Date of Award

2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Committee Chair

D. Keith Hollingsworth

Committee Member

Hugh Coleman

Committee Member

George Nelson

Committee Member

Sarma Rani

Committee Member

Chien-Pin Chen

Subject(s)

Heat--Transmission., Heat engineering., Two-phase flow.

Abstract

Measurements were made in a highly confined gas/liquid diabatic flow in order to assess the hypothesis that mixing in the liquid phase in the bubble wakes is the dominant enhancement mechanism. Data were collected in a highly subcooled bubbly flow in a horizontal rectangular high-aspect-ratio minichannel of 1.29 to 1.48 mm channel spacing and a width of 23 mm. The channel was formed by an electrically heated metallic upper wall and an unheated transparent lower wall. Air bubbles were injected into a laminar liquid flow at either a single point or through a sintered metal plug on the lower wall. Liquid crystal thermography was used to measure the temperature distribution on the upper wall. High-speed images were recorded of the plan view of the bubble field through the lower wall, and images from a point downstream of the on-coming bubbles were acquired via a flexible micro-borescope. Air bubbles in the absence of phase change produced a heat transfer enhancement as high as 3.5 times the single-phase value. These heat transfer measurements were compared with the results of a previous study of naturally nucleated boiling in the same channel. Correlations were developed which collected both data sets to ± 26%. A computational model based on a transient conduction mechanism operating in the bubble wake collected both data sets to ± 18%. These conclusions support mixing in the liquid as the primary mechanism for heat transfer enhancement in a subcooled bubbly flow in a highly confined channel. Bubble images acquired with the micro-borescope are considered to be the first of their kind in the literature for a highly confined bubbly flow. These images document measurands that address bubble shape and confinement. The smallest bubbles were approximately spherical, medium-scale bubbles were asymmetric spheroids, and bubbles that spanned the channel were lozenge-shaped. A relationship between bubble height across the channel and diameter observed in the plane of the channel was demonstrated. All three bubble shapes displayed an approximately fixed ratio of bubble velocity to a scaling velocity based on the liquid velocity at the center of force of the bubble corrected for vapor blockage.

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