Session R07: Bubbles: Growth, Heat Transfer and Boiling

Conjugate Heat Transfer Coupling for Interface Capturing Simulation of Boiling Phenomena

Onset of film boiling is a very interesting and complex process, particularly for industrial applications such as chemical processing and energy production. To better understand this process, both high resolution experiments and simulations are valuable tools for research. However, very few flow solvers exist capable of large-scale simulations in complex domains. A good candidate for such work is the massively parallel multiphase CFD flow solver PHASTA which has been shown to perform wide variety of adiabatic interface capturing simulations in the past. Additional capabilities must be added to handle the film boiling phenomenon. The focus of the presented research is to develop and implement the Conjugate Heat Transfer (CHT) capabilities within the solver to be coupled with the Parallel Lattice Algorithm for Interphase Dynamics (PLAID) approach which is undergoing verification. A pool boiling simulation is presented intending to emulate the conditions of transition boiling, or surpassing critical heat flux, the simulation seeds multiple bubble nucleation sites on a wall with a transient, linearly varying heat flux applied to the wall. The major focus areas of the study are the implementation of CHT and its coupling with PLAID to deliver a reasonable result, though both algorithms are not mature enough to claim true accuracy. Limited testing of the CHT accuracy is performed and careful inspection of the behavior of both algorithms is described.   

Bubble Generation in Fluid Containing Gold Nanoparticles by Laser Irradiation. (1) Time-resolved visualization

Gold nanoparticles cause "localized surface plasmon resonance" for electromagnetic waves with a specific wavelength. The electrons cause resonant absorption and can be heated efficiently due to electron-phonon interaction. In particular, bubbles will be generated around them when irradiated by a strong enough laser in a liquid environment. The bubbles are known as "plasmonic bubbles." These phenomena are expected to be applied in various fields, such as medical and energy engineering. However, the mechanism of it is not yet fully understood. Therefore, we observed these generated bubbles individually at 900 kfps with a high-speed camera in this study. It was found that the bubbles exhibit complex behaviors, such as collapsing while oscillating their diameters. With the image processing technique, the time dependence of the bubble diameter was analyzed. We have shown that the generated bubbles collapse through two processes. The former is the steam condensation with a collapsing time scale of a few to several tens of microseconds. The latter is the re-dissolution of dissolved air with a collapsing time scale of hundreds to thousands of microseconds.   

Bubble Generation in Fluid Containing Gold Nanoparticles by Laser Irradiation. (2) Evaporation with a long temporal delay

A nanosecond pulse laser irradiated gold nanoparticles in the water, causing plasmonic bubbles around the nanoparticles. According to the simple numerical simulation, the water temperature surrounding the nanoparticles increases over boiling in several nanoseconds. The superheating water generates rapid bubble generation, with some external events, such as pressure pulse. The high-speed camera (360~900kfps) resolved the bubble generation and collapsed with a microsecond scale. We observed the bubble generation at the wall just after the laser irradiation. The gold nanoparticle stacked on the wall may generate a giant wall bubble. Then, at 50 microseconds after the laser irradiation, tiny bubbles were generated in the middle of the cell. This means the superheating water surrounding the nanoparticles remains stable for over 50 microseconds, following boiling and condensation in 10 microseconds. Superheating stabilization will be the key to such a long temporal delay phenomenon.   

Investigation of heat transfer improvement through nucleate boiling by applying the LIF technique

Thermal management has been a critical practice for efficient cooling in various applications, from power plants to electronic industries. Among different cooling strategies, nucleate boiling is one of the preferences due to high latent heat during phase change. A cyclic process initiation, growth, and departure is a core mechanism in nucleate boiling that effectively removes the overheat from the heated surface. Enhancing the performance of heat removal in nucleate boiling, therefore, requires increases in bubble size and departure frequency. This study investigates bubble growth and associated heat transfer characteristics of four engineered fins in water under a subcooled condition, including flat, dimpled, holed, and nanoparticle-coated surfaces. Time-dependent bubble size, departure diameter, growth rate, and departure frequency are estimated using high-speed imaging. A comprehensive study comparing bubble dynamics parameters allows us to determine the fin structure for the efficient mass and heat transfer during bubble growth, subsequently helping us optimize the heat sink for electronic cooling. With an array of optimized fins, a heat sink is designed and tested for the cooling of an electric battery.   

Impact of substrate wettability on Nucleate Boiling Heat Transfer in single and multiple bubble systems: a Direct Numerical Simulation analysis

Phase-change phenomena, especially boiling, are critical in many industrial applications, including power generation plants and thermal management of micro-devices. These devices, noted for their high heat power density and dissipation rates, require sophisticated thermal management systems, essential for both space applications in microgravity and ground applications such as radar systems. Boiling stands out as an efficient cooling method to ensure their reliability.

In this study, we examine the transition from small-scale (O(1)) to large-scale (O(1000)) nucleation sites. We perform direct numerical simulations using our custom TPLS solver, which employs the diffuse-interface method to track the liquid-vapor interface evolution. This technique removes the stress singularity at the three-phase contact line, allowing for the imposition of a contact angle boundary condition to define surface wettability. This approach aids in understanding the impact of surface wettability on the nucleate boiling heat transfer coefficient (NBHTC), as well as bubble growth and departure.

We investigate how wettability affects the heat transfer coefficient, an indicator of energy efficiency, and the interaction between neighbouring nucleated bubbles. Our simulations are validated by in-house nucleate boiling experiments using FC72 on silicon surfaces. We show that hydrophilic substrates improve the NBHTC and assess the residual volumes after bubble departures. To further explore the relationship between nucleation, wettability, and micro-layer formation, we use an enhanced hybrid-pseudopotential lattice Boltzmann method.   

Dynamics of bubble formation on superhydrophobic surface at quasi-static regime

We experimentally studied bubble formation on superhydrophobic surface (SHS) under a constant gas flow rate and at quasi-static regime. The radius of SHS R_SHS varied from 4.2 mm to 19.0 mm, and the gas flow rate varied from 1 to 150 ml/min. We measured the bubble volume, bubble geometrical parameters, contact angle, as well as forces acting on the bubble. We found that as increasing R_SHS, the bubble switched from Mode A where the bubble base pinned at the rim of SHS to Mode B where the contact line didn’t reach to the SHS boundary. Moreover, we found that Q had minor impacts on bubble shape but caused an increase of the bubble detached volume. After proper normalization, the relationship between V_d and Q agreed with these for bubbles detaching from hydrophilic and hydrophobic surfaces. During the necking process, the necking radius followed a similar power-law relation to that for a bubble necking at a nozzle, and the bubble volume was nearly a constant for small Q but increased significantly at large Q. Last, we found that the bubble growth is governed by a balance between one lifting force (pressure force) and two retaining forces (surface tension force and buoyancy force).