Conference Agenda

Microlayers beneath nucleating vapour bubbles are pivotal in enhancing bubble growth through evaporation during boiling, making their formation and depletion critical for accurate boiling heat transfer predictions. Recent studies employing advanced techniques such as Synchrotron X-ray imaging and Direct Numerical Simulations (DNS) have revealed significant morphological variations in microlayers during nucleate pool boiling on micro-structured surfaces. Bubble coalescence, a common phenomenon in nucleate pool boiling, further complicates microlayer dynamics. This study addresses a commonly observed but poorly understood coalescence event, where an ejecting bubble merges with a nucleating bubble on a micro-structured surface. Leveraging Synchrotron X-ray imaging of nucleate pool boiling and DNS of the bubble merging process, we report a jet formation mechanism induced by such a coalescence phenomenon, which leads to the rapid depletion of the microlayer. These findings provide essential insights for improving the accuracy of boiling heat transfer predictions.

Corrosion of structural materials is a critical issue for the nuclear industry. A major challenge concerns devices operating under boiling conditions, where corrosion can be influenced by various factors, in particular wall temperature, heat flux at the wall, and boiling. To enable an accurate modeling of corrosion in such industrial conditions, a comprehensive understanding of bubble behavior and associated thermal characteristics is imperative. This study aims to investigate the bubble dynamics and heat transfer through direct numerical simulations using the well-validated open-source code TRUST/TrioCFD. In this study, two-dimensional axisymmetric simulations are performed to investigate the growth and departure of bubbles originating from a single nucleation seed, focusing particularly on the effect of micro-region adjacent to the liquid-vapor-solid triple contact line and the transient conjugate heat transfer between the fluid and the adjacent solid wall. A multiscale modeling approach is adopted. The CFD-algorithm at the bubble-size scale is coupled to a sub-grid micro-region model. The micro-region model describes the partial wetting case. It takes the wall superheating and microscopic contact angle as inputs, and predicts the apparent contact angle and heat flux. Entire boiling cycles, including growth and departure phases followed by a waiting period, were simulated. We obtained detailed information on wall surface temperature and heat flux, directly applicable in corrosion models. Sensitivity analysis to the wall properties demonstrated that materials with higher thermal diffusivity exhibit a larger apparent contact angle, longer growth time, larger departure diameter, and shorter waiting time.

Dynamical System Scaling (DSS) is an innovative scaling methodology focused on incorporating transient behavior in the scaling criteria. This paper applies DSS to bubble dynamics by comparing experimental data to analytical solutions of the Rayleigh-Plesset equation. The Rayleigh-Plesset equation governs the motion of bubbles in an infinite body of fluid, and it is derived by simplifying the Navier-Stokes momentum equation with spherical symmetry. The motivation of this study is to evaluate a bubble growth correlation for rapid vaporization transients, which was identified in a previous DSS analysis with a peculiar initial growth rate. A simplified Rayleigh-Plesset equation is derived assuming that the bubble growth is solely inertially controlled. Atomic bomb test data is used for comparison purposes as it is presumed to have negligible heat transfer on the shockwave interface. DSS is employed to calculate the distortion between the experimental data and the analytical solution. Additionally, results will be compared to theoretical DSS work previously performed which applied DSS to bubble dynamics. This asymptotic analysis concludes that the correlation is unsupported, and the original test data does not cover the early stage. Therefore, DSS successfully identified that further improvement is needed to the existing correlation.

Small Modular Reactors (SMRs) constitute a significant advancement in nuclear technology, where Pressurized Water Reactors (PWRs) are extensively applied. Within PWRs, pressurizers play a critical role in maintaining pressure and help ensuring thermohydraulic safety. Nitrogen gas pressurizers offer advantages such as rapid response, compact design, and simplicity, rendering them more suitable for SMRs than steam pressurizers. Nonetheless, nitrogen may dissolve in cooling water and desorb as bubbles during pressure transients, leading to two-phase flow and damage reactor safety. Currently, a substantial gap exists in the accurate prediction of the conditions under where and when bubbles nucleate, how they evolute and impact on thermohydraulic safety. To establish a reliable predictive model and identify solutions for enhancing reactor safety, it is imperative to investigate the nucleation and growth behaviors of bubbles under supersaturated conditions during depressurization. This study aims to elucidate the supersaturation ratios at which bubble formation occurs and to characterize their evolution over time. We conducted microscopic experiments and numerical simulations of bubble dynamics at supersaturation ratios ranging from 0.1 to 3 on hydrophilic and hydrophobic surfaces. The results indicate that the initial bubble nuclei sizes are below 20 μm, significantly smaller than the conventional view of over 100 μm. The bubble growth behavior conforms to the Epstein-Plesset equation, and Ostwald ripening occurs at specific sites. Besides, surface wettability is proved to have significant influences on bubble nuclei size and density. These results provide experimental evidence that the existence of nanobubbles may contribute to inaccuracies in classical theoretical predictions.

The behavior of bubbles in multiphase flow systems is critical to many industrial applications, including nuclear reactors and separation processes. While significant research has been done on bubble dynamics, the effect of perforated plates in downward flow conditions remains less explored. This study aims to investigate the dynamics of air bubbles in a downward flow as they interact with perforated plates, focusing on bubble penetration probability and bubble residence time near the plate.

Preliminary results suggest that the perforated plate significantly interrupts the two-phase flow, with a smaller open area ratio leading to fewer bubbles passing through the plate. Experiments were conducted in a vertical water channel with a perforated plate positioned to obstruct the downward flow. To focus on the bubble motion, a single bubble was injected into the channel. The bubble motions were recorded using high-speed imaging, and varying flow rates and hole geometries were tested. The Bond number and other dimensionless parameters were analyzed to understand the characteristics of the flow and the air bubbles.

The study is expected to reveal how flow rate and perforation geometry influence bubble motion when the bubble encounters a perforated plate in downward flow. The findings from this research will contribute to a deeper understanding of bubble behavior in two-phase flow systems with perforated structures, which can inform the design of more efficient separation devices and reactors.

A series of experiments were conducted to investigate bubble characteristics in a vertical annular channel under a wide range of pressure conditions. For this purpose, a custom-designed 4-sensor optical fiber probe (4S-OFP) was developed to measure key local two-phase flow parameters, including local void fraction (α), bubble velocity (V_b), and Sauter mean diameter (D₃₂). The 4S-OFP demonstrated applicability in high-pressure, high-temperature environments up to 15 MPa and 350°C, with measurement uncertainties of approximately 2% for α, 5% for V_b, and 16% for D₃₂. The experimental conditions covered outlet pressures of 0.2–10.0 MPa, mass fluxes of 400–5,000 kg/m²s, inlet subcooling temperatures of 7–25°C, and heat fluxes of 300–620 kW/m². Local bubble parameters were systematically measured and analyzed under varying flow conditions, including mass flux, inlet subcooling, heat flux, and system pressure. The results showed that void fraction was strongly influenced by flow conditions, particularly heat flux and system pressure, while bubble velocity demonstrated a strong sensitivity to changes in mass flux. Additionally, the Sauter mean diameter (D₃₂) exhibited noticeable variations depending on both the inlet subcooling and the system pressure. These findings provide valuable experimental data for validating and improving existing thermal-hydraulic models, particularly under diverse pressure and subcooled boiling flow conditions. The dataset also highlights the importance of precise measurement techniques, such as the 4S-OFP, for advancing the understanding of local bubble dynamics in two-phase flow systems.