Bubble Growth Behavior Research Articles

Study of Orifice Design on Oleo-Pneumatic Shock Absorber

Aircraft oil-strut shock absorbers rely on orifice designs to control fluid flow and optimize damping performance. However, the complex nature of cavitating flows poses significant challenges in predicting the influence of orifice geometry on energy dissipation and system reliability. This study presents a comprehensive computational fluid dynamics (CFD) analysis of the effects of circular, rectangular, semicircular, and cutback orifice profiles on the internal flow characteristics and damping behavior of oleo-pneumatic shock absorbers. High-fidelity simulations reveal that the rectangular orifice generates higher damping pressures and velocity magnitude than those generated by others designs, while the semicircular shape reduces cavitation inception and exhibits a more gradual pressure recovery. Furthermore, the study highlights the importance of considering both geometric and thermodynamic factors in the design and analysis of cavitating flow systems, as liquid properties and vapor pressure significantly impact bubble growth and collapse behavior. Increasing the orifice length had a negligible impact on damping but moderately raised orifice velocities. This research provides valuable insights for optimizing shock absorber performance across a range of operating conditions, ultimately enhancing vehicle safety and passenger comfort.

Prediction of the Bubble Growth Behavior by Means of the Time-, Temperature-, Pressure- and Blowing Agent Concentration-Dependent Transient Elongational Viscosity Function of Polymers.

Bubble growth processes are highly complex processes, which are not only dependent on the foaming process parameters (temperature, pressure and blowing agent concentration) but also on the type and structure of the polymer used. Since the elongational viscosity at the bubble wall during bubble growth also depends on these influencing factors, the so-called transient elongational viscosity plays a key role in describing the gas bubble growth behavior in polymer melts. The model-based description of the transient elongational viscosity function is difficult due to its dependence on time, Hencky strain and strain rate. Therefore, representative viscosities or shear viscosity models are usually used in the literature to predict the bubble growth behavior. In this work, the transient equibiaxial elongational viscosity function at the bubble wall during bubble growth is described holistically for the first time. This is achieved by extending the so-called molecular stress function (MSF) model by superposition principles (temperature, pressure and blowing agent concentration) and by using the elongational deformation behavior (Hencky strain and strain rate) at the bubble wall during the initial, and thus viscosity-driven, bubble growth process. Therefore, transient uniaxial elongational viscosity measurements are performed and the non-linear MSF model parameters of the two investigated polymers PS (linear polymer chains) and PLA (long-chain branched polymer chains) are determined. By applying the superposition principles and by changing the strain mode parameter to the equibiaxial case in the MSF model, the transient equibiaxial viscosity master curve is obtained and used to describe the bubble growth process. The results show that the extended MSF model can fully predict the transient equibiaxial elongational viscosity function at the bubble wall during bubble growth processes. The bubble growth behavior over time can then be realistically described using the defined transient equibiaxial elongational viscosity function at the bubble wall. This is not possible, for example, with a representative viscosity and therefore clearly demonstrates the influence and importance of knowing the transient deformation behavior that prevails at the bubble wall during bubble growth processes.

Phase-field simulation on fission gas release behavior of large grain UO<sub>2</sub> fuel

In order to predict the release behavior of fission gas in large grain UO<sub>2</sub> fuel and provide support for the development of accident tolerant fuel, a phase-field model is used to simulate the release behavior of fission gas in the microstructure of UO<sub>2</sub> polycrystalline in this work. This model adopts a set of coupled Cahn-Hilliard equations and Allen-Cahn equations, using conserved field variables to represent the distribution of fission gas and vacancies, and distinguishing bubble phase from matrix phase by using order parameters. This model focuses on investigating the effects of different grain sizes, temperature conditions, and diffusion coefficients on the release behavior of fission gas, demonstrating the nucleation, growth, and fusion behavior of bubbles. Simulation results are obtained for fuel porosity, bubble coverage on grain boundaries, and average bubble radius at a certain degree of burnup. The results show that temperature and diffusion coefficient have a significant influence on porosity and bubble coverage on grain boundaries. When the diffusion coefficient is high, grain size also has a significant influence on fission gas release behavior. And when the diffusion coefficient is low, the influence of grain size is not significant. In addition, the distribution of fission gas bubbles under high burnup obtained through this model is also in good agreement with experimental result. The model can predict the behavior of fission gas release in large grain UO<sub>2</sub> fuel.

