Keywords

Thermal Management, Carbon dioxide utilization, Two-phase flow, Transonic flow, Compressible flow, Joule-Thomson effect

Subject Categories

Heat Transfer, Combustion | Mechanical Engineering

Abstract

High-power electronic and power-conversion devices now generate heat fluxes that exceed what conventional air- and single-phase-liquid cooling can dissipate, motivating the search for more capable coolants and cooling mechanisms. Carbon dioxide is an attractive candidate for this role: it is inexpensive, non-toxic, and environmentally friendly, and near its critical point its thermophysical properties change rapidly, which raises the heat-transfer coefficient. In addition, a large pressure drop produces strong cooling through the Joule–Thomson effect and phase change, so that the throttling element itself becomes a source of cooling. This dissertation studies the throttling, phase change, and choking of high-pressure carbon dioxide through microscale restrictions, and shows how these processes can be used to control two-phase flow and enhance cooling. All experiments are conducted on a closed-loop, high-pressure setup that delivers carbon dioxide to purpose-built microdevices under independently controlled inlet and outlet pressures. The mass flow rate, the pressures, the local temperatures from embedded and in-orifice resistive temperature detectors (RTDs), and high-speed visualization are recorded throughout. The diabatic experiments show that a microscale throttling element enhances cooling. The large pressure drop across a sudden contraction produces a strong downstream temperature drop, and the channel equipped with an orifice sustains lower surface temperatures and higher heat-transfer coefficients than an identical plain channel. The adiabatic experiments then characterize the choked, transcritical flow behind this behavior. The mass flow rate rises with pressure drop and approaches a limiting value at the onset of choking. A single nondimensional pressure ratio governs this onset for both liquid and gas inlet states, and the depressurization is accompanied by a large Joule–Thomson temperature drop of several tens of kelvin. Transient temperatures measured inside a micro-orifice, interpreted through a stagnation-enthalpy framework and supported by Mach number analysis, show that choking occurs inside the orifice. The flow there is governed by the coupled action of compressibility near the vena contracta, viscous dissipation, separation, the Joule–Thomson effect, and phase change. A liquid inlet produces cavitation and flashing, while a gas inlet produces condensation and an in-orifice normal shock. Optical diagnostics are also implemented to measure the fluid temperature and velocity directly.

Completion Date

2026

Semester

Summer

Committee Chair

Peles, Yoav

Degree

Doctor of Philosophy (Ph.D.)

College

College of Engineering and Computer Science

Department

Mechanical and Aerospace Engineering

Format

PDF

Document Type

Dissertation

Language

English

Download

Share

COinS
 

Accessibility Statement

This item was created or digitized prior to April 24, 2027, or is a reproduction of legacy media created before that date. It is preserved in its original, unmodified state specifically for research, reference, or historical recordkeeping. In accordance with the ADA Title II Final Rule, the University Libraries provides accessible versions of archival materials upon request. To request an accommodation for this item, please submit an accessibility request form.