Session Z09: Bubbles: Growth, Heat Transfer and Boiling

Coalescence-Induced Jumping Bubbles during Pool Boiling

As micrometric dew droplets grow on a nanostructured superhydrophobic surface, coalescence events trigger a capillary-inertial jumping-droplet effect.  Here, we explore how this concept of coalescence-induced jumping can be applied to vapor bubbles during pool boiling, as opposed to condensation.  Micro-cavities were laser cut into a slender aluminum substrate, which facilitated clear side-view imaging of microbubbles coalescing at small superheats.  Above a critical bubble diameter of ~100 µm, the surface energy released upon coalescence exceeded the work of adhesion of the pinning necks, enabling early bubble departure.  In contrast, an isolated bubble departing by buoyancy alone was ~1 mm in size.  In addition to the capillary-inertial jumping regime that is analogous to jumping-droplet condensation, we also observed a buoyant-inertial departure at larger sizes, where capillarity only serves to depin the necks and it is buoyancy that balances the inertia of lift-off.  

    

Bubble growth dynamics in a nano-pore via molecular dynamics and phase field simulations

We use phase-field (PF) and molecular dynamics (MD) to simulate the growth process of a single bubble nucleated in a nano-pore. We compare the growth rate and temperature distribution of PF predictions with those from MD simulations applied to a Lennard-Jones system. The domain is composed of a tethered solid wall, a cavity, and a liquid phase that transitions to vapour when the system temperature exceeds the saturation point. A uniform heating is applied at the bottom of the solid surface. The key physical parameters necessary to close the PF model are obtained by solving the inverse problem using the initial stages of the MD simulations. After this short period, both MD and PF simulations are carried out independently. The results show good agreement between the continuum model and the MD data. Interestingly, the transition between the initial stages of the bubble growth can be captured by the MD-PF model. The proposed methodology paves the way for a multi-scale modelling approach to study boiling from first principles including the highly complex phenomenon of nucleation.

    

Near-Wall Bubble Dynamics in Flow Boiling: Insights from Interface Capturing Simulation

Flow boiling allows achieve high heat transfer coefficients due to the combination of the forced convection process with the phase change. However, accurate modeling of this phenomenon is still a challenge. In the current work the flow behavior of HFE-301 in a vertical rectangular channel is modeled using interface capturing approach. Finite-element code PHASTA coupled with level-set method for interface capturing is utilized. To observe multiple nucleation events, nucleation cavities of varied sizes are introduced in the channel. The resulted bubble characteristics (bubble departure diameter, departure frequency and the amount of steam released) are compared against experimental data. Although the smallest cavity size considered is still much larger than the real one, it allows to observe bubble departure diameters close to the experimental values. As the cavity diameter decreases, a neck formed at the bottom of a bubble becomes narrower. Therefore, allowing smaller bubbles depart from a smaller cavity. It has been observed that as bubbles depart, they move toward the center of the channel, but then return back to the wall (bubble bouncing motion). It is noticed that smaller bubbles tend to bounce higher.

    

Effects of a Tube Diameter on Single Bubble Condensation in Subcooled Flow

Bubble condensation that involves an interaction between the bubble and subcooled liquid flow has a significant role in effectively controlling the heat for thermal devices. In the present study, the numerical simulations are performed by using the VOF (Volume of Fluid) multiphase flow model to investigate the effect of the tube diameter on the bubble condensation. As the tube diameter decreases, the condensing bubble persists for a long time, and it disappears at a higher location, indicating that the condensation rate decreases with a decrease of the tube diameter. At a small tube diameter, the heat transfer coefficient of the condensing bubble, which indicates the quantitative parameter of condensation rate, is observed to be smaller than the large tube diameter. Because the subcooled liquid incoming from the inlet does not sufficiently affect the side of the bubble due to the backflow occurring at a narrow gap between the side of the bubble and the tube wall, the heat transfer rate reduces at the small tube diameter.   

Multiphase Flow Vertical Aerated Column with Superhydrophobic Internals using X-ray Computed Tomography and Wire Mesh Sensors

Bubbly flow in bubble column reactors promotes mixing necessary for many chemical processes. We show that if superhydrophobic-coated material is introduced into a bubble column, there can be a substantial difference in gas holdup and earlier initiation of churn-turbulent flow which can alter larger-scale mixing without a need to change the superficial gas velocity. Additionally, pressurized water reactors operate in a single-phase flow. Localized nucleation sites arise to promote convectional heat transfer to the bulk liquid. During an event of flow loss or any condition that can result in bulk liquid temperatures exceeding saturation temperatures, bubbles will form more frequently, creating a boiling casualty. If a superhydrophobic-coated material is introduced into a fuel cell assembly, the vapor bubbles will show an affinity to the air layer in these coatings and therefore escape faster from the flow regime, resulting in a larger margin to reach Critical Heat Flux. As the flow becomes optically opaque at few percent gas phase volume fraction, we utilize two dual plane wire mesh sensors to obtain velocity profiles and bubble size distributions, in addition to the traditional pressure and level-based gas holdup measurements to calculate average phase fraction. A custom build photon-counting dual energy threshold X-ray computed tomography system is employed to get a higher resolution view of the time average phase fraction non-intrusively. We report satisfactory agreement between these gas holdup measurement techniques with differences arising for understood reasons, and use the insight thus yielded to discuss the effect of superhydrophobic surfaces on bubble column flow dynamics.   

