Flow condensation of low GWP refrigerants inside smooth and micro/nano-structured macro/mini channels

In-tube condensation of refrigerants plays a critical role in various systems, including domestic and industrial HVAC&R, high-power electronics, and automotive applications. Traditional techniques for enhancing refrigerant-side heat transfer coefficients involve micro-fins, ribbed surfaces, non-circular tube geometries, and microchannels. However, as heat source power densities increase, more scalable solutions with greater heat transfer potential are urgently needed. This dissertation investigates the use of micro- and nano-structured surfaces, fabricated on copper and aluminum surfaces, to enhance in-tube condensation heat transfer. The first section focuses on the fabrication and characterization of four micro- and nano-structured surfaces with varying roughness length scales. Literature suggests that pore size distribution and increased surface area at the micro- and nanometer scales directly contribute to improved two-phase heat transfer coefficients. Hence, a suite of advanced techniques is employed to assess the same. A detailed comparison of these methods highlights their accuracy across different length scales and structure types, revealing that the inability to resolve underlying features at a high resolution remains a major source of error. The next section introduces a novel surface characterization technique based on stereological data analysis. This method utilizes line roughness profiles obtained from Focused Ion Beam (FIB) Scanning Electron Microscope (SEM) images to estimate three-dimensional surface roughness. The technique achieves a resolution dictated by the imaging method, capable of resolving features down to ~10 nm. Additionally, the use of FIB cross-section imaging enables accurate inclusion of underlying porosity in micro- and nano-structured surfaces. The findings indicate that actual surface roughness can be up to five times higher than values measured using conventional techniques. Building on these insights, the structured surfaces are fabricated on the internal walls of copper and aluminum tubes and tested for heat transfer enhancements. Initial experiments demonstrate a heat transfer coefficient (HTC) increase of up to 150% at high vapor qualities, with a minimal pressure drop increase of ~8% in a 2.4 mm internal diameter (ID) tube using an etched aluminum surface. To explain these enhancements, a film-thinning theory is proposed, attributing the improvement to the wicking effect of the porous surface on the condensate layer. To further explore the limits of this mechanism, a comprehensive parametric study is conducted across a wider range of tube IDs, structured surfaces, and refrigerants. The final section discusses the challenges associated with testing mini-channels for thermal-hydraulic performance in flow condensation, identifying major sources of uncertainty such as saturation temperature drop, mass flux oscillations, and thermal resistance imbalances, along with solutions to mitigate these issues. Results indicate that while structured surfaces can achieve a maximum HTC enhancement of ~55%, this improvement is confined to a very narrow operational window. A detailed study of film thickness and flow regimes provides insights into the limitations of HTC enhancements using structured surfaces. Overall, this dissertation explores the potential of micro- and nano-structured surfaces for enhancing in-tube flow condensation of low-GWP refrigerants. It also critically evaluates state-of-the-art characterization methods and introduces a novel technique for improving surface roughness measurement accuracy by resolving the underlying porous layers of micro- and nano-structures. The findings contribute to advancing phase-change heat transfer research, paving the way for more efficient and reliable thermal management system design.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Singh, Bakhshish Preet

Contributors dc:contributor

• Miljkovic, Nenad
• Braun, Paul V.
• Wang, Xiaofei
• Cai, Lili

• condensation
• flow
• refrigerants
• low GWP
• GWP
• R1233ZDE
• R515B
• R1233ZD(E)
• stereology
• surface roughness
• roughness
• microstructured
• structured
• nano structured
• nano
• micro
• etched aluminum
• aluminum
• copper
• heat transfer
• enhancement
• refrigeration
• mercury intrusion porosimetry
• gas adsorption porosimetry
• micro CT
• SEM
• FIB
• AFM
• porosity
• laser confocal microscopy

Statement dc:rights

• Copyright 2025 Bakhshish Preet Singh

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/129551

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F129551