[CCoE Notice] Ph.D. Defense Announcement

[Khator, Suresh]

Ph. D. Dissertation Defense

A Combined Photographic/Thermographic Study of Highly Subcooled
Flow Boiling in a Narrow Channel

Arif Bilal Ozer

Department of Mechanical Engineering

9:00 am, December 1, 2010 (Wednesday)
Mechanical Engineering Large Conference Room (2nd floor)

Committee:
Dr. D. K. Hollingsworth (Chair), Dr. Larry C. Witte, Dr. Stanley J. Kleis, Dr. Dong Liu, Dr. Paul Ruchhoeft

ABSTRACT
Minichannels (those with channel spacing near 1 mm) offer an attractive scale range for technological applications of two-phase cooling. They are small enough to create large heat fluxes, but large enough to remove significant quantities of heat, and they do not present the same degree of fouling and maintenance issues anticipated for applications of true micro-scale channels.  This study applies liquid crystal thermography in concert with high-speed video imaging of a two-phase flow field to study highly subcooled bubbly flow within a millimeter-scale channel. The experiments were conducted with a high-aspect-ratio (1.2*23 mm) rectangular, horizontal channel with one wide wall forming a uniform-heat-generation boundary and the other designed for optical access to the flow field.  By using a thermochromic liquid crystal coating on the uniform-heat-generation boundary, wall temperature variations were obtained and the heat transfer characteristics were investigated. The experiments were focused on high inlet subcooling, typically 15-50 *C. The test surface temperature increases downstream until boiling initiates when a group of nucleation sites activate at a "boiling front".
It is commonly thought that cooling of the surface downstream of the front would be dominated by active nucleation sites.  However, few active sites were observed downstream of those near the current position of the boiling front.  This observation provides strong evidence that it is the action of bubbles sliding in the confined channel that causes the enhancement of heat transfer, not the extensive nucleation of bubbles. A quenching/diffusion analytical model was developed to describe the heat process associated with sliding bubbles.  The model is based on a transient conduction hypothesis.  It is able to capture the elevated heat transfer rates measured in the channel without the need to consider nucleate boiling from the surface or microlayer evaporation from the sliding bubbles.  Bubble speed and size enter the model through a Fourier number associated with the bubble passage frequency.
The insight gathered from this experimental investigation provides the basis for a better understanding of the physics of subcooled bubbly flow in minichannels, and this understanding should lead to new models of this regime.


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