Date of Award

1-1-2012

Document Type

Dissertation

Degree Name

Ph.D.

Department

Mechanical Engineering

First Advisor

Corinne S. Lengsfeld

Second Advisor

Phillip Danielson

Keywords

heat transfer improvement, microchannel coolers, ultrasonic nsonation

Abstract

The motivation for this work is the need to remove waste heat from laser diodes and high speed transistors in processes which are exponentially increasing past 1 kW/cm2 as anticipated by Moore's Law. The hypothesis guiding the work is that ultrasonic insonation of micro coolers employed to dissipate these heat loads can improve heat removal. It is thought that the mechanism promoting the benefit is enhancement of the ability of the coolant to remove latent heat in two-phase operation by managing entrained bubble size near the cooler's exit so as to forestall flow reduction or blockage caused by large bubbles, wedges and slugs accumulating there. Insonation experiments to prove the hypothesis have been done on several micro channel coolers in the range 4-80 kHz to quantify improvement in heat flux removal. In order to understand how insonation would produce benefit in heat removal, a research effort was undertaken to study the affect of 5-30 Pa acoustic fields on air bubbles rising in small aquariums. This involved developing a Faraday cage shielded acoustic probe, along with a force-beam calibration tool, for measuring field levels near a strongly electromagnetic-radiating ultrasonic source. Experiments were conducted on columns of pseudo monodisperse, sub-millimeter diameter air bubbles in water, and other fluids using bubble generators optimized for this purpose. A numerical analysis model based on energy balance of the acoustic work done on a bubble resulted in predicting mass transfer flux, and in quantifying bubble shrinkage and growth when irradiated on either side of its resonance. The model, and experiments show that bubble populations can be predictably altered by ultrasound. The research was concluded by identifying and quantifying micro channel cooler performance change when insonated in the range 4-80 kHz. It was discovered that 28 and 58 kHz radiation of exchangers having hydraulic diameters spanning 0.02 to 0.6 mm could produce heat flux removal improvements of 5 W/cm2 in devices normally removing less than 30 W/cm2, a factor of 17%. Peak thermal resistance improvement approaching 60 % has been observed.

Provenance

Recieved from ProQuest

Rights holder

Peter Webb Higgins

File size

162 p.

File format

application/pdf

Language

en

Discipline

Mechanical engineering

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