Motion Behaviors of Two Ascending Bubbles in a Wide Range of Viscous Liquid

Foaming devolatilization is a common phenomenon in a process of viscous solution. Bubble growth and motion behaviors depends on the flow conditions and the properties of liquid phase and that is closely related to heat and mass transfer efficiency in devolatilization process. Herein, experimental study of two ascending bubbles in a wide range of viscous fluid were conducted to achieve an understanding of the formation, coalescence and escape. As a supplement and verification, the motion behaviors rising bubbles were also investigated numerically. Results showed that the first bubble generated a tail vortex to affect the next bubble entering the region. The escape time also increased from 0.3 s to 65 s as the fluid viscosity charged from 0.05.Pa · s to 447 Pa · s, and the bubble broke and it could not formed completely with a high viscous fluid in large rate. It was expected the experimental escape time of bubble in different flow rate was in good agreement with the numerical results.

Experimental Study on the Effect of Microchannel Spacing and Fractal Angle on Bubble Growth Behavior

Bubble growth behavior significantly influences boiling heat transfer performance, and different microchannel structures and configurations affect bubble growth behavior. To explore the impact of microchannel structures and configurations on the growth behavior of boiling bubbles, two types of microchannel test plates were fabricated on copper substrates using laser machining technology. It was a parallel configuration plate with five different microchannel spacings and a blade vein configuration plate with four different fractal angles. The bubble growth behavior on these two types of surfaces was studied through visual experiments. The results show that smaller microchannel spacing leads to earlier bubble coalescence and departure times under the same degree of superheat. The 3.00 mm microchannel spacing is the critical interfering distance for the parallel configuration plates, while interference behavior occurs for the bubbles on the simulated vein configuration plates at any fractal angle. Furthermore, in different ranges of superheat, the bubble departure diameter increases with increasing superheat, and the frequency of bubble departure initially increases and then decreases with increasing superheat. This study provides experimental data support and design reference for the design of heat transfer plate structures.

Influence of the wettability dividing line on bubble growth and detachment behavior in a von Kármán swirling flow field

The contact-angle (CA) hysteresis generated by the difference between the advancing and receding CAs of a droplet on an inclined surface generates an adhesion force that can hold the droplet on the surface. Similarly, the CA hysteresis generated by a wettability dividing line (WDL) between the superhydrophobic and superhydrophilic regions can improve the anti-shear stability of bubbles in water, with a maximum range of more than 150° for the difference between the advancing and receding CAs. Based on this mechanism, the influence of a WDL on the growth and detachment behavior of bubbles at different Reynolds numbers (Re) in a von Kármán swirling flow field was experimentally studied. The results show that the WDL significantly improves the anti-shear stability of bubbles when compared to uniform superhydrophobic and hydrophilic surfaces. Furthermore, the detachment volume and maximum contact width decrease with increasing Re, while the tilt angle and asymmetry ratio increase significantly. The fluctuation amplitudes of these parameters gradually increase with rising Re. Based on variations in these parameters, bubbles' growth behavior can be classified into three modes: stabilization, transition, and oscillation. Additionally, the constraint mechanism of a WDL on a bubble was analyzed based on variations in the CA hysteresis generated by the WDL and the maximum contact width. Finally, the detachment dynamics of bubbles in the three modes were analyzed based on a force–balance model.

Depressurization induced vapor bubble growth in liquid methane during microgravity

Depressurization and re-pressurization maneuvers are used for cryogenic propellants in spacecraft tanks to mitigate the incoming heat flow and to maintain an appropriate net positive suction head (NPSH). A depressurization maneuver can be used to transfer thermal energy from liquid to vapor and thus allow for subcooling of the liquid by a following, quick pressurization. The depressurization will lead to phase change, caused by the superheat relative to the initial saturation conditions, and the following pressurization will subcool the bulk liquid. Phase change will occur at any of the free surfaces of the bulk liquid or at nucleation sites in cavities at a wall. The mechanism which governs phase change at nucleation sites differs from nucleate boiling due to the temperature distribution in the liquid. In this study, we investigate the behavior of a single nucleation site during a depressurization under microgravity. We observed the bubble growth and evaluated the bubble radius with time. A total of five experiments were performed. The thermodynamic conditions are known and therefore can be correlated to the vapor bubble growth behavior. The superheat in the system varies with time, thus simple models are not applicable but can be used to estimate the bubble size. The data can be used as a reference for further analytical or numerical investigations.

