Enhanced Flow Boiling Heat Transfer through Geometrical Wettability Patterning

Thermal management is a crucial aspect in various engineering and technological applications, where controlling the temperature of systems and components is vital for efficiency, safety, and reliability. Thermal management covers a wide range of techniques and approaches to manage heat generation and dissipation in devices and systems, from electronic gadgets to industrial machinery. One advanced method in thermal management is enhancing flow boiling heat transfer. Flow boiling is a process where a liquid undergoes phase change to vapor upon absorbing heat from a surface, commonly used in high-heat-flux applications like power generation and electronics cooling. The effectiveness of this process is critical in ensuring optimal performance and preventing overheating. Enhanced flow boiling heat transfer is particularly relevant in high-performance computing, where the demand for efficient cooling solutions is constantly growing due to the increasing power densities of modern CPUs and GPUs. It’s also important in renewable energy systems like concentrated solar power plants, where efficient heat transfer is vital for optimal operation.

To this account, a new study published in ACS Applied Materials & Interfaces Journal by Dr. Christopher Salmean and Professor Huihe Qiu from the Department of Mechanical and Aerospace Engineering at the Hong Kong University of Science and Technology conducted detailed experimental research to enhance flow boiling heat transfer. Their focus was on manipulating surface wettability patterns, specifically exploring the effects of superbiphilic wettability patterns with varied geometries and orientations.

The authors developed surfaces combining superhydrophobic and superhydrophilic areas, using silicon nanograss as the base material. The surfaces were treated to create specific geometrical patterns, such as circles, squares, diamonds, triangles, and a combination of rings and chevrons. The superhydrophobic areas were created using a chemical treatment, while the superhydrophilic areas were achieved through oxygen plasma etching. The authors’ experiments involved a flow boiling system where water was used as the working fluid. They used a unique setup to measure the boiling properties across a range of heat fluxes. High-speed cameras provided visual insights into the boiling process, complementing the quantitative data. The focus was on how bubbles formed, grew, and departed from various patterned surfaces. Two primary effects were observed: the influence of the local contact angle on bubble detachment from superhydrophobic areas and the trapping of droplets inside bubbles on ring-shaped patches, which enhanced the heat transfer.

The study demonstrated a significant enhancement in heat transfer efficiency. The boiling heat transfer coefficient and critical heat flux (CHF) of the heterogeneous surfaces were enhanced by 62% and 24%, respectively, compared to homogeneous surfaces. Salmean and Qiu demonstrated the ease of bubble departure from superhydrophobic patches to be dependent on the interaction between the local contact angle and the bubble’s tilt due to hydrodynamic drag. The ring-shaped superhydrophobic patches could trap droplets inside forming bubbles, thus augmenting the heat transfer. They also established and validated a general model to estimate the ease of bubble departure, incorporating geometric considerations into the analysis. Moreover, their study compared symmetrical (circular, square, diamond-shaped) and asymmetrical (triangular) patterns and found that the geometry and orientation of the patterns significantly influenced boiling performance. Ring and chevron patterns, created by inserting superhydrophilic cut-outs, demonstrated unique characteristics in terms of bubble formation and heat transfer enhancement. The findings highlights the critical role of wettability patterning in boiling heat transfer. By manipulating the surface properties at the microscale, the researchers were able to significantly enhance the boiling performance, opening new avenues in thermal management technologies.

The implications of the study by Salmean and Qiu are vast and transformative. By enhancing the heat transfer coefficient and critical heat flux of heterogeneous surfaces over homogeneous analogues by 62% and 24% respectively, the researchers have set a new benchmark in thermal management. The establishment of a general model to estimate the ease of bubble departure marks a significant stride in predictive capability in this domain. This advancement not only opens new avenues in efficient thermal management but also paves the way for further miniaturization and durability in two-phase cooling systems.

In a statement to Advances In engineering, the authors said: “Our methodology diverges from conventional practices by harnessing the thermal energy inherent in the system, thereby generating supplementary forces capable of expediting the expulsion of nascent bubbles. While the prevailing norm in our domain involves the application of external forces for such purposes, our innovative utilization of thermal energy signifies a departure from conventional paradigms in addressing the dissipation challenges associated with elevated heat fluxes. “