Synthesis of CRUD and its effects on pool and subcooled flow boiling

Abstract

This work is dedicated to studying the effects of synthetic CRUD (Chalk River Unidentified Deposits) on pool and subcooled flow boiling parameters. Previous pool boiling studies have demonstrated the potential of porous, hydrophilic surfaces to lead to more efficient boiling. CRUD is a naturally occurring porous, hydrophilic layer that forms on fuel rods during reactor operation. As such, CRUD deposition may have large effects on critical heat flux (CHF) and heat transfer coefficient (HTC). An investigation of such effects was conducted as part of the CASL project by creating well-defined and characterized synthetic CRUD with parameters representative of reactor CRUD on indium tin oxide-sapphire heaters. The effects of synthetic CRUD on boiling heat transfer were then experimentally studied, focusing on heat transfer coefficient (HTC), critical heat flux (CHF), nucleation site density, bubble departure frequency, and bubble departure diameter. These heaters were tested in pool and flow boiling facilities in MIT's Reactor Hydraulics Laboratory. Synthetic CRUD was created using layer-by-layer deposition of 100 nm silica nanoparticles to form porous, hydrophilic thick films. Photolithography was used to manufacture posts that were then dissolved to create characteristic boiling chimneys. Features such as thickness, wettability, pore size, and chimney diameter and pitch were verified to be representative of reactor CRUD. Silica nanoparticles were used as a surrogate for reactor CRUD nanoparticle materials (iron and nickel oxides) since they create more stable films. To ensure accurate modeling, independent of material, 10 nm silica nanoparticle and 10 nm iron oxide nanoparticle boiling tests were conducted and found to be similiar. During testing, IR thermography and high-speed video (HSV) are used to obtain two dimensional temperature profiles of the active heater area to quantify properties such as HTC, nucleation site density, bubble departure frequency, and bubble departure diameter. The bubble parameters follow expected trends with mass flux and heat flux. IR/HSV flow data (Chapter 6) has shown that HTC increases with the presence of chimneys, increasing thickness and increasing chimney diameter. However the HTC is relatively unaffected by the chimney pitch and is decreased by the presence of an LbL layer. The boiling curves and CHF data obtained from pool boiling experiments with iron oxide and silica oxide nanoparticles with and without chimneys also confirm these trends. The largest HTC is observed in the case of uncoated heaters, followed by heaters with chimneys, with heaters with an LbL layer without chimneys having the lowest HTC. From pool boiling data, the benefit of a CRUD layer is observed in the enhancement of CHF. The flow boiling trends are further supported by the combination of measured basic bubble parameters according to the heat flux partitioning model. The statistical significance of these trends varies with mass flux. The data generated here may inform advanced models of boiling heat transfer and/or validate existing models.