Date of Award


Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Mechanical Engineering-Engineering Mechanics (PhD)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Amitabh Narain

Committee Member 1

Sajjad Bigham

Committee Member 2

Kazuya Tajiri

Committee Member 3

Sunil Mehendale


Controlled but explosive growth in vaporization rates is made feasible by ultrasonic acoustothermal heating of the microlayers associated with micro-scale nucleating bubbles within the microstructured boiling surface/region of a millimeter-scale single-channel heat exchanger (HX) – part of a typically multi-channel heat-sink. The experiments illustrate the achievement of a remarkably high stable heat flux (10 – 80 W/cm2) for a partial flow-boiling-based cooling approach and exceptional efficiency through active/passive enhancement in the vaporization rates into the heterogeneously nucleated micro-bubbles (through acoustothermal heating of their microlayers) and their removal rates (typically within the passive microstructured region of boiling). A controlled but explosive growth in evaporation and departure rates into the nucleated micro-bubbles during flow-boiling of 3M Novec HFE 7000 is enabled by the use of inexpensive layers of woven copper mesh to form a microstructured boiling surface/region. These microstructured boiling regions are subjected to nano/micro-scale amplitude ultrasonic (~1 - 6 MHz) and sonic (< 2 kHz, typically) vibrations – induced by one or two very thin (0.5 – 1.5 mm) ultrasonic Piezoelectric-transducers (termed Piezos) that are placed and actuated from outside the heat-sink. These Piezos, powered by specialized drivers, are positioned externally to the mini-channel heat sink. The ultrasonic frequencies are for sub-structural micro vibrations whereas the lower sonic frequencies are for suitable resonant structural micro-vibrations that assist in bubble removal and liquid filling processes (a type of vibrations actuated process for the multi-layered mesh microstructure, whose single layers were Wenzell filling under static pre-bonded conditions). The flow and the Piezos’ actuation control allow an approximately 5-fold increase in heat transfer coefficient (HTC) value – going from about 9000 W/m2-°C associated with microstructured no-Piezos cases to 50000 W/m2-°C at a representative heat flux of about 25 W/cm2 while the saturated (or slightly sub-cooled) liquid at the test-section inlet boils between 40 - 60 ℃, while heat-sink exit pressures vary within the range of 0.9 – 1.5 bars. However, when striving for desired high heat fluxes, certain operational challenges arise. These include significant pressure drops and instabilities within the heat sink, ultimately leading to either Critical Heat Flux (CHF) instability or similar issues at the system level across its components. The CHF suppression issues are separately addressed (not part of this thesis). The flow-loop hardware has been strategically chosen. The flow-loop control further ensures that the millimeter-scale rectangular cross-section flow channel, with its bottom heated-surface microstructured, operates in a way that there are no vapor-compressibility-induced flow instabilities at the system level. The reported experimental results, use a newly revised flow loop, the setup has a boiling test section, three Coriolis flow meters, absolute pressure transducers, temperature sensors, cartridge heaters, power meters, air/liquid-cooled condensers, two gear pumps, a reservoir, copper tubing, superheater, one-way valves, two-way valves, shut-valves, filters, etc. and several other accessory devices - including bonded Piezos, Piezo drivers, etc. Furthermore, the need for substantial total inlet mass-flow rate and extensive vapor volume flow rates to support such (or higher) high heat-flux values results in system-level challenges, which if not properly addressed (as is done here), would result in impractical pumping power requirements and undesirable device-level performances (such as condenser instabilities, cavitation in and around gear pumps and Coriolis Meters), including flow instabilities (in the absence of suitable flow controls). To address these concerns and attain stable operations with reduced pressure drops, a new experimental setup has been devised and reported here.

Available for download on Tuesday, December 31, 2024