Flow Boiling in Meso-Scale Pin-Fin Coldplates

Coldplates are a critical component in various cooling applications, such as cooling of data centers, high-end central processing units (CPUs) and graphics processing units (GPUs), and the thermal management of power electronics. The rising demand for computational resources, high-power-density electronics, and compact-integrated systems is accompanied by thermal management challenges, necessitating the transition to more aggressive cooling technologies. This shift has led to a growing interest in two-phase coldplates/heat sinks as a promising solution due to their high heat transfer coefficients and improved temperature uniformity. Recent flow boiling studies have focused on fin-enhanced silicon microgaps and microchannels, given their importance in cooling high-power-density chips operating at ultra-high heat fluxes. However, the majority of the research on flow boiling in mini- and macro-scale configurations has been limited to channels and tubes, overlooking crucial geometries such as pin-fin-enhanced mini-channels. This literature gap, particularly the study of flow boiling in pin-fin-enhanced channels in the meso-scale, has limited the availability of physical insights, data, flow regime maps, and correlations that assist in the design of two-phase coldplates. Dielectric fluid flow boiling in a millimeter-scale coldplate was experimentally investigated under non-uniform heating conditions, representing realistic heat dissipation scenarios for high-power-density applications such as electronics cooling. Background heaters simulated low-dissipating-power components, while hotspot heaters mimicked high-heat-flux devices with heat fluxes reaching up to 1 kW/cm2. Flow visualization showed the role of localized nucleate boiling in enabling the two-phase coldplate to stably manage such high heat fluxes. High-speed visualizations provided detailed insights into the flow regimes and bubble dynamics, highlighting the effectiveness of two-phase coldplates in handling intense, localized heating. A parametric study on dielectric flow boiling in meso-scale pin-fin coldplates using HFE-7200 dielectric fluid was conducted to develop an in-depth understanding of the effects of pin-fin structure and flow parameters on two-phase heat transfer, pressure drop, critical heat flux (CHF), and flow regime transitions. The study covered seven pin-fin-enhanced geometries with hydraulic diameters ranging from 880 µm to 4.25 mm and provided valuable insights through over 840 high-speed visualization videos, identifying two distinct CHF mechanisms and three main flow regimes. The primary observed flow regimes included bubbly, slug, and annular/stratified flows. In meso-scale coldplates, buoyancy, viscous, inertial, and capillary forces all play substantial roles in shaping flow patterns and boiling behavior, highlighting the need to understand how these regimes differ from micro-scale geometries, where surface tension forces dominate. The influence of these forces on flow boiling patterns and characteristics is discussed in detail. A modeling framework was developed to assist in the design of two-phase pin-fin cold-plates by advancing the state-of-the-art Lee model. An enhanced phase-change Lee model, which accounts for both the degree of superheat required for the onset of nucleate boiling and the variation of saturation temperature with pressure, is presented. The computational predictions of this enhanced model were evaluated through experimental studies across scales, from dielectric fluid flow boiling in microgaps to millimeter-scale pin-fin-enhanced coldplates, with a focus on managing ultra-high heat fluxes and complex non-uniform heating conditions. The enhanced model’s predictions were initially compared to those of the original Lee model for flow boiling behavior in enhanced microgap. The enhanced model demonstrated better agreement with experimental data in the subcooled section of the microgap compared to the original model. In the mm-scale coldplate, the enhanced Lee model accurately predicted flow boiling under non-uniform heating conditions, effectively capturing the mountain-valley trend created by the hotspots. Additionally, the model successfully predicted the flow regimes, capturing the transition from stratified flow at low flow rates to bubbly and slug-churn flow at higher flow rates, aligning closely with experimental observations. The enhanced Lee model demonstrated high efficacy in predicting thermal-hydraulic performance and flow regimes. Experimental validation confirmed the model’s value as a powerful tool for designing two-phase coldplates, supporting the development of advanced thermal management solutions for high-power-density applications.

Date

2024-12-07

Resource Type

Text

Resource Subtype

Dissertation (PhD)

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