The motion of air bubbles in tubes filled with nanofluids is of practical interests. Thus, this study focuses on the dynamics of air bubbles... Show moreThe motion of air bubbles in tubes filled with nanofluids is of practical interests. Thus, this study focuses on the dynamics of air bubbles rising in tubes in nanofluids. Many authors experimentally and analytically proposed the rising air bubble velocity in vertical tubes in common liquids when Capillary number is large. We report here a systematic study of an air bubble rising in vertical tube filled with nanofluids when the Capillary number is small. The presence of the nanoparticles creates a significant change in the bubble velocity compared with the bubble rising in the common liquids. We observed a novel phenomenon of a step-wise decreases in the bubble rising velocity vs. bubble length for small Capillary number. The step-wise velocity increases is attributed to the nanoparticles self-layering phenomenon in the film adjacent to the tube wall. The effect of volume fraction of the nanoparticles and the tube diameters are investigated. Also, we measured the film thickness and calculated the film structural energy isotherm vs. the film thickness from the film meniscus contact angle measurement using the reflected light interferometric method. Based on the experimental measurement of the film thickness and the calculated values of the film structural energy barrier, we estimated the structural film viscosity vs. the number of nanoparticles/micelles. Due to thenanoparticle film self-layering phenomenon, we observed a gradual increasing the film viscosity with the decrease in the film thickness. But, we found a significant increase in the film viscosity accompanied by a step-wise decrease in the bubble velocity when the number of nanoparticles/micelles decreased from three to two particle layers due to the structural transition in the film. Bretherton analyzed the rise of a single long air bubble at a very small Capillary number under the effect of gravity in a vertical tube filled with common liquids with a thick and stable film. However, Bretherton equation cannot accurately predict the rate of the rise of the slow-moving long bubble in the vertical tube in nanofluids because it is valid only for very thick films and uses the bulk viscosity of the fluid. But, we demonstrate that the Bretherton equation can indeed be used for predicting the rate of the rise of the long single bubble through the vertical tube filled with the nanofluids by simply replacing the bulk viscosity with the proper structural nanofilm viscosity of the fluid. Ph.D. in Chemical Engineering, May 2018 Show less