Helium atom diffusion in titanium tritides (TiT2-x, −0.02 ≤ x ≤ 0.25): A first-principles and kinetic Monte Carlo study

The tritium (T, or 3H) and helium (3He) diffusion properties are critical to the helium bubble nucleation and growth behaviors in tritium storage materials such as titanium tritide (TiT2-x). Hereby, the possible atom migration paths of T and He in TiT2-x as well as their barriers and prefactors are calculated by density functional theory calculations, and are further applied in kinetic Monte Carlo simulations to determine the overall diffusion rates. The correlations of diffusion coefficients of the He atom with temperature in different TiT2-x (0 ≤ x ≤ 0.25) are shown as Arrhenius plots. The results show that two diffusional regimes, migration more via tetrahedral vacancies or octahedral interstitials, are presented depending on temperature and composition for both stoichiometric and non-stoichiometric tritides. Comparing with T, He exhibits similar diffusion mechanisms but faster diffusion rates. Our results suggest that the tritium (or vacancy) concentration, temperature, and axial strain are effective for tuning the diffusion mechanism and rate of a helium atom in titanium tritides.

Bubble growth and departure behavior in subcooled flow boiling regime

Understanding the detailed dynamics of bubble ebullition cycle is central to effectively exploit coolant phase change in subcooled flow boiling. In the present study, the bubble growth rate and the bubble departure mechanism in subcooled flow boiling conditions are investigated. The bubble growth rate is estimated by employing an energy balance model, which ensures that the applied wall heat flux contribution to components, such as (i) microlayer evaporation, (ii) conduction through superheated layer region, and (iii) condensation heat transfer, are accounted. The bubble departure diameter and the type of departure (i.e., sliding or lift-off) are predicted using a simple force balance model on the vapor bubble. The implemented models are thoroughly validated against the benchmark experimental data. Furthermore, the influence of operating conditions, viz., pressure (1–3 bar), mass flux (200–1000 kgm−2s−1), heat flux (200–500 kWm−2), and the degree of subcooling (20–30 K) on the bubble dynamics, is investigated for subcooled flow boiling conditions. Based on the parameters considered, a flow regime map is developed to identify the bubble departure type and diameter for a given set of operating conditions. It was noticed that the bubble departure diameter was maximum at low pressure (1 bar), low mass flux (200 kgm−2s−1), high heat flux (500 kWm−2), and low degree of subcooling (20 °C). For all the values of pressure and degree of subcooling, the sliding mode of departure was noticed at low and high mass flux values, whereas bubble departs by lift-off for moderate values of mass flux.

Single bubble nucleation in water-filled 59-nm nanochannel

Bubble nucleation, by surface heating, is simulated in a water-filled 59 nm height silicon-dioxide nanochannel using computational fluid dynamics. Disjoining pressure of water is integrated in the continuum-level simulations to capture the molecular effects of nanoscale confinement. Input parameters of imposed heat flux and phase-transition coefficient are widely varied to perform a set of 50 simulations. Bubble nucleation occurs within a narrow temperature range of 124.7 ± 2.5 °C, and in excellent agreement with prior experimental finding. Onset of bubble nucleation occurs at the center of the channel rather than at the surface due to the presence of disjoining pressure, i.e., no three-phase contact line forms and a water film is present between the bubble and the surface. Post nucleation, three distinct paths of bubble collapse, steady-state, and continuous growth are observed, with the last case leading to formation of three-phase contact line. Such behavior is attributed to a critical thickness of the water film and associated disjoining pressure. The present work sheds light on the importance of disjoining pressure and its dominance in governing the bubble onset and growth behavior. Such fundamental understanding can potentially advance the development of future thermal management devices, such as heat pipes.

