Heat sinks having extended surface area can substantially increase the heat dissipation limit of two-phase immersion cooling compared to the critical heat flux limit of flat surfaces. However, the highly nonlinear and boiling-regime-dependent surface boundary condition makes designing an optimal two-phase immersion heat sink a challenge, and no systematic design approach currently exists. In this work, we propose a density-based topology optimization method for designing heat sinks under pool boiling conditions. Boiling on a heat sink is modeled as a conduction problem with convective boundary condition, where the heat transfer coefficient is defined as a function of superheat and spans all boiling regimes (natural convection, nucleate boiling, transition boiling, and film boiling). The heat sink is represented as voxelized density distribution within the design domain, with the thermal conductivity modeled as a function of local density. The boiling boundary conditions are implicitly incorporated at the heat sink interface using an interpolation function of density and superheat. Penalizations are applied to both the thermal conductivity and the boiling boundary conditions to enforce a binary final design, while density filtering and Heaviside projection are applied to control the feature length scale. Optimized heat sink designs are generated iteratively using a multi-objective cost function, with the adjoint method applied to compute sensitivity at each optimization iteration. The resulting coral-like heat sink designs are predicted to effectively dissipate heat at a footprint-area-based power density significantly exceeding the critical heat flux of a flat surface, and are further shown to outperform conventional pin-fin and plate-fin arrays as benchmarks. A detailed analysis of heat sink superheat and heat transfer distributions further reveals that the optimized designs effectively spread the heat load and reduce the conduction resistance by maintaining the surface area with high heat transfer coefficients in the nucleate boiling regime close to the heat input region. The superheat-dependent boundary condition captures the drastic reduction in heat transfer coefficient with transition to film boiling and naturally penalizes operations in such unfavorable regimes, facilitating the generation of heat sinks that avoid thermal runaway. This study demonstrates the effectiveness of proposed topology optimization method in balancing the heat spreading and conduction resistances under a boiling boundary condition to achieve optimal two-phase immersion heat sink performance.

Wettability is a key surface characteristic that affects boiling behavior. Recent studies have shown that surface wettability as described by a static contact angle offers an incomplete description of this behavior, whereas the dynamic wettability described by advancing and receding contact angles is required for accurate description of ebullition dynamics. The observed influence of dynamic surface wettability on boiling performance has been previously explained based on arguments of single-bubble growth and departure; however, the mechanisms by which these dynamic wettability regimes influence boiling performance at high heat fluxes are still unknown. In this study we experimentally reveal the mechanisms by which ambiphilic surfaces (having receding contact angle <90 degrees and advancing angle >90 degrees) offer better boiling performance compared to hygrophilic and hygrophobic surfaces (for which both receding and advancing contact angles are below or above 90 degrees, respectively). These mechanisms are identified through analysis of surface temperature distributions acquired via high-speed infrared (IR) thermography, complemented with side-view visualizations. The low receding contact angle of ambiphilic and hygrophilic surfaces prevents dewetting during bubble growth, thereby preventing the premature critical heat flux (CHF) that occurs for hygrophobic surfaces. Furthermore, the high advancing contact angle of ambi-philic surfaces acts to significantly improve the nucleation rate per unit surface area, which is identified as the primary mechanism of nucleate boiling heat transfer coefficient enhancement over hygrophilic surfaces. These first-of-their-kind observations of the surface temperature distributions during boiling on surfaces spanning all regimes of dynamic wettability provide new understanding of the avenues available for performance enhance-ment by controlling the surface dynamic wettability.

A supercooled liquid droplet that freezes on a cold substrate interacts with the local surroundings through heat and mass exchange. Heat loss occurs to the substrate via conduction and at the droplet interface via evaporative cooling, diffusion, and convection. In a group of many droplets, these interactions are believed to be responsible for inter-droplet frost propagation and the evaporation of supercooled neighboring droplets. Furthermore, interactions between a standalone freezing droplet and its surroundings can lead to the formation of condensation halos and asymmetric solidification induced by external flows. This paper investigates droplet-to-droplet interactions via heat and mass exchange between a freezing droplet and a neighboring droplet, for which asymmetries are observed in the final shape of the frozen droplet. Side-view infrared (IR) thermography measurements of the surface temperature for a pair of freezing droplets, along with three-dimensional numerical simulations of the solidification process, are used to quantify the intensity and nature of these interactions. Two droplet-to-droplet interaction mechanisms causing asymmetric freezing are identified: (1) non-uniform evaporative cooling on the surface of the freezing droplet caused by vapor starvation in the air between the droplets; and (2) a non-uniform thermal resistance at the contact area of the freezing droplet caused by the heat conduction within the neighboring droplet. The combined experimental and numerical results show that the size of the freezing droplet relative to its neighbor can significantly impact the intensity of the interaction between the droplets and, therefore, the degree of asymmetry. A small droplet freezing in the presence of a large droplet, which blocks vapor from freely diffusing to the surface of the small droplet, causes substantial asymmetry in the solidification process. The droplet-to-droplet interactions investigated in this paper provide insights into the role of latent heat dissipation during condensation frosting.

When a liquid droplet freezes on a cooled substrate, the portion of latent heat released by ice formation that is not immediately absorbed by the supercooled liquid droplet is transferred to the solid substrate below the droplet and the surrounding air. It is important to quantify heat dissipation through these two pathways because they govern the propagation of frost between multiple droplets. In this paper, infrared (IR) thermography measurements of the surface of a freezing droplet are used to quantify the fraction of latent heat released to the substrate and the ambient air. These IR measurements also show that the crystallization dynamics are related to the size of the droplet, as the freezing front moves slower in larger droplets. Numerical simulations of the solidification process are performed using the IR temperature data at the contact line of the droplet as a boundary condition. These simulations, which have good agreement with experimentally measured freezing times, reveal that the heat transferred to the substrate through the base contact area of the droplet is best described by a time-dependent temperature boundary condition, contrary to the constant values of base temperature and rates of heat transfer assumed in previous numerical simulations reported in the literature. In further contrast to the highly simplified descriptions of the interaction between a droplet and its surrounding used in previous models, the model developed in the current work accounts for heat conduction, convection, and evaporative cooling at the droplet-air interface. The simulation results indicate that only a small fraction of heat is lost through the droplet-air interface via conduction and evaporative cooling. The heat transfer rate to the substrate of the droplet is shown to be at least one order of magnitude greater than the heat transferred to the ambient air. (C) 2020 Elsevier Ltd. All rights reserved.

Fluid-structure interaction (FSI) is an important fundamental problem with wide scientific and engineering applications. The immersed boundary method has proved to be an effective way to model the interaction between a moving solid and its surrounding fluid. In this study, a novel modeling approach based on the coupled immersed-boundary and finite-volume method is proposed to simulate fluid-structure interaction problems. With this approach, the whole computational domain is treated as fluid and discretized by only one set of Eulerian grids. The computational domain is divided into solid parts and fluid parts. A goal velocity is locally determined in each cell inside the solid part. At the same time, the hydrodynamic force exerted on the solid structure is calculated by integrating along the faces between the solid cells and fluid cells. In this way, the interaction between the solid and fluid is solved explicitly and the costly information transfer between Lagranian grids and Eulerian grids is avoided. The interface is sharply restricted into one single grid width throughout the iterations. The proposed modeling approach is validated by conducting several classic numerical experiments, including flow past static and freely rotatable square cylinders, and sedimentation of an ellipsoid in finite space. Throughout the three numerical experiments, satisfying agreements with literatures have been obtained, which demonstrate that the proposed modeling approach is accurate and robust for simulating FSI problems.