Boiling is a critical heat transfer mechanism in power generation, chemical processing, desalination, and electronics thermal management. Its performance is quantified by two key metrics: critical heat flux (CHF), the maximum heat removal rate before transition to film boiling, and heat transfer coefficient (HTC), the efficiency of heat transport per unit area per unit wall superheat. Traditional strategies to enhance CHF and HTC—such as surface micro/nanostructuring—often require intricate surface design and complex fabrication procedures and are not amenable in practical applications. This study investigates externally applied acoustic fields as a noninvasive, dynamically tunable method to control bubble departure during boiling. Acoustic actuation promotes vapor removal through capillary waves, streaming, and radiation pressure, enhancing bubble detachment and suppressing coalescence. Experiments conducted at three voltage levels reveal that moderate acoustic forcing (1 V) increases bubble departure frequency, reduces bubble size, and enhances microlayer rewetting, leading to a 42% increase in CHF and higher HTC across all superheats. In contrast, excessive forcing (6 V) disrupts microlayer stability, reducing HTC. High-speed imaging and heat transfer measurements corroborate these findings, establishing acoustic forcing as a tunable mechanism for optimizing boiling heat transfer, providing advantages over passive surface treatments in high-power-density applications. Copyright © 2025 by ASME

The rising power density in modern power electronic devices, especially those using wide-bandgap (WBG) semiconductors like gallium nitride (GaN) and silicon carbide (SiC), has led to significant thermal management challenges. Localized heat fluxes can exceed 1 kW/cm2, surpassing the capabilities of traditional cooling methods such as indirect liquid or air cooling. In this study, we evaluate direct immersion cooling using dielectric fluid as a scalable solution for managing extreme heat loads in SiC devices. Unlike conventional methods, immersion cooling provides direct access to the heat source, eliminating the need for heat spreaders and thermal interface layers. We experimentally assess the performance of immersion cooling with 3M FC-3283 fluid, employing a four-wire Kelvin setup to measure surface and fluid temperatures, and validate our results using ANSYS Icepak simulations. The system achieved a heat transfer coefficient (HTC) of up to 820 W/m2·K, a 65× improvement over natural convection air cooling (12.5 W/m2·K). Immersion cooling enabled power dissipation up to 21 W, far exceeding the 1.4 W limit of air cooling, while maintaining safe junction temperatures. Simulations showed agreement with experiments, with device temperatures remaining below the 150 °C threshold even at 70 W. These findings establish immersion cooling as a high-performance, scalable solution for next-generation power electronics and offer insights into optimizing fluid selection, PCB design, and packaging for SiC devices. Copyright © 2025 by ASME

Droplet nucleation, growth, and departure during dropwise condensation is not fully understood to date. A model that fully captures this energy dense and often highly stochastic phase change process and the associated energy release is currently missing. In fact, detailed analytical and numerical heat and mass transfer models use numerous hard-to-prove assumptions that require a closer look as more diagnostic tools become available with advances in science. This work revisits and critically analyzes one of the central assumptions that is routinely used in modeling dropwise condensation. We introduce an innovative outside-the-box characterizing technique that utilizes elastic scattering transmission spectroscopy to directly quantify nucleation site density, a major development that will have significant implications in current dropwise condensation models. By applying anomalous diffraction theory and inverse scattering analysis, our novel technique provides useful insight into the probability of nucleation events within a temperature- and pressure-controlled environment that is free from non-condensable gases. The results reported in this study mark the first application of elastic scattering transmission spectroscopy in fluid mechanics and heat transfer, providing an innovative state-of-the-art analytical tool for analyzing thermo-fluidic transport processes at submicron length scale, particularly those processes that lie beyond the resolution limits of conventional imaging techniques. Copyright © 2025 by ASME