Analysis of surface wettability effect on nucleate boiling with a diffuse interface method

Multi-phase systems with phase-change phenomena, in particular boiling, are common in many industrial applications, including power generation plants and thermal management of high-power and high-dissipation-rate micro-devices which would burn out if not cooled properly.

Due to the non-equilibrium thermodynamics and the complexity of coupling the heat and mass transfer in phase-change and surface processes, these systems are difficult to describe accurately. Although experiments have been conducted to study boiling, its mechanisms and heat transfer characteristics are still not understood completely.

We simulate pool boiling using the diffuse interface method (DIM) embedded in our home-grown “TPLS” solver. This method allows the imposition of a boundary condition to prescribe wettability removing the stress singularity at the three-phase contact line, thus enabling us to analyse the role of surface features on heat transfer coefficient, bubble growth and bubble departures. Our framework also allows simulation of populations of bubbles and analyse bubble interactions at varied bubble sizes for different wettabilities as a function of superheat. We compare our simulations with our nucleate boiling experiments using FC72 on silicon surfaces.

Our simulations show the importance of surface tension on departure conditions, suggesting a higher heat transfer coefficient in hydrophilic cases. Conversely, we have found limited bubble growth rate on hydrophobic surfaces. In hydrophobic cases, the larger amount of residual vapour left on the heater surface after bubble departure limits the coolability of the substrate but it might promote the growth of forming bubbles subsequently.   

A Study on Boiling inside Nanopores through Localized Joule Heating

We employ the nanopore bubble generation platform to study post-nucleation explosive boiling at the single cavity limit, having dimensions below the Abbe diffraction limit. When a voltage bias is applied across a thin nanopore filled with electrolyte solution, localized Joule heating accumulates superheat in the liquid within the pore volume. Nucleate, transition, and film boiling structures within nanopores are examined at nanosecond resolutions using acoustic and resistive pulse sensing. During nucleate boiling, a homogeneous bubble nucleates at the pore center followed by its departure. During film boiling, a heterogeneous nano-torus vapor bubble blankets the cylindrical pore surface and undergoes pinned volumetric oscillations in thermal resonance with the Joule heating. As the voltage is increased, nucleate boiling within nanopore transitions towards stable film boiling, wherein intermittent torus bubble oscillations are observed during the transition regime. However, beyond a critical voltage, the nanopore undergoes a reverse transition from film boiling to nucleate boiling. This contrasting boiling structure is theoretically explained through the pinning effect, wherein excess Joule heat production leads to destabilization and collapse of the nano-torus bubble.   

Universal Physical Descriptors of Boiling Heat Transfer from Unsupervised Learning

Understanding the mechanisms during boiling has been challenging due to the high dimensionality and stochasticity of the bubble dynamics. Conventional analysis of boiling images and data are limited to existing knowledge. Unsupervised machine learning is a powerful tool to uncover physical insights into the bubble dynamics during boiling without human supervision. This study demonstrates the universality of physical descriptors of boiling heat transfer extracted from pool boiling experimental images using principal component analysis (PCA), an unsupervised dimensionality reduction algorithm. The physical descriptors are interpretable when compared with conventional parameters such as the size and count of bubbles as well as vapor area fraction for a wide range of heater surface-working fluid pairs. The time-series principal components (PCs) are analyzed, where the correlation between dominant frequencies of the PCs and the heat flux is discovered, linking heat transfer performance with bubble nucleation and coalescence events. The approach works well for both low and high surface tension fluids, as well as diverse engineered heat transfer surfaces.

    

Elongated Cavitation Bubbles Induced by Long-Pulsed Lasers: A Computational Study

Laser-induced cavitation plays a significant role in many engineering and biomedical applications. If the duration of laser pulse is similar to or longer than the local acoustic time scale (i.e. long-pulsed laser), the dynamics of the cavitation bubble is determined not only by the initial event of bubble nucleation, but also by the continuation of laser absorption and phase transition. In this talk, we present a coupled photo-thermal-mechanical model to predict the formation of non-spherical, elongated bubbles due to long-pulsed laser radiation. Key components of the computational framework will be introduced, including the FInite Volume method with Exact multi-material Riemann problems (FIVER), an embedded boundary method for fluid-laser coupling, and a customized local level set method for interface tracking. The numerical results will be compared with high-speed images obtained from Ho:YAG and thulium fiber laser experiments. A parametric study will be presented to investigate the effects of laser beam properties on bubble shape and the efficiency of energy delivery.

    

Not Participating

Mass diffusion into and out of microbubbles is measured in a specialized facility, where bubbles are suspended and exposed to varying pressures and concentrations of non-condensable dissolved gas. The facility is a closed system with dissolved oxygen probe and pressure transducer measuring the dissolved gas content and pressure in the system over the duration of the experiment. The bubbles are pushed upward into a 2x10mm glass channel test section from an inhouse designed and made monodispersed bubble generator. A stepper motor pushes a syringe to inject downward flow into the test section, where the flow balances the buoyancy force of the rising bubble. Thus, the bubble is trapped in the glass test section and is recorded by time-resolved in-line holography. The holograms are reconstructed to calculate the growth rate of bubbles, which is in term is used to estimate the diffusion coefficient at low pressures. The presentation will cover the inhouse designed and made monodispersed bubble generator and the measured mass diffusion coefficient of non-condensable dissolved gas in water down to 10kPa.