11/19/2024 Taylor Parks
Written by Taylor Parks
MechSE fourth-year PhD student Jalal Inanlu and Founder Professor Nenad Miljkovic recently published in the prestigious journal Science Advances. Their study, “Unveiling the fundamentals of flow boiling heat transfer enhancement on structural surfaces,” which was coauthored by several other present and former members of Miljkovic’s research lab, is the first-ever flow boiling paper published in the journal.
Flow boiling refers to a heat transfer process that occurs when a liquid flows over a heated surface, causing the liquid to reach its boiling point and undergo a phase change to vapor. This type of boiling involves forced convection due to the fluid’s motion. Micro- and nano-structured surfaces offer the potential to enhance two-phase heat transfer, which is significant given the increased demand for efficient thermal management systems that accompanies industrial shifts to electrification. Systems that leverage the latent heat of the working fluid experience high energy density and relatively constant streamwise temperature, which is beneficial to many industries such as electronics, air conditioning and refrigeration, and space exploration.
“[Our paper] offers scalable methods for structuring industrially relevant materials using geometries of proper design that enhance heat transfer coefficients,” Miljkovic said. “It also uncovers the previously unknown mechanisms behind these enhancements through very challenging in-situ boroscopy visualizations, allowing engineers and material scientists to move beyond trial and error and create better surface designs for optimal thermal performance.”
Inserting and sealing a boroscope inside an industrial-scale tube (i.e., approximately one meter long) in a pressurized system presented its own challenges. “I had to do a lot of trial and error with different methods,” Inanlu said of arriving at the successful process described in the paper. Further challenges came during data processing,
“We experimented with deep learning-based computer vision codes, but they were mixing upstream bubbles with the bubbles that were generating at test sites,” Inanlu explained. “We had to do everything manually and process the frame by frame and pixel by pixel.”
The team’s extra efforts in investigating the microscale bubble dynamics paid off, as they achieved heat transfer coefficient enhancements up to 391% for aluminum tubing and 41% for copper. Furthermore, based on evidence from their visualizations, the team demonstrated microstructures to be significantly more effective than internal fins, which are currently widely used in industry, for enhancing flow boiling performance. Internal fins primarily improve the convective heat transfer of the liquid phase, offering comparatively limited impact on overall heat transfer enhancement. In contrast, microstructures amplify nucleate boiling, a process responsible for substantial heat removal during two-phase flow. Micro-structured tubes achieve these benefits with a notably lower pressure drop, which positions their design as an ideal alternative to conventional finned tubes.
In ongoing work, Inanlu and colleagues are developing new chemistries for copper etching to achieve the same heat transfer coefficient enhancement as for aluminum during flow boiling.
“Our work helps material scientists and engineers to discard the trial-and-error-based structuring mechanisms because we have an understanding of what kind of structures are beneficial for enhanced flow boiling,” Inanlu said.