Condensation is an essential phase change process in nature and in industry. In nature it provides life-sustaining water to plants and animals. In industry, it is central to numerous engineering processes including power generation, seawater desalination, distillation, environmental control, and electronics thermal management. Current knowledge base in the heat transfer community shows that, expedited droplet departure (i.e., higher departure frequency) during condensation is beneficial for improving the heat transfer rate in dropwise condensation. However, the proof and physical justification for this widely adopted belief is missing in literature. In this work, we critically re-examine and test the validity of this belief and understanding by conducting well-controlled condensation experiments in a temperature and pressure controlled environmental chamber. For this study, condensation on surfaces with varying contact angles are conducted under nearly identical subcooling conditions. The results are analyzed by comparing trends in heat flux, droplet departure frequency, average droplet departure radius, and the maximum droplet radius before departure in between successive sweeping events to put the aforementioned ambiguity to rest. Our results reveal that while condensate droplet departure frequency increases monotonically with contact angle, it does not follow the non-monotonic heat flux trend reported in dropwise condensation. In contrast, the maximum droplet radius on the surface exhibits strong correlation with the experimentally measured heat flux trend, suggesting that maximum droplet size on the condenser surface serves as a more reliable proxy for dropwise condensation heat transfer rate. To explore the underlying mechanism, condensate droplet growth rate behavior is examined across a range of equilibrium contact angles. Our results show that condensate droplet growth rate also increases with increasing contact angle. These observations point to a fundamental trade-off: as contact angle increases, droplets grow faster but remain on the surface for shorter durations. This competing interplay between growth rate and surface retention (or residence time) points to the existence of an optimal contact angle for dropwise condensation. Our findings not only challenge a long-standing belief regarding the direct correlation between departure frequency and heat transfer rate but offer a fresh perspective and new framework for the design and optimization of condenser surfaces. We anticipate this work to sow the seed for follow-up studies that re-examine and re-evaluate key assumptions that are embedded in the widely used dropwise condensation models. Copyright © 2025 by ASME

Since the advent of integrated circuits several decades ago, effective heat dissipation continues to be the primary factor limiting the performance and reliability of modern high-power analog and digital electronics. Due to its high latent heat of phase change, boiling has been considered a viable thermal management solution for electronics. Traditionally, boiling is limited by the critical heat flux (CHF), which is the maximum heating power per unit area that can be dissipated before boiling crisis leading to catastrophic thermal runaway. Here, we use state-of-the-art additive manufacturing to fabricate a boiling surface (15 mm × 15 mm) with slanted copper micropillars (75 µm × 75 µm) at 45°, 60°, 75°, and 90° tilt angle to increase CHF. In our experiments using ethanol, the CHF of the 75 μm tall three-dimensional (3D) printed pillars was ≈65 W/cm2, ≈57 W/cm2, ≈61 W/cm2, and ≈70 W/cm2 for 45°, 60°, 75°, and 90° slant angles. These CHF values are 20-50% higher than the CHF on flat copper plates (≈45 W/cm2). Importantly, we observed early onset of nucleate boiling (ONB) for the 3D printed copper micropillars at ≈2 K superheat as opposed to ≈14 K for the flat copper plates. The early ONB led to high heat transfer coefficient of ≈453 kW/m2·K, ≈154 kW/m2·K, ≈167 kW/m2·K, and ≈418 kW/m2·K for the slanted micropillars at 45°, 60°, 75°, and 90° tilt angles, respectively. This is orders of magnitude higher that the heat transfer coefficient for flat copper plates (19 kW/m2·K). We anticipate improved boiling performance by optimizing the pillar dimensions, arrangement, and tilt angle. The results reported in this study demonstrate the potential of additively manufactured copper micropillars for thermal management applications in electronics cooling. Copyright © 2025 by ASME

As an essential heat and mass transfer process, boiling is widely utilized in advanced cooling strategies, including immersion cooling for electronics. Surface microstructures can enhance boiling heat transfer by affecting bubble nucleation, growth, and departure from the heated surface. Past studies have shown that re-entrant surface structures can improve the boiling heat transfer coefficient. In this study, we investigate the effect of silicon dioxide capping layer thickness on the pool boiling performance of silicon surfaces with well-defined re‑entrant microcavity structures. Re‑entrant microcavities with oxide cap thicknesses of 2 µm, 1 µm, and 0.5 µm were fabricated on silicon surfaces using photolithography and plasma etching. Silicon surfaces with vertical microcavities and flat silicon surfaces were also tested for comparison. Pool boiling experiments were conducted in saturated de‑ionized water using platinum thin‑film heaters on the backside of silicon samples. The results show that the reentrant microcavities triggered nucleate boiling at a lower superheat than the vertical microcavities. The re‑entrant surfaces with a 2 µm thick cap achieved the highest critical heat flux (CHF) of ≈ 156 W/cm2, corresponding to a ≈ 117 % increase over vertical cavities and a 58 % increase over flat surfaces. The re‑entrant surfaces with 1 µm and 0.5 µm caps reached a CHF of ≈ 142 W/cm2 and ≈ 136 W/cm2, respectively. Increasing the capping layer thickness from 0.5 µm to 2 µm resulted in a ≈ 15 % increase in CHF. These results demonstrate that the re‑entrant microgeometry strongly enhances boiling heat transfer. Furthermore, increasing the capping layer thickness improves the CHF for re-entrant microcavity surfaces. This study provides insights into microscale surface modification for enhancing boiling in electronics thermal management. Copyright © 2025 by ASME