Graduation Semester and Year

2023

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering

Department

Mechanical and Aerospace Engineering

First Advisor

Dereje Agonafer

Abstract

The use of air-cooling as a thermal management technique in data centers has consistently maintained its importance, however its efficacy is limited to CPUs with lower power requirements. The expenses and energy use related to the air circulation process for high-power density racks are deemed excessively costly. The limits of air cooling have aroused concerns owing to its low specific heat capacity and poor thermal conductivity, which result in reduced effectiveness. Consequently, a substantial discourse has emerged about alternate and effective cooling methods that provide supplementary advantages, such as the recuperation of waste heat. In light of the limitations associated with conventional air-cooling approaches, several operators of data centers are embracing inventive methodologies to alleviate the consequences caused by escalating power densities. A solution gaining traction for high-power IT equipment is immersion cooling. A comparison between forced convection air cooling and single-phase liquid immersion cooling (Sp-LIC) highlights substantial advantages in the latter. The direct contact of dielectric fluids with components enables higher heat dissipation.. This technique enhances IT equipment reliability by safeguarding against pollutants and harsh conditions. Furthermore, Sp-LIC reduces capital expenditure (CapEx) and energy costs by eliminating the necessity for fans and computer room air handler units. The objective of this investigation are divided into two parts: thermal efficiency of immersed information technology equipment (ITE) and operational efficiency (overall life cycle of fluid, reliability of components, and serviceability). The thermal part of the study is further divided into 3 section: In first section, an in-depth numerical study is performed which explores multi-parametric optimization for heat sinks in forced convection within an open compute server design when immersed in single phase immersion fluid. Optimization at constant pumping power iterates pressure drop and thermal resistance minimization as objective functions. Varying fin count and fin thickness for a constant base thickness in aluminum heat sinks reveals the correlation between geometric parameters and objective functions. These results contribute to standard methodologies for optimizing heat sinks in immersion cooling. In the second section of the thermal study, a systematic empirical study is undertaken to examine the heat transfer and pressure loss characteristics of aluminum metal foams while immersed in a dielectric synthetic fluid. The research used metal foam specimens with core heights of 0.0127m (0.5 in), varying relative densities between 10.7 and 12.3 percent, and pores per inch (PPI) values of 5, 10, 20, and 40. The metal foams underwent exposure to several flow rates, heat fluxes and inlet fluid temperature. In the final section of the thermal study, the experimental methodology for obtaining the velocity fields using Particle Image Velocimetry (PIV) in a single-phase immersion tank is discussed. PIV is a non-intrusive optical measurement technique used to visualize and measure fluid flow velocity patterns. The velocity field data obtained from these experiments provides valuable insights into the fluid flow behavior and aids in understanding the thermal performance of the immersion cooling system. The second part of the research aims to investigate the impact of immersion cooling on server reliability. While immersion cooling excels in thermal energy management compared to conventional air-cooling, there have been limited studies on its reliability. The assessment of material characteristics, such as modulus and glass transition temperature, has substantial significance in the mechanical engineering of electronic components. The substrate, which is an essential element of an electronic package, significantly impacts the dependability and failure mechanisms of electronics, both at the package and board levels. The use of established material compatibility tests, such as ASTM 3455, is applied with appropriate adjustments in compliance with the design recommendations for immersion-cooled IT equipment outlined by the Open Compute Project (OCP). The main objective of this research is to investigate the effects of thermal aging on the thermo-mechanical properties of various substrate cores when immsered in single-phase dielectric fluids. This research examines the effects of aging on the substrate core by subjecting it to synthetic hydrocarbon fluid (EC100), Polyalphaolefin 6 (PAO 6), and ambient air for a duration of 720 hours. The aging process is conducted at two different temperatures, namely 85°C and 125°C. The complex modulus and the glass transition temperature of the substrate core are then determined and compared before and after the aging process.

Keywords

Data center cooling, Single phase immersion cooling, Heat sink, Metal foam, Thermomechanical properties, Particle Image Velocimetry (PIV)

Disciplines

Aerospace Engineering | Engineering | Mechanical Engineering

Comments

Degree granted by The University of Texas at Arlington

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Available for download on Thursday, August 01, 2024

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