Bubble departure characteristics in nanofluid flow boiling

In recent years, nanofluids have been an active area of research in many engineering applications, especially for nuclear reactor safety systems due to their enhanced thermal properties as a coolant. In this study, experiments were performed in subcooled flow boiling of water and water based alumina nanofluids with different nanoparticle concentrations (0.001,0.01 vol.%) in vertical annulus at heat fluxes varying from 30 to 230 kW/m² at 1 atmospheric inlet pressure and constant subcooling of 20°C to study the bubble dynamics by image processing technique. This paper presents the results of visualization experiments that were carried out to investigate the dynamics of vapor bubbles generated in water and nanofluid flow boiling. The experiments were performed under subcooled as well as nearly saturated conditions. Bubble images during flow boiling were captured with high speed visualization and analyzed by image processing software, IMAGE j. The experiments show that the bubbles depart from nucleating sites shortly after nucleation and slide along the heater surface. The bubbles grow while sliding, attain a maximum size, then lift from the surface sometime during condensation, and quickly vanish in the bulk liquid. (author)