bubbles were smaller and grew more slowly with the increase of liquid subcooling, and microbubble jets were observed at moderate heat flux regime.

Heat Transfer Modeling of Confined Bubble Evaporation in a Microchannel

AbstractIn this study, we model the heat transfer mechanism for a single nucleate vapor bubble which then expands to an elongated bubble during evaporation in a microchannel. The model is a logical mix of empirical correlations and analytical models and is defined for the two steps of bubble growth including: (a) partially confined growth, in which the bubble expands from nucleation site until it fills the entire cross‐section of the channel, and (b) fully confined growth, in which the bubble reaches the side walls and begins to elongate axially in downstream direction. To estimate the heat transfer characteristics of vapor bubble growth during evaporation in the microchannel, the time variation of liquid film thickness and bubble nose position are evaluated. Finally, a time‐averaged value of local heat transfer coefficient is obtained for a period of time using available heat transfer correlations for each heat transfer process separately.

A mathematical study of bubble dynamics in superheated sodium chloride solution

A mathematical study of bubble growth in the superheated binary solution with a non-volatile solute is conducted by solving the momentum, energy and species equations around the bubble. Additionally, the two-characteristic-parameter correlation (TCPC) model is used to identify the thermodynamic conditions at the bubble-solution interface. By considering a single bubble and ignoring the interactions between the bubbles, the bubble growth characteristics in the NaCl solution are analyzed in detail, and the effects of NaCl concentration and operating conditions on bubble growth are discussed. It is observed that the non-volatile solute of NaCl slows down the bubble growth process in the entire duration of the growth period due to the extension of delay time and reduction of the driving forces with increase of solute concentration. The effects of solute concentration on bubble growth cannot be entirely eliminated by modifying the operating conditions to give the same degree of superheat by compensating the boiling point elevation (BPE). The increase of solute concentration near the bubble at moderate or high superheated conditions has non-negligible influence on bubble growth. A revised Mikic-Rohsenow-Griffith (MRG) formula by considering the solute effect and the increase in solute concentration near the bubble surface is proposed to predict the bubble growth in salt solutions. The predictions from the revised formula agree well with the complete simulations of bubble growth at various salt concentrations.

Investigating effects of heating orientations on nucleate boiling heat transfer, bubble dynamics, and wall heat flux partition boiling model for pool boiling

Studying the nucleate pool boiling is an important research in the thermal science. Therefore, the pool boiling experiments are performed in this paper to investigate the effects of heating orientations on the characteristics of boiling heat transfer and bubble dynamics. Based on the measured data, the boiling heat transfer capability increases as the inclined angle of heating surface is elevated. This enhanced effect is pronounced in the lower heat flux region and is insignificant in the higher heat flux region, which is also reported in the previous works. The characteristics of bubble growth, departure, sliding, and coalescence as well as the corresponding timings can be clearly revealed in the frames taken from the high-speed videos. Based on the measurement and observation, the merged bubble diameter and departure frequency increase with the increasing heating orientation. However, the active nucleate site density is measured to be independent of the inclined angles. The relationship of bubble dynamics with the wall superheat and the heating orientation can be obtained using the least-square-error regression method. With the input of these regressed correlations, the wall heat flux partitioning model can be assessed with the present measured boiling curves under various heating orientations. Good agreement reveals that the measured results and the corresponding regression equations provided herein can assist in validation of other boiling heat transfer models. This is one of main contributions from the present work in addition to providing additional data to understand the boiling characteristics of pool boiling under variable heating orientations.