Thermal-hydraulic characterization of single- and two-phase immersion cooling for electronic modules

The continued growth in power density and integration level of electronic systems has created severe challenges for thermal management. High-performance processors, power electronics, and dense printed circuit board (PCB) assemblies routinely dissipate heat fluxes that exceed the capabilities of conventional air-cooled heat sinks and even many single-phase liquid cooling solutions. At the same time, reliability requirements and space constraints in data centers, defense systems, and transportation electronics demand compact, robust, and electrically safe cooling schemes. Immersion cooling in dielectric fluids has emerged as a promising approach to address these needs. By directly submerging electronic assemblies in a thermally stable, electrically insulating liquid, immersion cooling minimizes thermal interface resistances and enables both single-phase convective cooling and nucleate boiling at the device surfaces. However, the detailed thermal-hydraulic behavior of immersed, spatially distributed heat sources on a PCB remains poorly characterized, especially when complex flow geometries, buoyancy effects, and device-to-device interactions are present. This thesis experimentally investigates single- and two-phase immersion cooling of a representative multi-device PCB in a commercially available dielectric fluid. The board is housed inside a flow channel with adjustable wall spacing and operated in a closed loop that provides controlled flow rate, inlet temperature, and system pressure. Eight surface-mounted power devices are arranged in a rotated I-shape configuration, allowing the influence of upstream devices, channel width, and flow rate on junction temperature to be studied systematically. Single-phase experiments characterize the relationship between heat flux, temperature rise, and pressure drop over a range of Reynolds numbers and channel geometries. Two-phase tests extend this framework to boiling conditions, characterizing the onset of nucleate boiling, the progression of boiling along the device array, and qualitative limits where dryout or instability is observed. The results demonstrate that immersion cooling can provide low junction-to-fluid thermal resistance at moderate flow rates, but that the detailed flow topology and device placement strongly influence thermal uniformity and maximum temperature. In single-phase operation, decreasing the channel gap reduces thermal resistance but significantly increases pressure drop. Upstream devices tend to run warmer and influence downstream devices through both forced and buoyancy-driven recirculation. Under boiling conditions, the onset of nucleate boiling is highly non-uniform across devices, and local heat removal is enhanced where bubbly or churn flow is sustained. Based on these observations, the thesis proposes qualitative design guidelines for immersed board layouts, channel geometries, and operating conditions, and outlines directions for future work on more detailed flow visualization and modeling.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Mechanical Engineering

Grantor

University of Illinois Urbana-Champaign

Year dc:date

2025

Author dc:creator

• Farhat, Daniel

Contributors dc:contributor

• Miljkovic, Nenad

• Immersion cooling
• Flow boiling
• Two-phase heat transfer
• Power electronics cooling

Statement dc:rights

• Copyright 2025 Daniel Farhat

Language dc:language

en

Handle dc:identifier

https://hdl.handle.net/2142/132581

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/132581