Exploring the fundamental mechanisms of refrigerant flow boiling heat transfer enhancement on scalable micro- and nanostructured metal surfaces through the state-of-the-art borecopy method

Over the past decade, there has been a growing interest in the utilization of micro and nanostructured surfaces to improve the efficiency of two-phase refrigerant flow boiling systems, which has shown great potential for reducing energy consumption across various industries such as heating, ventilation, air conditioning, refrigeration, electronics cooling, power generation, and chemical processing. The aim of this research is to provide a comprehensive understanding of the underlying mechanisms of heat transfer enhancement in structured tubes during two-phase refrigerant flow boiling. The study involved conducting experiments on round tubes made of copper (Cu) and aluminum (Al) with 1-meter length and inner diameters of 4.57 and 4.72 millimeters. To gain an in-depth understanding of the enhancement mechanisms, an in-situ liquid borescopy method was implemented to directly observe boiling dynamics at different flow regimes ranging from bubbly to annular flow. The borescope was inserted perpendicularly to the flow at different lengths of the heated tubes. The structured tubes were compared to plain tubes based on their heat transfer coefficient (HTC) and pressure drop. The results indicated that the etched aluminum exhibited an impressive HTC enhancement of up to 420%, albeit with a 10% increase in pressure drop. Similarly, the etched copper showed an HTC enhancement of up to 50% with the same or lower pressure drop than the smooth tube. The study found that the primary mechanism responsible for the HTC enhancement is the increase in bubble nucleation sites that results from the introduction of structures on the surface. The size and depth of the structure pores were identified as critical factors that determine the performance of the various structures studied. Additionally, the surface structures were found to affect the flow regime, delaying the transition from bubbly-slug flow to annular flow, which was a secondary factor contributing to the observed HTC enhancements. In conclusion, this research presents promising surface structuring methods that can be applied to metallic tubes and channels to enhance refrigerant flow boiling with a reasonable pressure drop penalty. Furthermore, it develops a powerful borescopy method that can be extended for in-situ studies of flow boiling and flow condensation processes.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2023

Author dc:creator

• Inanlu, Mohammad Jalal

Contributors dc:contributor

• Miljkovic, Nenad

• Heat Transfer
• Flow Boiling
• Microstructures
• Nanostructures
• In-situ Borescopy

Statement dc:rights

• © 2023 Mohammad Jalal Inanlu

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/120525

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F120525