Dimensional analysis of vapor bubble growth considering bubble–bubble interactions in flash boiling microdroplets of highly volatile liquid electrofuels

Electrofuels (e-fuels) produced from renewable electricity and carbon sources have gained significant attention in recent years as promising alternatives to fossil fuels for the transportation sector. However, the highly volatile e-fuels, such as short-chain oxymethylene ethers (OMEx) are prone to flash vaporization phenomena, which is associated with the formation and growth of vapor bubbles, followed by explosive bursting of the liquid jet. This phenomenon is important in many practical applications, for example, superheated liquid sprays in gasoline direct injection engines as well as cryogenic engines. The simulation of a flash boiling spray of such highly volatile liquid fuels is numerically challenging due to several reasons, including (1) the complexity of the bubble growth process in the presence of multiple vapor bubbles and (2) the need to use an extremely small time step size to accurately capture the underlying physics associated with the flash boiling process. In this paper, we first present a bubble growth model in flash boiling microdroplets considering bubble–bubble interactions along with the finite droplet size effects. A dimensional analysis of the newly derived Rayleigh–Plesset equation (RPE) with bubble–bubble interactions is then performed for Reynolds numbers of different orders of magnitude to estimate the relative importance of different forces acting on the bubble surface. Based on this analysis, a simplified nondimensional semi-analytical solution for bubble growth, which also includes the bubble–bubble interactions, is derived to estimate the bubble growth behavior with reasonable accuracy using the larger time step sizes for a wide range of operating conditions. The derived semi-analytical solution is shown to be a good approximation for describing the bubble growth rate over the whole lifetime of the bubble, thus making it useful for simulations of superheated sprays with large numbers of droplets and even more bubbles. The bubble–bubble interactions are found to have a significant impact on the bubble growth dynamics and result in delaying the onset of droplet bursting due to the slower growth of the vapor bubble compared to the bubble growth without bubble–bubble interactions. Furthermore, in a comparison with DNS results, the proposed bubble growth model is shown to reasonably capture the impact of bubble interactions leading to smaller volumetric droplet expansion.

Heat Transfer Mechanism Investigation of Bubble Growth on the Superhydrophilic Nano-Structured Surface Using Moving Particle Semi-Implicit Method

The boiling behavior on nano-structured surfaces is a frontier research direction in nuclear engineering. However, the mechanism of boiling heat transfer on nano-structured surfaces is still unclear. In this study, a depletable micro-layer model and the nano-structure model are proposed based on the Moving Particle Semi-implicit (MPS) method coupled with Meshless-Advection using the Flow-directional Local-grid (MAFL) scheme, also known as the MPS-MAFL method. The developed method in this paper establishes a bridge between the nano-scale surface structure heat transfer and the macroscopic bubble boiling. Only by knowing the nanoparticle size, porosity, and thickness of the nano-structure, the heat transfer of the nano-structure can be considered into the macroscopic boiling bubble growth process. The accuracy of the approach is validated by benchmark cases and experiments, respectively. The present method quantitatively simulates the bubble growth behaviors on nano-structured surfaces for the first time. The results indicate that the heat transfer contribution of the micro-layer to bubble growth was not neglectable, while the proportion of heat transfer rate of the micro-layer on the bared surface was 40.55% at ΔTw = 7.22 ℃ and 32.23% at ΔTw = 10.15 ℃. The heat transfer contributions of the micro-layer and the wicked fluid to the bubble growth in the nano-structured heater were about 42.13%, the ratio of them was 14:11. The current study provides a fundamental base for further investigations.

Influence of bubble generation on the microchannel electrochemical gas evolution reaction

To understand the influence of bubble generation on electrochemical reaction in microchannel reactor, visualized experimental research was carried out to analyze the bubble growth and detachment behaviors on needle electrode and investigate the voltage–time diagrams of a typical electrochemical oxidation and hydrogen evolution reaction. Two kinds of gas–liquid flow patterns were observed in the microchannel reactor, which caused different current fluctuations in the linear sweep voltammetry experiment. Quickly bubble rupturing was important for maintaining stable electrochemical reaction by avoiding the block of ionic conduction between electrodes. The voltage fluctuation caused by periodic growth and detachment of bubble on the needle electrode clearly exhibited the variation of electrochemical reaction process. The diameter of the bubble shear-ruptured from the electrode was exponential with the capillary number of electrolyte solution, and the relative diameter of bubble and electrode was linear with the relative voltage fluctuation. The voltage fluctuation was mainly contributed by the bubble induced activation and ohmic overpotentials, which ratios were almost 50% respectively.

