Experimental investigation of effects of surface roughness, wettability and boiling-time on steady state and transient CHF for nanofluids

Abstract

Critical Heat Flux (CHF) is one of the primary design constraints in a nuclear reactor. Increasing the CHF of water can enhance the safety margins of the current fleet of Light Water Reactors (LWRs) and/or increase their power output. It has been shown that a suspension of nanoparticles called nanofluids in DI water enhances the CHF of water significantly. During boiling, nanoparticles in the nanofluid develop a coating on the heater surface, which is porous and hydrophylic, leading to a higher CH{F compared to water. One of the primary objectives of this thesis is to conduct an experimental investigation of the effects of three parameters (i.e. initial roughness, initial wettability and boiling time) on the steady-state CHF of nanofluids. Experiments with DI water served as the base case for CHF values, and experiments with nanofluid were done analyze their effects on CHF. Metallic heaters made of SS304 oriented vertically in a pool of test-fluid are used for experiments. The nanofluid used in the experiments has 0.01 v% ZnO nanoparticles in DI water. Multiple experiments were done to measure CHF of DI water (base-case) and test nanofluid for varied initial surface roughness (Ra), surface wettability and different preboiling times. Results indicate that compared to water, nanofluids enhanced CHF by an average of 77% (ranging from 25% to 150% for different surface and experimental conditions). It was also observed that the effect of nanofluids in increasing CHF was less pronounced if the initial heaters contained a superhydrophilic surface coating before use with nanofluids as opposed to the initial heaters being bare and uncoated. Additionally, the thickness of the nanocoating appeared to plateau after approximately 30 - 40 minutes of boiling time, and additional pre-boiling times of up to 8 hours did not have any effect on nanocoating development or CHF. The other objective of this work is to assess the applicability of nanofluids to accident scenarios in nuclear reactors, which are accompanied by rapid power transients. Such situations are simulated by rapidly increasing heat flux through the heater elements from 0 to CHF in short time frames of 1, 10 and 100 s. It was observed that for nanofluid tests, nanocoatings started to generate on the heater surface in as short time frames as 10 and 100 s, and the nanocoatings enhanced CHF, compared to DI water, by approximately 20%. However, for 1 s tests, nanofluids did not enhance the CHF, and nanocoatings were not detected at the heater surface. Additionally, pre-boiling the heater surfaces in the nanofluid caused a CHF enhancement for all three rates of power increase.