Nucleate boiling process in nanofluids is important because of its potential in enhanced heat transfer. However, it is difficult to observe the boiling phenomenon due to the indistinct image. In this investigation, stable nanofluids was prepared by α-Al2O3 nanoparticles, 30 nm in diameter, and ultrapure water. The bubble behaviors in water were observed by high-speed CCD camera. Unique bubble sweeping phenomenon, existing in the upper and/or lower part of the heated wire, emerged due to the existence of nanoparticles. The experiment shows that the bubble-top jet flow phenomenon only exists when the small bubble returned to the heated surface, which demonstrates that it was the vertical Marangoni convection along the bubble interface that induced the jet flow. Meanwhile, flocculent clustering of nanoparticles can be observed to swirl at the bubble-bottom for low-concentration nanofluid, when the heat flux was relatively small. The SEM images of the nanoparticle deposition layers indicated increased thermocapillarity, but it seemed to delay the detachment of small bubbles from the heated surface. While n-butanol was included as surfactant, it promoted the nanoparticle deposition for low heat flux condition. The bubble behaviors were consistent with those of pure fluids and no bubble circling phenomenon was observed. The boiling curves were then depicted for alumina nanofluid with or without n-butanol. The boiling heat transfer in water was enhanced with increasing nanoparticle concentration. The boiling curves shifted right when increased the surfactant concentration in the nanofluid. It appeared that the surfactant-induced inhibited bubble growth and enhanced nanoparticle clustering in the near-wall region were the main reason for the shifting.

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