Dynamics of growth and detachment of single hydrogen bubble on horizontal and vertical microelectrode surfaces considering liquid microlayer structure in water electrolysis

The dynamics of the growth and detachment of a single hydrogen bubble on both the horizontal and vertical microelectrode surfaces in water electrolysis were synthetically investigated by combining the numerical simulation, force balance analysis, and available experimental data. Approximately, multiple steady simulation cases with different bubble diameters for different growth instances were conducted to state the actual unsteady bubble growth and detachment behavior. The numerical simulations of the temperature distribution considering the heat transfer caused by the liquid microlayer and induced Marangoni convection effects were performed. Then, a force balance model for predicting the bubble detachment diameter was developed by fully utilizing the simulated multi-physical field parameters and the experimental results of some key bubble geometric parameters. The presented numerical model and the force balance model were validated by comparing them with previous experimental data on the potential and the bubble detachment diameter, respectively. The simulation results indicate a significantly larger potential value occurs within the microlayer, and hence, the Joule heat of the electrolyte is mainly generated in the microlayer and then transferred to the bulk flow region. Obviously, the temperature gradient distribution is formed at the bubble interface, causing unstable Marangoni convection structure. The distribution patterns and evolutions of the electrolyte temperature, Marangoni convection velocity, and microlayer thickness for the horizontal and vertical microelectrode systems are significantly different. The present force balance model presents higher prediction accuracy for the bubble detachment diameters. Moreover, the in-depth force analysis results reveal that some dominant forces influence the bubble growth and detachment.

Experimental and LBM simulation study on the bubble dynamic behaviors in subcooled flow boiling

Bubble dynamic behavior is a fundamental issue in comprehending flow boiling process. In this paper, the bubble characteristics are studied in depth through experiment in conjunction with the Lattice Boltzmann method (LBM). Firstly, the visualized experiment in rectangular channels with double heating plates is performed during subcooled flow boiling. High-speed camera is employed to directly capture the macroscopic bubble dynamic behaviors under different working conditions. Furthermore, LBM model with a novel hybrid boundary scheme is developed to investigate mesoscopic bubble characteristics, including contact diameter, microlayer thickness profile and superheated layer thickness. The bubble diameter from LBM model is compared to experimental data with a mean relative error of 10.8%. The results of the current research find that the bubbles nucleating on the inside channel have smaller contact diameter ratio and larger inclined angle. Finally, combining macroscopic experiments and mesoscopic LBM studies, a model of the bubble growth rate is proposed for predicting bubble growth behavior. In the process of model development, three aspects for deciding the bubble growth rate are considered, including microlayer evaporation, heat diffusion from the superheated liquid layer, and condensation at bubble dome caused by subcooled flow boiling. The prediction model agrees well with the experimental results, with a mean relative error of 14.7%. The established bubble growth model will help to predict the heat transfer process during flow boiling.

Numerical Study of Growth of a Vapor Bubble in Superheated Seawater with Time-Varying Pressure

Flash evaporation is used in a wide range of applications from seawater desalination to nuclear and space applications. The fundamental mechanism in flash evaporation is mechanical and thermal non-equilibrium which results in liquid being superheated and subsequent bubble growth. Therefore, study of bubble growth is essential to accurately predict the vapor generation rate during flashing. Accordingly, this article provides a comprehensive numerical study of growth of a single vapor bubble during flash evaporation in seawater. Effects of system pressure, liquid superheat, salt concentration, and time-varying pressure during flash evaporation on bubble growth behavior are studied. The Rayleigh-Plesset equation with energy and salt concentration equations is solved numerically with 4th-order discretization schemes. Excellent agreement between the results of the present study and reported experimental results is found. It is observed that during bubble growth in seawater, salt continuously accumulates at the vapor-liquid interface which consequently reduces the growth rate and degrades thermal performance characterized by Nusselt number. However, it elongates the inertia-controlled regime during which the bubble radius is directly proportional to time. It is shown that properly handling the non-uniform distribution of solute in the liquid boundary layer is key to modeling bubble growth in seawater and binary solutions.

