Conference Dates

July 1-6, 2007

First Page

275

Abstract

Studies of iron oxide deposition on Alloy-800 heatexchanger tubes have been part of a continuing research program at the University of New Brunswick; the present work formulates mechanisms for the effect of bubbles on deposition in water under boiling conditions. To supplement results from earlier deposition experiments in a fouling loop at UNB, measurements of bubble frequency and departure diameter as a function of heat flux were performed. High-speed movies of bubbling air/water systems indicated that a pumping action moved particles from adjacent areas at the surface to bubble nucleation sites. To explain the observations, the model considers deposition and concomitant removal. Deposition includes microlayer evaporation and filtration through the porous deposit. The deposit is sparse in the first stage, when the dominant process is microlayer evaporation including particle trapping and pumping, creating spots of deposit. Filtration becomes more important as the deposit thickens to a stage when microlayer evaporation becomes negligible. Chimney effects then control. Turbulence due to detaching and collapsing bubbles affects removal. In sub-cooled boiling, collapsing bubbles generate enough turbulence to maintain much of the deposit labile while in bulk boiling bubble detachment from the nucleation site is dominant and a smaller portion of the deposit is labile and subject to removal. Model predictions are presented and shown to agree quite well with experimental data.

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