Review on bubble dynamic of subcooled flow boiling-part b: Behavior and models

Subcooled flow boiling is extensively applied in engineering systems and greatly improves efficiency owing to bubbles' existence and associated heat exchange enhancement. However, accidents may occur as a result of the lack of understanding and control over subcooled flow boiling, which may lead to unnecessary losses. In this respect, bubble dynamic of subcooled flow boiling disposes of an important research significance, which pushes further the understanding of the boiling mechanism. The purpose of this review is to discuss bubble dynamic involved in subcooled flow boiling and summarize current research status on the so-far-utilized models and applications in this field. In this respect, this review discussed the bubble behavior of a bubble life cycle briefly, where key bubble dynamic parameters have also been introduced. In addition, bubble growth, departure, and lift-off behavior, as well as influencing factors were discussed. Finally, recent advances in bubble sliding and the application of bubble dynamic in boiling simulation have been presented. This article's content deeply contributes to a further understanding of subcooled flow boiling, and the application of bubble dynamic to realize the heat transfer enhancement, subsequently allowing for gains in system efficiency and safety margins.

Simulation of single bubble dynamic process in pool boiling process under microgravity based on phase field method

We use the phase field method to track the gas–liquid interface based on the gas–liquid two-phase flow in the pool boiling process, and study the bubble nucleation, growth, deformation, departure and other dynamic behaviors on the heating surface under microgravity. By simulating the correlation between liquid undercooling and bubble dynamics, we find that the bubble growth time increases with the increase of liquid undercooling, but the effect of liquid undercooling on bubble height is not significant. Meanwhile, the gas–liquid–solid three-phase contact angle and the gravity level will also have an effect on the bubble growth time and bubble height. With the increase of the contact angle, the bubble growth time and bubble height when the bubble departs also increase. While the effect of gravity level is on the contrary, the smaller the gravity level is, the larger the bubble height and bubble growth time when the bubble separates.

Numerical investigation of phase change heat transfer in a confined micro-space by lattice Boltzmann method

Phase-change heat transfer has attracted wide attention in thermal management of electronic infrastructures, such as the data center and 5G base station antenna. It possesses the characteristics of high equivalent thermal conductivity, rapid heat diffusion and good temperature uniformity. However, the existing thermal solution to advanced high-performance devices becomes more challenging with high heat flux and small heat dissipation area. Current surface modification technology has been applied for enhancing phase-change means in energy-related fields. In this paper, the hybrid lattice Boltzmann (LB) method was utilized to explore vapor-liquid phase-change mechanism and its enhancement in a confined micro-space. Different modified surfaces’ effects on bubble growth behavior and interfacial phase-change heat transfer were respectively discussed. Based on the pseudopotential LB approach and energy equation, the boiling and condensation regimes were quantitatively evaluated with the heat transfer coefficient and transient heat flux. The numerical results indicated that the wettability possessed significant impacts on the primary characteristics of phase-change heat transfer. It was found that hydrophilic contact angle promoted the initial boiling, while hydrophobic one helped to facilitate drop-wise condensation. The hybrid surfaces possess the best performance for the boiling heat transfer enhancement. Both the modified hybrid and wettability gradient surfaces have positive contributions to the condensation heat transfer enhancement. This study is expected to provide a reference for improving phase-change heat transfer technology for sustainable energy applications.

Numerical simulation of single bubble growth in vertical rectangular narrow flow channels

The narrow rectangular channels of compact type reactors rely on boiling to generate bubbles to transfer a large amount of heat. On the one hand, the growth behavior of bubbles is closely related to the heat-mass conversion efficiency. On the other hand, the resulting flow instability and critical heat flow also affect operation safety. We set up a test bench with a visible flow channel to investigate the bubble growth in vertical rectangular narrow channels. The bubbles at the ONB (Onset of Nucleate Boiling) site were observed at 1 atm in subcooled flow boiling water. Two typical bubble growth processes were picked out to simulate numerically. A high-speed camera and thermocouples were used to record bubble contours and wall temperatures in the experiment. The heat flow was obtained based on assumptions and theoretical analysis. A numerical simulation of the whole process was conducted, where the bubble contours and temperature fields can be obtained. According to the simulation results of two examples, the microlayer evaporation at the bubble bottom accounts for about 30% of the total evaporation amount.